passlabs a40 project..question

[tone]

if i were to omit the Q11 N channel Jfet..to be replace with a 10k resistor, how do connect it.? from R9 to which pin G,S or D.using the original circuit broad from passlabs article.thanks

 

[MikeW]

From the base of Q3 to ground.

 

[jam]

Tone,

You could also replace the resistor with a current diode. (The fet in the A-40 article is strapped like a current diode). This gives you a 20db improvement in rail rejection.
Failing that you could split the 10k resistor into two 5k resistors and attach a cap to ground (say 200uf), observe polarity.

Jam

 

[ElectroNick]

Hello

I've got another question about the A40. We've just completed the amps and got 'm running. Only 30 mV DC offset, so that looks OK. Maybe in the future a bit of tweaking and we could reach 0 V.

But... the tantalum caps in the amp were replaced with solid aluminium types. It lookes like the amp is a bit more quit in the high frequencies (compared to my amp that is..) Could those caps be responsible for this ???

Has anybody experimented with those caps (C2,3) to improve the sound.
Any suggestions ??? Or are we not patient enough and should we wait till the amp has played for a few days ?

Grtz,
Nick

 

[Eric]

Nick: My amp has approx 30mV offset on one channel and closter to 45-50mV offset on the other channel. I don't find that this creates any problem at all. I didn't use the tantalum caps either; instead, I put 220uF 25v BlackGate FK caps in their place. I can't really compare the tants to the BG, but I am very happy thus far with the amp.

I have not yet had the opportunity for a critical comparison between the a-40 and my Marantz MA500 monoblocks that I've been using for years now.

Jam: You've recommended using a current diode in this place before. Since I'm still somewhat of a newbie, could I trouble you for a more detailed account of how to pull this off? Thanks!

My a-40 project - pictures soon.

Eric

 

[jam]

Eric,

Look for a 3 to 4 ml current diode. (I believe Motorola makes them). Solder the banded end to the top of the diode stack (base of current source transistor for differential pair) and the other end to ground.

Check

http://www.fosu.edu.cn/fas/Library/chips/1n2n/1N5292.pdf

Jam

 

[jam]

Nick,

I have not noticed much difference between the types of cap used but I have a slight bias (no pun intended) to black gates in that position. 30 to 40 mV offset is not bad and I would not worry about it. If this still bugs you could adjust the current source to null the offset.

Jam

 

[Eric]

Thanks, Jam!

 

[colwaiting]

A-40 parts list

The small signal trans are a bit unihqe
would a mps 18 ( higher gain) or a 2n 4401 (general purpose lower voltage) be ok? And how about newer versions for those drivers--Are we working in the 35-100 gain range? (there are some nice linear panasonics@ digikey)

 

[colwaiting]

parts sub

Nelson answers this at passdiy.com
and capably, i might add.>>Doug

 

[MikeW]

Rails

What is the rail voltage +/-42 volts or +/-34 volts?

 

[jam]

+/- 32volts give or take a volt or two.

Regards,
Jam

 

[MikeW]

One more question. Does anyone have a cross for Q11 that has the correct pinout? Can I use an IRF9610?

 

[jam]

In short, the answer is no. The IRF9610 is a mosfet Q11 is a Jfet.
Use a current diode.

Regards,
Jam