Help needed: odd DC offset on UCD400.

[maxlorenz]

Hi everybody:
I need help. I have two UCD400ST based monoblocks. My left amp has allways had two "issues": audible "tump" on power on and HF distorion (wich I thought was due to DAC or PS wiring). I have 5 other UCD amps and no one makes a "tump".

Yesterday I make measurements and I got -90mV offset from this amp (at speaker's posts). Wether source is ON or OFF makes no difference on this measured value. His sibling reads +17mV .

Both modules are stock, but DC coupled (signal input cap removed). My DAC is AC coupled (read 0V) and my "pre" is a stepdown tansformer. Reading at amp's input is at the limit of the multimeter capacity: around +/-0.3V, wich may be OV, I guess.

I have double checked the PS (and signal) wiring polarity and it's OK.

What can be wrong?
Please, any tip is welcome.

Another question I have is, what is the function of the trimmer resistor on the module PCB?
I hope it is useful to solve this offset problem

Many thanks in advance.
Mauricio

[IVX]

maxlorenz, that pot on the main PCB for the dead time adjusting. If you want adjust DC you must do it on the modulator plugin board, which covered by compound! Maybe new versions have DC adj. pot, yet, but it's will on the modulator plugin PCB anyway.Try to ask here -Jan-Peter van Amerongen <info@hypex.nl>.

[maxlorenz]

Thanx, IVX

M.

[mazurek]

One of my ucd 180 channels has a dc offset which is not proportional to the dc input also. I think thats just the way it is.

[maxlorenz]

Yeah, but mine has this HF noise and I want to make sure that this is not the cause (or one accompaning symptom) of the noise.


Thanks
M

[RX5]

Hello there

since you mentioned removing the ac coupling caps, try re fitting them again and see if the problem (DC offset) is fixxed... if it does, its probably in the opamp buffer board...

[classd4sure]

Is it just the one module with the HF noise?

Maybe check solder connections on the entire input stage, from the signal pins to the op amp.

I have found I've totally eliminated any HF type cross talk noise. Gone. At the source like I said before, kill it before it happens, which is probably easy with the UCD's.

Take your DAC out of the equation test with something known good, that includes MP3's..

Maybe try replacing the bootstrap cap just for kicks.

[maxlorenz]

Thanks for the tips!

Sincerelly,
M

[maxlorenz]

Update:
Last night I checked the signal input circuit: it looked OK to me. Anyway, I resoldered the signal wire to the PCB.

Chris:
I did not have a suitable exact value cap to replace the bootstrap cap. Did you hear improvement with this mod?

While I was there I tried to change the small low voltage decoupling electros near the transistors (whose exact type I can't read). I aborted because the underside of the PCB was too crowded.
Instead, I paralelled there a couple a 100nf film caps.
I remember that I bought some Black Gate N type for that task, but I used them on other project. It's funny so few appear to have tweaked that low voltage PS! Instead we all are dreaming of a dedicated external PS.

Today I think the HF improved significantly: in fact now this channel sounds opaque compared with the other one. I think it is still burning-in. DC offset is still there...
Today I will try this on the other amp: Panasonic FC 47uf?/50V //Wima MKS 100nf. If it is any good I will buy some small Tantalums "á la ClassD"

Appart that, I received a kind reply from Hypex
I will attack this DC problem with both backups.

Regards
M

[classd4sure]

Hi,

So what did Hypex have to say on the matter?

So it's the ones closest to the transistors your messed with and wound up bypassing. There's a good chance your tweak did more harm than good. Try re opening your mind and give it a careful listen again, see if perhaps you've fooled yourself. Saying it sounds more "opaque".... like a wall of sound? That's, a bad amp, with a flattened soundstage and no imaging.

Your bypassing could have had the opposite effect desired... parasitics/resonances etc.

When you're being bothered by HF hiss it's easy to make a "tweak" that chops off the HF air/microdetail (very bad tweak) and think it's an improvement because you noticed a reduction in the hiss, but you also reduced all the good stuff in that frequency range, which makes for a very dull amp. Sounds to me like this is the trap you've fallen into.

Reconsider your tweak, look for the source of the hiss to cure it. Maybe have another look at your layout/wiring again.

Don't just jump in changing those electrolytics with low impedance ones either, for they're also there to act as dampers, being considered the equivalent of more elaborate bypass+snubbers. See Bruno's old posts around somewhere, hotrod thread maybe.

I did change the bootstrap on the 180's, I did in conjuction with the bigger decoupling caps so I can't comment on how much or little they actually do for the sound. I just thought it may be leaking a bit much perhaps causing your DC problem. I guess the same could be true for the input stage caps. How do those rails measure?