TIM and all that

Subjectophiles don't want to believe measurements, particularly THD measurements, but there is no good reason for this.  (Of course, you can't just go by specs, which may be lies, but actual measurements at different power levels and frequencies, etc.)

Some of the rampant distrust of measurements began with audio engineers who did a lot of them.  Matti Otala was a designer for Harman Kardon who designed the great Citation XX and many other good amplifiers.  He determined that conventional THD measurements were incomplete, and a new kind of measurement was needed he called TIM, and the way to achieve TIM was by eliminating or reducing global feedback.

This whole theory is still believed by many, but is wrong.

It has been determined that TIM is nothing unique, and especially that it does not require a particular kind of design (low global feedback) to achieve it.  It has been proven many times over that high feedback amps can have vanishingly low TIM as well as other kinds of distortion, simply by being designed for low THD under all conditions.

One Huge Gap in Surround

Problem with the 7.1 analog inputs on my Yamaha HTR-5790 is that there is no way to make them do any processing, they only go straight through.  Therefore, I am stuck with whatever functions my Oppo BDP-95 has, and those are limited to volume, delay, and mixdown.  The Oppo offers no functions to convert 5.1 to 7.1 using Dolby Pro Logic II x, for example.  With an AC3 connection, I should be able to select whatever processing I like, but then I have a compressed signal, and therefore am losing the full resolution of DVD-Audio and SACD recordings.  I can see why the receiver is like this, it must only have 2 ADC's inside, and it would need 6 or 8 to handle processing of analog multichannel inputs.  Are there any receivers that DO handle such processing???

This makes one of the fundamental issues in Surround, the inherent incompatibility of 5.1 and 7.1, much worse.  To explain, requires a history lesson.

I was becoming an audiophile just as the original Quadraphonic wars were running.  Despite their differences in implementation, all Quadraphonic systems simply assumed a 4 channel speaker layout, 2 speakers in front, and 2 in back as mirror image.

It turns out that this 4 speaker configuration is fairly worthless.  Humans do not have very good front-to-back auditory sensitivity.  Things in back are somewhat muted, but it's hard to have any idea of where they are unless you turn your head.

When this was all re-thought for Home Theater, it was fundamentally changed.  Instead of Quadraphonic (because more is better or something) we got Surround, in which an ambient soundfield is created arournd the listener, starting with SIDE speakers.  This actually has some perceptual utility, maybe (though I think that often multichannel listeners don't appreciate the forward depth of stereo systems, which is often superior to multichannel in practice for various reasons, one of them being the confounding effects of the center channel, which IMO should be eliminated).

But somehow, the sense that "back" was needed still  seemed important to some people.  So, the 5.1 specification fudges the side speaker position so that they are also somewhat behind the listener.   Often the specification is something like 90 degrees to 110 degrees, but most people, especially the relatively uniformed, put their side surrounds much further back that 110 degrees.

When you get to 7.1, finally it is done correctly, with both sides and back speakers.  But now, the sides are supposed to be at 90 degrees, as close as possible, right on the sides of the listener.

So if you set up your system for 7.1, as I have finally done in my kitchen system (sans center channel, which is being automatically added back into the front stereo pair), it will not play 5.1 recordings, which are the vast majority, in the way that most 5.1 systems will play them.  There is no back signal at all.

Effectively you need some kind of processing like Dolby Pro Logic II x (which, btw, my Yamaha receiver has, it says so right on the front, but it is ONLY applied to digital inputs).  Although Dolby Pro Logic is mostly known for generating fake surround from stereo, it also features a matrix for converting 5.1 to 7.1.  Although you could simply duplicate the side signals in the backs, and some schemes actually do that, it's best handled through a matrix.  What I've heard described in a matrix where the sameness between the sides is subtracted from the backs.  It might also benefit from some kind of steering which leaves things alone if they appear to be concentrated in one speaker.

Now that we have endless named proprietary schemes for doing matrixing we don't have any plain vanilla matrix boxes to fool around with as in the good old days.  Stuck with the fact I can't actually use the Pro Logic II x on my Yamaha, or find a simple 5.1 to 7.1 outboard converter I could stick in between my Oppo and Yamaha, I now dream of building such matrix boxes myself.

Update:  I may already have the answer to this problem in an unexpected place.

Two years ago I started buying Integra Research RDC-7 surround preamps simply as stereo DACs because they have dual differential 1704's, something I was pursuing at the time.  For various reasons, I decided not to use this unit, and the low 1.0V voltage for I/O would be problematic in my high end living room stereo.  I had reached the point where I was thinking of recycling at least one of these beasts (the beat up looking one) and possibly both because of no perceived need.

Sadly, the manual for the RDC-7 is not actually available, what one finds online is the RDC-7.1, a different beast.

Reading the manual for the RDC-7.1 it is clear that Dolby Pro Logic II x can be applied to the multichannel input (it lists the various options for multichannel inputs, and that is one of them).

I suspect either this or something similar is available on my RDC-7, especially because it has an "Upgraded" tag on the front.  It appears the THX Ultra 2 has a "Cinema" mode which also expands 5.1 to 7.1 surround.  In any case, this $4500 surround controller is not limited to "straight through" or "pure" for the multichannel inputs as the Yamaha HTR-5790 is.  Perhaps that's the deal, to do the cool stuff you need to get the actual preamp/controller.
  
Since the time this was made and now, generally all Home Theater Receivers and Surround Processors have been eliminating analog multichannel inputs, which  I think is a bad idea.  In addition the the compressed AC3 "Dolby Digital" signal, newer products transfer multichannel through HDMI, or at least HDMI connectors.  This may have the advantage that a digital signal is being transferred, and a layer of DAC and ADC may therefore be avoided.  I think that issue is extremely overrated--surprisingly digital coversion, especially to 24/96 and above, is essentially lossless and about as good as a pure analog interface--the kind high end audio has been built upon from the beginning.  Plus, analog signals do not have complex clocking issues or cause RFI from high speed pulses.  I am also not sure you will always get a full resolution signal via HDMI from all kinds of discs.  I have heard HDMI clocking may be problematic (but Oppo fixed this in the BDP-205) also. 

However, if it turns out I can't use the Integra Research to do my 5.1 to 7.1 expansion, any HTR with HDMI multichannel inputs and Dolby Pro Logic II x (or perhaps THX Ultra 2) will do.

I will first see if my RDC-7 *can* convert 5.1 to 7.1 through the multichannel inputs, and if so, begin working on incorporating it into my kitchen system.   My Yamaha HTR would then do nicely as a multichannel amplifier, possibly supplemented by a high end stereo amp for the front channels.

Addendum: Chesky put forward an alternative use for 6 channels in which the back surrounds are removed (they aren't THAT big a deal anyway) and replaced with front speakers at the widest possible positioning.  (Stereo can't use the widest positioning because that would destroy the phantom center image.)  Yahama and others used the extra two channels as "Presence" speakers which are above the stereo speakers.  I didn't mean to necessarily disparage these ideas, I have just found it more convenient as well as more typical and supported to have surround back speakers.

Dolby Pro Logic II x is not the only process to convert 5.1 to 7.1.  I already made that clear, but in the minds of many, all that older stuff has now been replaced by Dolby Atmos.  I think thats way over the top for home theater, and particularly for 5.1 to 7.1 conversion.  I would stay away from any process that has mandatory automatic room correction, though I'm not sure Atomos actually mandates that.

By all acounts (though none might be informed) the 5.1 to 7.1 conversion in Dolby Pro Logic II x is a matrix applied to the surrounds ONLY.  It does not reference the front or center speakers.  So, in principle, you could do this or a similar conversion with two inputs and three outputs.  This is a relatively simple thing, and for a 7.1 system, perhaps just what is needed.  As I said, Atomos looks way over the top.

In addition to "surround" formats, there are also some special formats like Amiphonics and Ambisonics.  These may require stereo back speakers to work.  These formats are somewhat controversial and I still wonder how well they actually work.  However, I purchased a few recordings of this kind, and that is another use for back speakers.  In this case, replugging connections may be the best way of handling it, plugging the 5.1 surround outputs into the back.

Measuring the FPB 300 damping factor [Updated Jan 13 2019]

With sadness but hope, I've decided to mothball the Krell FPB 300.  Though it has been very entertaining to try to work out the bugs, after 3 perfect repairs, and 3 years of experiments to get it to work properly, I've decided to hang it up until I have the spare time to disassemble and repair, rebuild, or re-engineer myself.  Which may never happen, but at least I'll have time someday to take it apart which would be worth something to me.  I can't bring myself to sell it after all this time and money invested.  As they say, as long as there's money, there's hope.  I also have hope that eventually suitable replacement transistors will be available.  I remember the way this went with tubes, which have never been more available than today.

Meanwhile, I have measured the distortion performance in both channels at 4 and 8 ohms, and the numbers are excellent in the good channel, at all power levels, basically at spect at 8 ohms and slightly worse than 8 ohm spec at 4 (which is not specified).  The numbers are not quite as good in the "bad" channel, but 0.04% THD at 8 ohms taint that bad.  I actually measured 0.09% THD at full power at 10kHz after a full warmup in that "bad" channel, beating the spec IIRC.  (Now I can't seem to warmup anymore...)

The Hafler might measure slightly better than the Krell good channel into 8 ohms, I hadn't measured 4, up to significantly above rated power.  It seemed to have peak output of about 60V, which surprised me.

The Aragon hasn't measured quite as well, however, I haven't done a full set of measurments only a quick check while tuning the bias, and distortion at final bias was never checked, and actually the final bias may not be the final final bias either.  Still, I got the Aragon down to 0.07% at 8 ohms at modest levels at 1khz, in bias fiddling a couple of years ago.

Tonight I was able to measure the Krell at 220 Hz at 8, 4, and 2 ohms, and unloaded.  (This is the "bad" right channel.)  I put the load on one set of outputs and a 6 foot cable to DVM on the other output.

1 watt nominal level into 8 ohms, about 2.88V

220 Hz

8 ohms  2.797V (warmer)

4 ohms  2.785V (coldest)

2 ohms  2.762V (cold)

noload   2.812-2.810(warmer)

DF(220hz,8ohms,1w) = Eunl / (Eunl-Eload) = 2.810 / (2.810-2.797) = 216
DF(220hz,4ohms,1w) = 2.812/(2.812 - 2.785) = 104
DF(220hz,2ohms,1w) = 2.811/(2.811 - 2.762) = 57.4

(Note: the DF should scale as half for each halving of load.  8 ohm damping factor is the one normally specified.)

The amp did not run long enough to actually get very warm, but even during the 10 minutes or so of operation there was improvement from 28.12 to 28.10 in the unloaded output voltage.  I could not go back and remeasure 2 and 4 ohms because the amp shut down twice when doing these measurements, and I didn't want to push it any more than that.

I was also able to measure 10kHz at 8 ohms:

10kHz

8 ohms 2.201

noload  2.213

DF(10kHz,8ohms,1w) = 2.213/(2.213/2.201) = 184

Very little change in damping factor from 220Hz to 10khz.

The apparent HF loss of -4.5dB compared with 220 Hz is probably mostly being caused by my crossover and EQ....more than I expected but it achieves the desired result here (flat response).  Also a dB or so HF loss in the Fluke 8060 meter I am using at 10khz.  I know from previous measurements, specs, and reports, that the Krell is virtually flat well beyond 10kHz, but that is not the question here, so I'm not going to turn off the crossover, bring out a better meter, etc., to measure this better. Turning off the crossover requires a reboot of the DEQ's and has messed me up in the past (when I forgot to turn it back on, etc).

****

In the "good" channel, that runs cooler with no suspicious hotspot and seemingly does not trigger shutdowns, I measured slightly better low frequency damping factor, but more than just slight improvement in the change in damping factor at 10kHz.

DF 8 ohms, 1w, 220 Hz, 222 (equivalent to 0.036 ohms)
DF 8 ohms, 1w, 880 Hz, 207 (equivalent to 0.039 ohms)
DF 8 ohms, 1w, 10kHz, 204 (equivalent to 0.039 ohms)

This time I normalized all measurements to 1 watt by adjusting the volume to 2.88v in all cases, overridding what are mainly affects from my crossover on the frequency response.

The improvement is consistent  with a theory that that feedback and servo are keeping the bad channel in line, but also reducing the ability to cope with changes in load at high frequencies, if only by a small degree, since it's "resources" are already being used to fight the faulty transistor(s).

Though perhaps there was some improvement due to change in measurement strategy.

And you might imagine if there really were a transistor in very bad shape, the result would be much worse.  Differences like this might occur from differential aging as well as manufacturing and repair histories and usage histories.

I was hoping to see a bigger difference, so that the good channel might have damping factor like 500 or something, and there being something "obviously" wrong with the bad channel, more than just a slight variation, even if that variation is consistent with the theory that the bad channel is slightly faulty.  The distortion measurements showed the same thing, with the good channel meeting spec and the bad channel being slightly off spec.

The damping factor for the FPB 300 is not specified, actually.

Unobtanium Transistors

The principal problem with maintaining amplifiers like my Krell FPB 300 is that the power output transistors, MJ1302 and MJ3281, have long been unavailable.

Motorola discontinued these transistors while Krell was still making amplifiers with them, so Krell bought the entire remaining inventory in the mid 1990's.  They used them in many different amplifier designs, and according to legend, Krell freely installed new ones during amp servicing.  That was wonderful while it lasted, until about 2009 or so.

Now, to keep the amplifiers running with authentic parts, it is necessary to scavenge transistors from parts unit amplifiers, similar amplifiers that might be even more valuable than the one you are trying to fix.  There is no public place where such scavenged transistors can be safely purchased, maybe you know someone you trust (and, do you really trust them).

MJ1302A and MJ3281A are sold on Ebay and through Asian distributors are almost certainly fake parts.  I bought some of these in the hopes they may be OK now that Chinese fabs have been making them for well over a decade.

Similar silicon is still available in the TO-247 case in the MJW1302AG by ON Semiconductor, the Motorola spin-off which still manufactures basic transistors for linear applications.

The problem is that the FPB amplifier heatsinks were designed for TO-3 transistors.  To use TO-247 would require complete re-engineering of the chassis and heatsinks.  One might as well buy a D'Agostino amp.

Meanwhile, ON Semiconductor does still make TO-3 power transistors, including the MJ15024G, which others now use in power amplifiers.

What defines Audiophile?

Struggle.  It's not High End audio if it doesn't take some work.  Contrary to what audio store mavens would tell you, you can't simply buy musical enjoyment.  It's not "all about the music."  It's about getting the sound that makes it sing, understanding how to get there, and traveling at least some small part on your own feet.  As Euclid said about Geometry, there is no royal road to being an Audiophile.  Not only is it not possible to get their without your own work (which could be just...actually serious listening...if you have totally obsequious dealer you trust and tireless elves to help daily) it's not the same thing.  To be an audiophile is to be endlessly deciding how to do audio, but hopefully not obsessing too much over these decisions, but hopefully making better ones as time goes on, that lead to more rather than less enjoyment overall.

To be an audiophile is to at least nearly always be on the edge of Audiophilia Nervosa.  That is the dark side, which we must stay as far away from as possible, I believe.  Some guidelines for avoiding that:

1) There is no one right way to do things, starting from the fact there is no one right set of goals to be aspiring to, including the nature and extent of one's unpaid personal labor and buying in audio, and when and what to listen to and how.

1a) The enjoyment people seem to be getting from their audio systems often seems inversely proportional to how good they are by various objective standards.  Why can't we who take audio seriously have it both ways?  Have high standards when we're tightening connections, and then, just enjoy?

1b) I believe 1a also includes that...it doesn't take much audio capability to make audio enjoyable.  The main thing is in fact...the frame of mind about it all.  To be able to accept things as they are now.

1c) Once one has banished unquestionably horrid sound, most audio systems are capable of communicating sufficient information for a fully expansive experience each time.  In fact, this doesn't take much.  The truly great systems have even more information to choose from, but both are sufficient.

So, avoiding obsession includes accepting the fact that the present will never meet the highest standard, or even sometimes ones that were recently being met, but still the envelope of expending experiences must continue.

Antenna Pointers

Ground Loop from Outside Antenna feeds (which themselves have proper outside bonded grounds and suppressors)

Despite ground bonding, I was getting significant ground loop hum from the tuner connected to my outside antenna.  I tried various ways to fix this.  Two things "worked":

1) Running antenna lead to F connector surge protector on my Monster power bar, then to tuner.  This reduced ground loop hum a lot, but not entirely.  I think having suppressor outside and inside as well on antenna feeds is good practice for safety reasons.  I did not notice signal deterioration.

2) Running antenna feed through "Magic" isolator.  I got Magic isolators for TV use almost 20 years ago.  I was worried they might not work for FM, but the case says nothing about having an FM trap, so I went ahead, and noticed no signal reduction.  In combination with #1, this eliminated ground loop hum entirely.  The 60 hz hum at the input to my Behringer DEQ 2496 connected to the Sub Out line signal from my Yahama THX receiver dropped to -140dB.   Before doing #1, the level was worse than -90dB.  As I did this after #1, I did not try it without #1.

Ground loop reduction became the order of the day when I started running my subwoofer signal through the Behringer DEQ.  Isolating the signal between the receiver and the DEQ with a Jensen Isomax transformer actually made matters worse.  The Behringer has 3 wire AC input whereas the Yamaha is only 2 wire.  The ground of the Yamaha is therefore determined by what it plugged into it, or not.

However, until I discovered antenna grounding and isolating, I was forced to use a Jensen Isomax transformer on the audio output of the tuner.  Listening to radio without that, hum was terrible, like -50dB, and other sources not much better than -60dB.  Once I applied the antenna grounding and isolating steps, the isolation transformer was not only no longer needed...it was introducing a little hum by itself on the radio signal and was better removed.

NB: Ground loops did not appear until I started using the DEQ because nothing else in my kitchen audio system is grounded.

Before doing any of this, I tried measuring the ground voltage and current running through the coax.  It was extremely tiny.  But that's all it takes to induce -60dB hum.  I decided in itself it was "safe," not representing a ground failure, simply a ground loop.

Folded Dipole Antennas made from Twin Lead must have Soldered Ends

My biggest discovery of the year, the lack of soldered ends explained recurrent issues I had been having since 2002 when I made a special antenna tuned for TV Channel 9 which I am still using.  In the end, the folded dipole with unsoldered ends became a Channel 9 blocker for other antennas, so I couldn't get Channel 9 on anything.  Soldering the ends fixed everything, and I get perfect reception now.  Bare copper is prone to getting semi-conducting oxidation which does very weird things.

UHF Antenna location is extremely important, down to millimeters.

As I hoped, I was able to find a location on the south side of the kitchen for a UHF TV antenna that picks up all local stations perfectly.

The problematic multipath-prone TV signals were coming from the south.   Previously I had both UHF and VHF antennas on the north wall, but the UHF signals for Channel 4 (which actually broadcasts on Channel 48) and Channel 5 (which uses another UHF channel) had multipath issues with recurrent pixelization multiple times per minute.

I first figured the south wall would be better because I could position the antenna above the bathroom mirror on the other side of the wall which was probably not helpful.  It would also then have a clear path to a southern originating UHF transmission, instead of being blocked by HVAC and other metal objects around kitchen.

But even on the south wall, the precise position proved to be extremely important.  Too far one way and I would get dropouts on channel 29, which comes from the north!  Too far the other way, and I was still getting multipath issues on channels 4 and 5.  There was a happy spot right in the middle so I stuck my Clearstream Eclipse antenna with "permanent" adhesive strip right there and enjoyed perfect UHF TV all night on local channels.

Overnight the Eclipse had unstuck from the wall and slipped down just over an inch.  The performance was now abysmal again.  I now stapled the antenna back into place.  But making some error, I stapled it about 2mm lower than before (fortunately, I had marked the original position).

This was not as good, I determined that afternoon.  I was not getting as many dropouts, but I was still getting them.

Back on the wall, I found that positions above the original position I had found were even better.  In fact, the signal strength and freedom from multipath kept increasing as I raised the antenna all the way to the ceiling, so I stapled it there.

Now reception is perfect on all local UHF channels, with no dropouts.