Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Stereomour => Topic started by: M42 on February 04, 2014, 01:31:10 PM
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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
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Check the hum balance.
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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
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Yes, the hum pot setting scan change with the tubes burning in.
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Hmm, can you feel the transformer vibrating while it hums?
How are the voltages in the amp after this noise presented itself?
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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.
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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
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1mV is very, very good for a the hum level. What speakers are you using?
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Doc,
Presently I'm using NST Super Zero 2.1's.
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OK, when you say the hum reduces does it actually go away, or just drop a little?
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No, it just reduces...it does not go silent.
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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?
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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.
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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.
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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.
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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
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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.
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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
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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.
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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
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At what distance are you when you listen for the noise from the speakers. At the listening position, or up close?
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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
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Are there any Bottleheads in or near Ottawa Canada?
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At this point I would try a different 12AT7. Sounds like you might have a noisy one.
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Thanks Doc...I'll try the new tube.
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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
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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.
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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
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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.
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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.
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Once I have a little more time I will move the components as you have suggested Grainger and report back. Thank you for your continued input, it's much appreciated. Fabian, in answer to your question....I have both, hiss and hum even with the volume control at minimum settings. As far as I can remember, the amp was dead silent (at the speakers) at lower volume settings. The hum/hiss seemed to become audible at the speakers at about the 100 hour mark or so. The amp has just under 200 hours on it now. What perplexes me most at this point is the continued hum at the transformer, which disappears when you touch the transformer bell (not the transformer laminations). Nothing is loose. Additionally on startup a humming noise is heard through the speakers for about six to eight seconds then abruptly drops to a much lower level. I've obviously stuffed something up on the build...I'm determined to find it and fix it.
Jeff
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Hum for a few seconds after startup is normal. Transformer bell hum is usually fixed by tightening the mounting screws a little. Hiss coming on after a hundred hours may just be a funky tube.
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Hi Jeff, M42
You asked the question who lives in Ottawa. There are a couple of us. I live in Stittsville and I believe a member mchurch does too, although I haven't heard from him in awhile.
I am a junior Bottleheader. I have built and upgraded a Crack along with a upgraded Quickie. If you need a second set of eyes or other components to try to narrow down your focus I would be willing to help.
As you can see in your thread, this forum is wonderful and the people hear will get you up and running. You don't have to worry about that. I am also thinking of getting either a Sex or Stereomour. I would love to hear yours.
Send me a PM if you would like.
Take care....Dave
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Hi Dave,
I have to agree, the forum and its members are a wonderful resource indeed. Where else can you get, in many instances, almost real time responses to your questions...and often from the designers themselves! It's nice to hear there are at least a few of us here in the Ottawa area. You're more than welcome to have a listen to my amp...despite my perceived shortcomings with my build, the thing still sounds amazing! Thanks for your offer to help...I'll send you a pm.
Jeff