Stereomour Teething Pains!

[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

[Doc B.]

At what distance are you when you listen for the noise from the speakers. At the listening position, or up close?

[M42]

I have the speakers set up on my desk top in a near field configuration...a little more than a metre from my position in my chair.

Jeff

[M42]

Are there any Bottleheads in or near Ottawa Canada?

[Doc B.]

At this point I would try a different 12AT7. Sounds like you might have a noisy one.

[M42]

Thanks Doc...I'll try the new tube.

[M42]

I've installed a new Electro-Harmonix 12AT7 gold pin in place of the stock tube. Unfortunately this did not resolve the issue. Is it possible that one of the 2A3's could be the problem?

Jeff

[Grainger49]

Jeff,

You are in luck, you have two 2A3s in your amp.  Try swapping them from left to right. 

The likelihood of you having two bad 2A3s is slim.

[M42]

I did swap the 2a3's earlier in my troubleshooting efforts...the problem remained. Would it be advise able to start the amp with just one of the tubes installed to see if a potentially bad one can be isolated/identified?

Jeff

[Grainger49]

Jeff,

I suppose you have determined the hum is in both channels.  Also that you have tweaked the hum pots down (after swapping tubes) to a minimum AC reading. 

So if it is in both channels swapping and starting with one out doesn't make sense... to me. 

Nearfield listening you can hear hum that isn't obvious at 5-6 feet.  But with inefficient speakers it should be very low.

I think there are other Bottleheads in Ottawa.  But you need to start a new thread in the General folder asking for local help.

Just to check, move all components apart.  The turntable the farthest away from the Stereomour, the Reduction at the full length of the interconnect from the Stereomour and the Crack unplugged and not connected to the system at all. 

We are trying to eliminate all variables.  Eliminate the Crack all together, move the transformers, and motor as far away from each other as possible.

I admit, I am grasping at straws.  You have followed all other instructions to the letter and this is baffling. 

[ffivaz]

Just a question : is it a hum or some hiss ? I have no hum, except the around 1.4 mV I can here when I put my ears against the speakers. But I have clearly some audible hiss, a white noise, that I can't get rid of and that is audible in the whole room, especially at night when there isn't another noise around.

Grainger has a FAQ post about the difference between hiss and hum.

I tried changing my 12AT7 tube without any luck. Reading around, here is some information with a workaround with hiss, by PJ : http://www.bottlehead.com/smf/index.php?topic=4521.0

Best wishes.