Noise from DIY Amp

[Keryn O'Shea]

G'day gents,
An amp l threw together has always had a slight noise once both inputs are connected to the DSP, it sounds like a mosquito. With one input plugged in, its silent, with both, that's when you can hear it. Used with the DR280s, the noise is just noticeable, however l plugged SLAPs in yesterday and the noise is very noticeable.

After trying to diagnose, (insulating leads, lifting grounds, changing power points, leads, amp, ect) l found using a second power source eliminated the noise.

I'm aiming to use the SLAPs as DJ monitors in four weeks, and when you're that close, the noise is annoying! Apart from running a second external power source, is there something which l can try? I'm hoping a magical resistor in the right place does it!

[Tom Smit]

I wish I had an answer for you.

Side note: I got confused when I saw that the white speaker wires were on the positve terminal of the board. Here in NA, white goes to negative, and black to positive. To add some confusion, on NA commercial highway trailers, white is the ground, yet, on GM vehicles (back in the day) it was the black that was negative.

[Keryn O'Shea]

I wish I had an answer for you.

Thanks Tom
I'm wondering if the noise is more prominent with the SLAP as it doesn't have a woofer LP filter? I'll test with an OT8 tomorrow.

[Bill Fitzmaurice]

It could be a ground problem. When there's more than one path to ground on the inputs a ground loop can cause noise. Since the amps appear to be chassis grounded via the power supply the input wires should not be grounded at both ends of the input wires.

[Keryn O'Shea]

It could be a ground problem...

Right.. l removed the amp mounting screws which had the ground symbol

And lifted the ground on the power supply, which cut the noise in half, sweet!
It was still annoying me though, so l reconnected the grounds and checked the voltage out of the power supply, it was 47.8v which is surely acceptable..
Not being able to leave things alone, l tweaked the plastic voltage adjustment screw

The noise lowered it's pitch all the way to zero as it approached 48.0v , then a hiss developed on the way to 49v. So at 48.5v, there's a totally acceptable, quiet and normal amplifier noise
I'll leave it playing music for a couple of days & see if the amps continue to be happy with an extra 0.5v.

[Keryn O'Shea]

When there's more than one path to ground on the inputs a ground loop can cause noise. Since the amps appear to be chassis grounded via the power supply the input wires should not be grounded at both ends of the input wires.

Bill, would this apply to an amp with balanced input signals? Because l have another DIY amp with a similar problem, and it has balanced inputs, the first amp pictured is unbalanced.
If we could nail that amps noise, that would be excellent!

[Tom Smit]

It could be a ground problem. When there's more than one path to ground on the inputs a ground loop can cause noise. Since the amps appear to be chassis grounded via the power supply the input wires should not be grounded at both ends of the input wires.

Bill, are you saying that the input negative wires inside the amp case should be on pin 3 rather than pin 1? And, that the RCA terminals should be isolated from the case?
(I am using this as a reference.... https://www.ranecommercial.com/legacy/note110.html )

[Keryn O'Shea]

And, that the RCA terminals should be isolated from the case?

Luckily these sockets have insulating washers