Stereomour II, Hum, Noise and Balance Issues [resolved]

[ompv62]

Hello,

I just completed my Stereomour II build today (my first ever attempt at something like this). All voltage and resistance checks are good. I very much enjoyed building the amp - the instructions are perfect!

I listened to the amp for about 30 minutes and it sounds glorious!

However, there appears to be a moderate hum and I'm getting some intermittent ticks and pops on the right channel. Furthermore, it appears that I have a channel imbalance because I have to turn the balance knob approx 3/4's towards the right channel to center the stereo image (on music I know very well with the vocals dead center). My speakers are Omega Super 3XRS (94.5db).

I've been looking at the solder joints with a magnifying glass on the balance pot and volume pots and around the A socket but cannot find anything that looks like it needs to be reworked. I assume the A socket is the right channel?

Any suggestions on what I should do to hunt down this problem?

I read Mike B's post about adjusting the hum pots:

http://bottlehead.com/smf/index.php?topic=5168.0

I do not understand this:

...
3)  Short out the input to one channel
4)  Put AC voltmeter on that channel outputs
5)  Adjust humpot for lowest reading on voltmeter
...

Can someone please explain this to a novice?

Thank you in advance,

[Paul Joppa]

To short the input, we normally advocate making a shorting plug - an RCA plug whose"hot" and "ground" are soldered together. Install the shorting plug on an input and set the selector to that input. A simpler procedure is to turn the volume control all the way down.

The outputs that are mentioned are the speaker binding posts. Put one lead of your voltmeter on the black post, and the other lead on the red post. Set the meter to AC volts, on the most sensitive scale if it's not an auto-ranging meter.

I assume the last instruction is self-evident  :^)

Not all meters are sensitive enough to indicate the hum, which should be only a few mV. In that case, connect the amp to speakers, put your ear next to the woofer and adjust for the lowest hum.

[Jamier]

I had some similiar popping in the first hour or two of life with my Stereomour II. I solved it by working the selector switch a dozen or so times. I decided, when this cured the problem, that minor oxide or corrosion deposits on the contacts of the switch just needed to be worn off. Next, I adjusted the balance pot by centering the image then centering the knob. I suppose you probably did this initially also, so maybe you balance problem has occurred since you initially did this?I adjusted my hum with the ear to the woofer approach. I got it as low as possible with the hum pots and then I was able to lower it still more by using a very small ammount of contact enhancer on each pin of the 12AT7s that are actively used( I will need to treat all of them when the shunt reg. upgrade comes out).If you do this be very certain to use a very small amount and wipe the excess from the receptacle, especially if it contains silver as mine does.

[ompv62]

Thanks for the info. I used my ear against the woofer and got the hum under control. I'm still getting a few pops and ticks on the right channel  - I'm going to give the amp of few days of break in before tackling that one. Perhaps as you said, there are some oxide or corrosion deposits to work out.

[Rocketman248]

Try swapping the tubes.  I had some minor popping in one channel when I built mine, and when I swapped the tubes, the popping switched sides.

 

[ompv62]

Thanks for all of your input. The ticks and pops have resolved. I guess it was  just a quick break-in period. The amp sounds breathtaking. It is hard to believe the weight of the bass that is coming from my single 4.5 inch drivers - this while maintaining the resolution and delicacy of the mid range and high end.