Hum, buzz, and microphonics. [resolved]

[Paul Joppa]

These observations suggest the source may be radio waves from some external device, which is picked up by the input wiring and/or the tube itself. The most common culprits for steady hum/buzz of that sort are lighting ballasts or  dimmers, and computer power supplies. But many, many other sources have been identified over the years. They can be very frustrating to identify if that's the case.

The possibility of ground noise remains viable though. Here are a few random things to check if you have not done so already: Does the Onkyo power amp have a 2-wire or 3-wire power cord? Is it plugged into the same outlet or power strip as the preamp? Were the screws at terminal 6 (the signal/chassis ground connection) properly tightened? Have you re-flowed the solder on all the ground buss terminals?

[Rublyow]

Hi Paul, thanks for the reply. I've double checked that the screws are all on tight, and I will go back and touch-up the the ground buss wiring tomorrow, maybe even just redo the wiring altogether to be safe.

The Onkyo is a 2-prong plug -- maybe this is the issue? I've tried plugging the Smash into a separate outlet, but this didn't change anything. I've also tried turning off and unplugging every light fixture and piece of electronics I can find, but this didn't help. We do have dimmer switches in the house, but they are 2 rooms away, and have no effect whether they are off or on.  I also tried RCA shorting plugs on all the inputs, to no effect.

One more observation, if I leave the Smash on for a while, I notice a faint crackle every minute or so. My guess would be just a little bit of tube noise, but figured i'd mention it on the off chance it helps track things down.

[Paul Birkeland]

The 2-prong plug isn't going to help anything.  You can run a wire from the chassis of the Smash to the chassis of the Onkyo to see if that calms things down a bit.

-PB

[Rublyow]

I tried running a ground wire to the Only with no success.  I also borrowed a power amplifier with a grounded plug from a friend. The hum is still there, but this amp has a balance pot, and if I turn down the gain on the power amp to about 10% then the hum is almost totally gone (though not completely) and I still have plenty of volume with the Smash volume control. Is this just masking the issue? Or is it indicative of what's going on?

I totally redid the ground buss wiring, and otherwise I am thinking of doing a total redo of the volume and balance pots,  and maybe the 220uf capacitors and all associated connections in case something is still lose. But I guess at this point I'm wondering if this is just a fool's errand, and the hum is most likely external in nature? 

[Paul Birkeland]

I tried running a ground wire to the Only with no success.  I also borrowed a power amplifier with a grounded plug from a friend. The hum is still there, but this amp has a balance pot, and if I turn down the gain on the power amp to about 10% then the hum is almost totally gone (though not completely) and I still have plenty of volume with the Smash volume control. Is this just masking the issue? Or is it indicative of what's going on?

This is fairly indicative of a system with a lot of sensitivity.  I would buy two pairs of -12dB inline attenuators to use at the input of your power amplifier, as this will give you roughly the same results on any amp you wish to use.

-PB

[Rublyow]

Alright I'll give that a try. When eventually transitioning to an amp with much lower output (e.g., the Seductor or Stereomour) will this present a problem? That is, will I still need the attenuators to reduce the hum, and will this in turn affect the ability of the amp to drive the speakers at adequate volume?

[Paul Birkeland]

When eventually transitioning to an amp with much lower output (e.g., the Seductor or Stereomour) will this present a problem? That is, will I still need the attenuators to reduce the hum

What you're really after is an amplifier with lower gain.  To get hundreds of Watts out of an amplifier with only a volt or so of signal at the input requires much less gain than getting four Watts out of the same volt at the input.

, and will this in turn affect the ability of the amp to drive the speakers at adequate volume?

This is a function of mostly the sensitivity of the loudspeakers in conjunction with the available power from your amplifier.

[Rublyow]

Just an update -- I borrowed a Quicksand from a friend to see if there way any benefit of having the power amplifier be totally out of the ground loop situations. It didn't really help things, there's still a hum (though sounds a bit more buzzy through the Quicksand). Interestingly, when I listen through the headphones with the speaker wires plugged in, there is no hum in the heapdhones, but when I unplug the speaker wires from the Quicksand, the hum becomes very loud through the headphones. Maybe it's just an issue of the heaphones and speakers "splitting" some of the signal, but still a bit weird.

Anyway, still working on tracking things down. Are there any good ways to isolate if it's an issue with the Smash wiring versus an issue with the wiring in my house (i.e., a light dimmer). Any other thoughts would be appreciated. I'm starting to run out of ideas. Maybe it's time for a full rewire....

[Paul Birkeland]

Try a pair of these and let us know what happens:

http://www.parts-express.com/harrison-labs-12-db-rca-line-level-audio-attenuator-pair--266-244

[Rublyow]

Ok, some progress. I was able to try the Quicksand with the -12 db attenuators and that basically fixed the problem. There is a tiny bit of hum when you're right next to the speakers, but nothing beyond what I'd expect from a DHT amp.

But using the attenuators did drop the max volume from the speaker by a huge mount, basically such that even at max volume it's well below "comfortably loud". I know the Quicksand isn't really meant for large speakers, even if they are efficient (98 db sensitivity), but how might I go about avoiding this issue in the future? (Sorry if this is a really uniformed question!) Should I be cautious about adding more gain to the preamp section of my rig? For example, would doing the Smash-up upgrade be ill-advised since it's going to increase gain even more? Or are there ways around this, like adding a passive attenuator between the Smash and my amp? Or, lastly, is it just an issue with what power amp I use? I was thinking about trying the Seductor next, but now I'm a bit concerned that this isn't a good match since the Seductor has no gain control.

Thanks!

[2wo]

They make a 6db version, you could try that...John

[Rublyow]

That makes sense, but still seems like a temporary patch to me. If I add more gain to the preamp stage, again, for example, with the Smash-up upgrade, or by upgrading my phono stage with more gain, then I would expect the issue to get even worse, correct? Is there a better way to deal with this issue besides jerry-rigging in-line attenuators? And what would I look for in a power amp to to determine if its likely to exacerbate the problem or make it better?

[Paul Birkeland]

Smash-up may improve your situation.

Try clip leading A6 and C6 to ground, then listen for noise. (Grounds are terminal 6 and 7)

[Rublyow]

Hi Paul, I finally had a chance to try clipping A6 and C6 to ground, but no change at all to the hum. One odd thing that was a bit noticeable before but is fairly striking now, when I turn the volume knob all the way up (with no source connected) the hum totally disappears, and is loudest when the volume know is all the way down. Anyway, just one last ditch effort to diagnose things. I'm going to give it one more solder touch-up job this weekend as well.

I should also so that overall things sounds great when music is playing. It's light years ahead of my old solid state... just a bit annoying during the quiet passages and between tracks.

[Rublyow]

Just an update. I installed the Smash-up upgrade and that definitely helped things. The hum is probably about 30% of what it was before, and is totally gone with -12 db attenuators. The extra gain also gives me a bit more headroom, so even with the attenuate I can reach decent volumes with low-level input sources. Definitely an improvement.