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Old 30th September 2007, 02:21 PM   #1
ncc is offline ncc
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Default Help me to solve my UcD400s Problem

After study the forum and read comments by other member. I've build a power amp using a pair of UcD400AD for a year now, the PSU is SPS80 by Coldamp (PSU voltage is 60v which I think a little bit high but within the amp spec).
It worked very well but recently I've run into one problem: After running the amp for about 20min, it suddenly gave out a very loud hum/buzz, so loud that I had to turn it off immediately for fear of damaging my loud speakers
From yesterday I could not run it anymore because everytime I try to turn it on, it gave out a very loud hum/buzz, to isolate the problem, I disconnected my preamp but the problem still there.
I try to measure the output and the DC offset is very low

I'm not an EE so I'm lost, not knowing what to do. Please help me if you could.

Thanks in advance
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Old 30th September 2007, 02:44 PM   #2
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Sorry I forgot to mention in my previous post that if I remove the interconnect cable from preamp to power amp, the hum/buzz was reduced to a small level.
If I reconnect the interconnect (in this case even the preamp is turned off, the hum/buzz is very loud. I disconnected the interconnect and plug a piece of wire (1m) to the input jack in the power amp, I got the same kind of hum/buzz
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Old 30th September 2007, 07:58 PM   #3
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Hi

SMPS is not durable as linear power supply and it can easily build up hum and noise. Have you check the voltage rial or try your UcD with linear power supply.

Kittikun
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Old 30th September 2007, 08:12 PM   #4
Thago is offline Thago  Belgium
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Hi
I' m not a major specialist too, but this sounds like a ground loop and/or earth problem to me. I wonder if your circuit ground is connected to earthing? If so, I would try to deconnect the circuit ground from (case) earth. Never ever remove the case earth, this could be dangerous!
Were there any changes to your house earthing circuitry, or did you connect any new equipment? Mostly RCA cables are connected to earth (or ground?) at one side, so adding new equipment could cause a ground loop to occur.

Kind regards
Dirk
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Old 30th September 2007, 08:49 PM   #5
ssanmor is offline ssanmor  Spain
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Quote:
SMPS is not durable as linear power supply and it can easily build up hum and noise.
Sorry, but this is what I call a superficial statement

Ncc,
are there any visible damages in the PSU or the amplifier modules? Do you have the possibility to measure the voltage rails with an oscilloscope?
If your problem persists, please send us an e-mail to coldamp and we will try to help you determine the cause of the problem. As with any system, any of the components may be the cause of the problem: the amplifiers, something related to the connection (quite possible now that I read your second post), or, why not, the PSU.

But such a simple statement like kittikun's one denotes very little knowledge of state of the art and a lot of prejudices. I am sure that comments like this one were very common also when people started to use Class-D amplifiers. Are they also less durable than Class-A, B, etc?

Best regards
Sergio
www.coldamp.com
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Old 30th September 2007, 10:29 PM   #6
Javin5 is offline Javin5  Switzerland
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ncc, here are some quick thoughts:

If the hum dissapears (well, almost) if you remove the cable, this could indicate that the cable shield somehow has lost contact with the connector. Try to replace the cable, or check its connectors for reliable contact with the shield.

Another possibility would be that the cinch input terminal has bad contact with signal ground.

Regarding the power supply, are the dc output voltages still ok?

Regards -- Kurt
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Old 1st October 2007, 06:24 AM   #7
ncc is offline ncc
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Thanks for you all for your response to my problem

+To Thago: I don't think that I have a ground loop problem since my amp working so well for almost a year without any problem, my circuit ground is not connected to earthing (amp case) and there's no changes to my house earthing circuitry and I don't have any new equipment.

+To Javin5: I've check and replace the cable but the problem persists, also the cinch input terminal has good contact with signal ground
As for PSU voltage, I still measure 60V DC voltage for each rail

+ To ssanmor: I don't have an electronic oscillator so I cannot measure the output, I'll contact coldamp for assistance
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Old 2nd October 2007, 06:21 PM   #8
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ncc,

Have you tried using linear power supply?

Kittikun
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Old 3rd October 2007, 07:21 AM   #9
ncc is offline ncc
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To kittikun:
I'm planning to try linear supply but I was too busy during the last few days, so may be I'll try to assemble a linear supply in the next few days, after that I'll test my UcDs with linear supply to see if they're working OK or not, if they're working OK then the SMPS PSU is broken, if not then the problem is probably from the UcDs modules.
I'll let you know the result later
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