Stereomour Teething Pains!

[M42]

Hi,

I am having some noise issues with my recently completed Stereomour. The amp presently has just over one hundred hours on it. From the outset on power up the power transformer would make a distinctly audible hum which would dissipate somewhat after a couple of seconds. I could then hear a persistent but quieter residual hum. Oddly enough the hum could be silenced by placing my hand on the top of the transformer bell. I have installed all the required fibre shoulder washers, and all the mounting hardware is checked secure.

Initially, the only noise I would hear from the speakers would be some hiss only when the volume was adjusted to about seventy five percent of the travel of the knob. Last night I began to hear a hissing sound and background hum from both speakers. The

[Doc B.]

Check the hum balance.

[M42]

Hi Doc,

When I completed the amp I did adjust the pots...I was able to adjust them both to one millivolt (the lowest value my meter is capable of reading?). Can/will these settings change as hours are put on the amp?

Jeff

[Doc B.]

Yes, the hum pot setting scan change with the tubes burning in.

[Paul Birkeland]

Hmm, can you feel the transformer vibrating while it hums?

How are the voltages in the amp after this noise presented itself?

[M42]

Hi Paul,

No...I can't say that I feel the transformer bell vibrating. I will trim the hum pots again as Doc has suggested and check the voltages again. I'll report back with the results.

[M42]

Hi All,

I have checked all my voltages again and they remain essentially unchanged so I have not bothered to list them all at this time. I did however miss the power supply PC board pads "HV+" which originally measured at 392 vdc compared to the specified 380. I adjusted the hum pots back to a reading of .001 on both sides. Unfortunately the problem remains. Now however, the hum is audible through the speakers when the amp is started and reduces in volume as the hum noise subsides at the transformer.

Jeff

[Doc B.]

1mV is very, very good for a the hum level. What speakers are you using?

[M42]

Doc,

Presently I'm using NST Super Zero 2.1's.

[Doc B.]

OK, when you say the hum reduces does it actually go away, or just drop a little?

[M42]

No, it just reduces...it does not go silent.

[Doc B.]

OK, so something doesn't add up. NHT super zeros are very inefficient, which means you shouldn't be able to hear 1mV of hum through them. So somehow you are hearing more than 1mV and the possible sources would be unlikely to be coming from inside the amp if you can get it to measure 1mV. So that residual hum is maybe being pickup up by the amp from another source. Is there another piece of gear with a power transformer near the amp?

[M42]

The amp sits between my Thorens turntable and Reduction . My Crack amp sits next to the Reduction. However the noise is present when the other amps are off.

[Doc B.]

Well, it might be that the 12AT7 is introducing some hum due to heater-cathode leakage, but that would most likely increase that 1mV reading. The other possibility would something like a cold solder joint on the last filter cap connections creating some extra ripple from the high voltage supply, but again that would probably show on the meter. Did you have speakers connected when you measured the 1mV, and if so could you hear the hum? That might indicate that the meter is not sensitive to higher hum frequencies, i.e. 120Hz and 180Hz.

[Grainger49]

Try this, when you hear hum, stop the music and measure the output binder posts with your meter.  Don't adjust anything or short the input. 

Post what you measure.  It might be coming into the Stereomour.