Noise in opamp circuits?

[Coriolis]

Don't know if this souldn't be in the lounge, but:

I'm helping a friend out with his old Roland Jazzchorus amp. It has huge amounts of selfnoise and is almost unplayable.
The question is, what to replace to get rid of it? It has 4558's in the preamp, reverb section and chorus/vibrato sections, and those are supposed to be noisy, but this is extreme!
My question is, do opamps go bad, like electrolytics do? They don't, do they?
What if we tried changing the electrolytics, should we go for the ones in the powersupply (can they add noise when they're not in the signal path) or smaller ones in the signal path?
Could it be resistors? and which ones? Should we test feedback resistors or what?

A lot of questions...

[bioroids]

Hi!

You should read this article at Geofex:
http://www.geofex.com/circuits/when_good_opamps_go_bad.htm

I dont know much about amps, but there should be a way to "audio probe" it to check wheter the noise appears at a particular place, or if its the sum of little noises all along the signal path.

Luck

Miguel

[Paul Marossy]

Has it always been this way, or has it developed this problem over time?

[Processaurus]

Hi,  I play through a JC-120 and it has a good amount of hiss at low volumes too, I've coped with it a little by using the higher gain input and turning down the volume on the channel I'm not using.  I remember reading a technicaly minded reviewer's tips addressing the hiss issue at the JC-120's review page at Harmony Central.  Apparently older ones don't have the hiss.  They didn't have a solution, but they said roland redesigned the JC-120 after they couldn't get some of the transistors, especially power transistors that are used in the older USA made ones, and the hiss is a side effect from that redesign.  It seems to me like the problem is after the volume control, because turning it up to where guitar will be brutally loud adds little noise, but the hiss is there with everything turned down.

On your friends you might try plugging directly into both the power amp inputs (because its stereo) at the same time and you'll be able to tell if the problem is in the preamp or power amp section.

If you find anything more out about this problem I would be really interested to know, because its impossible to use the amp for recording, unless you crank it up to overcome the background noise.

[toneman]

Yes,
It's not about DIY stompboxes,
so, similar subjects  should be in the lounge
IMO
Or, take it to an amp forum...

[Paul Marossy]

My intuition was telling me "transistors". Maybe if you see what's in there and replace them with some low noise types, it would be an improvement? You could also try some low-noise opamps like the NE5532/34 series.

[Coriolis]

Well I guess this particular amp is one of the redesigned ones, cause it has opamps where, on another schem I saw, there are transistors. My friend bought it secondhand (but years ago) and it has been this way ever since.
Seems strange that it should be the power trannies, but I don't know much about that.

Toneman, I guess you're right. The reason I posted here was that I was looking for a more "generic" solution to noise problems.
I know 4558's are supposed to be noisy, but I couldn't believe it was bad enough to cause that much noise. So I thought there might be other factors.

Gonna try audioprobing/switching opamps/etc...

And will return with my findings in the Lounge!

C

[gez]

And will return with my findings in the Lounge!

Personally I think you should post here.  Amp forums tend to be biased (pun intended) towards valve stuff...they know bugger all about op-amps and trannies (joke)!  There's also a lot of crossover (pun intended) between trannie and stompbox stuff, so what the hell.

I used to play through one of these amps (didn't own it though) and I hated the noise levels...