Hum, buzz, and microphonics. [resolved]

[Rublyow]

I just recently completed this build and, oh man, is something not right. With the volume all the way down, there is a loud hum, a bit of buzz, and crazy microphonics -- to the point that it starts to feedback through the speakers and get louder and louder. Also when I turn the volume knob, there is a loud scratchy sound. But when playing music, things generally sound ok (despite the noise in the background), however, even when the volume knob is totally maxed out the listening volume never gets above moderate levels.

I've already looked over all of the wiring and everything visually looks ok, but I'm going to go back and re-solder anything that looks even mildly suspicious. All of the measurements checked out fine, though most of the terminals marked with an asterisk gave a resistance of "1" on my ohmmeter (but not a zero reading) with no fluctuations. All of the voltage readings were within spec, though the readings at terminals 4, 9, 20 were a bit high (160 vdc rather than 150). Is there anything else I should be looking for while re-soldering?

I've read elsewhere that hum/microphonics can be an issue with high gain and high sensitivity speakers. My speakers are DIY, about 98 db sensitivity, and I'm running though the power amplifier stage of an old integrated Onkyo A10 amp, at 85 watts at 8 ohms, 50 kohms input impedance. Any thoughts?

Thanks!

 

[Paul Birkeland]

Does the 0D3 give you a nice, purple glow?

 

[Rublyow]

It does -- the glow is quite striking compared to most tubes.  In general, all of the tubes and LEDs light up.

 

[Paul Birkeland]

Are you running the amp/speakers in your signature?  If you had something like 105dB sensitive horns on a 1500 Watt amplifier, there's a strategy to dealing with that.

If not, I would be suspicious of flaky solder joints in the preamp.

-PB

 

[Rublyow]

I'm just running a cd line-level source through the Smash, then into the Onkyo A10 power amp stage, and out of Tempest Fusion speakers. It's a lot less than 105 db horns and 1500 watt.

 

[Paul Birkeland]

Your speakers and the amplifier section of the A10 being driven by the Smash should get to uncomfortable listening levels quite easily. 

Based on that piece of information, along with having hum in the circuit, I would be looking for poor ground connections.

-PB

 

[Paul Joppa]

The spec I found online was 0.15v input sensitivity for the Onkyo A10 amp. That's very sensitive, considering you are putting 2.00 volts into the preamp. Unfortunately it did not specify if that was for full output or some other measure.

 

[Rublyow]

So I went back and touched up all of the joints. It's a bit of an improvement -- the microphonics are down to normal levels, but the hum is still quite loud (my guess would be 120 hz). When I hook up the Eros phono the volume is great (it gets to uncomfortable levels at about 3 out of 10), though with the line-level CD player it's still is pretty weak even at max volumes, but maybe this is just an issue with the gain of the cd player. I've checked and rechecked for any mis-wiring and can't find anything.

One thing I did notic on recheck, the A4 and C4 terminals are supposed to measure 1.57 volts but are actually measuring 3.68 volts each. Is this within normal range, or does this indicate an issue somewhere?

Paul -- I've attached the spec sheet for the Onkyo A-10 amp. It has the input voltage as 1.5 volts. Is this the same thing as input sensitivity?

 

[fullheadofnothing]

Your CD player is not putting out a line level signal. Does it have its own volume control? Have you used it before alongside the Eros with another preamp? Were the levels comparable?

Is the hum a problem when you use the Eros, or is it just on the CD?

 

[Rublyow]

The hum problem exists regardless of what's plugged in, and doesn't change when I adjust the volume, or even if I  have a source plugged into the Smash at all. As long as the Smash is on and connected to the amp, there's a hum. I tried switching wall outlets, but no change.

The CD player is just a crappy old discman, but I just tried plugging in my ipod and it's the same general volume, though maybe a tad louder. I use this setup 99% of the time for vinyl, so I don't really have any other sources to plug in....

 

[Paul Joppa]

...Paul -- I've attached the spec sheet for the Onkyo A-10 amp. It has the input voltage as 1.5 volts. Is this the same thing as input sensitivity?

Thanks, that's better information. The source I found listed only the preamp sensitivity, not the power amp section sensitivity.

 

[Paul Birkeland]

One thing I did notic on recheck, the A4 and C4 terminals are supposed to measure 1.57 volts but are actually measuring 3.68 volts each. Is this within normal range, or does this indicate an issue somewhere?

That's very, very abnormal.  This still tends to point to a very flaky ground connection, or a missing ground connection all together.  I also believe that your noise could be a loose 220uF/250V power supply capacitor.  With the amplifier off, they should feel nice and tight if you wiggle them.

The 3.68V measurement is enough to say that the preamp has not passed its voltage checks and should not be used.  Can you recheck all your other voltages to see what you have?  I am guessing there is at least one other voltage for each channel that is way off.

 

[Rublyow]

Alright, went back again, rechecked all of the wirings step-by-step, and touched all of the joints up again. I rechecked the voltages, and I made a mistake reporting the previous voltages -- the 3.68 vdc is actually on the A7  and C7 terminals. The A4, C4, A8, C8 all read about 1.6. When I turn up the precision on the ohmmeter, terminals A6 and C6 read -0.22 and -0.1 rather than exactly zero, but that was the largest deviation I could find from the specified voltages. The full readings are below.

I just fired it up again, and no change at all to the hum. One thing I noticed, with no source plugged in, the hum is loudest when the volume is all the way down, and drops to almost bearable levels when the volume pot is at about 50% volume, then rises again when the volume pot is at 100%. Not sure if this helps diagnose the issue at all.

Also, I double and triple checked the 220uf/250V capacitors and they are definitely stuck on solid.

So how else can I trouble shoot this problem? Is there any way to verify it's not an issue with one of the tube, or is there any other way to diagnose where/if there is a flaky ground connection other than continually re-solding the joints?

Not that I image it's terribly helpful, but I've also attached a couple of images in case there are any glaring errors that I keep missing.

Thanks!



terminal VDC
1 0
2 0
3 0
4 152
6 0
7 0
9 153
10 0
11 0
12 0
13 0
15 49
16 49
17 214
18 0
19 0
20 153
21 49
22 49
24 0
25 0
28 0
29 214
30 0
31 0
34 0
35 230
36 0
A2 46
A3 49
A4 1.61
A5 0
A6 -0.22
A8 1.61
C2 46
C3 49
C4 1.5
C5 0
C6 -0.1
C8 1.6
B2 0
B3 214
B5 153
B7 213

 

[fullheadofnothing]

The fact that the hum level is affected by the volume control suggests that it is being picked up by the input wiring. Since you're not shorting the inputs, it's not as conclusive, but it's pointing in a direction.

Is it the same on all inputs?

Have you tried it in another room with different equipment?

 

[Rublyow]

Yeah, the hum is the same on all inputs, regardless of if I have anything plugged into them or not. I unfortunately don't have another setup to try. I did plug in my Quickie just for comparison and that is pretty much dead quiet. And for a point of reference on what the hum sounds like, if I touch a tube on the Quickie while on (or even put my hand really close to a tube) it causes basically the identical hum I get out of the Smash. I did try some -12 db attenuators and that definitely made the hum quieter, but didn't really fix the issue.

Is it worth just rewiring the inputs and the volume/balance/selector switch, or is there a better way to trouble shoot the issue?

 

[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