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Old 28th May 2006, 03:55 AM   #11
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Klaus,

At the risk of repeating what others have said in previous threads the caveats of matching the input pair on both sides of the lower output stage are important.

For example in my baby X Aleph I matched the input pair within 5 mv and the outputs within 10mv. On testing it started out at about 8 volts and quickly dropped but I was not happy about the variations.

On my next effort I used jig with one 9610 as a biased reference and was able to match under 1mv because of the improved resolution of the meter.(measuring the absolute difference in Vgs)

I also carefully matched and selected matched sets IRFP240's for the lower output halfs and matched the bias and current gains and bolted the input pair together with a solid block of aluminium.

In the chassis the variation is typically under 100 mv after an hours warm up. I used to spend all night tweaking it to get it right on 0mv until it started to annoy my chickie babe!

The output resisters to ground are the important ones and have the most effect. I doubt however that even a 1 volt would cause a concern under dynamic conditions.
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Old 28th May 2006, 04:17 AM   #12
moe29 is offline moe29  United States
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i had problems until i hyper-matched the diff pair in a working circuit, and then i mounted them back to back. I also "hyper-matched" the
big FETS in the working circuit... then things worked out fine.
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Old 28th May 2006, 07:44 AM   #13
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Hi Macka,


when you say "under a 100mV after an hour of warmup" then what is the absolute dc startup value?

William
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Old 28th May 2006, 08:43 AM   #14
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William,

Its been ages since I turned them off so i am sorry I dont remember. If l turned them off I would have to call the power station to turn them on again! The cat would also hate me.

I think is was something like 5-6 volts and it very quickly dropped to under 2 volts and then went down slowly.

regards

Macka

Ps. You know we do have a sense of humour down here in The Land Downunder.
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Old 28th May 2006, 04:26 PM   #15
Klaus is offline Klaus  Germany
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Hi Maka,

thanks for your matching hints! Well, I am sure to have "hyper matched", too. We did a group buy and I had many FETs to find really perfect pairs with ongiong production serial number from the same lots. So I do not see a main reason here.

I will try to analyse in detail which device has which temperature drift. A first step towards constant conditions should be to bring the diff pair also onto the main cooling plates - so they also see refernce temp and heat up faster. Second step could be to play around with NTC or PTC bypasses around important Rs or to use cole Rs with negative temp drift (?).

I want to get rid of R47 and 48 - they can not be to helpful soundwise. Feedback to the bias, that is not so nice...

Regards especially to downunder

Klaus
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Old 28th May 2006, 04:48 PM   #16
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Hi Klaus,

I tried a few things too to look for temp influence. It cost almost a whole can of ice spray but I found out that (in my amp):

cooling the diff pair had no influence on abs. dc
cooling the heatsinks of the output fets had no influence
cooling the current source for the diff pair had a big influence
cooling the zener had influence

I donīt think I tried the BC550 for the main current sources but I will do that next time.

william
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Old 28th May 2006, 07:50 PM   #17
Klaus is offline Klaus  Germany
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Quote:
Originally posted by wuffwaff
Hi Klaus,

I tried a few things too to look for temp influence. It cost almost a whole can of ice spray but I found out that (in my amp):

cooling the diff pair had no influence on abs. dc
cooling the heatsinks of the output fets had no influence
cooling the current source for the diff pair had a big influence
cooling the zener had influence

I donīt think I tried the BC550 for the main current sources but I will do that next time.

william
Hi William,

thanks, very helpfull!!

I will try first do move the CCS FET for the diff pair from the PCB to the cold housing bottom plate. This should give a reference temp and reduce drift significantly. Afterwards I will play around with the cooling of the diff pair.
Zener drifts are not existing in my amp by using the LM329 voltage reference with low drift of just 50 or even 20ppm/K.

Regards

Klaus
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Old 28th May 2006, 07:54 PM   #18
Netlist is offline Netlist  Belgium
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I build my CCS with the TL431 and I'm very happy with it.
The first prototype was with the classic IRF9610 and I found it harder to make it behave.

/Hugo
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Old 29th May 2006, 04:30 PM   #19
moe29 is offline moe29  United States
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i saw this on another thread:

From Nelson

1) Match parts for absolute and differential DC offset

2) Trim values with potentiometers for absolute and differential
DC offset

3) Common mode DC offset feedback from the outputs back to
the sources of the input diff pairs (for absolute DC offset)

4) Differential mode DC offset feedback (regular X feedback) for
differential DC offset

5) Resistive loading at the outputs to ground (about 100 ohms
or so) which minimizes absolute offset drift

Works like glue....
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Old 4th January 2009, 02:56 PM   #20
Telstar is offline Telstar  Italy
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Klaus can you please send me an email?
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