S.E.X buzzing

[Paul Birkeland]

I'm seeing about 800-900 microvolts of hum on our demo unit here on the 8 Ohm tap unloaded.  That number drops to 300-400 microvolts with a 10 Ohm load (most meters get a little inaccurate down this low).

This residual hum will decrease if you move to the 4 Ohm tap, which is a very useful setting if you plan to use high sensitivity headphones, and of course a little bit of burn in time on a fresh set of tubes won't hurt either.

[thomas27]

Thanks PB - am reassured, this is exactly in line with what I'm getting and should only get better over time

[physicsmajor]

Also potentially worth noting: I hear a pretty significant difference in noise if the RCA jacks are unconnected vs. if they are hooked up to a source component.

Without a source - notable hum, tracks with volume pot.
With a source (how you are always going to use the amp, obviously) - this disappears. We're talking at least 6dB of hum just gone, and this is repeatable. What remains is a tiny bit of noise at the highest volume settings which I am fairly certain is the source, not the amp.

I mention this in case people are testing for hum by plugging headphones in to a source-less amp. Connect a source to the RCAs first! It doesn't have to be playing.

[Doc B.]

The best way to do it a power amp or other component without a volume control is actually to short the input. Even if you have a source plugged in you can still get noise from the source component and bad cables. Of course since the S.E.x. amp has a volum control just turning down the volume all the way will do the same thing.

[mcandmar]

Done a bit more testing this evening and discovered the hum starts immediately when i power on, then a few seconds later when both tubes warm up and start working it doesn't change in volume. Therefore am i right in assuming that rules out the audio path / amplification?

It also sounds like its ~100hz in frequency,  so wouldn't that imply the problem is somewhere around the main power supply section?

[Jim R.]

Have you thoroughly checked the whole power supply terminal strip and components -- making sure all diodes are in the correct orientation and are not damaged, filter cap for dc filament supply ok?, and especially the soldering around the joints where the thick wires all come together from the big diodes -- those can suck up a lot of heat and can be difficult to solder well?  Also, too much heat here without heatsinks on the component leads can damage the diodes and cap.

HTH,

Jim

[Doc B.]

Can you measure the hum level? It's very difficult to help someone determine whether residual hum is normal or not without knowing what the level is.

[mcandmar]

I'm not entirely sure how to do this correctly so i measured both ways.

Measuring between the chassis plate and L/R at the headphone jack i see .49v/.48v AC on the multimeter. Between the ground point on the jack and L/R i see .002v AC, i think that's as low as my meter goes so i'll need a better one.

I got the scope out (old analog) and with the earth lead clipped onto the chassis plate i see an AC waveform of ~.34v @20ms (50hz). Between the ground point on the jack and L/R i'm way down at 500mv/div and see an odd looking seesawing ripple ~200mv peak to peak in 10ms divisions (100hz).

Hope i did that right, and thanks again for the help..

Mark

[Paul Birkeland]

Done a bit more testing this evening and discovered the hum starts immediately when i power on,

Sure sounds a lot like a bad solder joint in the power supply.

[Doc B.]

Are all the filter capacitors oriented properly?

[mcandmar]

Ok so i assume those readings aren't right, and point to an issue in the main power supply section.  Everything looks ok and has been reflowed but to be 100% i'll use a solder sucker this time and disassemble/rebuild that entire section.

Thanks again,

Mark

[mcandmar]

Stripped and rebuilt the HV power supply section (terminal 1-5), and the two power feed posts 10 & 20 which seemed to improved things a little.  Good news is the power on noise is much better, after the initial bwong noise of the transformer energizing it all goes quite before the tubes start working so we are making progress.

I think i am now down to tube noise with a light hum in one channel, and a slightly louder buzzing sound in the the other, but the noises switch channels when i swap the tubes around.  The question is if the light hum is from the tube or the amp so i will source a replacement set of tubes and work from there.

Cheers,

Mark

[Mach2]

I have a similar buzzing problem after finishing my S.E.X build yesterday. My readings are quite similar to the other users' here,  3.83V @ C5, and -2.38V @ C4, and I have hum noises from both channels. I can live with the (barely audible) noise from one channel, however the noise from the other channel is very audible and annoying, especially with a low impedance phones such as my ATH-AD2000.

After tearing off half of my beard and going through and checking that all the soldering joints all over again, I swapped the tubes and the hum switches channels, so I guess the tubes are the problem here. I'll let my S.E.X play for a while and see if burn in will solve the humming problem. I hope those 6DN7 tubes are easy to find if I happened to have a bad tube.

PH

[Grainger49]

PH,

If this kit was a recent purchase Bottlehead will send you a new tube if the noise doesn't settle down after 50 to 100 hours of burn in.  You can just leave it on.

[physicsmajor]

I have a similar buzzing problem after finishing my S.E.X build yesterday. My readings are quite similar to the other users' here,  3.83V @ C5, and -2.38V @ C4, and I have hum noises from both channels. I can live with the (barely audible) noise from one channel, however the noise from the other channel is very audible and annoying, especially with a low impedance phones such as my ATH-AD2000.

After tearing off half of my beard and going through and checking that all the soldering joints all over again, I swapped the tubes and the hum switches channels, so I guess the tubes are the problem here. I'll let my S.E.X play for a while and see if burn in will solve the humming problem. I hope those 6DN7 tubes are easy to find if I happened to have a bad tube.

PH

If you're using low impedance phones, did you wire it up using the 4 ohm taps? If not, that's probably the easiest way to reduce your noise floor. At least on the lower noise channel, where the tube probably isn't the issue.