Sublime amp buzz/whine/noise

[Omacore]

Good morning,

I'm experiencing a bit of a buzz, or maybe even a whine, coming from the Sublime amp when it's powered up.

The build went well with no issues. DC measures right on 24.0VDC. All three LEDs are lit up. The amps do function properly in the sense that they receive an input from a BeePre2 and amplify it to the speaker, but they do produce audible noise.

I've been using the BeePre2 for the last month, which typically feeds a Kaiju then into a pair of floor standing speakers. Those are Klipsch RP-8000F II with a nominal impedance of 8 ohms. This whole system is dead quiet to my ears. I don't have a passive subwoofer yet. Building a pair of those is going to be next month's project. These speakers are just being used for testing purposes of the Sublime for the time being. The test is using the Sublime amp instead of the Kaiju as I had been previously.

I have built two of these Sublime amps and they are both doing exact thing. The noise isn't loud, but it is definitely loud enough to be heard, especially with no input. The noise appears when the Sublime turns on and is connected to the speakers. It does not matter if there is an input connected. The noise does not change at all if I turn the pots for the crossover or the volume control.

When the device turns on, the buzz comes down from a higher frequency to it's steady state buzz very quickly (less than a second). When I turn it off, the buzz rises in frequency for a couple seconds, then returns to the steady state frequency and rises again before it goes quiet.

Using my cell phone mic, I see that most of the noise is at 480hz, with a smaller peak at 960hz. There are other peaks, but they're a fair bit lower.

Is there something can be done about this noise?

[Doc B.]

Hmm. All the ones I've built have been dead quiet so we'll need to do a bit of sleuthing. Try disconnecting the heavier red and black wires from the screw terminals that connect the output of the control PC board to the amp PC board. Fire it up and see if the noise is still there.

[Omacore]

I disconnected the heavier red and black wires as described and the noise went away.

I reseated the wires a couple times and whenever they are re-connected, the noise persists.

I removed the RCA input wires to the preamp board, not expecting it to matter. Sure enough, it did not change anything when I removed them.

I don't own an oscilloscope, so I can't really see what's going on. I tried putting my multimeter on VAC to see if it would detected anything. When the power is active, it does detect  0.02 VAC on the V+ and V- of the power supply. That's not a lot, and I don't think this multimeter is really the right tool for something that small. It's a EEVblog BM235 Multimeter.

After I power off the device, the 0.02 VAC shoots up a bit for a few seconds to around 0.3 VAC, then slowly dissipates to 0.000 VAC on the meter. The amount of time that it takes to do that is definitely longer than what I hear audibly, but perhaps the load allows it to drain faster? I repeated that test removing both boards from the power supply unit and the same effect happens, it just takes 30-40 seconds instead of 5-10 seconds. Not sure if there is something here, of this is just a red herring.

[Doc B.]

You really don't want to run a switching supply with no load. Since the noise went away with just the preamp/control board disconnected it sounds like that is where the problem may lie. We can send out another board to try. Shoot Eileen an email at queen at bottlehead dot com with your shipping particulars and we'll get another one on the way.