need guru help: not your usual bad cord/reversed wire humming

[madsientistjj]

im building an unusual mad scientist combo using two "preamps" that are actually 10w amps with the blown op amp power chips removed. i just use what was the signal imput to those op amps as my line out to a power amp. generally works pretty good. in this case im trying to use two of them to make a 2ch and a/b between them BUT theyre not getting along. im also using an old starforce power amp section from another dead amp.

when its all hooked up to test it im getting a loud open ground type hum when i touch the strings of (any) guitar. it only does it with one preamp and not the other. its not the cord or the guutar and ive tried swapping the ac mains and lifting grounds etc etc and can only make it worse.

the preamps are a squire champ 15 and a fender frontman. the frontman is the humnmer but the champ and the starforce have given me compatibility noise issues before.

anybody have a clue what im talking about?

[ArtieToo]

Re: need guru help: not your usual bad cord/reversed wire humming

Well, I had to bite my tongue to not make some smartass remark, but since your new here, I won't. So, welcome to the forum.

Anyway, if I understand your post correctly, you're trying to patch three "blown" units together to make one working unit. It's a recipe for disaster. Certainly it can be done, but you'll run into exactly the kind of problems you're experiencing. It would be almost impossible for anyone in a forum to help with a problem like this. Obviously, you want to check your grounding scheme, and then grab an o'scope and start tracing the signal.

Good luck, but that's about the best I can do in this forum.

Artie

[devastone]

Re: need guru help: not your usual bad cord/reversed wire humming

A) Welcome to the forum.

B) What Artie said. Sounds like the kind of science project I would have created for myself decades ago, but unfortunately it's not something that is easy to troubleshoot, especially via the interwebs. Granted if you figure it out, you'll probably have learned something, and as Artie said, check your grounds and make sure they are all connected since it sounds like you have multiple circuits going on.

[madsientistjj]

Re: need guru help: not your usual bad cord/reversed wire humming

a. not a warm welcome fellas and b. you guys are no help and no fun. the blown op amp are removed. the power amps original preamp fell victim to too much dust in the pots. ive been using all three for quite some time. im just having issues getting all three to get along as a team. im sure a 4pdt switch will solve the issue but i was trying to kearn something as to why they cant be simply parralleled together. thanks anyway i guess.

[ArtieToo]

Re: need guru help: not your usual bad cord/reversed wire humming

Uh . . . we both said "welcome to the forum".

[devastone]

Re: need guru help: not your usual bad cord/reversed wire humming

Yes, we did say welcome, and honestly meant it.

Not trying to be "no fun", the only place I can thing of to start with noise issues and the limited amount of info is, make sure there is a common ground. If I had time to get a list of the amps you are talking about and go look up the schematics I might be able to give you better advice on where to break and join the circuits, but, still, running 2 preamps in parallel (if that is truly what you mean) will take additional circuitry to deal with impedance mismatches and mixing.

If you have lots of time on your hands, it might be worth looking up electronic basics and going from there. I'll be glad to help with that where/when I can.

[ArtieToo]

Re: need guru help: not your usual bad cord/reversed wire humming

Here's something else to consider. Within the middle of a "system", there's often a DC offset, or bias. It will be removed in the final stage by a cap. If you tap into the middle of that system, you may have some DC voltage that will cause the hum/noise/buzz that you get in the next section. Try inserting a 10uf cap between your "noisy" section and the power section. Or, put a meter on that preamp section and see if you have some DC offset there.

[DankStar]

Re: need guru help: not your usual bad cord/reversed wire humming

I'd consider using 2 power supplies, one for the power amp and one for the preamps. Might eliminate some ground loops. Might be more flexible that way anyway, more like modules.