P20 causing amps to hum

I know this has been posted before, and I read the threads but still don’t see an answer. I upgraded recently from a p10 to a p20 and this resulted in my amps (D’agostino momentum 300 mono blocks) humming loudly enough to hear from across the room. I unplugged all components except the amps from the p20 and still had the hum. Also switched out power cables but still had hum. Plugging amps directly into outlet there is no hum. I have a dedicated 20 amp circuit using 15 amp plug to p20.

Talked to PSA and they sent me a new unit. I upgraded outlet to furutech and upgraded power cord to transparent 20 amp cable to use 20 amp plug on p20. Still have hum. The hum is louder in sine wave mode than multi wave and decreases when I lower the voltage selector down to 110v.

Any ideas on how to fix this issue? Is this happening on all p20s?


Sorry to hear. I don’t have any hum with my P20. I have a mainly tube setup. Noise actually decreased compared to the P5 I had before.

So with your P10 you didn’t have hum? Something else must have changed. Are your power cords coming close to your interconnects or speaker cables?

No hum with the P10.
The amps hum even after I disconnect speaker and all other cables except power cord. Have also tried several different power cords, both on the amps and P20. The same with both P20’s. I have no electronics background so not sure what else to try. A friend recommended using a 2 prong plug with no ground prong on the amp power cord to see if its a ground loop but I am going to call D’agostino before trying this as I don’t want to cause any damage to the equipment.

Yikes. That’s not good. Just to confirm, the hum is coming from inside the amplifiers themselves. Right? You had written you’ve disconnected the speaker cables.

If that is the case then it seems you might have DC coming out of the P20, though that is really rare. And you’ve had two P20s? This doesn’t seem right.

What is the distortion in and out as measured on the front panel display?

THD in is 3.3
THD out is 0.1

Yes. It is the second p20. I have a friend nearby who has the exact same set up as me. A P20 Driving two momentum 300 D’Agostino amps. Although the P 20 is on the heavy side I think I can get it over to his house and see if my P 20 causes his amps to hum. Any other suggestions prior to doing that?

Sadly, just went to friends house and his amps hum plugged into his p20 also. :thinking:

The good news is the equipment is not malfunctioning.

Given the P20 does not cause other amps to hum (at least of which I am aware) this implies there is something unique about the D’Agostino amps.

Please let us know what D’agostino suggests as well as their reaction to a cheater plug.

I’m not sure of the exact mechanism but my (cheaper) Dan amp also hums quite frequently (Straight from the wall). I think my dealer implied that my mains might be too high for the fitted transformer. Something like the windings are getting saturated or some such nonsense. Does the hum go away if you reduce the input voltage a bit?

The hum does go away If I reduce the voltage to 110, down from 120. Not sure that’s great for the amps though. Also, there are threads from others (Google search) also reporting their P 20s causing their amps to hum. Don’t remember which amps they had but they weren’t Dagostino. I do agree though. The 2 P20s I’ve tried as well as my friends P 20 we’re all driving D’Agostino amps. I did talk to one of the engineers at D’Agostino. He recommended that I switch to the 20 amp circuit on the P 20 because he didn’t think the 15 amp circuit was providing enough power. I did that but it didn’t help at all. Certainly now well beyond my level of expertise in figuring out what’s going on. Waiting to hear back from PSA. Hope they can figure this out.

Paul, yes. The hum is coming directly from the amps.

It’s the same hum you hear from fluorescent lights or a refrigerator.

Right. No. Switching inputs from 15 to 20 wouldn’t matter to the P20’s output.

I too have seen threads where Dagastino’s aren’t happy with the pure sine wave output and I don’t know why. Have you tried changing the phase calibration on the P20?

Sorry, I live in the uk so don’t know American voltages, but isn’t it supposed to be 110v, not 120v

Reading a bit more, valid US voltage is 110V to 120V, so if your amp’s transformer hums at 120V but doesn’t at 110V then I would say you amp prefers 110V.

By phase calibration Do you mean sine wave versus multi wave. If so, it sounds better on multi wave with less hum but does not matter which multi wave setting. I’ve also done the auto tune which doesn’t change anything. I will check with D’agostino on Monday and see whether it’s OK to bring the P 20 down to 110 V output. It is currently set on 120 which I assume is the default because I did not change that when it arrived.

I find this article helpful. 110V is acceptable at the utilization point for line voltage.

In addition to auto tune, you can vary phase calibration manually as well. I think Paul is suggesting to try changing it on your own and listen to what happens. You can vary phase by plus or minus 10.

If your amps are unhappy at 120V, but then are quiet and presumably not unhappy at 110V, how can that be bad? Do they sound different at 110?

I will try 110v. Thank you for the article Elk.