Sony vFET Amplifier Part 2

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#2,523
Gyuri said:
Hey, John,
I have an idea what do you check, assuming that your amp's DC voltages are O.K..
Put a about 2.2VAC 50Hz signal on input.
An audiocard output can be good, and there are online signal generators, like this:
Online Tone Generator - Free, Simple and Easy to Use.
Choosen 50Hz, because it can be measured most of the DMMs.
(In my case it is about 2.2VAC RMS, because this is the level, where my amp started to clipping.)
Measure AC signal at T18, there should be about 17.3VAC.
While output shows about 14VAC RMS.
It is almost exactly 16dB, as Papa stated in the article.
This is a triumph of science and engineering, as I used to say.
Kind regards,
Gyuri
Click to expand...

Thanks Gyuri, I'll see what I can sort out.
Regards
 
#2,525
Hi John,
As I can see, your measurements compared to mine, shows significant difference voltages between T7-10 and T9-T12.
Yours was:
T7 to T10 = 3.93V
T9 to T12 = -3.67V
Mine is:
T7 to T10 = -2.024 and T7 to T10 = -2.041
T9 to T12 = 2.063 and T9 to T12 = 2.019
Don't ask, what it means, I'm just saying. 😉
 
#2,526
The difference may be due to connected the instrument polarity.
But the value almost twice of my measurements.
I've always joined minus pole to the first testpoint, the plus pole to the second test point.
I hope, this was understandable. 🙂
 
#2,533
It is a better place: 🙂

I made a little mistake, because I wanted to do it well (we know it, the road to hell is paved with good intentions) as I haven't used those little heatsink for 2SK2013/2SJ313.
Rather I coupled them to the main heatsink with aluminium blocks.
So I must have to forget that front end would reach its working temperature on its own.
It resulted that I must have had to lowering bias current at front end, as SONYs heated up the whole thing.
It caused some trouble for me, as this kicked off the offset balance
Although bias is exactly 100mA at R32, and offset on output around 1mV at each channel, and amp sound extremly well, I'm not satisfied.
My question is:
How can be set all of the operating points to a correct state, when SONYs at their places?
Some measurements here:
http://www.diyaudio.com/forums/pass-labs/295311-gyuris-pass-sony-vfet2-build-9.html#post4954842
 
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