Diagnosing noise from power amp of Ecoplate 2

[Janalex]

Hi everyone,

I have 2 Ecoplates IIs in my studio. One is noisier than the other. The noise is mechanically heard standing next to one of the amplifiers. It’s subtle but it’s later amplified by the pickups and would like both amps to be equal in noise if possible. The noisier amp had what looked to be a replaced power transformer. I purchased a NOS Stancor P-8671 so now both units have the same model. I don’t think it helped much but it was cheap.

Fuses are correct at 1 amp. And transformer doesn’t seem to be cooking. The exact measurements are +17.96 and -17.58 vdc. One regulator is Motorola and the other is a replacement from Korea in the noisier unit.

On the quieter one it’s 18.48 and -17.52 vdc. They are a different brand regulator in the quiet one altogether

Send and returns are set identical.

The only obvious difference in the power amp is there are bigger caps in the noisier amp. 2x 2200uf pairs as opposed to 2x 1000uf pairs in the quiet amp. The working theory is maybe there is more current being drawn in the noisier unit. Could this be contributing?

Any other thoughts are appreciated.

Jan

 

[dmp]

The noise is mechanically heard standing next to one of the amplifiers.

Can you explain this better? Do you mean you literally hear some noise when standing next to the plate?
In that case it would be from the driver. Can you describe what frequency? 60 Hz, 120 Hz, white noise?

 

[JohnRoberts]

Mechanical noise from a transformer vibrating at mains frequency (50 or 60 Hz) does not automatically show up in the audio outputs. Transformer noise can be from a faulty transformer. I have heard noise from a severely overloaded transformer, but such a transformer would be also generating noticeable heat (cooking?).

Have you tried measuring the ripple voltage across the two power supplies? You might need to hoke up a capacitor coupled probe to measure that.

If you have one noisy one and one quiet one, you could try to compare them, maybe swap some components.

JR

 

[Janalex]

Yes when you turn on one amp it can be heard standing next to it as opposed to other. It’s 60hz. I don’t think it’s the transformer being faulty cause its brand new and prior transformer was doing the same. If I disconnect the driver it still persists. JR mentions that overload has to be quite severe to make it audible in the output and that’s not the case as it’s not cooking. Probably if I didn’t have the second quieter plate I wouldn’t have even complained but it kind of frustrates me to have this discrepancy. I guess the only question is would there be any benefit to changing the value of the smoothing caps pictured so they match? Would there be any value in changing brand of voltage regulator so they match? Interestingly the behavior when turning the amps on an off differs too. One has a loud pop. Not sure if any relation.

Also I am not aware how to measure ripple current.

 

[JohnRoberts]

The noisy transformer may not be associated with the noisy plate.

What happens if you put the noisy transformer into the quiet plate, does it get noisy or stay quiet?

JR

 

[JohnRoberts]

Ripple "voltage" is the ac voltage riding on top of the unregulated supply (looks like a sawtooth). Ripple "current" can be calculated from Vp-p of the ripple voltage and the capacitance value, but I don't really care about the ripple current.

JR

 

[Janalex]

Looks like I erroneously attributed the low end hum to the power supply however upon further investigation the hum comes from channel 2 on the amp only. Channels 1 and 2 on the other amp and channel 1 on this amp all have similar hum when connected direct to a powered speaker. A single channel has much more hum. I already swapped pickups and that didn’t swap the hum. I also swapped opamps. This is the schematic. The electrolytic caps are new….

 

[Whoops]

Hi Janalex,
from what I understand from your posts the problem you have is an acoustic/physical noise and not noise in the audio path. Although as the pickups pick vibration the noise is pickup up by the pickups.
Is that correct?

Normally in this type of situation the noise would be from a power transformer core vibrating, it could be because a transformer is overloaded, but sometime it could be because of loose lams or lack of enough varnish impregnation.

But if you changed the transformer for a new one and the same noise persists then It's probably not related to the transformer.

Can you open the Amp, or bring it outside the case so that you try to understand what component of part of the amp is making that noise?

if I unserstood correctly your posts, the problem might be something simple like some loose screws allowing something to vibrate inside the amp

By the way if you post schematic photos, you should rotate them to proper orientation before posting, with the wrong orientation you are making it difficult for someone to help you

 

[scott2000]

Is it possible that the orientation of the power transformer is affecting anything?
Here's the flipped version..lol

 

[Janalex]

Just want to restate that I no longer believe the issue to be the power supply. Although there is mechanical noise I don’t think that’s what’s causing the electrical noise as it’s really only affecting channel 2. I need to do some measurements comparing output 1 and 2 and see what’s different

 

[Janalex]

Fixed it. At some point when the LED was replaced the solder blob from the resistor in line with the LED contacted a resistor in channel 2.