Pass Aleph P 1.7 preamp - distortion

[xmerlin]

Hi to all,
I'm finishing my 6 channel based on nelson pass aleph p1.7 but I've some distortion problems with some modules.

Mosfet are matched with an agilent 34401a ... < 1 mvolt

First module is ok , the the other have this issue:

I've misured THD with keithley 2015 thd ...and the issue is only over 4 volt rms on the output.

Do you have any idea? ...I've changed also pairs ...with the same result.

Best regards,
Christian

 

[Zen Mod]

pics and dc measurements

 

[xmerlin]

psu voltage: 61,67v

IN 2v RMS
OUT 9,70v RMS
THD 3,351 @1Khz

Q15 Q16 Q17 Q20 Q19 Q18 gate-source voltages
29,60 19,83 10,82 10,68 20 29,23

---

psu voltage: 61,67v

IN 2v RMS
OUT 13,2v RMS
THD 0,519 @1Khz

Q15 Q16 Q17 Q20 Q19 Q18 gate-source voltages
29,78 19,72 10,75 10,69 19,99 29,39

some gain values lower drastically THD ...I've mached the mosfet with the nelson pass procedure ...but probably they are not well matched.

thanks,
Christian

 

[Zen Mod]

if everything is made identically on good and bad channels , problem is in mosfets

 

[xmerlin]

As I can see also good channels have strange THD with some gain levels.

THD is going up and down ...with little changes on the gain level.

All the channels are made with the same components ...same pcb and so on.

 

[xmerlin]

I've lowered the output of the signal generator to 0.5v (1v before) and now the problem is gone.

I'm driving the aleph with unbalanced signal and I've connected - to GND on INPUT and OUTPUT

Could it be related to some clipping?

Thanks,
Christian

 

[Zen Mod]

of course it can be