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-   -   Help identifying the signal path (http://www.diyaudio.com/forums/solid-state/626-help-identifying-signal-path.html)

Eric 22nd August 2001 08:43 PM

I'm looking into placing high quality resistors in the signal path of my a40 amp (<a href="http://www.passlabs.com/images/projects/a40f6.jpg">Schematic here</a>).

I want to be sure that I have correctly identified all of the resistors that are in the signal path. From my limited experience here, it looks like only the following resistors are directly in the signal path:

R1, R2, R4, R5, R13, R14, R15

Have I missed any?
Have in mistakenly included any that are not in the signal path?

Thanks for your help!

Hoffmeyer 22nd August 2001 08:53 PM

Hi Eric.
R3 is missing, and so is R6, and R16-20.

Eric 22nd August 2001 09:21 PM

Hoffmeyer, thanks for the additions. I know I left out R16-19. These are wire wound resistors that I've already chosen as MILLS MRA-5 1% 5w.

So, now the list of resistors in the signal path are:

R1, R2, R3, R4, R5, R6, R13, R14, R15, R16, R17, R18, R19, R20

Damn, thought I was going to save myself some money here. It looks like 14 out of the 21 resistors per channel are in the critical path... guess that's what Nelson means when he talks about simplicity of design - not much extra in there...

Thanks for the help!

djk 23rd August 2001 12:17 PM

The only resistors that are NOT going to have a large impact on the signal are R8,9,11,12,13

Eric 23rd August 2001 01:53 PM

Hmmm... I'd be inclined to agree that R8, R9, R11, and R12 are not in the direct signal path, but it appears that R13 is part of the feedback loop... Wouldn't R13 then be a critical value?

Thanks for the help!

Hoffmeyer 23rd August 2001 04:13 PM

Eric.
Now that you're replacing resistors in your A40
1: does that mean, that it's the original layout/partlist?
2: have you considered replacing C2 and C3 with Sanyo Oscon electrolytics? They will fit into the original layout.

And now to your question:
Yes, R13 is in the feedback-loop, as described by mr. Pass himself, and must therefore be as critical as the rest of the resistors in the signalpath.

Eric 23rd August 2001 04:20 PM

Hoffmeyer: yes, I'm using the original parts list and layout from the Pass labs web page. I've managed to locate all of the originally specified parts with the exception of the output transistors (see my <a href="http://www.facstaff.bucknell.edu/esantane/movies/a40.html">parts list</a> here). For those, I have MJ11015&16's from OnSemi. However, I have not built the amp yet - too many other projects in the way, so I've just been slowly collecting parts. I don't suspect that I'll begin construction until around December.

For C2 and C3, I was thinking of using Black Gates FK series polarized caps. I haven't read anything about the Sanyos...

Thanks for the confirmation of R13!

djk 24th August 2001 08:10 AM

"The addition of D3 and R13 provide feedback, treating the base of the transistor as the (-) input of a summing amplifier. Inasmuch as the differential voltage between the emitters of Q7 and Q6 is relatively constant in this class A amplifier, the bias circuit rejects the AC signal, and what does creep through is removed by C3. Thus the bias circuit watches DC bias and ignores the signal." I'm going to change my mind and add R7,10 to my list as they only see DC that is both constant in voltage and current.


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