Pass-tharagard A-75 upgrade - diyAudio
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Old 31st December 2005, 03:09 PM   #1
daly2k is offline daly2k  United States
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Default Pass-tharagard A-75 upgrade

Amigos, I recently put my Aleph 1.2's on the shelf to add some new line filters and to tackle some xformer buzzing issues and I fired up my 1993 Pass/Tharagard A-75 to run my maggie 3.6r's. However I had to make some politicially correct revisions. I replaced my two power xformers (one per channel) which were taken from some dyanaco 400 amps. They buzzed and ran hot at the bias levels required.
I replaced them with a torroid set up for 4 ohm speakers. It is almost twice the rated current of the one suggested in Nelson's and Norm's article. I added a 2nd regualtor for the driver board so as to maintain dual mono operation and a balanced volume control.

The sound is awesome, quiet, clean, and matches the Aleph 1.2"s for soundstage but does not have their warmth or power. When I reread Nelson's and Norm's article, I noticed the reference to folded cascode operation! Did anyone use this option and did it make a difference? I used the schematic as shown.

I also set the feedback direct from the outputs to the driver board as suggested in the article and the square waves look perfect. Did anyone try feedback at 50% or at other values then suggested? I don't want to dig back into the amp since it is sounding very good. But when I get my Aleph 1.2's modded I may want to expeirment. Thanks in advance for any ideas.
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Old 31st December 2005, 07:26 PM   #2
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If you go with the folded cascode or lesser feedback you may
get some of the warmth, but it's interesting that you did not
experience as much power, as the A75 would have similar
power capacity into the lower impedance of the Maggies,
I believe.
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Old 31st December 2005, 08:00 PM   #3
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I built my A75s with 100R in R21 and R22 I intended to experiment by switching them in and out, but liked it enough in folded cascode operation that I never bothered. I also used a jumper for R81. The sound is a bit more liquid and warmer than my Leach amps. Hope this helps.
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Old 1st January 2006, 03:58 PM   #4
daly2k is offline daly2k  United States
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Nelson and Bob Thanks for the reply,
Regarding the power output of the A-75; when I tested it using a 5ohm 200 watt resistor, I got over 160 watts output/ch with no signs of clipping. I measured the dc on the output rails of both channels and it was a rock solid +/- 41 volts. Regulators were at 50 volts.
The tranny I put in isn't loading down since it is two sizes larger than the spec't (800VA-I think). My Maggies don't put out a lot of bass and my big criticism of them vs other speakers is their dynamics. I have them in a big room 15 ft from the rear walls and 10 or more feet from the side walls. (Basement converted to listiening rec room. The A-75 does sound more authoratative in the bass than the Alephs. My comment on power was total watts produced, my Aleph 1.2's will output 300watts into 4 ohms and measure 64 volts on the rails. Both amps image fantastically and the tonal balance is whatever is on the recording with the Aleph's a touch warmer like tubes. Maggies can't do drum kits like horns. By the way I am using Buggtussel subs set at 24db-50hz. The maggies are really power hungry and will suck up as much power as you can give them.

The A-75 to me sounds better than the McIntosh 402 which I had for a while. That amp (402) was exceptionally clean sounding and had the dual balanced design. Of course the 402 had all the protection features and more power, I just put a vol. control on the Pass so I can mute the amp safely (I have a tube preamp).

I am building some line arrays (12 ribbons and 12-5" mids per side). 6ohm impedance, 1500hz 24db crossover, and I hope 96db efficient. Will the Pass first watt work with these? Or will damping be an issue?

I read the el-pipo article and was even tempted to do a smaller version but my wife suggested a permanent room in the garage.

I will give the folded cascode a try after my Aleph's are back on line. I haven't heard any amps commercially that I would use or buy to replace what I have. Of course I have not seen or heard the Pass XA's. These are excellent circuits. dave
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Old 1st January 2006, 09:40 PM   #5
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You will find some personal observations on the sound of my A75 versus feedback and folded cascode here. In short, experimentation seems to be very worthwhile. Remember that folding cascode reduces the amplifier's gain. Cheers . Pierre
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Old 2nd January 2006, 12:47 AM   #6
daly2k is offline daly2k  United States
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Thanks Pierre for the feedback, I liked the suggestion of an external switch to try different resistor values for global feedback.

When I first built the amp almost 13 years ago, I remember testing it without any global feedback, and the square waves etc did not look very clean so I never listened to it this way. I was also a bit worried about oscillation without any feedback but it never occurred during the testing. My input diff j-fets are matched closely but not manically close and my offset is 20mv or less after warm up at the recommended bias. The amp is dead quiet-no noise even with my ear attached to the speakers.

I do think that 100% global feedback does give ithe amp a clean distortion free sound, and I can hear the echo affects in the recordings clearly. I am interested to try the folded cascode.
Saludos, dave
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