KSA-150 and KSA-250

It is unclear whether the KSA-150 and KSA-250 used a managed variable bias system.  Reading the manuals carefully, it sounds as if there may be some kind of bias management, but not as sophisticated as Sustained Plateau Bias which officially appeared with the KSA "S" models.

If KSA-250 has multiple bias levels, John Atkinson's famous calculation of the Class A power of a KSA-250 is only true for the lowest bias level, not the higher ones.   (In the first version of this post, I was sure KSA-250 had multiple bias levels.  Now not at all sure.)  I'm guessing the relatively cool running 250 had variable bias whereas the much hotter idling 150 might not have.

The Mark Levinson amplifiers like 331, 332, and 333 definitely use some kind of managed bias system to provide more Class A power on demand.  It says clearly says so in their manuals but only calls it "proprietary" and "developed by Madrigal Labs" and doesn't give it a fancy marketing name.

The KSA-250 and KSA-300S were much larger than the FPB 300 and approaching the FPB 600 in transformer capacity and weight.  The FBP 300 chassis actually seems like the successor to the KSA-200S chassis.  So the successive models appear to be in two groups (though within each group, models do get slightly heavier and certainly more sophisticated over time):

KSA-150

KSA-200S

FPB 300

FPB 400cx

(this group always had something like 3kW transformer)

and

KSA-250

KSA-300S

FPB 600

FPB 700cx

(this group always had something like 6kW transformer)

As time went on, Krell was either getting or at least advertising more power from almost the same amount of iron and aluminum.  The old 150 could double down all the way to 1 ohm and get 1200W.  The FPB 300 doubles down to 2 ohms…where it gets 1200W.  Actually the FPB is said to get more power into 1 ohm, but the 1 ohm power is not rated and it is certainly not doubling down to 2400W into 1 ohm.  I think I've heard that something like 1600W may be available into 1 ohm.  Ultimately, the FBP 300 and FBP 400cx have the same transformer, output transistor assembly, and chassis and the 400cx is rated at 1600W into 2 ohms.

If Krell ever advertised Sustained Plateau Bias for the Evolution models..they don't seem to do so any more.  I thought they did at first, perhaps when Dan was still with the company.  It doesn't seem like they were doing full rail regulation anymore, just a fancy kind of filtering.

I think the sustained plateau bias on my FPB 300 is great, though it could be smarter still and I'd still like a manual indication and control as with the "S" models.  The S models lacked the sophisticated rail voltage regulation, and I think they had slightly higher levels of distortion than FPB, but I like the ability to control bias levels.  If I could preset/hold the lowest bias level, I wouldn't need a second "summer or background music" amplifier.

200 degrees C

The schematic for the Krell 200S indicates MJ15024 and MJ15025 power transistors, although I have read that for best results you need the custom versions Krell actually ordered from Motorola which are only similar to MJ15024/25 but better.

The specs say the 15024 has a maximum Normal Operating Temperature of 200 degrees C.  (That is like 2.4 times higher than any temperature I've actually measured on the heatsinks.)

Device dissipation is 250W per device, and 16 amps.  The derating for power is 1.43 watts per degrees C, so at 80 degrees C the max dissipation would be 171 watts.

I suspect the transistors used in my FPB 300 are at least this good, and I believe there are at least 10 transistors per channel (and maybe 20 or more).  Therefore, even at 80 degrees C, one channel could be dissipating at least 1700W (and that's not counting the power consumed in regulated power supplies, drivers, etc).  and there are 20 outputs devices in each channel.  Therefore, even at 80 degrees C, each channel is capable of dissipating more power than the power supply could possibly deliver, 3400 watts.  And that's not counting the power supply and driver transistors, etc.

Bottom line is that this amplifier is safe to operate at the full maximum temperature of 80 degrees C, which is about 180 degrees F, and still deliver full power.

In my most recent testing, it does not seem like the left channel is rising to the next plateau.  Instead, it looks like it is simply drifting high.  The plateau bias starts it at the right level, but then it slowly drifts up, more like 50-100W than 150W.  Of course if it ultimately drifts up to 80 degrees C, plateau bias computer puts it back down again.  After an hour of idling with inputs connected to the Audio GD Dac, I was seeing about 500W consumption, with one channel being about 145F and the cool one being 115F.  Actually, the hotter channel is probably in a better temperature range for good sound.  After a few hours of playing the amp was happily consuming about 600W, which is perfectly fine, with  155F/135F temperatures, also fine, once again, the drifty hot channel actually being more linear.  Transistors tend to get more linear when they are hotter and/or run with greater current.

If the amp is kept playing music, it might cycle through the 180F max but then settle down to 155/135F with about 600-700W continuous consumption, which is about optimal.

If I could set the bias level to level 2 (600W) that would be nice, but I'm effectively getting about the same thing, just with an occasional higher reach that warms the amp up quicker.

That Class A Feeling

I've been getting that feeling.  That same feeling I got when, as a kid, I wandered into Mel Shillings' fabled Music and Sound, which had relocated…into my neighborhood in Southern California!  They had the huge amps, the planar speakers that created extraordinary stereo images (about the first decent stereo I had ever heard), and something else I forgot about until just now…

Air Conditioning!  Running my Krell with typical power consumption in the 600-900W range is generating a lot of heat.  And, since my listening room is the living room, which happens to have the thermostat, almost as soon as I turn the amp on the Air Conditioning comes and and pretty much stays on as long as the Krell is running.

Well I'm sure the Krell loves being the center of attention again, right in the center of the room, getting the main blast of cooled air from the ceiling vent on the other side of the room.  But while it's bracing to sit down into the stream of cold air, and brings back memories, it's also expensive and it even seems to make me a little sick.  And when I'm done listening the rest of the house has been chilled about 3 degrees below normal.

The problem here that I hadn't quite solved when I tried moving the Krell into the corner--shielded from cold air by the couch and then thought it was overheating--is that the thermostat is right on the opposite side of the same room, and the fully air conditioned air hits the Krell and practically bounces right back to the thermostat.

Back then I did get a new thermostat installed…but it never quite satisfied me wrt programming (it was basically impossible to program on the wall, that's another story for my Computer Critical blog).  What I really wanted then, and now, was a portable "personal" thermostat I could locate to another room.  The perfect other room would be my bedroom, which often doesn't get warm enough in winter, but isn't much bothered by heat from the Krell.

Well they do make such a thermostat system now, the Prestige IAQ 2.0 System, which has a portable thermostat called the Portable Comfort Control.

OK, I could just turn the thermostat up, but I hate it when I forget to do that.  And what if I'm running the Krell all day???  I think thermostat in the master bedroom is the perfect place…I could just leave it there always except during parties I'll put in back in the living room.  Or if a friend is doing lots of baking in the kitchen.

When the AC isn't running, the Krell could still use some air movement, so I just bought the Econo-Cooler from Cool Components (these aren't sold by many online stores btw).  I've generally hate fans in equipment.  But I'm OK with fans I can dial down to the lowest speed myself…and if it's noisy I'll try something else.  I only wanted to start with one Econo-Cooler (CP-EC) but it didn't seem like I could order just one and get the full ps-300 power supply which has many possible voltage levels.  They also have control modules with digital temp readout, thermostatic control, and triggers, but I'm worried about the difficulty of getting the thermostatic control to work properly.  I don't want to put it directly on the Krell--that might fry it (the thermostat).

Another thing is to have connections for my "backup" amplifier, the Aragon 8008 BB.  It doesn't generate quite as much heat in operation (about 1/2 the amount of idle heat) and it has now been moved into the corner where it probably won't affect the thermostat as much.  I'm thinking that in the long run I want to have it hooked up through my QSC ABX box (but not directly) so I can actually do ABX tests.  But I don't trust the speaker relays in the ABX box (nor the connectors, nor the box itself) so what I'd do is use the box to switch a DC current which would then control a pair of 300V 40A DBDT relays right behind each speaker.  For decent ABX tests, I might eventually have to have my own computer control that "works" the relays a bunch of times so you can't figure out A/B just on the relay sound (even the QSC ABX box itself has this problem a bit…with some practice you can hear the difference in the relay sound).

Well I looked at lots of things, starting with pins that could be directly inserted into the cheap 5-way binding posts at the back of the Acoustats (still one of the best options).  Finally I decided to use a familiar old solution--banana plugs.  I'll "stack" the plugs (plug one plug into the holes on the other) to connect the speaker to the A amplifier or the B amplifier.  The Acoustats will remain connected with bare wire that goes to one plug, and so will the Krell.

(BTW, no use in plugging bananas directly into the 5 way posts on the Acoustats.  Heavy banana plugs don't take long to fall out of those cheap posts, and are often hanging part way out.)

The only local Radio Shack had hardly anything anymore (sad!!!) so I ordered these from Parts Express.

OK Class, how much Class A Power ?

I've mentioned before in this blog how manufacturers routinely inflate Class A power claims by various technically correct but misleading means.  I've also discussed the correct calculation, which is 2RII where I is actual idle bias and R is the load.

The Krell FPB 300 has different idle bias depending on situation.  Temporarily I have seen it consume about 1200W continuously for 30 minutes or so after turn on.  But over a whole day, it can't sustain much more than 700W, which seems to rest on an second bias plateau around 600W.  The bias circuit kicks everything down when the heatsinks start getting above 180F.

For 600W idle, that's 300W per channel, and about 2 amps across the 170V maximum peak-to-peak voltage.  So 2RII gives us 32W into 8 ohms and 16W into 4 ohms.

An amplifier with 1/2 the voltage swing but dissipating the same power would be running 4 amps in consuming that 300W, and therefore have 4 times as much Class A power.  The price for having more maximum power on tap given the same dissipating capacity is that you cannot have as much Class A power.  People would like to ignore this but it is basic math.  If you want more peak watts from the same quiescent consumption, you have to give up the Class A Watts.  Also, Class A Watts do not easily double down as they are limited by current in most amplifiers.  So Class A Watts will halve into each halving of impedance.  The famous Mark Levinson ML2 did not do this because the amplifier was voltage limited at each lower impedance down to 2 ohms--so it did indeed "double down."  But it was not practical for many people, and especially me, because the maximum power into 4 ohms was 50 watts (25 watts at 8 ohms).  And you get that 50W from a single channel mono block about as big and heavy as my Krell.

Here are the Pass Labs .8 specifications.  My Krell can continuously dissipate about the same as Pair of Pass Labs XA60.8's, which have a combined ship weight of 214 pounds.

Cook that Krell

I was all worried that one channel of my FPB 300 was running hotter than the other.  I shouldn't have been, my Krell, which was sitting in the corner of the living room for 4 years (during my recent building boom) waiting for me to get around to prepare for getting fixed, was operating normally.

The FPB *has* to heat up to the normal max temperature to trigger something like recalibration.  It then sets itself back down to lower, usually in the 600W bias plateau, but if the amp has been driven hard enough it will get set back to the 300W minimum bias plateau.  (My wattages are the actual amplifier power consumption…what is available as Class A power is different and will require a long explanation later.  What has revolutionized my understanding of the FPB amplifier is watching it's power consumption from moment to moment over hours of listening.  Now I think I understand it.  It makes very good use of the ultimate limited resource--the heatsink area of the amplifier.  You could say it "optimizes" the resource for the requirements of the situation.  I am happy to be even more a fan of Sustained Plateau Bias.  It's a way to approximate a much larger non-adjusting Class A amplifier, compare my FPB 300 with the metal of a pair of Pass Labs XS300's, which have similar power into 2 ohms, but use about 6 times as much metal.)

As you are driving the amp hard, it will hit the top normal temperature, which appears to be about 165 degrees F, then get set back, and this may happen over and over.  Ultimately, when driven hard enough, the amplifier becomes a high bias Class AB, with 300W minimum bias, and there is no more setback, consumption just falls immediately after every peak.  Many people may not drive the amp that hard--I have the least sensitive loudspeakers ever sold, and with the most difficult to drive load.

Once both channels have hit max temperature once or twice and been set back, they begin to have fairly identically, so I am wondering if hitting max temperature is a necessary thing to keep the amp working right.  Moving the amplifier to a different location is going to cause differential air flows…which has a profound effect on passively cooled heatsinks, and a new recalibration--hitting max temperature--will be necessary before things are balanced again.

As I was using my amplifier for awhile, the left heatsink had it's bias setback, and before long it was the cool heat sink compared to the right, which took it's time to get to 165F, but after some harder playing it did too, and then the right channel got the bias setback.  The power consumption fell to no less than 600W, but during ever more brief pauses in the music it would ramp down to 300W.

I am just guessing that a recalibration occurs when the bias is set back.  It would make sense for the amp to do its recalibration on thermal setback, as then it is changing the bias in a big jump anyway.

165F is a fairly high temperature for audio amplifiers, but it is nothing in the world of power inverters, for example, which require similar parts.  The Krell was specifically designed to run this hot as part of it's normal behavior, and as a way to get more class A power from a limited amount of metal, even if it's not usually considered a limited amount of metal.

Update: I'm seeing 175F after a day of hard playing.  I think the ultimate setback point is 180F as it was for the KSA-x00s amplifiers.  It the sensors see that, the amp kicks down the bias.  When the heatsinks have been cooler, they don't measure the temperature as quickly as the sensors do, so the rise in setback point actually demonstrates the sensors read quicker than my measurement of heatsink temps, as well as the actual temperature being higher than I thought.

It's only to be expected that if there is a setback point, eventually things will hover around it.  So 180F is a normal Krell temperature.  If the amp shuts down, or if I see 200F, I should call Krell.

Power is in the 700's with more than a second silence dropping to 600W.  Still takes about 20 seconds to get pushed down to 300W, so the nominal bias level is 600W consumption now.  So this is a long term sustainable bias level.  Actual wattmeter readings are typically in the 700's.

That bias level is comparable to A pair of Pass Labs mono XA60.8's, each of which weighs about the same as the Krell, and continuously dissipates 375W per mono block chassis.  So Krell is made to cook at max temperature, Nelson Pass designs his amps to have a more moderate temperature--at the cost of twice as much metal for similar performance.  And of course the XA60.8 has no where near the voltage or current capability of the Krell either, even as a heavier pair of amps.

The first XA amp which beats the FPB voltage spec of 60V is the XA200.8, which has 62 volts.  Only the XA160.8 and XA200.8 (and XS300) have 50A current rating.  Krell doesn't give a current spec, but Martin Colloms measured the FPB 600 as having 117A and described it as arc welding.  The XA200.8 weighs 177 pounds (per mono block) and dissipates 750 watts per channel.  I pushed my FPB to 1400+ watts draw, but only for a dozen seconds as it was already warm.

Krell FPB amps are the poor man's Pass Labs…   The Eco-Freak Pass Labs…

People will say Pass Labs is fundamentally different, all about simplicity, etc.  The FPB amplifiers have a no-feedback output which is simply an array of emitter followers in parallel and differential push-pull.  That's simple and elegant. Stuff ahead of that is a bit complicated, of course, including but not limited to the bias system.  But nevertheless the FPB specs bandwidth at 240kHz and 0.1Hz.  The distortion is spec'd at 0.02% at 1kHz and 0.15% at 20kHz and that's for full power.  Pass typically specifies 100kHz and 1% distortion.  (Of course that means he's too indifferent to specs to bother to measure distortion just before it begins to rise near clipping.  But it won't out-do the Krell either.)

I keep forgetting this, but Krell specs the full output (300W) input level as 2.35V RMS, but I continue to believe that more input is possible to reach the full peak output implied by 60V, 450 watts into 8 ohms.

Thinking about Sustained Plateau Bias

I am a big fan of the Krell Amplifiers with Sustained Plateau Bias, especially the later models which implemented fully regulated power, such as my FPB 300 and on up.

But how useful is this SPB, really?  It's not clear how much this has been actually copied (of course, SPB and other acronyms are Krell trademarks, and different from the underlying patent, which might be licensed so long as it is not named the same…and could be in widespread use as far as I know).  It does seem possible that many big audiophile transistor amps use a managed bias scheme of some sort, not unlike how the FPB does it with a microprocessor, whether they fall within the claims of the patent or not.

It's incredible in my FPB 300!  Reasonably intelligent.  It gets standby and idle bias perfect from the get go.  It holds on to the 300W consumption idle for low background music.  Then the next bump up is at least 450…if both channels 600W consumption.  Then consumption seems to go up in about 100W increments as needed up to 1400.  I can't test any higher than 1400.  It jumps up quickly, takes longer to go back down.

If the amp is cold, it may hang on to 600W.  If amp is warm, it gets assertive about pushing consumption back down.  It amp gets too hot--above 165C--it pushes down consumption fast even if the loudness hasn't changed.  But in my testing, it has never pushed consumption down below 600 during loud passages for this reason…only during low levels or silence.  During silence when it wasn't too hot, bias lowers back to idle bias in less than a minute, how fast depending on how hot it was.

It is precisely during episodes of the amp pushing down bias due to high temp that it tends to make the chassis ping noise.  Or sometimes if amp is just left to cool off after hard playing.  But it has been quite quiet playing tonight.

So while it's not "adaptive" (so says Krell in owners manual) it is actually somewhat adaptive.  There are limits to what you get…and they are not necessarily preset limits such as buying a 30W Class A amplifier and having 30W into 8 ohms, 15W into 4 ohms, etc., of Class A power.  No with the KPB 300 you can get, momentarily, tons of Class A power.  But if it heats up too much, you get less and less Class A margin, and ultimately you may be forced to endure Class AB.

This is a pretty good deal, actually.  In fact it makes a lot of sense for the amp to work this way, it gives you all the Class A power the heat sinks can support, and all the Class AB power the transformer can support.

By letting the bias go lower during less demand, and at idle, heat sink margin is saved for the louder passages for the amplifier to remain in Class A.

It's a great idea I think.  It's only a disappointment if you believed every word of the hype, that you were getting 300W of Class A power into 8 ohms (and 1200W of Class A power into 2 ohms), and as much as you want at any time, always Class A power.

That would be impossible, without the actual old fashioned Class A amplifier that dissipates 3-4 times the Class A power and has no more power at all than that (because if it has more Class AB power above that, the 3-4 multiplier becomes even larger).

So 1200W in two channels is 2400W means the amplifier would have to dissipate at least 7200W to be this hype dream amplifier.

Actually the FPB 300 heatsinks seem pretty comfortable in my air conditioned room dissipating about 1000W continuously (with normal breaks).  Above that too much, and I get the thermal system cutting back the dissipation to 600W or so.  Then it would gradually climb back up to 1000 again.

So with the thermal setback…it actually is adaptive, and with many levels on tap, it adapts to any room conditions, giving you the maximum Class A power within the limits of the situation.  It doesn't happen to be close to 1200W per channel for sure…but it's not chopped liver either.

It will be interesting to do that calculation…what actual Class A power into 8 ohms (and half as much into 4 ohms) from the apparent comfortable maximum of 1000W continuous dissipation.

Earlier today, I was enjoying mine right now at about 305W AC consumption, FM radio station KPAC playing local artists, level set to 70 (-33.8dB but other reductions apply in home distribution), heat sinks r/l 113/116F.  At idle power consumption was 75W, heat sinks r/l 87/83, ambient 76 with AC running (blowing right over amp).  Basically now it's a high bias AB amp with likely less than 10W Class A power into 4 ohms, but if power exceeds something it rises to higher levels of class A power as much as it needs to and can sustain.  I'd be consuming more power with a pair of XA30's I believe, and though they have more Class A power than I do now continuously, I can have way more on demand, and way more power overall.  With a low level playing it can be very stable at 300W consumption.

Krell and it's bias

My Krell FBP 300 seemed to be approaching a stable 114 degrees F or so at idle in both channels over a couple hours.  That didn't seem to change playing KPAC at Tact level 60 (about -44dB).  When I cranked the level up to 80, one channel rose to about 122 but the left channel shot up above 150 degrees F.

I rolled the volume back but the hot channel didn't seem to cool down much.

I reversed both input connections and speaker connections.  The problem was same as before and in the same channel of the amplifier (left).  This time, when I turned the level down, at first the left seemed to get lower, down to about 140.  But then later it shot up to 155 at the hottest point.

This is probably wrong, there shouldn't be that imbalance.  But what is the correct temperature?   Apparently many Class A amps run their heatsinks around 120-130F.  IIRC Nelson Pass advises the low end of that for the homebuilder (45-50C…the F5 is designed to run 25 above ambient or typically 50C, while Pass Labs amps are spec'd to run at 53C, where obviously everything has been carefully engineered for that and higher).  My Aragon 8008 BB used to idle in around 132 until I put the Super feet on it, now it idles about 115F, and it's not even considered a Class A amplifier.

But lookee here!  The Krell KSA-100S manual describes Sustained Plateau Bias and says that if the heatsinks go above 80C (!) the amplifier cuts out the two highest bias levels until it falls back to "normal" temperature (not defined).

So that's saying in a way that 80C is a kind of "normal" condition that the amplifier adapts to (I believe many other amplifier would shut down at 80C or even lower).  Well we see at least one way Dan the Man demanded special versions of his power transistors from Motorola.

If a 100S can rock with 80C so can my 300 I believe.  So this isn't any reason to panic, but nor is it reason not to get the amp fixed asap.  The temperature difference shouldn't be there.  Actually the cold channel might even be better a bit hotter…but that was specifically the channel repaired last time (IIRC) for distortion.  It's probably optimized and so should the other channel be.  When optimal the FPB amplifiers have amazing low distortion at all levels and all audible frequencies.  The FPB 300 was measured by Martin Colloms as having a 300kHz power bandwidth.

On this blog here I was posting about the temperature imbalance in September 2010, after having had it repaired in 2009.   The same blog mentions microprocessors on each channel's main board, and how they need to be reset, and how the procedure is simple if only someone would reveal it.   The consensus seems to be Get the Amp Fixed!  I wasn't actually the original poster, but chimed in with my own similar problems.

On this blog here a person reports 79.6F in standby mode, idling/background music at 97F, 30 minutes serious listening as 132F for the FPB 300.  Lots of other interesting comments, including the guy who says the normal range for inverters is up to 80C, and the unit shuts down at 90C.

When is Class A "Real"

OK, after much heart wrenching deliberation, I'm planning to keep the Krell FPB 300.  I will get it fixed.

I'd really decided this on my own, as I was taking pictures of the Beauty it all came back to me.  But I've been reading blogs to shore up my decision.

And one of the things that ticks me off is how glibly people declare that their KSA-250 or whatever is a True Class A amp, and the Sustained Plateau Bias amps like the FPB 300 (mine), the 300c, and the 400cx are Not True Class A.

People will even gloat about their KSA-250 "250 Class A watts into 8 ohms, 500 Class A watts into 4 ohms, 1000 Class A watts into 2 ohms", etc.

But THEY are the ones who have it all wrong.  Actually the FPB 300 is far more of a True Class A amp than the KSA-250.

John Atkinson did the calculations long ago based on the bias current.  The KSA-250 puts out 25 watts of Class A power into 8 ohms.  So indeed it is a True Class A amplifier if you never use more than 25 watts and your impedance is no less than 8 ohms.  But that's not how people think about it, they're thinking 250/500/1000.

Furthermore, if the amp puts out 25 watts of Class A power into 8 ohms, and that's limited by the bias current and not the max voltage, then it necessarily puts out only 12.5 watts of Class A power into 4 ohms, 6.25 watts of Class A power into 2 ohms, and 3.125 watts of Class A power into 2 ohms.  And we're talking about a monstrous 180 pound amplifier.

Correctly speaking, the KSA-250 is a High Bias Class AB Amplifier.  The FPB 300 is a Sustained Plateau Bias Class A amplifier.  It is always and by necessity operating its output stage in Class A, even if the Class A Bias level is continually changing.

Operating the output stage in Class A (as compared to Class AB) has certain key advantages.  For one, it permits the amplifier to operate well without feedback on the output stage.  Class AB amps cannot do that.

Operating without feedback means that reactive loads cannot induce ringing or similar interactions with the amplifier.  I think this is key for me, as my electrostatic speakers are a very reactive load.

Sorry but at the end of the day, I see nothing especially interesting about large Class AB amplifiers, and notably the Krell ones.  If I were to get a large Class AB amplifier, I'd likely get a JC1.  John Curl knows that game as well as anyone, and even his lowest priced amplifiers are excellent designs for their cost.

Sadly I feel the same way about Dan D'Agostino's new Momentum amplifiers.  Just another large Class AB amplifier, as far as I can tell from published technical info (which may well be the limitation of the published info which may be limited for trademark reasons…and the 1W low idle power suggests a managed bias system of some kind…and also makes one wonder if Dan isn't seeking to reverse his earlier crimes).  It may be a fine amplifier…but I have no information that technically says it would be better on my 1+1's and one little factoid that suggests it might be worse…it appears to have "some" feedback.  The FPB 300 is claimed to have no loop feedback around the output stage.

If the Momentum isn't a Class A amp, I don't know of any reason to not prefer a pair of JC1's (which have much better and more published specs), except for the copper chassis (which I think is cool, but not so much the meter).  And neither of those amps qualifies like the FPB on the Zero Feedback score.

Given the most reactive load of all, something like the FPB with zero feedback is called for.  And in fact the FPB sounds terrific on my Acoustats.  All the edginess and harshness is gone, but the detail and layering increases.  The FPB is unique among all other amplifiers I have tried in this way.  Some people describe the FPB as sounding "dark."  What I feel is that other amplifiers on my 1+1's have glare, and the FPB uniquely removes that.  I'm not sure if any other amplifier would do as well, at any cost.  So I'd better keep it and repair it until I find something else as good…there might not be…though I don't have any reason to believe a 300c or 400cx wouldn't be still better.  Though on that score, I wouldn't think the improvement to be all that necessary from those even pricier models, since those don't especially have to do with the zero feedback output stage (which remains about the same, though the 400cx increases the available power…mainly by programming it in I've heard…it was just limited by the microprocessor).  And there is no way in which the FPB 300 is a slouch in it's voltage amplification given the low distortion, low noise, and bandwidth.  I know Martin Colloms rates the improvements in each Krell model as having been monumental--even more monumental than the first.  But I think this is hype.  The FPB was a new and better kind of amp for power sucking super capacitive Acoustat speakers.  After that the company made some improvements in it's input amplification and power programming, and these improvements would be nice to have, but no way do they compare with the initial advance.  And then the FBP series of amplifier were replaced with EVO, which appears entirely different (and I'm not sure if useful to me), and now those may not even be available as a new product.

But beyond the little world of FPB and it's c and cx descendants, I don't know of any other amplifiers that would serve my needs as well, despite costing many thousands of dollars more, and the shockingly high cost of the FPB amplifiers when it was introduced is now dwarfed by the charge for any audiophile blessed tin can.

Meanwhile, the Sustained Plateau Bias is not the only kind of variable bias system devised.  James Bongiorno patented the Sliding Bias Class A used in his Sumo Nine amplifier.  That amplifier has always sound intriguing to me, I've often lusted for one, but the fan makes it useless for me.  If only he had built it out with heatsinks into something like the fan-less FPB 300.

Early Threshold amplifiers also use a kind of trick Class A bias.  Back when I worked at a store selling them, it was often called "sliding bias" but that was not exactly correct.  James Bongiorno got the patent for sliding bias, and that was after the Threshold amps had been made.  I believe the Threshold amps clamp the transistors so that they always remain in the ON state.  Well that's better than turning on and off as most Class AB amps do, but it's not gaining the linearity advantage even of sliding or plateau bias, and I'd doubt it could be used without feedback either.

Sadly the Sliding Bias invented by Bongiorno may have hardly been used, precisely because it was patented.  Bongiorno himself had bad luck running audio companies, and others didn't want to use something that might be restricted or that they might have to pay royalties on.  So this may be another case of how patents slow progress rather than promote progress.

Sliding Bias might even work better than Plateau Bias, though the latter sounds more sexy.  Krell got it's own patent on Sustained Plateau Bias.

Now if your whole thing is electronic constancy, simplicity, etc., then Sustained Plateau Bias is going to sound like pure evil, and nothing like Class A normally does.  But all the neo- Class A designs still have the key benefit of making the output more linear before feedback, and/or with all the devices always in the on state and ready to respond to reacting loads.  And those are objectively measurable aspects of behavior.  The benefits, if any, of simplicity on sonic performance--it's possible it's there but it has absolutely no backing in objective audio science today, given similar outcomes in THD (better would be THD weighted for harmonic detectability) it wouldn't matter so much whether they derived from a simpler process or a more complex one.  As much as I'd love to have a Pass Amp (though I'd need at least a pair of XA 200.8's for my Acoustat 1+1's, go figure) nothing of the thinking behind their special circuit simplicity has not been confirmed by DBT.  DBT says you might as well just have another full-feedback Class AB amp.  That would apply to me too, but I wouldn't know if they tested Acoustats--I think there's a better chance of amplifier feedback audibility with the Acoustats than just about any other speaker.  But there's no reason to believe the simplicity thing applies more to Acoustats than other speakers, at least other speakers which are about equally transparent or reasonably close.

(LOVE Pass Amps anyway, love isn't science, but obviously a pair of XA200.8's is out of the question.  I'm wondering what kind of Class A amp to get to replace the HCA-1000A on the super tweeters.  Several of the First Watt amps would do, F5, J2.  Look at what they cost!  And now Pass is not freely giving out schematics on some models.  Certainly the old Nelson way was nicer.  However I can't complain as it appears the F5 build information still appears to be available.  I think I'd build an amp before paying $5000 for a 25 watt amp.  $5000 for a 1600 watt Class A amp (used FPB 400cx in 2 ohms--which is Acoustats at 20k) wouldn't be bad, if that's all you pay.  But no good having 1600 watt amp on the supertweeters.  But would I ever get around to building a Class A amp?  The HCA-1000A is an excellent amp already, surprisingly hard to beat (and DBT would say you can't) but I'd love to have a small solid state class A amp, a field which is actually fairly limited, First Watt, Sugden, Pioneer Series 20 M-22, Pass Labs Aleph, Early Krell, Mark Levinson ML2.  I think the best of the First Watt designs are head and shoulders above the rest, and with modern parts.  Arc welding and Acoustats may require a different strategy.)

I think amplifiers of the sliding bias/plateau bias types are needed for very inefficient but very resolving loudspeakers, and it's unfortunate the drive to simplicity--which I concede has benefits--has relegated such things and thinking of them to obscurity and worse.

One other idea I've had is the construction of the FPB and subsequent amps marked a clear break from the past (or it might have occurred slightly earlier).  The earliest Krell amplifiers have a slightly casual internal construction.  The later ones look mil spec.  Now the FPB 300 looks very professional inside..but also lacks ostentatious audiophile parts and fancy construction techniques.  It may be multilayered board and it is quality looking for sure but it's not a special engineered plastic like used in Mark Levinson's top gear.)

You expect the FPB 300 to behave like a monster but it behaves like a kitten.  There's no drama on turn on or anything else, unless drama is demanded at the input jacks.

An example of a rare if not unique Class A amplifier whose Class A power doubles down into lower impedances is the Mark Levinson ML2.  This is a 25W (into 8 ohms) which dissipates 400W at idle.  The 400W idle suggests 100W Class A.  Indeed the rated power is 50W at 4 ohms and 100W into 2 ohms.  It's possible it has 100W of Class A power into 8 ohms.  But remember it only has 25W (rated power anyway) into 8 ohms and weighs 65 pounds AND DISSIPATES 400 W !  Part of that dissipation is because the output stage has regulated power, very rare in audio amplifiers, but that set includes the FPB amplifiers.

If your amplifier is going to have 1000W of Class A power at 2 ohms it will be needing to dissipate between 3000W and 4000W at idle.

Krell for sale pictures

It's the most beautiful piece of equipment I've ever owned, the heaviest, the most amazing design, and the best sounding amplifier I've ever heard in my system.  But I'm afraid of having continuing to repair it at $1000+ each time, or even having a repair unavailable, it is truly unique and requires factory knowledge.

[Update: I've decided to persist in getting the Krell fixed and back into use.]

Using the Audio-GD DAC 19

Given that I have one tri-amped system and one bi-amped system, all using digital crossovers and EQ, I need 5 DAC's just for those.  These should all be PCM DAC's, according to my current thinking, and I plan to phase out the use of Behringer DCX 2496's and use all Behringer DEQ 2496's, since the DEQ's have digital output and allow use of external DAC's.  I can program the DEQ's to act just like one side of a crossover.  Currently I use two of the DCX's, and they provide 4 out of 5 of the DAC's that I need for my multiamped systems.  The one and only separate DAC currently in use is in the #1 most important location--driving the amplifier which drives the Acoustat 1+1's in my living room system.  Ever since I figured out how to use the DEQ as a crossover last year, I have been using an Onkyo RDV-1 as my DAC (it has a front panel switch which enables DAC-only mode) for the extremely transparent 1+1's.  The RDV-1 was a $4000 DVD-Audio player with PCM 1704 DACs, clock circuitry from Apogee, and built to very high audiophile jewelry standards (for example, all capacitors are a custom version of Nichicon Gold Tune…Onkyo apparently had a special relationship with Nichicon).  It has been compared with some of the best DACs available today, and with the PCM DAC it avoids the sigma delta mist.  When I switched from using the DCX to drive the 1+1's to using the RDV-1, it was a revelation, however it was not a very controlled experiment because the DCX has this positively charming 10V RMS output for 0dB, which wastes at least 9dB of potential dynamic range into a typical system like mine, since the amplifier only requires about 3V for peak output…and I basically never even approach peak output either.  So there were other reasons why switching from the delta sigma DAC's in the DCX 2496 to the RDV-1 made things much better.

Well that leaves 4 DAC slots to fill with new PCM DAC's.  One of the most highly touted online (and an incredible bargain) has been the Audio-GD DAC 19.  That was one of the very first new production 1704 DAC's I read about, and remained highest on my list.  Lite Audio's offerings seem positively pedestrian from a design standpoint compared with the Audio-GD.  Audio-GD pushes the limits of design in what seem like good ways (though I might argue with some).  It brings to mind the design ideas of James Bongiorno, John Curl, Dan D'Agostino, and Nelson Pass.  Minimalist FET circuitry, direct coupled, with no feedback.  (Personally, I'd be happy with the best technology opamp, the TI OPA 211.  It's extremely hard to top that with discrete circuitry.)  Actually the RCA outputs may not be direct coupled, the website only says that the current mode outputs (an interesting idea I may use in the future) are direct coupled.  If there are coupling caps, they are either some nice looking electrolytics, some very small WIMA's, or both.  I don't have the schematic.  I'm happy to have a fully solid state unit since I think tube DAC's are faddish and noisy.  In principle a decent tube DAC could be made, but it seems no commercial company bothers to design optimal tube products.  (And especially not Audio Research in it's Golden Age preamps including up to SP-10.  It's amazing these are so poorly optimized, yet so highly regarded.)  Where tubes are used, it's most often for a kind of sonic nostalgia.  But Lite Audio does make a highly regarded solid state unit that is more expensive than this, the model 83, and I'd consider it, if not for the fact it seems to have soft input switching, and if that loses mode during power outages, I'd scratch it from my list, at least for the bedroom DACs where there is no UPS (certainly don't want any "sealed" but actually H2 leaky batteries in the bedroom).

So when the Audio-G_D DAC 19 came back on the market as the Anniversary Edition, my only question was how many to order.  I decided to play it safe and just order one.  What if it didn't meet my expectations in some way?  What if it had the oily chemical smell as I have found with many products made in China?

Update 7/3/15:  The very slight and not especially unpleasant oily smell that I could smell through the vent holes in the DAC 19 has disappeared after a couple months of continuous usage.  I would not have used it previously in the bedroom.  But even initially it was absolutely nothing like a few previous products from China that required return or discard due to strong chemical smell.  Well only very slightly in that it also, initially, had an oily chemical smell.  Nevertheless I love this DAC and might get another, though I retain some qualms about moving one to the bedroom.  Now returning to my story of smelly products (and, btw, I do think that strangely enough, I have an especially sensitive sense):

And therein is a long story.  I'm not sure when I first noticed this special oily-chemical smell on Chinese products, but it has seemed unique to Chinese-made products, especially heavier metal and/or rubber ones, and not unlike WD-40 but darker and dirtier and with a tinge of something like diesel fuel.  It was certainly apparent with the Weider weight set I bought through Sears Roebuck in 2005.  To keep that smell, which may be less toxic than it smells--but still one wonders, out of my house I first discarded all the packing (which was deeply infused with the smell, and parts of the packing materials looked positively yellow-brown oily), I then hosed everything off, I then washed everything with dish soap and toweled dry--several times and discarding cleaning and drying cloths each time.  When I was done, I had nearly removed all of the smell from everything, but certain rubber pieces still had some smell.  I decided that was good enough for the exercise room.

The next thing I remember, especially, was the Chinese-made KLH receiver I bought at Best Buy about the same time (2005) for $89.  I thought this cheap stereo receiver looked cool, and as I had just gotten used to digital volume controls with the TOTL Yamaha receiver I had just gotten for the kitchen, it might be cool to play with that.  I actually didn't have a specific use in mind.  But not only did this receiver work remarkably poorly (it had very high noise level on line inputs) it had the terrible smell, Especially When it was running, but even when it was not running.  Unwilling to let someone else suffer as a result of this, the final disposition of this receiver was to be boxed, wrapped in double heavy duty trash bags, and discarded to the trash (landfill).  Sad, but that was the only alternative for me.  As a matter of principle I do not unload unsafe or even potentially unsafe products onto other people, even for free.  Also I had not realized how bad the smell was until the return period had expired.

Next to have the bad smell was an IR Thermometer.  It wasn't a Fluke, but it was Fluke's second label brand.  This was a sophisticated $200 electronic product.  It had strong smell.  I was very saddened by this--it got the same treatment as the KLH receiver.  I wrote a bad review that was published by Amazon that has drawn periodic criticism (it appears, some from Chinese).  I don't know if the company fixed the smell problem, but my sample was unarguably smell-defective.

All through this time I carefully check out most little gizmos, and major pieces like Behringer.  By and large nearly everything has been smell-free, and increasingly so.  I've never had a Behringer piece which had oily-chemical smell, even when disassembled.  And needless to say my home is full of little gizmos made in China.  So the problem does not at all affect everything made in China.  And it most certainly has never affected anything sold by Apple (in my experience and expectation).

After I filed that bad review of the IR thermometer, I waited about a year before I bought my next thermometer, and this time I bought the Sears brand so I could return it to the store.  It looked very similar to the one I had previously purchased--but no smell.  I was hopeful my review had turned the tide, that the Chinese now were cleaning up their act.  But how?

Which leads to speculation about where this smell comes from.  Is it a preservative added to rubber and vinyl?  Is it an insecticide?  Is it caused by local air pollution in the factory, it's surroundings, or shipping facilities, even the shipping boats themselves?  I don't know, and it would be very interesting to know.  If perhaps it is caused by air pollution, was it fixed by cleaning up the pollution or instead by forcing workers to wash everything off with alcohol?  In the latter case, are perhaps the smell-free goods the ones which are the greater human catastrophe?

I bring this up not just to describe my thinking and why I purchased only one Audio-GD Dac 19.  Unfortunately the DAC 19 does indeed have a trace of the oily-chemical smell I have associated with certain Chinese products.

It's far less bad than the KLH Receiver, and not as bad as the IR thermometer either.  I can basically (so far) only smell it with my nose within a few inches of the vent holes on the top of the DAC 19.  But it is there, unambiguously, and it hasn't significantly gone away in the two days I've had the DAC 19 powered.  Because of the vent holes on top and bottom, air is constantly circulating through the unit (not something I like in principle) which you can feel, and bringing that oily-chemical smell with it--constantly.  I was thinking the air current might eventually carry the smell away, but it hasn't yet in 3 days.

The packaging material was pristine and has no smell.  The outside of the unit is pristine and has no smell.  There is only something inside which emits a bad smell through the ventilation holes, and somewhat moreso when the unit is running.

I was also worried about the lack of a muting device on this DAC.  How would that work straight into my big power amp driving highly capacitive electrostatic speakers.  Just after ordering, I noticed that fact and immediately had buyer's remorse.  Then I planned to use the DAC as the bedroom monitor speaker DAC.  I feel fairly sure the HCA-1500A amplifier wouldn't hurt the Revel M20 speakers with a few spurious pops.   But on this score, the DAC 19 has worked fine.  The lack of a muting relay has not yet been a problem, in fact it seems to have less issue with changing digital sources than the muting-laden Behringer devices (which can often be driven or switched to make particularly loud pops).

As a result of the smell, I will not be using this DAC in the bedroom, where air ventilation might sometimes be limited to a closed door.  I think it's probably just fine in the living room, where there is constant ventilation.

And in fact I may be using this as the next DAC for my Acoustat 1+1's.  It sounds at least as and perhaps even more transparent than the RDV-1 DAC.  The bass seems deeper and more well controlled also (despite this just being used now down to 80 Hz).  It has a relaxed yet transparent sound.  (Caveat: this is merely a casual observation.  As is my practice: this is not based on any kind of controlled experiment, not even A/B.  On Sunday I simply hooked up the new DAC before powering the system on the first time.  Only the most rigorous tests are of any value, nobody but DBT types bothers to do them, and they basically don't show any differences between DACs that have been substantiated to my knowledge and I wouldn't be the first because this is damned hard work I am not up to.  Anything but the most rigorous test may induce superstitions that are hard to shake.  So I test not-at-all and no listening description is verifiable except as something I remembered feeling.  Did I say my living room system is generally sounding fabulous--though most recently until the Audio-GD I had been obsessing about the harshness on some recordings.  Well on later listening Saturday night it was totally transparent and not harsh, before changing DAC's, playing the Reiner recording of Concerto for Orchestra, except perhaps a tad harsh in a later track where I quit to go to bed.  This is a very challenging piece for all to play, record, and play back.)

Did I also mention it has hard-switched inputs, a knob that stays fixed to one position or another until you turn it by hand?  Thats exactly what I'd like in all my DAC's.  I hate soft-switching inputs which may lose their state during power outages.  It also has another a very nice plus, a front panel polarity switch.  If I were using these DACs for each way in a multi-way system, I could just press all the buttons to reverse polarity--and that would be cool.  Or if I were using it as a headphone DAC.  As things are, it's of relatively little use to me.

It's solidly built, and an incredible value.  Perhaps one of the best DACs there is, at 1/200 the cost of some others.  If they could only get rid of that oily smell.  I'm going to ask.

AF/RF Spectrum Analyzer

I've already got one (<50kHz) HP spectrum analyzer.  But now I think what I'd like would be an 8566B, which has range from 100Hz to 22GHz, and minimum 3Hz bandwidth.

BTW, HP instruments became Agilent when HP sold off it's original instrumentation business to become a PC maker.  Agilent later reorganized to separate out medical devices and now the instrumentation business is called Keysight Technologies.

Modex Modules

Modex modules from RPG are available tuned to 40, 50, 63, 80, 100, and 125 Hz.

Even Ethan Winer (designer of RealTraps) says they are good, but expensive for what you get, and he claims you'd need a lot of the low frequency units to actually do much good at low frequencies, just like his absorbers.  It's Surface Area, Surface Area, and Surface Area.  (As I was reading a blog about bass absorption which led me to the Modex, I noticed that Mr Winer has apparently been banned from WhatsBestForum.  I wonder what led to that and I still give him and the Forum considerable if not unlimited respect.)

But it seems to me that the Modex modules are fundamentally different from RealTraps.  RealTraps are broadband absorbers with reflectors that prevent higher frequencies from being absorbed.  Modex modules are tuned to specific frequency ranges (and therefore are as much like resonators as absorbers).  I would expect that tuned units like the 40 Hz Modex (exactly what I need) would be much more efficient at absorbing the lowest frequencies than the RealTraps (which have no special tuning).  And most things sold as "bass traps" now aren't really resonators (as the "trap" would suggest) but really broadband absorbers very much like RealTraps.  GIK, for example, makes similar absorbers, possibly better than RealTraps in some applications, but still only an absorber, not a tuned resonator.

UPDATE:  GIK actually now makes both untuned and tuned bass traps, and they are indeed an incredible bargain compared to RPG, even if RPG is better (which I'm not sure…and may not be).  In untuned traps, which GIK recommends as the starting point, GIK sells about 8 sq ft of bass trap for $139.  GIK's 40 Hz tuned trap comes in 24x24 for $239--compared with RPG's $684 for the same size.  GIK will even custom tune, and their stock lowest trap is 40 (I might need 45).  Neither GIK nor RPG are making true Helmhost resonators--at least in the models I am considering.  GIK looks like they know what they are doing, and are pressing forward too.  I think I will get some GIK traps soon.  I'll need to make more good measurements first.  Actually most if not all of the measurements I've posted here are with my existing equalizers in place.  What has to be done in designing room treatments is to measure the actual room response.  And I will do that in the prime listening spot and elsewhere, including the Bad Corner near the first bedroom.  That corner seems to concentrate the bass worse than any other location, and it leads to boomyness in bedrooms (though, when the sound absorbing doors are shut, all is attenuated a lot) which is highly undesirable also.  That's the corner where I intend to put the first bass traps.


UPDATE 2: Well, there isn't enough room in the Bad Corner.  I may have to use a Bag End eTrap there…and it might be more successful anyway.  Looking at GIK's before and after measurements with tuned traps…the tuned traps mainly suppress the decay, rather than the initial amplitude.  The untuned traps do go more after the initial amplitude.  Hmmn, along with the eTrap, I might need some damping, but it would have to be made to 15" sides, 15x30" would do.  That would be the top, and strap on the bottom.  A trap on the bottom would cover up the electrical outlet, while the eTrap would not.  eTrap was always my plan.  A GIK Scopus would also work, actually, since it can be hung flat against the wall.  One way or another, the combination of tuned and untuned traps in the Bad Corner would work.  But I could use a full size trap in the corner next to the kitchen door.  I need 3' of coverage on top, so I could get the standard two 24" trap package, and have one cut to 12".  It's basically the only wide open corner now.

Anyway, back to the original post...

Even Modex may not actually be Helmholtz Resonators…that terminology does not appear in anyones description.  But at least they are tuned and therefore resonator-like.

B&H Photo sells (special order) a pair of Modex 40Hz corner traps for $684.  Each has 4 sq ft front area (24x24), though relatively shallow for corner traps.  That's only a little more expensive (perhaps 1.5-2x) than traps made by GTK and RealTraps which are not tuned.

This is looking like a good option for the corner of the hallway adjacent to the living room.  The shallow depth is a blessing because there isn't much space.  The Modex might be no more objectionable than the Bag End eTrap I had been thinking of putting there.

Winer's charge about Modex being expensive would definitely apply to the Modex Metal Plate absorbers for $1944.  Those are claimed to have uniform absorption from 35-100 Hz.  They are thin but  have even more surface area: 60x40.  Strangely while B&H says they are 4" thick, there is a plot showing effectiveness vs depth.  The 4" depth has peak absorption just over 100 Hz, and almost nothing below 60Hz.  The 10" depth has peak absorption at 60 Hz.  So this doesn't match the description at all, and I'd be worried without doing more research.  But I won't be buying a $2000 absorber anyway.