Gallien Krueger 400RB Hiss

[Delta362]

I'm working on an early version of the 400RB. If all controls are up, there is considerable hiss. Turning down any of the controls lowers the hiss so all seem to have an effect on it, as does the -10dB pushbutton and other voicing filter pushbuttons. I've cleaned all the pots and jacks, plus the 3 pin connector that goes between the preamp and power amp board. No noticeable change.

Plugging a cable into the effects return loop with the other end being shorted results in the boost and master volume pots only changing the hiss (as expected), so it seems that it is being propagated throughout the preamp section.

I probed a signal tracer on the output of U1, but am I not picking up any noticeable noise there. I do, however hear some hiss on the +/-15VDC rails. Would hiss on the low voltage power supply manifest itself thru all of the opamps? I have not checked the +/- 60V rails for noise yet. Given the age of the amp, would new power supply filter caps help, or is this more likely a result of noisy resistors from that era?

[Jon Snell]

Assuming you are familiar with working in the elctronics industry, master volume to zero, has the his gone? If it has the hiss is purely pre amp based if not it is probably U1 in the power amp side.
You mentioned the attenuator pad switch reduces the hiss? if it does the his is coming from U1 on the preamp board.
Hiss on the rails rarely causes issues on the output stage or opamps in general.

[Delta362]

Thanks, Jon! I ended up replacing all three of the LF353N in the preamp section. Seems to have lowered the hiss quite a bit. While it is still there, especially with all knobs on max, at normal settings it is very much improved.

[Jon Snell]

Good to hear it, (pun intended).
Personally I would choose a TL072 over the older LF353. Slightly lower noise and both are JFet input so high impedance on the inputs with a similar frequency response so it won't screw up the signal and go unstable.
I seem to remember, they were always a bit hissy.

[Delta362]

Thanks for the recommendation! I opened it back up and swapped out the LF353 with TL072 and it made a very nice improvement. I didn't have any TL071 on hand (only a few NE5534 which I elected not to use in this case) to replace the LF351 in the power amp and the customer wanted it back today for a gig tomorrow, so I couldn't get it even more quiet before I had to button it up.

[g1]

I seem to remember, they were always a bit hissy.

Agree, and especially when using the 'boost' control.