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herodote 20th July 2006 04:37 PM

PASS A40 - bias
I've build the A40 with lambda BJT from german supplier (

While setting the bias with R11 i wondered how do one knows that the bias is correctly set up ?

Mr Nelson Pass writes
- 1.5A per PMD trail (so 0.75A per transistor), p. 70 of the a40.pdf file on passdiy
- as well as to measure 1 volt (DC?) between Q7 and Q9.
Measuring 1 volt get 0.72 (1.44 for the trail)

An other article ( states 0.8A per ransistor (so 1.6 per trail). Is there a difference (except the heat).

What shall be the target value ?

What does trigger the DC offset ? I have 40mV is that acceptable ?

Nelson Pass 20th July 2006 06:37 PM

Not much difference between 1.5 and 1.6A.

40 mV is OK in my book, but you could trim it lower if you
want by adjusting the resistor off the Collector of Q1.

herodote 21st July 2006 01:17 PM

Trimming DC offset
Thanks for your answer, only to make sure. to adjust the resistor value OFF colector of Q1 in order to reduce the DC offset means adjusting the value of R6 which is currently of 680ohms ? Is it a correct understanding ?

Small question : A40 is driving a home made loudspeaker based on 4 fostex motors with a 103db w/m, so fairly sensitive - which is good because no need of lots of power -. At the start of the amplifier they is a slight 'bump' in the loudspeaker (i connected another pair 93db w/m nothing can be heard). Its there a way to avoid this 'bump'


Eric 21st July 2006 02:10 PM

That's my web page that you've referenced above. At the bottom of this page, I have listed a number of different (but very close) bias points that I experimented with. These differences are pretty small, and make only minor changes in power consumption and heat dissipation. The main reason that I tweaked this a little was to achieve a bias setting that is closer to Nelson's article, since I used different transistors.

I did not detect any changes in sound quality as a result of the different bias levels.

Enjoy your new amp!


herodote 24th July 2006 10:56 AM

Thanks Mr Pass the advise cured the problem. I still have a slight 'bump' in the loudspeakers at the amp start. Can that be cured ?

Your web site (passlabs, passdiy and firstwatt are real pleasure (or treasure) to read for non expert.

Thanks to Eric, the article was indeed usefull.

Eric 24th July 2006 01:30 PM

I don't recall if my amp has a bump at turn on or not... Either it is very small and I've gotten used to it, or its not there - I'll check tonight. One thing that might help is an inrush thermistor on the power supply. I think that Nelson has recommended a CL-60 for this or other of his amps...


amp_guy 25th July 2006 04:45 AM

cl60 works fine in A40
Ihave an A-40 that has run daily for years.
it has a cl-60 inrush limiter with around 200,00 uf of caps.
it is essential ! you will smoke your power switch and probably the bridge rectifiers as well

It would be interseting to know what output transistors everyone else has used (succesfully of course)
mine has motorola 2N6051 and 2N6058
with the bias adjusted to about 1.5 amps.

Eric 25th July 2006 02:23 PM

There is a very small turn-on thump with my a40 - small enough that it doesn't bother me in any way. The thump might be less pronounced because I'm running a small set of bookshelf speakers that don't have a great deal of bass output. With a more "full range" speaker, the thump *might* be a little more prevalent, but I don't know.

I have about 240,000uF of caps for both channels together, but do not have a thermistor installed. I suspect if I installed a thermistor, it would solve the small power on thump and also prevent the momentary dip in the room lights that happens at power-on. My a40 has been running for about 3 years...


Edit to add: I'm running MJ11015 and MJ11016 transistors from On-Semi in my output stage. If you substitute this part only, the bias comes out very slightly low compared to what Nelson describes in the article. I've adjusted R11 a little to bump the bias back to where it is specified in the article.

Nelson Pass 25th July 2006 07:15 PM

If you put a large cap across the bias transistor and reversed
diodes from the + and - of that cap to the output it will tend to
kill the turn on thump.

lumanauw 26th July 2006 02:32 AM

2 Attachment(s)
Thanks Mr.Pass, for the tips :D

I don't understand about the purpose of D604a and D603a (ISS82). They are from bias generator, but connected not connected to output node. Are they for "turn on thump" problem too?

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