A40 help needed - almost working

[Eric]

Hi Folks,

I am having trouble with my A40 amp and hoping someone can lend some insight. The amp works - it produces sound- but something isn't quite right. Here is what I've done:

I got everything wired together and powered it up. Immediately after power up, the smoke escaped from R15. A little bit of playing around revealed that I mixed up my NPN and PNP transistors for the output stage (rookie mistake, I know). So, I swapped Q7 and Q8 for Q9 and Q10. I powered it back up and my R15 did not smoke. I made all of the measurements described in the article and they are all reasonably close, but the sound was terrible - lots of distortion. Further poking revealed that my mixup also toasted output resistors R16 and R17, so I replaced them also. Although I think they are fine, Q7, Q8, Q9, and Q10 have also been replaced.

Now I have an amp that makes music that sounds nice (on my crappy test speaker) but still seems a little "off". Further checking revealed that almost unmeasurable voltage (probably in the mili-volt range on my analog meter) across each of R16, R17, R18, and R19. This, together with the output transistors being completely cold after 30+ minutes of running, seems to indicate that there is virtually no bias current for the output stage. This would seem to explain the less than robust sound from the amp.

So, given my screw-up, what else is likely to have been toasted when I powered up the first time?? I'm thinking that I should replace Q6 (controls bias) and maybe also Q4 as both are connected to R15 that completely fried.

Any insights or advice??

Thanks!

[ElectroNick]

Hi Eric,

I too had problems with firing up the A40 for the first time.
Because Q11 couldn't be found I used a subsite. But... this one had a different pinout. So you guess what happened. A big flash and smoke
Replaced that Q11 and still no effect. Because both diodes D1 and D2 blew also. Replaced them too and that seemed a lot better. A few mv's AC at the output, no DC.

Be sure to check the voltages mentioned in the http://www.passdiy.com/pdf/a40.pdf . If these are correct you can safely connect it like i did.


Good luck,
Nick

[jam]

Eric,

Replace Q11 with a 4ml current diode, failing that you could use a 10k ohm resistor till you get it working.

Jam

[Eric]

Nick, all of the voltages mentioned in the article are present. What I'm afraid that I have now is a trial and error replacement of parts until it works.

Jam, thanks for the advice! I'll give it a try. Right now, my main limitation is that I've run out of spare parts. When I ordered all of my original parts, I ordered a few extra of each, with the exception of the MPSL01... I'll check on Q11 and probably have to re-order some more transistors.

Thanks! I'll keep working!

[Eric]

OK, here is what I found. Jam, I replaced Q11 (had a spare) and there was no difference in performance. On a hunch concerning the low bias, I replaced Q6 with a similar transistor I stole from the other channel.

On powerup, all voltage measures are correct and my output transistors started heating up pretty quickly. A quick measure across the output resistor now shows approx 0.45 to 0.5V and the sound is much improved.

So the upshot is that because I mixed up my NPN and PNP output transistors, I blew R15, R16, R17 and Q6. Now I have to order a few more replacements for Q6 to get the other channel to work!

On a final note, I've got a low level buzz in the working channel, audible from about 3ft from the speaker. Sounds like it could be in the neighborhood of 120Hz or so... Would a choke on the power supply cure this? Or is this a result of using unidentifiable surplus caps in my power supply? I have bypassed the bridge with small ceramic caps...

Thanks for your help and support!

Eric

[woody]

Seems like the bias network isn't working. Have you checked the
transistor orientation, the resistor values in this network and
the polarity of the bypass cap ?

good luck,

bob12345678

[jam]

Eric,

Glad you got it working. I have had Q11 fail on me several times when I had the amplifier. When I replaced it with a current diode, no more problems.
I found the best way to improve the circuit was to isolate the front end (pc board) with a diode aand a capacitor for each rail.
A 1N4002 diode and about 2,000 uF of capacitance worked well, be sure to check the polarity of your diodes and capacitors if you install this mod.
Grounding can be an issue, and may be the cause of the hum you hear, if it is not related to the power supply. I found that the input jack needed to be located next to each other and both grounds soldered to each other with a short piece of wire, but this could change with your chassis layout.

Jam

[Eric]

Jam, To isolate the front end PCB, I am assuming that you have the main power supply caps, then the diode, then the smaller cap and hook this to the PCB, one for + rail and one for - rail?

What I have now is a star grounding scheme for each channel (currently, I am operating only a single channel). The input ground, the output ground, the center tap from the transformer, the PCB ground, and the input shield all go to ground which is a copper strap that connects the ground leg of the two power supply caps (one for + rail, one for - rail).

From the + rail, I have a dedicated line to each output transistor, and another line to the PCB (kind of like a star power supply). I have the same configuration for the - rail. I am using a single bridge rectifier for each channel, and the ground of both channels are tied together using a bridge configuration from the latest Zen design.

I'm wondering if using 2 bridges per channel will reduce the hum also...

thanks for the help - Eric

[jam]

Eric,

Correct you will be surprised how much of an improvement you get.

I dont think two bridges will help the hum problem but it is a better way to build the supply. I could be wrong but I think the problem lies in how you ground the inputs to the star ground, I spent many hours correcting my ground problem, but that wes years ago.

Jam

[tone]

Hi eric,
im interested in building this A40 amp also, just wonder what v VA and voltage value of transformer u are using.. thanks

rgds,
tone