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Fabian 21st November 2007 04:24 PM

UCD700 and SPS80 HV - noise problem
Hi together.

Iím having troubles with an UCD-AMP Iíve just built. Iíve got some terrible noise, hum and jet engine sound on one unit. I guess itís some issue with earth /GND connection, but Iím not sure how to cure this.

Let me describe my setup.

Iíve built two mono amps, each consisting of an UCD700HG, a SMPS80HV (switching power supply, from ďcoldampĒ) and an additional 15V supply for the UCD driver stage.
Since I suppose itís a ground problem, let me explain my connections.

Supply Wiring
1. Mains ďEarthĒ is not connected to anywhere inside the amp. Only ďPhaseĒ and ďNeutralĒ are connected to the inputs of the switching and the additional power supply.
2. The SMPS provides a high and low voltage output. High Voltage output is +90VDC, GND, -90VDC. Low voltage output is +12VDC, GND, +12VDC. The GNDs of those two outputs are not connected in inside the SMPS. Those are pulled together at the UCD 700 board.
3. The GND of the additional power supply is referenced to the negative high voltage input at the UCD700 board.

Signal Wiring
I use RCA and XLR inputs at the same time, just to be able to listen to any source.
This is how Iíve connected it.
1. Again, no GND connection to the chassis.
2. All connectors are isolated from the chassis.
3. RCA signal is connected to XLR Audio Hot (pin2).
RCA GND is connected to XLR Audio Shield (pin1).
4. XLR Audio Hot (pin2) is connected to UCD700 non inverting input.
XLR Audio Cold (inverted signal, pin3)) is connected to UCD700 inverting input.
XLR Audio Shield (pin1) is connected to UCD700 GND.
5. Inverted Audio Input and GND are connected (shorted) on the UCD700 board.

Now the point is that both amps are clones of each other. One behaves nicely the other one doesnít. Once when Iíve shorted the inputs, the bad noise, hum and whistle has almost completely gone. :confused:
Does anybody of you have an idea of what Iíve might have done wrong or can suggest away out?

In the link below youíll find a the wiring scheme and some pictures of the real amp to see details that might help to find the devil.

Thanks a lot in for any advice.
Best regards,

Link to pictures:

alvinlim 21st November 2007 10:56 PM

15v ground?
from the diagram, your 15v supply GND is connected to the -50v rail? is that correct?

Else, it sounds like an SMPS problem.

ncc 22nd November 2007 02:58 PM

Hi Fabian,

Your problem is just like mine. I also use ColdAmp SPS80 as PSU, My 2 UcD400 modules had been working perfectly until 2 month ago then they started to make terrible noise, hum and jet engine sound, I posted my problem in the forum and received some answer, most of those considered my problem is ground loop but like you I could not find the ground loop.
Another member suggested that it's the SPS80, to single out the problem I had to put together a linear PSU to test the modules to find out if it's the SPS80 or the modules themself, without the SPS80 the problem still persisted so SPS80 is not the culprit, then I started to think that may be a kind of high frequency oscillation, maybe someone in the neighbour just installed a wifi station or something that make the modules oscillate, so I thought maybe a kind of low pass filter may help, to test the theory I installed a 100 ohm resistor after the signal input jack then a 100pF capacitor connect the other end of the resistor to ground (of course the signal to the module is connected after the resistor)
Much to my suprise, the problem disapeared and the amp working so far so good.

You could try my solution to see if it could cure your problem and let me know the result.

BWRX 22nd November 2007 03:24 PM

I thought the UcD modules already had low pass RF filters for the inputs on the board?

Fabian 22nd November 2007 06:32 PM

Thanks for the replies so far.

Yes, the GND of the additional 15V supply has to be connected to the negative supply pin of the UCD board.

Your remark about interference with some kind of wireless gear is interesting. I always have a squeeze box where I listen to music. I will check that.

One other thing that came to my mind is this: What shall I do with the cabinet. It is made of anodized aluminum, whose surface is electrically non conducting and i did not connect it to any point of the amplifier. I just googled for "amplifier grounding case" and found the following statement:

"Connect the case and pot housing to signal ground. This helps shield the amp from hum and noise. Most cases are tied to signal ground and/or AC ground. Do not connect V+ or V- to the case instead of signal ground as it will short the rail if it touches another case, cable plug, or other grounded conductor. Do not connect AC ground to signal ground or the case."

Is it correct that I then should be sure to electrically connect all parts of the cabinet and connect them somewhere to GND (signal ground)? Maybe this would stop all the noise and maybe prevent the amp the amp from picking up any wireless noise.

What do you think?


ncc 23rd November 2007 01:32 AM


Originally posted by BWRX
I thought the UcD modules already had low pass RF filters for the inputs on the board?
I don't know if the UcD modules already had low pass RF filter for the input or not but in my case if I did not connect the modules to the interconnect cable the hum is small, if I connected the interconnect cable to the modules the hum was very loud, so I though it's the interconnect cable that acted as an antena and picked up high frequency interference and the is amplified by the modules.

Forgive me if I'm wrong since I'm not an EE

ackcheng 24th November 2007 05:11 PM

I have the same setup. My SPS80HV supply output is floated and the UCD is all floated. The mains ground is connected to the SPS80 input ground (for safety). The +/- 12V ground is connected to the mains ground

Fabian 25th November 2007 08:43 AM

Hi all,

I've found one bad point in my amp. I just removed the cabling from my XLR input connector and the bad noise is almost gone. Before I did that, I've connected all housing parts and connected them to audio GND. This brought a just a little Improvement.

But now I can check different setups. Leave everything floating, tie the case to audio GND, to Mains Earth or both together.

Ackcheng, how did you connect GND of the +/-12VDC supply to mains earth? If I'll connect mains earth on the SPS80, I think I will have an earth connection via the UCD700 and there shouldn't be another connection necessary, or am I wrong?

Did you try other cabling setups? And did you hear an improvement when do changing something?


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