Chasing noise, how: hum and/or buzz

[Kenrod]

So i'm looking into noise issues on a 30 year old Peavey Studio Pro 40.

i replaced the main filter caps in the PS, and while i'm sure it reduced the noise some, I'm looking to get this thing quieter.

It starts to get noisy above half way on pre gain, post gain and reverb control. Does that help?

I have a simple DIY audio probe (capacitor based) and a scope. Can I simply trace the circuit and listen (probe) or look (scope) for the noise to pinpoint the repair needed?

I'm a decent component replacer, but still working on these trouble shooting skills.

Any help is appreciated, including pointing me to resources to read up on. I like to learn, in what spare time i have.

thanks in advance

[SteveJ]

There is a lot of useful information here to get you started. Tube Amplifier Debugging Page

[Kenrod]

Thanks for the link. I'm working on a SS amp, but some of those tips looked useful in my situation. i'll clean all my jacks ASAP.

[mikeydee77]

So i'm looking into noise issues on a 30 year old Peavey Studio Pro 40.

i replaced the main filter caps in the PS, and while i'm sure it reduced the noise some, I'm looking to get this thing quieter.

It starts to get noisy above half way on pre gain, post gain and reverb control. Does that help?

I have a simple DIY audio probe (capacitor based) and a scope. Can I simply trace the circuit and listen (probe) or look (scope) for the noise to pinpoint the repair needed?

I'm a decent component replacer, but still working on these trouble shooting skills.

Any help is appreciated, including pointing me to resources to read up on. I like to learn, in what spare time i have.

thanks in advance

Some ideas

Sometimes a chopstick is a faster tool for locating an intermittent problem. Just tapping away looking for the place where tapping effects the noise. I also use a wooden spoon and/or brush sometimes and also rubber Mallet for a chassis bump test.

For permanent and distinctive noise I prefer the audio probe and for general signal tracing I like the scope.

What you should aim to do is isolate the fault to find out where is it introduced. Try to work out if it is present in the pre amp or just the power amp? Some amps have jacks separating amp stages; fx loop or pre/power amp connectors.

If you don't have the jacks or you need to break down further get the schematic and work from the output back towards the input.

I try to divide the amp into halves and then identify the earliest stage with the fault, continue sub dividing until getting close.

Common causes of noise you will learn in time can be connectors, pots and op amps just to start with.

Also the amp's controls can hint to where the fault is introduced. Like if noise was made worse by ch1 tone control, then the noise is introduced there or by an earlier stage.

If the noise is everywhere, then suspect common sections like power supply or power amp.

Don't forget that multiple faults are all too common.

[52 Bill]

A better description of the noise would be helpful. While a silent amp is possible, most are not, especially when the gain is turned up.

Some noise is created in the solid state devices themselves (ICs and transistors) and some is amplified noise from resistors and caps and more commonly from bad lead dress of internal wiring.

If you are serious about quieting down this amp, you should start by trying to isolate where the noise is being generated.

[Usable Thought]

I have a simple DIY audio probe (capacitor based) and a scope. Can I simply trace the circuit and listen (probe) or look (scope) for the noise to pinpoint the repair needed?

You didn't say whether you're using a spectrograph with the audio probe? If not, it's a great tool - so just in case I will describe it.

Effective & inexpensive spectrograph apps are available for computers & tablets, etc. You can run the probe output into a USB sound card & thence into your computer/gadget of choice.

The only thing to watch for is that yes, the cap on your probe will take out the DC, but on a tube amp at least it is possible to kill a sound card by putting the volume too high when looking at something like ripple. I mistakenly blew up the little Radio Shack battery powered amp I was using in front of the soundcard by (somewhat stupidly) putting the battery powered amp's volume on full when the probe was on the output of an HT power supply with about 4V P2P of ripple on it. Not my finest moment. I don't know what solid state AC voltages you would be looking at, but obviously the same caution would apply.

Anyway the combination of audio probe plus spectrograph is particularly effective for hunting down "permanent and distinctive noise," as mikeydee77 puts it. With a scope it's hard to get a good enough look at the frequency profile for a given noise, whereas the spectrograph is perfect for that. In my case I started with putting a mic on the spectrograph to listen to the noise as it came out the speaker, and was then able to use the probe plus spectrograph to pretty quickly determine that the exact same noise profile was associated with a noisy PT and PT secondaries. That narrowed the issue down & made a quick solution possible.

[Mick Bailey]

As per previous replies, you need to qualify the noise - is it LF hum or general hiss?

Plugging a guitar into the power amp in socket disconnects the preamp. With the guitar volume on zero, do you still have noise?

[Kenrod]

Sorry for the late reply. I may have messed up my notification thing.
I haven't looked at it much over the last week, except to take the preamp out signal to another quiet guitar amp.
The noise was amplified by the test amp. So it appears to be in the preamp stage. I'll also check the power amp by inserting a quiet signal their and see what I hear.
The noise seems to be mostly a "buzz" at this point.
I'll try to take a short video this evening and post a link to it.

[dhart67]

did you ever find the cause of noise?