RF noise - Hypex UcD400HG

Zeustek

Member
2020-02-15 2:16 am
Hi all, I'm new here.

I have a Micromega PW400 power amp which has a pair of Hypex UcD400HG modules and I'm getting some pretty severe RF noise. It is much worse when the unit is just powering on.

Occasionally I'll be listening and I wil start to hear a kind of wooshing noise that varies in amplitude and frequency. It sounds like RF interference but it seems to be coming from the amp itself as I have isolated it from everything and tried a battery powered DAC (Chord Hugo 2) and more or less get the exact same problem.

I'm running a pair of Q Acoustics Concept 500's and using unbalanced input.

I'm thinking about maybe reducing the gain, do I need to mod the amps? or just turn the surface mounted POTS?
 
In order to pick up radio, an oscillator circuit controlled by an incoming signal, and a rectifier circuit, are required.

This often happens in defective op-amp circuits. The gain of the op-amp when it is not properly compensated can do the RF oscillator, the RF signal comes through audio wires, the input stage of the op-amp does the rectification.

The product is probably defective.
 

Zeustek

Member
2020-02-15 2:16 am
Not quite sure what is going on. I've been fiddling with this thing for a while now.

I bought XLR cables and switched to the balanced input and the bizzare sounds have stopped. The input is still unbalanced but I'm using an RCA to balanced cable and it seems to be resolved, for the time being at least. This amp has a switch on the bottom (XLR/RCA) but it didn't seem to have any effect when using the RCA input.

Does anyone have any clue what the sound could be? It sounds like a high pitched swooshing/squeeling noise which would only manifest when the RCA was connected.
 
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Some possibilities: The single-ended inputs are somehow allowing the two class D modules to cross-feed some switching frequency (being separate modules with separate power supplies they are likely oscillating at different frequencies, not slaved from each other).


Or there is feedback from the outputs that is much more efficiently picked up by unbalanced inputs than the balanced inputs, causing RF injection into the modulator which then wreaks havoc somehow.



It might be that the two modules are deliberately supposed to be set to different switching frequencies with enough gap between them, but somehow aren't far enough apart.


Its worth listening carefully to each channel in isolation to see if one is misbehaving at all, it
could be a confluence of several issues.


If its completely OK with differential inputs, it might be that the source impedance presented to the RCS jacks is pushing the behaviour over the edge somehow, perhaps by leaving the differential inputs floating?
 
In order to pick up radio, an oscillator circuit controlled by an incoming signal, and a rectifier circuit, are required.

Actually, no oscillator is required. The RF signal can be rectified (i.e. detected) in the gate or base circuits of just about any transistor in the circuit, including those in op-amps. All you need is an RF signal of sufficient amplitude.

It's called "Audio Rectification"...

The quick test for this is to get a kid's walkie talkie and sweep the antenna near the amp (etc) and see what happens.

Some easy solutions are also possible through avoidance... move wireless devices as far from the amp as feasable, use hard wired networking as much as you can, turn off cellphones when near the device, etc.

The problem of locating the accidental detector within a circuit is going to be tricky at best. A device --amplifier in this case-- not designed with good RF filtering can be a major problem to troubleshoot as there are likely to be multiple points of entry for the RF signal.
 

Zeustek

Member
2020-02-15 2:16 am
Thanks for the tips guys.

I have noticed that the right channel is 'noisy' when compared to the left. The left sounds like a normal hiss, whereas the right channel now has a clicking noise, as well as some other odd sounds. It is drastically different after switching to the XLR plugs though and I can't hear anything from the PLP.

I don't have a kids walkie, but I do have access to the real deal as my father in law has a farm, I'll give one a try and see what I pick up.
 

Zeustek

Member
2020-02-15 2:16 am
I also physically moved the amp, and powered off every other device nearby to try and eliminate any possible interference, then used my Chord Hugo 2 to test it and ended up with the same result (massive amounts of noise when using the RCA inputs), although I was connecting to the Hugo with bluetooth. I didn't even think about bluetooth causing issues. Maybe I should try a dumb old CD player.