A Hum [solved]

[danox574]

Hello - I've built my Paramounts and everything is in spec plus or minus a little bit.  Passed all tests and both worked very well right from the beginning.  Great experience.  Have been listening for around 3 hours, so I know that they have a way to go.  They are being fed from the BeePre.  During these three hours, they were pretty quiet and I didn't notice this.

One channel is clean and quiet, but the other channel has a developed a hum after power up.  It's intermittent - comes and goes every few seconds, or minute.  It's a low hum, not like a ground loop.  The audio file attached is two minutes, and it is there at the beginning, changes in tone a bit, goes away around 14 seconds, starts to fade back in around 25sec, is continuous until 1:48, and then cuts in and out for a bit until I stopped recording @ 2min.  I was holding my iPhone about 1" from the speaker.

Varying the source input or volume controls on the BeePre change nothing, so I assume it's in the Paramount.

I'm probably going to go reflow every bit of solder, but I'm hoping that hearing it could help you help me target the right area of the amplifier.

This is a m4a file zipped attached - I couldn't find an audio format that I could attach directly to the message...

[fullheadofnothing]

Does the noise follow the tube? Does the noise subside after warmup (say, a half hour or so)?

[danox574]

It does not move with the tube, I have confirmed that after swapping both the 300B and 5670.

It's very consistent on power up and once the amp has been on for a while - I don't think I made it to a half hour, but after 10-15 minutes it still randomly appears and goes away.  It seems to be getting louder too, when I started this, it was barely noticeable from across the room, but it's getting more prominent with each power cycle.  Or, maybe I'm paying more attention.

Music is produced without any issue while the noise is present, the amp does work.

[danox574]

Check that - it's coming in from the BeePre.  Unplugging from the BeePre from the amp made it go away, swapping the cables between the two sides resulted in the sound moving to the other channel.  I can re-post this over there once I do the basic BeePre troubleshooting.

[danox574]

So here's the updated version of what's going on:

Hello - I've built my BeePre and it has about 5 hours on it, now that I have my Paramounts built I am running it more.  It has developed a hum in the left channel which does not move to the right channel when the tubes are swapped.

It's intermittent - comes and goes every few seconds, or minute.  It's a low hum, not like a ground loop.  The audio file attached is two minutes, and it is there at the beginning, changes in tone a bit, goes away around 14 seconds, starts to fade back in around 25sec, is continuous until 1:48, and then cuts in and out for a bit until I stopped recording @ 2min.  I was holding my iPhone about 1" from the speaker.  This was recorded about 10 minutes after power up with no audio coming in.  Varying the volume controls, or input, does nothing to the output hum, it remains consistent.  The amp works otherwise - the quality of music amplification is not degraded during hum, it's just added to the sound.

I don't believe it did this early on, but I was always playing music - I was configuring a sequenced power up from a Panamax unit for this and the Paramounts when I spotted it, so I was powered up for a while without audio.  It very well could have been there this whole time.

This is a m4a file zipped attached - I couldn't find an audio format that I could attach directly to the message...

I'm hoping that hearing the noise will help you help me target the right area of the amp for troubleshooting.

[Paul Birkeland]

I moved the topic for you, and I was sort of thinking about the issue of Preamp vs. Power amp as you were writing your last response. 

Tell us more about your BeePre.  Is it all stock?  Stock tubes? Etc.

What's your line voltage? 

Periodic hum like that could be something simple like a loose wire, it could be a loose 1085 regulator not making good contact with its heatsink, or something more menacing like low line voltage.

-PB

[danox574]

stock build, no parts changed, right out of the box and manual.

The left channel did have a problem before with two of the LEDs not coming up - I reflowed the 4007s and magically it worked.  Details here:

http://bottlehead.com/smf/index.php?topic=6835.15

Paul felt reflow on the 4007s was not what brought me up, he felt I nudged the 12V diodes while doing so - maybe I need to actually reflow terminals 28-31. 

Line voltage here is 118-120v according to my meter.
-Dan

[Paul Birkeland]

Yeah, I bet if you measure the POS-IN voltage while the amp is humming, you'll find that it is sagging. 

Pop the filament regulator board off on the offending side and give the two fat diodes a tug with your needlenose pliers, then tug on those 10,000uF caps.  My bet is that something in that cluster of parts (and wires) isn't quite soldered all the way. 

-PB

[danox574]

They held on the tug, but I reflowed and added solder to 28-31, top and bottom, as well as the lug on the transformer that the big diodes connect to.  Also checked the POS IN and NEG IN on the board itself, reflowed positive, it wasn't really great looking.  Fired it up and everything was nice and quiet for about a half hour.  Powered down.

Brought it back up again and the left channel hum is back - it's coming and going just as before, changing harmonics like it did in the file that I attached.  I had one good half hour but I don't really feel did anything that had any effect.

Any suggestions are appreciated - I can do any measurements or troubleshooting you recommend.

Edit: I just did the top row opposite the two caps, NEG IN looked a little grey instead of silver.  A few minor touch-ups and we're back to silence - at least for the moment...but with intermittent problems you never know, so advice is still appreciated.

[Paul Joppa]

My advice is to take PB's advice, and measure the POS-IN voltage while it is humming (and not). This will tell us whether it is a low supply voltage that causes the humming, or something else.

If you have an oscilloscope, then monitoring this voltage will also reveal whether there is an increased ripple causing the hum even if the average voltage is correct. That would help determine whether it's the filter caps or the diodes - or, if both measurements are correct and steady, whether there might be some more subtle problem.

The more data there is, the easier it is to identify a cause.

[danox574]

Well, I am 2 days with no hum (about 6 hours or so of usage) - and the most direct correlation I can find was kind of an ugly, grey solder pad on the Neg In on the left channel.  I also reflowed 22-25 but they shouldn't have had an effect, my guess since I was being pointed to 28-31 and the Neg In connects to 28 - I had a proximate connection but not a conductive solder bridge.  I didn't get a chance to put the scope on it during the issue, as checking those terminal has resulted in no more issues, but if it returns I will no doubt get the scope on it and verify your suggestion Paul.

I think we are tidy for the moment, and I really appreciate the feedback and help.  The Paramounts are 100% so far as it turns out, so out of a build on the Crack with Speedball, Stereomour, Seduction phono, mainline, paramounts and BeePre, I've had a total of one problem which you quickly directed me to a target area to resolve, and I think that's really impressive. 

[Paul Birkeland]

so out of a build on the Crack with Speedball, Stereomour, Seduction phono, mainline, paramounts and BeePre, I've had a total of one problem which you quickly directed me to a target area to resolve, and I think that's really impressive.

Your build success is certainly also impressive!

We'll be here if that hum comes back.

-PB