UCD180 questions

[peranders]

Originally posted by Bruno Putzeys

Whoa. That's what happens when correct observation (=listening test) is incorrectly interpreted (simply looking at the output stage).

The hidden truth revolves around PSRR. An op amp always uses one rail as an implicit reference. The PSRR of this rail is zero, with only loop gain standing in for rejection. Thus, great improvement is to be had if you can make sure that rail only sees a constant current (thus making it cleaner). Tying a current source across the output transistor that ties to the "bad rail" turns it off and replaces it by the constant current source.
This has nothing to do with "class A" operation, even though for reasons of simplicity we call it that.
Most bipolar IC processes have p-jfets but no n-jfets. For this reason, the majority of IC jfet op amps has the VAS on the negative rail.

Bruno, you may be right here but I wonder if anyone has done any real tests, both with instruments and by hearing? I have this current source to V- and used LT1028 in my QSXM3 phono amp and OPA627, AD8610 and OPA134 in my headphone amps and I can't here a thing in difference. Some people claim they can really pick this but I'm skeptical.

[ewildgoose]

Bruno, hmm ok, I think you are saying that I am going to be much better off going single ended then? However, I suspect that in practice the volume control will be usually at 9 oclock ish, and only rarely turned all the way down. So if I am only clipping off, say, 20db from max signal levels then I might hope that this will be enough to give a useful boost to signal quality?

In practice, low level listening will only be useful in rare conditions (my wife for example likes some music on quietly if having 40 winks on the sofa.) So I think having lower quality audio at these levels will be fine.

What is your basic design for pre-amp then? PGA?

Oh, one other last question which is probably relevant here: earthing through the passive pre... Assuming balanced cabling still, multiple physical amplifiers in seperate earthed chassis, and a single multichannel source component. Now in the pre-amp is it best to just connect earth wires from each input direct to each output, or should all the input and output earth lines be tied to the chassis earth in the preamp? I think from what you said before that tying them all together is the correct technique, but I am anxious to avoid earth loops and later problems after what will probably be a long soldering session...

Thanks

[classd4sure]

Originally posted by peranders

Bruno, you may be right here but I wonder if anyone has done any real tests, both with instruments and by hearing? I have this current source to V- and used LT1028 in my QSXM3 phono amp and OPA627, AD8610 and OPA134 in my headphone amps and I can't here a thing in difference. Some people claim they can really pick this but I'm skeptical.

Hi Peranders,

That's certainly a fair question.
I wonder, maybe your supplies are just too clean for it to make a difference? Tests would be good though.

[Bruno Putzeys]

Originally posted by classd4sure
I wonder, maybe your supplies are just too clean for it to make a difference? Tests would be good though.

Precisely. If you have excellent supplies the difference goes away.

[peranders]

Still, how much good does this circuit do that the internet rumour says makes all the difference I don't have the answer

[classd4sure]

Originally posted by peranders

Bruno, you may be right here but I wonder if anyone has done any real tests, both with instruments and by hearing? I have this current source to V- and used LT1028 in my QSXM3 phono amp and OPA627, AD8610 and OPA134 in my headphone amps and I can't here a thing in difference. Some people claim they can really pick this but I'm skeptical.

Hi, I just double checked the data sheets for opa134 and opa627, I don't know about the others you've mentioned but these show worse PSRR on the positive rail, so if this trick indeed works as discussed here it wouldn't have helped being tied to the negative rail anyway.

Even though there's a thread on this forum which raves about the improvement with the opa627 being tied to the negative rail via a simple resistor.. I think we should probably look towards the instrumentation for proper test results on this one.

[stef1777]

Basic questions.

Can I replace the two 22UF coupling cap with 47UF / 25V? I have some nice Black Gate in stock of this value.

Thanks in advance.

Stef...

[Jan-Peter]

Stef,

If you mean the 22uF coupling caps in the UcD180, this is no problem.

Are the Black Gates not too big?

Jan-Peter

ww.hypex.nl

[stef1777]

Originally posted by Jan-Peter
Stef,

If you mean the 22uF coupling caps in the UcD180, this is no problem.

Are the Black Gates not too big?

Jan-Peter

ww.hypex.nl

Hi Jan-Peter,

The box well arrived this morning.

Yes, the coupling caps (juste after the NE5532).

I have some BG PK serie 47UF 25V in stock. They are 6,3mm diameter. I should have some 47UF 35V ELNA Silmic II somewhere, but their are biggest (10mm).

I plan to test the modules first without mod...

[pburke]

Originally posted by stef1777



I have some BG PK serie 47UF 25V in stock.

for coupling you really should use the Blackgate N series. May be too big, though.

Peter