Rickenbacker RB55 maybe

[mtlbasslad]

Oh I promise to use it - baby steps...

[mtlbasslad]

Ok Enzo, be proud of me - read through the 90 page user manual for the scope... here is a pic of what I found. 0.2v 100Hz input - I see a lot of noise but no crossover notch distortion...

[g1]

Looks good as far as the bias is concerned, no evident crossover notch.
It does look a bit noisy, but some of the more modern scopes are more prone to choppy displays like that, especially at low signal levels.
Suggest comparing to a known good quiet amp, at similar level to see how clean a waveform this scope will give.

[mtlbasslad]

I tried a few amps, all displayed dirty signals. So here is a pic of the display directly from the signal generator - still looks dirty. Is that my cheap signal generator or do I have a dirty power line?
Should I be looking for a power line conditioner?

[Helmholtz]

Cheaper digital scopes often have considerable noise at mV settings.

Are you using a well shielded probe?

[Enzo]

And where is your scope probe ground clip?


Any dirt on your power line is not likely to come out from your generator. That power line is turned into a power supply which is filtered, and then the circuitry creates the test wave form. SO line noise does not shoot straight through the unit.

[g1]

At least you have verified that it is not the amp that is giving the noise, so I think you can consider it good.
The more 'noisy' looking display of digital vs analog scopes is a fairly common complaint, and partly due to the limitations of the analog.

A bit of info on some settings that may contribute here: https://www.rs-online.com/designspar...ope-look-noisy

[Greg Robinson]

Use an isolated DI/reamp or similar box between computer and amp. The switching supplies on many computers induce noise on the ground-referenced audio outs. Fine if you're running into class-II (non-grounded) speakers or headphones or something, but cause havoc when connected to class-I appliances. There's a reason why there are fancy "music-grade" soundcards, and even some of those are not immune to this sort of problem.

[mtlbasslad]

To answer some questions...

The probe is what came with the scope.
Scope probe ground clip is on the negative from signal gen.

Interesting reading, thanks g1.

Mr Robinson, I don't own either a DI or Reamp - they might well solve the noise problem, but I'll just use a more modern amp (we have several) The Fishman & G-K are immune. Thanks for the info.

I'll reinstall in enclosure later & hope my clumsy fingers haven't messed up anything...

[mtlbasslad]

Well, amp is back together & I didn't break anything!
Still a bit of noise but nothing like before - only thing I did was cleaning all jacks & tighten 2 loose ground wires. Most noise comes from Netflix, almost none from ITunes...

Once again thanks to all for the suggestions, and once again I've learned a few things.