Help, my pre amp "hisses"

[IdahoSteel]

Greetings to all, I've been a member for a couple of months but haven't posted anything until now. I have a PAS that I've upgraded extensively. It has the SDS labs (triode electronics) power supply board and the Curcio Audio Linestage and a new selector switch. This all is a couple months old and has sounded really good until yesterday when it started making noise in the left channel, the worst of the noise stopped ( kind of like a NOS tube burning in) but now I have a pretty good amount of hiss in that side. This is unaffected by the volume control, unchanged by the input source, same into different amps and same with or without the tube installed. I have checked all of the wiring, no problem there. I put the meter on all of the resistors, everything looks good. I had a backup set of output coupling caps so I replaced those, noise maybe a bit less but still there. The meter says that the remaining caps have the right value, (2 Blackgate Electrolytics and 4 generic MKP types from Digikey that are .1 uf 160v I think) This linestage has a couple of JFets and two small transistors as well as the 12at7. I've swapped out the tube as well, no luck there either. I don't know much about electronics, but have had great luck building a number of projects, all work great until now. I just don't know what else to check. Is it possible for the semi-conductors to make noise like this? I could replace the caps, they aren't that expensive and it's no problem to get to them, but they test OK. I haven't been able to contact Joe Curcio, he doesn't answer his phone anymore and hasn't responded to my e-mail. I'm so BUMMED, just finished my VTA upgraded ST70 and it all sounded so good. Any help would be much appreciated. I guess I could box the thing up and send it to someone who has an O-Scope but the thing doesn't have that many parts, seems like I should be able to fix it if I could just figure out what is wrong. Last thing, the sound of the music is unchanged except for the beautiful black background is gone, I can hardly listen to it. Any thoughts, suggestions on where to go from here?
Thanks in advance, this is a great forum lots of good people here.
IdahoSteel

[BrassTeacher]

Does the amount of hiss vary with the time of day?

If the answer to this question is "yes", you may have an RFI problem, and something isn't shielded well.

Is the hiss consistent sounding, or sort of like someone wrinkling cellophane softly? If steady, and there's a voltage regulator of any sort, look there. If not consistent, or changes volume as the preamp gets warmer, take a look at those JFETs, or any other signal path semiconductor.

Very carefully, and with one hand only, with the system up and running, and the cover off the pre, place a fingertip near (often, touching is not required) each of the coupling caps, and see if a "hum" starts developing. Any that do, take them out and reverse the leads, and re-solder. If you move a finger near any of the JFETs, and similar happens, try re-soldering any that react. If that doesn't cure it, you may need a new JFET.

Also, try wiggling the tubes, one at a time, with everything powered up. If any of them make noise, you have a dirty tube socket. If any of the tube sockets look like they have had excessive heat, like they are "toasted" or noticeably darker in color on top than on the sides, you may need to replace the tube socket.

Good luck!

[IdahoSteel]

Thanks a lot Don, no the time of day has no effect, the noise started out like the crinkling cellophane but now is a very steady hiss, but is only in one channel,s o I don't think the voltage regualator would apply, I can touch and wiggle everything in the amp without affecting the hiss at all and it is only on the one side. I take it from your comments though, that it is possible for the JFET to make this kind of noise? I already replaced the output coupling caps with some spares, this had no effect on the noise. I'm wondering if I should just go ahead and replace the remaining caps? The semi's are the hardest things to source,
I really appreciate the help,
Byron

[audiodyssey]

My experience with hiss is RESISTORS are usually the cause, not capacitors, if it is a passive component at all. That's just my 2 cents... Overheating heating a resistor while soldering is a good way to do add some hiss into your system. Try re-flowing solder on all the tube pins if they are PCB mounted types. Then I would try replacing resistors in the stage systematically. Without test equipment you'll have to use my method of repair...trial and error.

[audiodyssey]

PS...I'd reflow ALL solder connections, another good cause of hiss is a slightly cold joint.

[Bruce221]

audiodyssey is right-resesters are prob-the problem-or a cold solder joint -in that channel if l-or r-relpace from input in though-trial and error -if only 1 chan do the same -can be very time consumming-but worth it

[IdahoSteel]

Thanks to all for responding, the thing about this is that the preamp worked and sounded great for two months before developing this issue quite suddenly, this makes me tend to discount the cold solder joint, not that I can't make a mistake but very unlikely(joke), or a resistor. I did get an e-mail back from Joe and he thinks one of the semi's (probably the Mos-Fet) to be the culprit. I have ordered a bunch of new parts and we shall see. I'll let everyone know the outcome so that we can see who has the bragging rights. I really appreciate the help.
Byron

[grateful]

If it is steady suspect an active component. if intermittant suspect mechanical connection or a tube socket

[BrassTeacher]

Thanks a lot Don, no the time of day has no effect, the noise started out like the crinkling cellophane but now is a very steady hiss, but is only in one channel,s o I don't think the voltage regualator would apply, I can touch and wiggle everything in the amp without affecting the hiss at all and it is only on the one side. I take it from your comments though, that it is possible for the JFET to make this kind of noise? I already replaced the output coupling caps with some spares, this had no effect on the noise. I'm wondering if I should just go ahead and replace the remaining caps? The semi's are the hardest things to source,
I really appreciate the help,
Byron

Yes, a slowly-dying transistor (or tube even) can make all kinds of noises, but they mostly sound like a 'crinkling' or 'rustling' sound ("rustling" is the word i couldn't remember the other night), or a hissing sound. I've even heard transistors make a combination of both sounds at the same time.

Sometimes, but not always, you can hit a noisy transistor with "freeze spray" and the noise will stop, get softer, or change in pitch, but only temporarily. If you are familiar with how that's done, it may be worth a try to find the offending transistor, if there is one. It's not always going to find a bad one though.

[positron]

A few thoughts, not exhaustive, and not in order of importance. Be Very careful as there are lethal voltages present.

1) Cold solder connection or even dirty pins.

2) If you can obtain some cooling spray, spray one resistor or part at a time (not the tube). Sometimes a noisy part responds to cooling, thus aiding in finding the problem.

3) Although a remote possibility, an RF oscillation in a stage can give a similar impression. Might check each stage for a number one grid RF suppression resistor. If there is no RF grid suppression resistor (I have seen designs this way), a resistor will solve the problem. Usually a couple of hundred to 680 ohms, 1/4 or 1/2 watt, will solve the RF oscillation problem.

Hope this helps. :music:

[IdahoSteel]

Thanks to all who responded, I got the replacement parts from Mouser and changing out the power MosFets tixed the problem completely, so the designer, Joe Curcio had it right. I want to say here that his designs sound absolutely great, and he does follow up to provide the support when needed, he's just a bit slow, but I'm small potatoes and I'm sure he's really busy. I have AB'd this preamp against a much more expensive ( over $2000) and it more than holds its own. I have about $600 into it. Once again thanks to all,
Byron