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Old 23rd July 2011, 02:18 PM   #6651
RCruz is offline RCruz  Switzerland
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I am now prepared to make some measurements in order to understand if the circuit is acording to your plan.

Would you point me the most important values I should be looking for ?

First measurement:
Q2 Drain to GND
R = 7.3v
L = 6.9V

These should be 10~11v... what must I change ? Rs on se3cond stage ?

PSU = 28V
R6 (Rs) = 33r in both channels.
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Old 23rd July 2011, 03:13 PM   #6652
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Up R6 until you get 10-11V drain to ground. That will control the gain more dominantly also for both channels at that stage. It will also bias the output follower harder, do check that its current on its resistor to ground will not top IDSS. If it does, up that resistor too.
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Old 23rd July 2011, 06:59 PM   #6653
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Analysing left channel, as Vdrop on R10 (Rload = 4k7r) is 20.3V current should be around 0.0043.

To get Q2 D from 7 to 10V, current should go down to 0.0037.

So R6 (Rs) should be raised from 33r to 39r right ?
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Old 23rd July 2011, 08:16 PM   #6654
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It is so in the pdf too.
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Old 23rd July 2011, 09:13 PM   #6655
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Quote:
Originally Posted by Salas View Post
It is so in the pdf too.
Indeed.... I overlooked it. I am working on an old build.

I also found another issue.... I am using 820r for R7 (buffer Rs)... As R7 Vdrop is 7.9V, current on Q5 is 9.6mA (Over Q5 8.5mA Idss)... I wonder why it works ?
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Old 23rd July 2011, 09:20 PM   #6656
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It works, won't break but it can be altering the THD profile. Use 1.5K there if you will go 39R or about for 10-11V.
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Old 24th July 2011, 09:26 PM   #6657
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I replaced buffer Rs by a 1k2r and altered second stage Rs from 33r to +- 43r. (trimmer)

Now I can set drain Q2 to 9.75v in both channels.

Sound is much more airy with better transients but I did not solve the gain difference.

A good mod soundwise anyway
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Old 25th July 2011, 12:26 AM   #6658
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It looks like you are having high enough pinch off to need 43R. Its not a problem. Having both at 43R it will keep them in same balance. The idea was to have control to differentiate them for gain. Try replace the Jfet in the hissier channel's' head-amp first its still a better tactic if the difference starts there..
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Old 25th July 2011, 12:35 AM   #6659
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Try replace the Jfet in the hissier channel's' head-amp.
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Old 25th July 2011, 07:37 AM   #6660
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I will do that. This is an old build that was already subjected to severall mods and I am not sure the head amp jfets are matched.

Looking carefully, I found the BC550C to be different also (different makes).

The hissier channel plays louder.... Can this be due to using a higher Idss jfet there ?
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