Audiolab 8000a Background noise/hiss

[freiburg]

Dear all

I have a Audiolab 8000A here. It sounds great, works great but is no longer low noise. There is a constant background noise/hiss. This is present also when no input signal is applied and the volume is at zero.

On the power rails (measured on the big 10000uF caps) I also see this noise in addition to the ripple. It's just everywhere.

I usually repair Leslie Tube Amps and Hammond Organs. There, bad caps usually make hum.

Of course this Audiolab Amp is probably old enough to change the electrolytics anyway, but could this be the reason for this kind of noise?

Any hints would be very welcome and thanks in advance.

Christian

 

[JonSnell Electronic]

Both channels? Does it vary with volume setting? The volume control is before the tone control, may be an issue with a noisy transistor in the tone/line driver stage, before the main amplifier.
If you temporally remove C314/315, that will isolate the power amplifiers from the tone controls.

Doubtful to be power supply related.

 

[freiburg]

No variation with anything you can switch or turn on the front panel.....

 

[JonSnell Electronic]

Have you isolated the amplifier parts as I suggested?

 

[DF96]

Parasitic oscillation? Incoming RF interference?

 

[freiburg]

Have you isolated the amplifier parts as I suggested?

I am going to do this. Will have time for that tomorrow.

But indeed it is exactly the sound of a noisy transistor.....

 

[Mooly]

And is the noise on both channels, as Jon asked earlier?

 

[Mooly]

Another thought relates to your speakers. Are they very high sensitivity such as horn types? Anything like that would accentuate normal circuit noise.

 

[scottjoplin]

Is there any way to post the noise, visual and/or audio?

 

[freiburg]

Thank you for your replies:

1) Yes it is on both channels equally.
2) It is not speaker related.
3) Yes I could post the noise but not before tomorrow night. It sounds like the noise you get out of a noisy transistor, indeed.

 

[Mooly]

Thanks, I just wondered on the speaker question. It is odd that both channels are affected as well.

Try isolating the power amps as Jon mentioned, that's the first line of attack. If you can actually see noise on the rails to the power amps (what amplitude?) then that is very strange indeed. Make sure you are using the correct circuit ground point when you check that (not the chassis) because I'm wondering if the chassis is floating in some way... dunno … you need to do more tests and narrow things down a bit.

 

[freiburg]

Thanks, I just wondered on the speaker question. It is odd that both channels are affected as well.
.

Yes, that's why I thought the cause might somehow be a noisy power supply...

I just fixed an Oberheim OB8 Synthesizer. There, also a lot of noise on the power rails was an issue. Recapping the power supply and exchanging the bridge rectifier and the regulator improved the situation considerably.

Tomorrow night I will have time to proceed, so you will hear from me on Saturday....

 

[Mooly]

I wouldn't like to second guess this one tbh, deteriorated caps in themselves would not allow HF noise to appear on the rails, you would just expect an increase in ripple... however could an increase in supply impedance (deteriorated cap) cause the amps to become unstable in some way.

Very odd. Lets wait on the results of test and measurement.

 

[freiburg]

Hmmm, does anyone have a service manual that covers the serial number of the amp I am working on:

207E115228

On the PCB I find 207-017 Issue 7 and 207-016 Issue 7 and 207-038 Issue 2

 

[freiburg]

Tonights findings

Ok, here is what I found out so far:

1) I do not have the correct schematics for my PCB Issues, partnumber and values differ. However I managed to remove the correct parts

2) Removing 47u capacitors in the line/tone amplifier circuit (C315/C316 in the commonly available schematics, C508/C608 in this unit) reduces the noise by a factor of 5:

Measurements - Google Photos

3) Reinserting any of the two caps brings back the noise level on the according channel

4) Removing the 4u7 cap after the volume control (input of this stage) increases the noise level considerably

5) The noise level on the +30V power rail is more than 2 Vpp !!!!!

So I still think it is the noise on the power rails, this also explains why I see this noise everywere..... There are many transistors in the power supply, so tomorrow I will look there.....

 

[scottjoplin]

Have you tried HiFi Engine for the manual?

 

[freiburg]

Have you tried HiFi Engine for the manual?

no will try thanks.

 

[Mooly]

Firstly make sure that you are using the correct ground for the measurements.

 

[freiburg]

Firstly make sure that you are using the correct ground for the measurements.

Thanks, I did.

On the collectors of Q602 and Q601 I have noise of 9Vpp.

An externally hosted image should be here but it was not working when we last tested it.

 

[Mooly]

That's a crazy amount of noise to see.

You should only be seeing around a volt of so of ripple on the main unregulated 44v rail that feeds this regulator. You need to see what is showing across C601, the main 10,000uF cap.