PASS A40 - bias

[lumanauw]

C3 (bias capacitor) changed from 220uF to 2200uF.
Upper and lower diode different position. Upper diode before base stopper 68R, lower diode after base stopper 68R.

[Babowana]

Originally posted by lumanauw

Upper and lower diode different position.

?!?!

The diodes look like two one-way bridges
from the output to the +/- of the cap. . .

[herodote]

Image http://www.diyaudio.com/forums/attac...amp=1153941051 are worth a thousand words (for dummies like myself for sure). Thanks

Diode can be a a zener like 1N4148 ?

[Babowana]

Originally posted by herodote

Diode can be a a zener like 1N4148 ?

Zener? No, no, no!
Diode. IN4004 would work.

[Nelson Pass]

Any diode will work.

[herodote]

So 1N4148 would be fine ?

(Sorry I'm not fluent enjough to understand the difference between a diode and a zener) I reckon Zener Is a Diode but there are diodes that are not zener. I was a bit suprise by the 1N4004 - seems oversized (maybe Babowana JH wanted to have a good time) Thanks anyway

[Nelson Pass]

a zener diode is just a diode with a low breakdown voltage.
In this case you could use some 5.6V zeners easily enough.
There are diodes you wouldn't use, like a 100 amp rectifier.

[Babowana]

Originally posted by herodote

(maybe Babowana JH wanted to have a good time)

I just wanted to have normal diodes as on the drawing, safely
avoiding my ignorance. I casually thought about 1N4004, which
I use most time.

Anyway, thanks to Nelson, you have full answer. . .

[herodote]

Thanks to all for their contribution.

What could be the cause, now that the DC offset trimmed down by the R6 values is set to +1mv -1mv and the bias via R11 is set to 1.55A, after an 1.5 hours the amplifier start giving a hum, like a ground loop in the speaker.

[ggrovlanson]

Hello,
In the early eighties a firend built an amplifier which he viewed as state of the art at that time for its schematic. Unfortunately he passed away and recently the amplifier did the same. I ended up opening the box and searching the web to find where / how this amplifier (which I like the sound very much) could be brought back to function. (ok this is almost pathetic..since i know little to nothing about electronic) Searching the web I found the passdiy and this forum sites which are full of all information I needed to get the amp fixed (almost). I'd like to thanks Mr Pass as well as all others who by their questions and answers have teached me a lot and fix the amp.

Beeing far far having a good understanding, one issue remained unsolved yet and even after several searches, I haven't found how to address it. Maybe some of you would have the answer, please be thanked again. Here it is :

There is some hum in the speaker.

I've removed the transformer away to no results.
I've applied Eric (http://www.facstaff.bucknell.edu/esa...ovies/a40.html) recommendations as well as Mr Pass (http://www.diyaudio.com/forums/showt...953#post969953) although there i haven't found 2200uf tantalum so far (car we use electrolitics instead ?) But to no avail.

Here are the measurements (if that helps)
Right channel
+DC 31.20; -DC 31.20
DC offset 2.8mv
Bias 1.56Amp per trail
Voltage drop
- D1 - D2 1,270V
- R7 0.642V
- R6 0.592V
- Q7 - Q9 emitter 1.089V
- R10 0.636V
- start up thump 0.184V
- light hum yes

Left channel
+DC 31.46; -DC 31.46
DC offset 0.1mv
Bias 1.64Amp
Voltage drop
- D1 - D2 1,286V
- R7 0.656V
- R6 0.589V
- Q7 - Q9 emitter 1.026V
- R10 0.649V
- start up thump 0.343V
- light hum yes

1°) What can be the cause of the hum in the speaker ?
2°) I don't understand why the difference in the start up thump.

The only difference I'm able to noticed with the schema on passdiy is the transformer. It is a secondary 1kVA at 24Volt DC with 8 wires 4 are connected to two rectifier of 50Amp 600V and the 4 other a fixed on the ground. I don't know if that makes a difference. Many thanks anyway.