Aleph J new build testing

[RainfallSky]

here is the grounding, note although i have dual power supply tx and boards, the mains power is common IEC.

Is it Western Electric wire at the center? Or is it Jupiter Condenser Co. wire?
If it's Jupiter wire then isulation is just a lacquered cotton.
I think it's not safe for mains..

 

[Wang1]

Thanks for heads up, you are correct, i changed already👍

 

[Wang1]

Ran into a problem, cant reduce dc offset below 180mv or so using R7, (on both channels), bias is ok, can adjust without problem, before when i checked seemed ok, would appreciate any pointers

 

[Zen Mod]

post exact schmtc of your build

 

[Wang1]

thanks

 

[Dennis Hui]

Did you install both the 2k pot and the 1K fixed resistor for the R7 position?
If so, get rid of the fixed 1K.

 

[Wang1]

yes

 

[Dennis Hui]

Then get rid of the fixed 1K and just trim with the 2k pot.
With the 1K the adjustment range is too small.

 

[Wang1]

I can try, but for my understanding what is going on?

 

[Zen Mod]

do what Dennis said

not enough voltage across that drain resistance, if both trimpot and resistor are populated

remove 1K, prior to that back down with trimpot setting

not enough voltage there, not enough Ugs for lower mosfet, thus not opened enough, resulting in positive DC Offset

 

[Wang1]

thanks for the explanation, will try

 

[Wang1]

Thanks, that did the trick and now i understand why, but why bother with the 1k resistor in the first place?

 

[Dennis Hui]

You use one or the other for R7 but not both. Your schematics probably should have made that clearer. Using both is a mistake.

 

[Wang1]

And what voltage should the Vgs be aproximately

 

[Wang1]

Thanks Dennis,

Actually, i asked this question some time ago when i was designing the pcb, i must have misunderstood the reply and kept R7, now all is clear 👍

I see now Vgs should be about 4.3v, is that correct?

R7- Is the fixed 1k resistor removed in place of the pot? Then once the offset is fixed is the pot replaced with a fixed resistor of the required value? (Same question R27 re bias)

 

[andynor]

Ranges will vary from batch to batch. But IME, 3V9-ish to 4V3 is perfectly normal.

Since your devices Vgs is unknown to you, I presume you bought matched transistors. If not, you have some work to do 🙂

Regards,
Andy

 

[Wang1]

they were bought from diyaudio shop

 

[Dennis Hui]

Nominally about 4.3V across R7.