Help With Hiss

[Daver]

All the preamp grounds now have separate wires going to the input jack. The cathodes, volumes, tone stack and filter cap. The presence cap and resistor (it's a long tail PI now) go directly to the PI filter negative eyelet. A ground wire from there goes to the output jack with the rest of the HV and power amp grounds. Changing around those grounds in any possible configuration does not change the hiss at all. Not worse, not better. I've separated the V2 cathodes and tried grounding them in various spots. I have replaced the cathode bypass cap for V2 as well as remove it. EVERY resistor or cap associated with V2 has been replaced. All the solder joints on the board and tube socket have been reflowed. I'm baffled that I can't even make it worse let alone better. Thanks, Chuck!
Dave

[mac dillard]

There was a builder over on AX84 that had problems with hiss on one of his builds. Did everything he could think of to control it. No luck..Turned out that he had a bad pot that was causing the problem..Just something else that you might check.

[Daver]

Mac, that's something I hadn't considered. So I've already eliminated the volume pots previously by having them out of the circuit. I just bypassed the tone stack and the hiss remains. So pots aren't the problem. Thanks for the help. I appreciate it.
Dave

[chinrest]

I'm diggin this one up. Did the amp go into the landfill? I've had a few stubborn noise makers and found that the fix was to play VERY loud for 20 minutes. After that, all the hum, hiss, buzz, and other trivial nuances magically disappear.

[Chuck H]

That post should be followed by a winky emoticon

Sometimes the eardrums just need a good bludgeoning!

[km6xz]

It is interesting to note that none of the many component changes made a difference in sound, yet some people insist that even changing copper $0.05 a foot wire by silver $1.40 a foot wire, or $18 caps in place of $1.20 caps turn any amp from sludge to golden. Oh well....

The sound character of the hiss could have given some clues. Was it really random in amplitude and frequency or constant amplitude and random frequency, or centered in some bandpass. Did it have white/pink noise character or was there snits and little pops embedded.

I was given an amp to "repair" by a home builder recently and everything that could be done wrong was in construction and in circuit(he combined two front ends, a Bassman, and Marshall 800 and hybrid switchable tone circuit. His complaint was hum and hiss. I got too far into it, but did not want to stop and ended up redesigning it and rewiring it. He said it sounded good before the repair but it just proved to me how little some people really listen to their own work. One cause of hiss was a 800khz oscillation in the lead side, which got back into both since the filters has too high inductance to have much effect on 800khz. The hum was coming in everywhere and took the longest to track down. When he picked up, I refused to charge him because I did not want to "own" it and be responsible for his tinkering, and did not want to give him the feeling that my time was worth so little that he could bring anything like that and have it done for $40-50. It took many hours to turn that pile of parts into a working channel switching amp. I have since created a policy that I do not "repair" home made gear but if they really want me to work on it, I will redesign it so it can work but at normal contract engineering prices, which would be thousands of $$. I give free advice and even offer to test performance for free however.

[Daver]

No randomness to the hiss. After comparing the amp to other amps I've built as well as other boutique, old Fender tweed and current major builder amps, I came to the conclusion that it wasn't that bad. I had many other people listen to it including guys at a local music store without me saying anything at first and nobody noticed the hiss. Maybe the amp broke in a bit. Maybe the tubes? At any rate it seems to be fine. I'm really sensitive to that stuff and kind of anal about noise in stuff I build. So, all is well so far. I sold an amp through the music store recently that has about the same level of hiss. The new owner loves it. I guess it's just me. Thanks.
Dave

[Chuck H]

Working on a build right now that seems to have an obscene amount of hiss in the reverb circuit. It also has an obscene amount of reverb I'm working on a happy compromise between noise and effect right now.

If you avoid large series resistances, carbon comp resistors, noisy tubes and not trying to amplify too much in a single stage, the rest of the hiss is simply unavoidable. It comes with the territory.

[Steve Conner]

Chuck, maybe it's time to go to the dark side and use a JFET/tube cascode in the reverb recovery stage?
More Phono Preamp Circuits

I suggest you ignore his comments about noise, I think he is getting confused between noise and PSRR. PSRR can be improved by filtering the power supply, but the stage's self-noise can't. The basic idea of connecting a JFET in the cathode of a tube stage is valid, and Ken Gilbert found that it had considerably lower noise than the same tube used alone.