Solid state amp hiss

[Axtman]

I have an old off brand name solid state bass amp that I am working on. It has a serious amount of annoying hiss. Since it has discrete components the ICs can't be noisy. The resistors look like metal composition not carbon comp, so I assume the resistors aren't noisy.

Any help on tracking down the hiss would be appreciated. BTW, it is a hiss noise not a hum.

Thanks!

[Jazz P Bass]

First item on the check list: the power supply.
A schematic would help.

[Axtman]

Thanks Jazz,

I have the schematic. I would think that bad filter caps in the power supply would make it hum not hiss. What percent ripple is acceptable?

Would bad electrolytic caps in the preamp section make it hiss? BTW the amp uses FETs in the preamp.

[Jazz P Bass]

Can you scan & post the schematic.
The reason I ask is there are methods to divide the problem down to the preamp or poweramp.

[Axtman]

[Axtman]

[Jazz P Bass]

Nice schematics!
I love the way they drew the transistors up to the heat sink.
Anyway, do any controls make the hiss better or worse?
If you ground out the input to the power amp module (point 2,3,4) that should pretty much kill anything coming in from the preamp.
If it still hisses then it is a power supply issue or it is in the power module itself.

[gbono]

I would "guess" that any on the input FETs have seen their fair share of ESD and would likey be the first thing I would look at. You might also try the old "shot gun freeze spary" approach and see if different components, in a given stage, change their level of hiss with cool spray. I have noticed that the "better" vintage SS gear used transistors sockets for the critical low level signal components.

[Enzo]

Yeah, I'd be betting on a noisy JFET.

Isolate the problem. You have two channels, does the noise affect both? Does turning ALL controls to zero kill the noise or does it remain? Then does turning up one channel but not the other bring up the noise? You have tone controls. DO ANY of them affect the tone of the hiss in ANY way?

[Enzo]

Yeah, I'd be betting on a noisy JFET.

Isolate the problem. You have two channels, does the noise affect both? Does turning ALL controls to zero kill the noise or does it remain? Then does turning up one channel but not the other bring up the noise? You have tone controls. DO ANY of them affect the tone of the hiss in ANY way?

[Axtman]

Okay I fiddled with the controls here is what I have found out. Along with the hiss noise there is a slight hum noise.

There is noise on both channels as I will explain later. Grounding out points 2 or 3 or 4 silences the amp. These are the connections where the preamp board connects to the power amp board.

Now the characteristics of the controls.

Normal Channel:
Turning the Volume control up increases hiss and hum until about 75% up then the hum quiets a bit.
Turning the Treble control up increases the hiss.
Turning the Bass control up does not effect the noise.
Turning the Midrange control increases the hum.
There is a popping noise when I flip on the bright switch.

Bass Channel:
Exactly the same as the Normal Channel.

Any ideas?

[Enzo]

I'd expect those bright switches to pop, the caps on them are not terminated.

I see some DC voltages on that schematic, are yours close?

SInce your tone controls affect the hiss, it is coming from the first stage, I still suspect the JFETs.

You have more than one source of hum. Probably a bit of power supply ripple, and maybe some grounding hum.

[pinkjimiphoton]

hello, sorry to bump a zombie, but a friend asked me to take a look at his RMC b350, and i found this thread.
in his case, it's got very low output.
is there any possibility someone may still have a copy of the schematics that are attached to this thread they could re-post please?
when i try to download or open, all i get is a "clear gif" even if i try to wayback them.
thanks so much!

[Jazz P Bass]

Here is a repost of the files.

EMC 350.zip

[pinkjimiphoton]

Here is a repost of the files.

[ATTACH]41088[/ATTACH]

thanks so much, my friend will be psyched. rock on!