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Old 15th January 2013, 09:29 AM   #2211
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Quote:
Originally Posted by Salomon View Post
Now you find more pictures and several infomations about my Aleph-X here on this website:
Class A monster: Aleph-X by Horst Wolf Audio Tweak
Hi Salomon,

very nice amp!
On the website you mention a "special thermal offset correction"

Would you care to tell a bit more about it? I would be very interested.

William
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Old 19th January 2013, 01:41 PM   #2212
Salomon is offline Salomon  Germany
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Hello William,

sorry for the long wait!
I donít know the exact values of my modification because it was more than two years ago and I did not make a documentation of it.

I think I have made the following thinks:

You have to take two pieces of the PTC KTY81-110 and solder in both parts parallel instead of the 475 R source resistor of the constant current source.
Be sure to make a good thermal coupling of both parts (per channel) to the heat sink. Now you have to re-adjust the offset with V2 (200R)
You will get a low offset in a much shorter time than with a normal 475R Resistor. The two PTC have soldered parallel in cold state a value of 500R.

I use this modification since more than two years together with modified DC-protection boards (see the pictures in the left and right back side of the amp) who are switching the output pins to ground and at the same time switch completely off the amp.

I do not use relays in the signal path, because they reduce the sound quality.
The protection boards are controlling each of the four output Pins of the Aleph-X amp separately.
It has not been necessary to re-adjust the amp since building the amp.
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Old 19th January 2013, 04:30 PM   #2213
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Hello,

thanks for the reply. I will try that. I remember looking for a positive tempo zener but that was a bit difficult.

William
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Old 28th January 2013, 01:10 AM   #2214
Eric is offline Eric  United States
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Looks like an interesting solution to offset! I'll have to give this a try. Currently, my amps live in an unfinished section of my basement that ranges from a low temp of 13c during winter to 20c during summer. As a result, my DC offset drifts as much as 2.5v on the completed amps. Heat sinks show a 35c temp rise.

I wonder if this will keep offset under better control...
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Old 28th January 2013, 09:03 AM   #2215
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Hi Eric,

it should. I did a few calculation on which resistors to use (R24/26 and VR2) to get the best result. Will try this when I get back home. Will report as soon as I have tried it. Now the bias through the diff pair in my amp changes almost 3% from cold to warm. With the PTC it becomes more like 0,2% (theoretically).

William
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Old 28th January 2013, 07:02 PM   #2216
Eric is offline Eric  United States
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Looking forward to the results, William! This seems like a relatively quick and easy mod. After this, I still want to replace my 9610 input differential with JFETs - so many projects, so little time.

Eric
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Old 30th January 2013, 04:31 PM   #2217
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Hi Eric,

maybe it would be better to replace the 9610's first as my changes and values will be based upon a Jfet input......
But I will try to create a sort of spreadsheet to calculate the resistor values (R24/26 and VR2) based upon the PTC, bias through the input pair and Vgs over Temp of the current source Fet.

William
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Old 30th January 2013, 06:56 PM   #2218
Eric is offline Eric  United States
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William,

I have a few matched quads of JFETs, but don't know where to go from there to replace my input differential. If you could give me a rundown of any resistor value changes and how to go about replacing the 9610's I would be very appreciative!

Eric
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Old 30th January 2013, 08:34 PM   #2219
gl is offline gl  United States
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Hi Eric,

Here you go. I have since removed the 15 ohm JFET source resistors. Matched 2SJ74's will do just fine in place of the 2SJ109's.

AX100 100W Aleph-X Monoblocks

You will be very pleased with the improvement in the sound of your amps.

Graeme

P.S. Please note NP's follow-on post and comment.

Last edited by gl; 30th January 2013 at 08:38 PM.
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Old 30th January 2013, 09:25 PM   #2220
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As an added note, I had to put the 4.7uf input coupling caps back into the circuit to get the amplifiers to work properly.

Graeme
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