Monday, October 17, 2016

The Guyz Behind Mondial

I put my Aragon 8008BB back online on Sunday, after a week of A/B/X testing with a Parasound HCA-1500A and a Carver TFM-24.  I've got an RMA to return my Krell FPB 300 to Krell for repair.  I miss the sweet sounds of my Krell FPB 300 and want it back online soon.  But that's no excuse for downtime.  The good audiophile must make do as well as possible with the best that is on hand.  The show must go on.  I have no patience for audiophile nervosa.

Just last week I discovered an obituary for Paul Rosenberg, the Mondial co-founder who along with Tony Federici were responsible for the great Aragon and Acurus components of the 1990's. This was also my discovery of the audio journalist Ken Kessler.  Paul and Tony sound familiar to me I think I once met them.

Ken affirms by telling the story of the big Mondial amplifiers having been designed by Dan D'Agostino.  In an alternative accounting I find more believable, the circuit and circuit board design are credited to Mike Kusiak, whose initials are one the board.  Robbii Wessen did the industrial design.

Coming from the pen of Ken Kessler, who appears to have actually had some good times with Paul and Tony, who probably heard it from them, this may be the true story, or at least "true enough" in the sense that D'Agostino might not deny it--there may have been collaboration involved.  I still find it hard to believe he got deeply into the details, I would more imagine that Dan may have proposed the basic circuit (on a cocktail napkin or something like that) and Mike and others did the layout and fine tuning.

The Aragon is a nicely made amplifier.  However, from a cold start it sounds more than a little strident.  It appears to need about 60 minutes of warm up to sound best, and even then it may be strongly on the Yang side (like the Guyz themselves?), not unlike a lot of high end audio products.  My actual Krell, however, sounds sweeter.

I have a strange memory which may not be entirely true.  I recall someone like Tony demonstrating an Aragon amp at a meeting of the San Diego Audio Society, possibly following a Krell demonstration, sometime in the mid 1980's.

Tony heard my thoughts on the Krell and said to me, "I have an amp just like what you want."  He said.  "There's no fan, it's not as expensive, has lots of Class A power, and sounds warm like the Krell."  So he led me and a few others into a smaller room, where he played some music on the amp.

It wasn't bad but was sounding a bit too bright.  Tony agreed and wasn't happy.  Then he noticed the tiptoes the store owner had put under the amp.  "I told them not to do that," he said.  "I remember Brian bragging he could put anything on top of those."  "But not my amp," he said.  "My amp needs to get warm, and it only gets warm down on the low feet.  That gives it the warm sound," he added.  "Here, I'll just put this towel on top of the amp.  That'll help it warm up."  We listened to it later and agreed it sounded more warm and better.   But I was bothered afterwards about Tony just leaving the object on top of the amp.  "Oh, you can't hurt it," he said.  "It would shut off it it got too hot.  It's got that notch too", he grinned.  I would not recommend doing this, btw.  The temperature regulation circuit might fail to act in time, especially in an ancient unit.  Nor do I strongly remember any particular person saying exactly this, but it was something that still left me a bit worried, especially using tube amps whose misuse can be very dangerous.  Now I hear others recall Paul and Tony as care free, so it seems consistent that Tony might be pretty laid back about warming up his amp.  And it now does seem they're sensitive to airflow like this, due to relatively weak bias regulation, at least in my unit (which was an early Connecticut unit, and also features the very robust original transistors).

I wasn't that tempted.  I used tube amps for midrange and bass mostly through the 1980s, which I had obtained for very low prices, and the high cost of even value amps like the Aragon seemed a bit out of my league, though by that time I could have afforded one, even with my barely affordable house payments.  At the same time, my Citation II tube amp, which sounded wonderful, often had a frying sound, and I wanted desperately to get rid of it and get something else.  But I couldn't get myself to do that either.  Finally when I was out of a job and selling my house I sold the Citation II and basically have never much looked back at tube amps.  Maybe just because I got sick of them?  I confess now I have regained a bit of interest in tube and other amplifiers recently, coincident with running various different backup amps.  Even if I can't prove I even hear a difference, as in one session of ABX testing.

When I got finally got an Aragon in 2010 as a backup for the Krell, it had very low feet, not original, actually just 1/4 inch stick on rubberized felt pads.  I ran it on top of a flat board (NOT the carpet!).  Still, it ran much hotter, 180W at idle.  When I put the Aragon on the brass carpet spikes from Mapleshade, this elevated the amp about 1.5 inches above the carpet.  The ventilation was vastly improved, and the amp temperature went down from 135F to 115F.  But part of that change was from the bias dropping.  The idle was now 120W, which BTW is close to specs.

The bias circuit is a simple transistor, no IC.  I think it has a function that runs the amp hotter when it is cold, then slows down as the amp heats up, reaching an equilibrium of sorts.  But perhaps on the tall feet it is not getting hot enough to get going.

"I told them not to put the amp on tiptoes," Tony said in my extended memory.  "This amp, mine, is biased to sound just right on soft feet.  That makes it sound more relaxed, just like a real Krell."  It does turn out that my Aragon was early production, made in Connecticut.  This memory makes me wonder whether my copy is the same one I encountered before.

After listening to the Krell for months, and then testing the Parasound and Carver amps on my Acoustats, the Aragon sounded beyond just bright, it sounded strident.  I found one passage about 10 minutes into Pathetique on RCA Living Stereo Hybrid SACD that was screechy.

I was determined to find out why the amplifier sounded different, and badly so at that.  So I first measured the Carver amp which I will soon be returning to its owner, and then my Aragon.  The Carver looked pretty good.  The Aragon looked horrible in the right channel, precisely where I had heard the screechiness.   It was showing 0.18% THD at 1kHz and 0.84% IM.  The left channel looked ok at 0.012% THD.  These were measured, in fact I did all my Aragon measurements with the actual Acoustat speaker loads.  I did the Carver measurements earlier using an 8 ohm dummy load.  The Carver measured 0.0029% THD.  I used RMAA to do the measurements with a Juli@ card on an old PC, the measurements were very contaminated in the case of the Aragon by ground loops because it has a grounded plug, the Carver only has an attached two wire cord.  I spent hours unsuccessfully trying to fix the ground loop, first by re-routing the AC connections to a single power strip, and then by bypassing the Juli@ card itself with an external optical input DAC from Schiit.  Nothing worked, the very complicated setup using the DAC only made things worse because there is still a ground loop formed by the outputs of the amp to the inputs of the Juli@.

I was about to cut out some new felt feet and do that heat raising trick by lowering the bottom of the amplifier a fraction of an inch from a flat board to block ventilation somewhat but not completely.  But then I thought, why not tackle this bias thing head on, by adjusting the bias?  Better to run higher bias WITH better ventilation also, for the best of both.  In fact, the improved ventilation made possible by the taller feet means I can run even higher bias more safely than a lower bias with deliberately blocked ventilation.

I had avoided doing this before because measuring the bias on the left channel is all but impossible, though very easy for the right channel which is on the side of the chassis.  But since the bias problem appeared to be only in the right channel, this would be a piece of cake (well, as bias adjusting goes...it's a long multi-hour process).

So I went ahead.  It turns out it is very easy to adjust the bias on the Aragon 8008BB, even on the left channel.  There is only one potentiometer on the entire amplifier circuit board, right in the center.  It's a multiturn sealed pot which couldn't be nicer to work with.  It took several attempts to turn the screw enough even to make any difference, then I discovered I could turn it around several times to achieve the desired result.

You measure the voltage in mV across the bias resistor for every transistor.  Aragon specs put these at 12mV for the inner channel, and 8mV for the outer channel.  At first this bugged me a lot, but now I understand...you set the bias so both channels reach the same temperature.  That's actually very important when you have a stereo amplifier.  When the channels are thermally balanced they will help keep each other at the correct point.  When they are not thermally balanced there will be a thermal bias oscillation where one channel gets hotter then cools itself down, followed by the other doing the same thing.  The larger the thermal imbalance between the two channels, the greater the thermal oscillation.

Only after much turning of the bias pot on the right channel did I get around to hooking up the now quite complicated measurement apparatus, which also took very precise location of the spike-footed amplifier to reach all the necessary cables, especially the speaker load through the Canare Four Cross speaker wire.  I aimed to achieve a bias like I had before of about 0.26mV, with a heatsink temperature which would now be lower than the 135F the amp had idled at with the original low feet.

The measurements came back excellent!  The THD distortion dropped from 0.18% to 0.0048% and the IM dropped from 0.84% to 0.036%.  Once again I think the real numbers might be even better if I could remove the ground loop problem.

Now however I noticed that the left channel wasn't getting warm enough any more, and sure enough its distortion numbers had gone way up.  So I had to readjust the left channel up to get the distortion down again there, though it seemed that once I got the left channel THD down to 0.86% simply adding more bias wasnt help much.

So I removed the cover of the right side wondering how hard it would actually be to change the bias, if not measure it.  It turned out to be remarkably easy to change the bias by turning the sealed multiturn pot in the center of the board.  At this point, I didn't need to check bias, I could measure the distortion.

Finally I idled the unit for hours, and decided several times to back down when the temperature in one channel or the other exceeded 125F, or if they appeared to differ.  I tried to adjust both channels to the same long term operating temperature.

After a 3 hour idle test, temps ranged 120-130 in the right channel, and 120-127 in the left channel, with low temps in front and highest temps in the middle.

I decided that was close enough, perhaps hotter than desired...but I need the Class A for good sound.

Which is indeed enormously improved.  This is still a Yang sounding amplifier, but very clean now.  Not as warm as the Krell but a lot closer.



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