K5881 hum / buzz

[Tom How]

Fitting suppressor caps seems like a low-cost route to eliminating another potential source.

One extra detail I've noticed. The amp is fed from a passive pot volume control. With the source (cd/dac) powered off, turning the passive volume knob up increases the volume of the buzz. Not quite sure what that means Unplugging the input leads entirely seems to result quite low levels of buzz, but it is still there.
Powering up the source doesn't seem to make much difference. I guess unplugging the leads is effectively the same as putting the volume knob at low.

EDIT Actually that's not strictly true. The source is powered up but not playing. The buzz is low (2mV AC on speaker posts) at minimum volume and also low at maximum volume, but appears to peak at mid volume (20mV AC on speaker posts)
Granted my multimeter is not the best at low voltage AC and only shows 0.002v, but you get the idea.

Possibly I am starting to chase my tail.

[Richard]

No the sound you hear is real
Take the passive control out of circuit. Connect the amp input to the dac directly with the interconnects and do not power up the dac. Power up just the amp, does it still buzz now?
You'll see we're trying to establish if the buzz is coming in through the input or if it's from the amp circuit. The connected but not powered dac is effectively shorting/blocking the input. If you have any RCA shorting plugs made up you could use them instead.

[Tom How]

Directly connected the amp inputs to the DAC outputs. DAC off, Amp on, and yes, high levels of buzz, about 15mV AC on the speaker posts.

[Richard]

Re your posts 11 and 13, it would be best to try shorting the amp inputs as close to the amp as possible, as it seems the volume control was attenuating a buzz picked up before it. You can make shorting plugs from an old i/c cut close to the plugs with the signal and outer braid connected together.

If you have any other interconnects it would be worth trying them in the set up too, as it seems likely the buzz is coming in the amp input before the volume control. This may be from the dac or via the i/c themselves. Are they standard shielded coax type or unshielded twisted pairs etc?

[Tom How]

with amp inputs shorted I can hear the buzz, but it is fairly low.

Interconnects are shielded, not twisted. I was worried the mains cabling was too close to the i/cs put moving cables around doesn't make much difference

[Tom How]

And my final extra confusion for tonight: I think maybe I have 2 different buzzes.

There is a low level buzz most of the time, same as when the amp inputs are shorted and all other equipment turned off.

However I can make it do a higher level and higher pitched buzz by, get this, physically touching the shaft of the volume pot. Earthing the shaft of the volume pot prevents this. The shaft is bolted to a small aluminium L which is bolted to a block of wood, there is no circuit between this any anything else. I guess I am acting as a big aerial and injecting white noise.

Therefore I think my observations about the volume pot position are muddied by the fact i might or might not having been touching the shaft.

I shall go and listen to a nice quiet string quartet and decide if the buzz is really annoying me or not.

[Phil Y]

Generally, the volume pot would be mounted on the metal chassis and its case therefore earthed. If it needs to stay on the piece of wood, I would be tempted to connect a wire to the pot (trapped under the nut would do) and connect it to earth.

Also, are all the signal and negative feedback cables in the amp routed where specified in the build instructions? If routed badly they could cause some pickup.

Phil.

[Richard]

It's a very sensitive amp at 75mV for full output but it should not buzz. You can vary the buzz changing the input but it's still there with the inputs shorted.

Is it on both channels?

Possibly just tired PS caps or maybe an all-invading RFI through the mains or ether.
I'd tackle what needs to be done anyway; change that first dual PS cap and the diodes and fit the suppressor caps.
Tidy the first stage/input wiring away from any mains wiring.
Use coax for internal input wiring and the feedback wires from the OPTXs.

[Tom How]

Originally Posted by Phil Y

I would be tempted to connect a wire to the pot (trapped under the nut would do) and connect it to earth.

Phil.

I have done exactly this

Originally Posted by Phil Y

Also, are all the signal and negative feedback cables in the amp routed where specified in the build instructions? If routed badly they could cause some pickup.

Phil.

Well, I haven't seen the build instructions for 20 years, but the signal cables are the other side of the case from the mains. Feedback path... this deserves looking at.

[Tom How]

Originally Posted by Richard

Is it on both channels?

yup

Originally Posted by Richard

change that first dual PS cap and the diodes and fit the suppressor caps.
Tidy the first stage/input wiring away from any mains wiring.
Use coax for internal input wiring and the feedback wires from the OPTXs.

PSU & sup caps are on order and will fit (and the diodes) when they arrive.

Input signal wiring is screened. Feedback wires I'm fairly sure are not screened, so i can look at that.