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    hum in a SP14

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    Marvins

    Posts : 2
    Join date : 2016-09-13

    hum in a SP14

    Post by Marvins on Sat Oct 01, 2016 12:44 pm

    Looking for someone to point me in the right direction . I have purchased Bobs ST120 a few a years back and it went together with out a hitch. I love the amp. I have a pair of home built K-horns and horns love tubes . I was running and old SS preamp with the St120. It was great , but I decided this past summer to mate it with Roy,s SP-14 .
    I took my time and the summer to build it. I had a couple of errors along the way , like the back on the front and attenuators upside down . Lol . Ordered more wire , tore it apart and made it right. But I had a hum I could not find and finally threw in the towel and sent the board back to Roy . I had two sets of resistors mixed up as I had either marked them wrong or measured them wrong with the meter. It was , I think 280 Ohms verses 280 k ohms ? Anyway Roy changed them , tested the board and sent it back.
    But the problem still persists . Rotating the volume attenautor from low to high ........ There is distinct hum at the low end , this lessens or disappears by mid point. At the extreme end of the dial ( or max ) the hum will re-appear. This is with no input running of course . Plays music sounds ok at mid volume. But to me I don't get as much volume as I would like either ?
    I used a SS preamp prior too and it had plenty of room . System is in the basement and I crank it sometimes when we are upstairs.
    I had to unsolder and turn the attenuators around , so not sure if they could be over heated them and cause me problems. That will be my next step. Order a new one and run directly to it from one of the jacks.
    This what I have done so far:
    All voltages check out as per Roy's instructions. I have tried new tubes. I have wired directly from one set of jacks to the attenuators , thus bypassing 90 percent of the wiring and switches with in . I ran a ground from the board to the ground buss to try that and nothing changes.
    We will assume the board is fine as Roy tested it . Either I have missed something or a transformer? Not sure how you could check that as I don't have an oscilloscope .
    And yes I probably should have just sent the whole thing back , but I was sure it was the board and cheaper to send it as I live in Canada.

    I see Roy is away. So any help would be much appreciated.

    deepee99

    Posts : 1333
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: hum in a SP14

    Post by deepee99 on Sat Oct 01, 2016 2:00 pm

    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.

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: hum in a SP14

    Post by j beede on Sun Oct 02, 2016 8:12 pm

    Hello Marvins,

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

    ...j

    deepee99

    Posts : 1333
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: hum in a SP14

    Post by deepee99 on Sun Oct 02, 2016 8:36 pm

    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.

    Marvins

    Posts : 2
    Join date : 2016-09-13

    Re: hum in a SP14

    Post by Marvins on Sun Oct 02, 2016 9:02 pm

    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

    deepee99

    Posts : 1333
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: hum in a SP14

    Post by deepee99 on Mon Oct 03, 2016 12:45 pm

    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.

    Zimmer64

    Posts : 113
    Join date : 2013-01-29
    Age : 52
    Location : Switzerland

    Re: hum in a SP14

    Post by Zimmer64 on Mon Oct 03, 2016 2:26 pm

    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

    jkaukas

    Posts : 1
    Join date : 2015-06-07

    Re: hum in a SP14

    Post by jkaukas on Tue Oct 04, 2016 6:49 am

    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.

    wanders

    Posts : 10
    Join date : 2016-03-16

    Re: hum in a SP14

    Post by wanders on Fri Nov 11, 2016 11:38 am

    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.

    deepee99

    Posts : 1333
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: hum in a SP14

    Post by deepee99 on Fri Nov 11, 2016 12:04 pm

    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.


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