hum in a SP14

I'm wondering if the coupling caps in your SP-14 are able to match the lower input impedance of the s/s power amp. You might find some beefier ones.
If you've done a tube swap, might also look to external wiring as well. Coax too close to AC lines (unless they're crossing at 90 degrees) can induce a lot of hum.

Hello Marvins,

Sounds like a ground loop. Does either your amplifier or preamp have a three prong power plug?

...j

j beede wrote:Hello Marvins,

Sounds like a ground loop. Does either your amplifier or preamp have a three prong power plug?

...j

Agree with beede, and ground loops can be extremely loud. Try using a cheater plug to lift the ground(s) and see if that does the trick.

The pre SP14 has a 3 prong plug and the ST120 has a 2 pronged plugged. No cheater plugs in Canada , but I can just remove the ground inside the Sp14 to try that. I would have thought it would have been designed to handle that . I did separate the two to deferent plugins but that did not work .

Thanks for the response

Marvins wrote:The pre SP14 has a 3 prong plug and the ST120 has a 2 pronged plugged. No cheater plugs in Canada , but I can just remove the ground inside the Sp14 to try that. I would have thought it would have been designed to handle that . I did separate the two to deferent plugins but that did not work .

Thanks  for the response

Marvins, if you'll PM me, I have a drawer full of cheater plugs) and would be pleased to mail you a couple of them. They're like $1 apiece. But forget that. Every old house needs a few of them. Having lived on both sides of the border most of my life, I'm quite adept at the art of smuggling. When in Vancouver I always pick up a big jug of AC&Cs, which you need a scrip for down here, and take Melatonin northwards which you can buy OTC here but not up there. Go figger.

Marvins wrote:The pre SP14 has a 3 prong plug and the ST120 has a 2 pronged plugged. No cheater plugs in Canada , but I can just remove the ground inside the Sp14 to try that. I would have thought it would have been designed to handle that . I did separate the two to deferent plugins but that did not work .

Thanks  for the response

I think you can try running a cable from chassis to chassis (or from RCA to RCA) and see if your hum goes away. Cured it once for me.

Kind regards

Michael

Not sure this will help your issue or not, but I was able to resolve a hum issue on my SP14 by doing the following. This was after ruling other issues such as wiring and soldering mistakes, voltage issues and inductances from wiring proximity. I followed the standard wiring instructions exactly but for the inputs but made the following changes. I removed the ground bridge jumper wire on the volume control between channels (as indicated in the instructions) and I also wired both of the grounds from the volume control to the input grounds on the pcb. On the output side, I still kept the single ground wire from the output to the PCB as per instructions. This change worked with the basic potentiometers that came with the kit, and possibly my issue was related to a ground loop created with the channel bridge from the volume control.

Had two hum/noise issues in building my sp-14: grounding and wiring. If you built the kit, then grounding should be straightforward, just follow the directions. I did not build the kit and had to try numerous iterations before getting it right. As to wiring, I originally wired with 24 awg, silver plated, twisted pair, silver shielded, teflon jacketed wire. (essentially the Don Sachs wiring). Not only was the wire cumbersome to work with, I ended up with hum that was noticeable at about half volume with no play back - not noticeable with music playing. I pulled all of that wire out and replaced it with Mogami 2330 mini-coax. The preamp is now absolutely, 100% dead quiet, to the ear and on the scope, at any volume level. You can get that wire at Markertek for $0.25/foot with free shipping. It's pretty delicate to work, however.

Hum is an invented word because all of the genuine cuss-words, including "golf" were already taken. Curing it is largely a process of elimination. If you've got one noisy channel, the problem is likely a tube on that side, or the wiring between the pre-amp and power amp on that side. If there's hum on both sides you can pretty well rule out a lame tube and the odds that two (one on each channel) have gone south are exponential.
I deal with this mess every time I swap out a component and nine times out of 10 it's the cabling. Keep the AC wiring as far as possible away from the low-level stuff (carried by the RCA cables). Or you could have an external ground loop involving the AC, whereupon at your own peril you lift the ground from one of the components.
Or, you could take up golf and learn all their words for hum.

Problem fixed , I had to stare at it for awhile . I was missing one ground from left side gain control to center attenuator. Hummmmmm free.
Don't think its loud enough though ?? I will ponder that one.

I was just going to say it sounds from your descriptions that maybe the volume control(s) aren't grounded, but seems you maybe fixed that (or PART of that problem).
If you are using the two gain controls also, those also need to be grounded. I will email the correct hookup again (as was in the SP14 build manual)
and also photos of the wiring.