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?