Sony VFET Amplifier Part 2

and now i have twins :cool: any my brother has his own vfet baby.

second time biasing was easier, best tip was to first bring t6/t7 and t8/t9 up to 1.4v with p3/p4, offest t18 around +-100mv and warm it up, wait 10min.

readjust p3/p4 for offset t18 around +-100mv then bring r32 up to 80mv with p1/p2, output offset +-50mv. wait 15 min.

now adjusting all 4 pots p1/p2 - p3/4 in cycle up to 1.45v at t6/t7 and t8/t9 and r32 to 90mv (check first t18), againg offset around t18 +-75mv and output offset +-50mv. baby steps needed or it run away fast (even more with one turn pots). let it heat up then for at last 30 min.

after this time r32 was a little bit under 100mv. go for 1.5v at t6/t7 and t8/9 and r32 to 100mv, check offest's. readjust again in cycle all 4 pots for the target bias and offsets. now with baby baby steps :). wait after every adjustment some seconds, you get the feeling when the circuit get into a stable state. dont touch the small heatsinks, the offest over t18 acts at once. i done the final fine adjustment via p1/p2.

at the end i had a bias of 101 mv, offest t18 floating around +-50mv at t18 (1.51v) and output offsets +- 20mv.

remember everything up to +-200 mv over t18 is ok and +-50mv output offset is ok, +-50mv over t18 and +-20mv output offset is perfect for me. the amp needs 1 hour to get stable. floating around a bit is normal.

this way was a lot easier then put t6/t7 and t8/t9 direct to 1.5v, hope that helps.

don't rush :).

Can I just confirm that this is how we are setting the bias? Ie: in terms of testing the offset, we are checking the offset between Ground and T18, AND also the output offset - ie: at the speaker positive output?

I've so far spent two days checking these - getting close - but I check the output offset, I am taking baby steps, I go back to T18, and Im at 2-4V, I correct that, then go back to the speaker outputs, which are at about 2-4V, and so on, and so on......

Cant help feeling like I am going around in circles. I am being very patient. I have had a listen and it sounds GREAT! However otherwise I havent had this much fun since I lost all my limbs in a deadly fireworks factory explosion in '68.

Just confirming I am doing this correctly?

thanks
 
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Yes, you measure T28 to ground and between speaker positive and negative, and try to get both near or at zero. Rumina's instructions are good.

It is best done with two or even three meters. One each for the above mentioned offsets and one for checking the voltage drop across R32, which should be at 0.1V (bias measurement - .1V drop across 0.1 ohm equals 1.0A)

Make small adjustments to the pots, and pay attention to the changes to the meter readings and try to get a sense of what happens to the readings when you adjust the pots. If a pot adjustment causes a large movement in voltage, back it off to reduce the large movement. So small adjustments mean small pot rotations and small voltage changes.

Also the component temperatures affect the measurements. So put the amplifier cover on between adjustments and try not to take too long making adjustments. When I first set up my amp, I left the cover off during the setup period and when I thought I was done, I put on the cover and the resultant increase in internal temperature put everything out of spec.
 
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Just opened the Gerbers. They are the single-pair output version.
I am interested in the two-pair output version as well.

Ah, then that's causing the confusion. If I remember right, Nelson has posted the gerbers for the 4 output version also, some time ago. Haven't found it in this big thread yet.

Maybe we can ask Tea-bag to run another group buy of the boards.

I have some three output versions of an output stage that can be used in all the VFET designed posted.

I am trying out a creative way to mount the boards and devices, so it might not work for everyone, but I'm glad to share the boards that I have already made.
 

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I have some three output versions of an output stage that can be used in all the VFET designed posted.

I am trying out a creative way to mount the boards and devices, so it might not work for everyone, but I'm glad to share the boards that I have already made.

If I understand correctly your board can be built with 2 pairs of Vfets only right?

What creative solutions are you looking into for mounting into the heatsinks?