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NicMac 9th March 2010 11:05 PM

How to bypass the Buffer stage on UcD180 modules?
Sorry if I might have asked the same question in another thread some time ago.....
I would like to make an integrated amp using the Buffalo II DAC with IVY III I/V stage (balanced output, close to zero DC offset and very low output impedance). I would like to pair this with a set of Hypex UcD180ST modules that I have coming in.
Can anybody please tell me how to bypass the coupling caps and buffer stage on the UcD180ST module? I don't need the gain, I have no or little DC offset and no real impedance issues.

Koifarm 10th March 2010 06:23 PM

Hai, just look here:

I removed the decoppling caps of 22uf from the UCD pcb.
They were between the opampinputbuffer and the UCD powerstage. Then i connect the input sym.+ en sym - to the new points on the PCB according the picture below.

NicMac 10th March 2010 10:28 PM

Hi Koifarm,
Thanks!!! Just the kind of info I was looking for. So the coupling caps are between the buffer and power stages and not directly on the inputs..... Do you know if this is also the way it is connected on the UcD180HG modules?
As you have done it the, mod is really simple and and pretty reversible.
I will try it. How did it work for you (sonically)?
Thanks again,

Koifarm 11th March 2010 11:41 AM

Nic, with the opampbuffer i had a little noise( hiss ) in the speaker. Without the opampbuffer there was no hiss. The sound improved in detail, staging and dynamics.

Important is that your preamp can deliver enough signal to drive the UCD.

I have no pictures of the UCD180HG, because if you bypass his opampbuffer then you can better buy the "ST" version. The biggest difference between "HG" and "ST" version is the opampbuffer you gonna bypass. Except for two powerrail capacitors.


NicMac 11th March 2010 01:48 PM

Hi Ronny,
I have a set of the ST coming in and I will tweak these. I guess the difference of the power rail caps between the HG and ST are relatively irrelevant as I'm using a regulated supply.
I will drive the modules directly of a Buffalo II DAC that I believe to be a perfect match (balanced, low output impedance, low DC offset, 4VRMS if in dual mono, attenuation in the digital domain).
Can't wait:D

Koifarm 12th March 2010 08:55 AM

Buffalo Dac's are great. So the combination will be great.


NicMac 24th March 2010 11:34 PM

What is the input impedance of a UcD180 with bypassed buffer stage? Searching the fora I find 1k8 but I think this refer to a SE connection configuration. What about balanced?

Koifarm 25th March 2010 02:08 PM

The impedance is the same. 1k8 for the "-" signal to earth and 1k8 for the "+" signal to earth.


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