High pitched sound from amplifier

[dj_holmes]

Hi Guys,

Wondering if anyone has advice on what could be causing a high pitched sound from an amplifier. A friend mentioned he was getting this high pitched sound which is exactly like another amplifier I have that makes the same high pitched sound.

I uploaded the video and it can be seen with the link below:

Dropbox - VID_20200104_180455541.mp4 - Simplify your life

He mentioned that if he removed the links at the back the problem is coming from the pre section. My amplifier doesnt have those links and is a completely different brand yet I get the same high pitched sound. My amplifier has a complete recap too yet the other has fairly decent old caps.

I have a cheap oscilloscope that I tried to measure the sound and it was telling me it was 50HZ but when I looked online it was not a 50HZ hum. The sound is mostly coming from the tweeter. Any advice would be appreciated. Thanks

 

[Mark Tillotson]

I see a fluctuating sound around 12kHz on the 'scope, perhaps there is ultrasonic oscillation and this is an artifact from it, perhaps its interference getting into the amp, either from mains or as RFI.


What is the sound source, or is it indepedent of that?

 

[al schmucker]

Sorry new guy just brousing at the moment.

Sounds to me like it is oscillaton.

Al

 

[dj_holmes]

Hi Guys, there is no source!

It also occurred to me it might be oscillation so I tried cap between the power pins of the opamp but no joy!

I also tried bypassing the power switch.

 

[Mooly]

Hard to say really and a plot of the spectrum shows a lot going on. The second FFT is from 6 to 8 seconds (the quiet bit). Could be anything... start by fitting shorting plugs to the inputs and have nothing but speakers connected.

 

[dj_holmes]

Hi Mooly,

The quiet passage is when the phone is moved away from the speaker to demonstrate the noise coming from the amplifier/speaker.

What you asked was done. Only the left side input was terminated, CD input was selected and left input was terminated and exactly the same noise, no difference!

 

[H713]

It would be really nice to see some oscilloscope images. You might be surprised by how small the amplitude is though- it doesn't take much to be able to hear it through a speaker.

I agree, it does sound like it could be some type of oscillation.

 

[Mooly]

Some commercial designs in the past (can't remember specific examples by name) have had issues with RF affecting front end stages and the manufacturers had various mods to try and overcome the issue.

As you mention in post #1 that it is front end related it really would be a matter of looking with a scope and also trying to break the signal path into stages to see where the problem is coming from.

 

[avtech]

What make / model are we talking about?

 

[U130421]

Maybe a problem of earth. Link the chassis to the mains earth with a separate wire.

 

[al schmucker]

I may have something for you. I assume you are from UK. Some amps available to you have a design that are prone to oscillation
Linear 200 Series II, 400, British Fidelity A1 and B&O 4500 Beomaster amp stages and these oscillated before they ever reach clipping. The PL 200 was known for instability because of it's design.

Also check bais on the finals. There is alot of information about the clipping of the finals and under biased.

Hope this helps.

There seems to be alot of info on Audiokarma.com

Al

 

[nigelwright7557]

My ears are too old to hear the high pitch signal at all.
I suspect its oscillation.
Try increasing VAS capacitor a little.
It can sometimes be output driver transistors driving into a output base without a 10R in series.

 

[dj_holmes]

Hi guys thanks for all the input. What is VAS capacitor? I will try the earth cable too but that looks fine.

This amplifier I had help with previously by removing the top end frequency. Link is below:

NE5534 circuit. I need more bass. Help!!

Thanks again for all the input.

I do have an oscilloscope but its a cheap digital DIY DSO138