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.

 

[M42]

Hi Grainger,

The hum always remains a background issue...it can't be heard when music is playing, except through quieter passages. The hum remains audible from the transformer at the amp. I'm afraid my technical/troubleshooting skills are virtually nonexistent. So somewhat red-faced I'd like to confirm a couple of things. When measuring the voltages to trim the hum pots...only one of any input RCA jack needs to be shorted. The measurements are taken from the respective right and left channel speaker binding posts?

In your post you advised me to adjust nothing and don't short the input....then measure the output (speaker) binding post. I presume the speaker cables must be disconnected to obtain the correct reading.

Regards,

Jeff

 

[Grainger49]

Jeff,

My thought was that there is something else happening to cause the hum.  It can be a matter of a bad solder joint on your interconnects.

Since you adjusted the amp to 1mV with the hum pots.  Any intrinsic hum in the amp would have been measured.

So measure from + to - on each speaker post with the speakers attached.  The voltage you hear through your speakers should be present.  And as Dan says, you shouldn't hear 1mV through your speakers (not sensitive enough). 

There must be some hum coming in that is being amplified to be heard.  I want to see how much voltage it is.

 

[M42]

Hi Grainger,

The interconnect from the Reduction to the Stereomour was supplied by BlueJeans cable as were the speaker cables. I have some other interconnects that I'll swap out and see if anything changes. I checked the soldered connections on the rca plugs from my turntable, from what I can see they look good. With the system on I measured at the speakers the following voltages:

Left Channel      Right Channel

.001 - .007        .001 - .014    (the voltages did not settle on any one value)

The binding posts on the amp read:

.001 - .010        .001 - .014

The meter is set to auto-range and read ac volts.

Thanks,

Jeff

 

[Paul Joppa]

So, you ARE getting more than 0.001 volt at the speaker terminals in the system. Useful data!

The next step is to make the same measurement with the volume control turned all the way down. If you still get excess hum, then remove the Reduction interconnects from the Stereomour. That will help sort out whether the cables are picking up something, or the Reduction is picking up something.

 

[M42]

Hi Paul,

With the volume turned all the way down with the turntable running, I measure .001 at the speaker terminals and the binding posts on the amp. I still hear the hiss/hum through the speakers, which seems unaffected by volume setting. When I removed the interconnect from the Reduction nothing changed. The result was the same when I disconnected the turntable from the Reduction.

Jeff