A few questions on a PCM1793-based USB DAC

[njepitt]

jkeny: I have a few caps in my parts box that look like MKT caps to me. (plastic boxes, right?) I have a coupl of 1uF, and a 3.3uF. I don't think it would be too tough to stick one of these in and see what difference it makes, so I'll give it a go. However, these are pretty old (and used!) so any results should be taken with a pinch of salt. Since I don't have any really high-quality film caps I can't help there, but I' love to hear about any experiences.

I also feel I need a better output stage first - just pushing everything through the cheap film caps I have at the end may mask any improvement anyhow... Did you have a chance to take a look at the circuit I linked to?
I am attaching it below. (I hope noone minds my doing this, but I can't get a direct link to work, and in any event, I think all appropriate copyright info is there.) Any thoughts on this circuit?

Cheers

Nigel

[jkeny]

Sorry, Nigel, I can't analyse a circuit from it's schematic, others here can!

[njepitt]

I decided to go ahead and build the "SACD enhancer" output stage I linked to above, and finish off the DAC. I put the output stage on a separate board, so it can be switched out without too much trouble if I feel like it. Three power supplies - one transformer/bridge rectifier combo feeds separate 3.3v regulators for the PCM2707 and the digital part of the PCM1793, another feeds a 5v regulator for the analogue side of the PCM1793, and a third runs a separate PSU for the output stage.

I made a couple of alterations due to the parts I had on hand - the output caps are two 22uF bipolars I had lying around, and I decided to put a couple of old 4n7 paper-in-oil capacitors in parallel with them, to see what would happen. (These appear to be really old, but have never been soldered, and don't appear to have leaked,so I'm guessing they're OK. I'm also translating "a oleo" as "paper-in-oil", but for all I know it's something-else-in-oil).

Since I don't have a distortion analyser (hell, I don't even have an o-scope...) I can only use my ears to minimise distortion by adjusting the trimpot. I already have it sounding *much* better than the temporary setup I had going before, and with a little effrot I can probably tweak it a little more. We'll see.

jkeny: If using a cap on only half of the balanced output gives you good results please post - I'd like to compare experiences. I'm only sorry I can't get hold of transformers to try out, but the circuit I used is dead simple, and I think it's worth your time and effort to give it a try. (I made no attempt to match transistors - anyone think it would make a difference here?)

All comments welcome, as always. Long-term plan is to use this experience in planning and building another DAC based on a PCM1794, this time with optical input as well as USB, so please poast any and all suggestions.

Cheers

Nigel

[jkeny]

Hi Nigel,
Good work, looks neat. You've really gone high res on the pics now - can even see your fingerprint on the white table

You like the SACD enhancer better than the cap output? I'll go back to listening to cap output on one differential leg when I get a chance & give you my impressions - I've just so many variables to nail down here at the moment -remind me if I forget

[njepitt]

Originally Posted by jkeny

Hi Nigel,
Good work, looks neat. You've really gone high res on the pics now - can even see your fingerprint on the white table

You like the SACD enhancer better than the cap output? I'll go back to listening to cap output on one differential leg when I get a chance & give you my impressions - I've just so many variables to nail down here at the moment -remind me if I forget

I definitely like this SACD enhancer *much* better than the cap output. Having said that, I don't have access to big audiophile-quality caps, or indeed anything other than completely standard stuff, so I'd be interested to hear what you hear when you find the time to try it.

Let me repeat my suggestion to give this SACD enhancer a try. I'd also like to hear from anyone who tried the jfet circuit I linked to above.

Glad the resolution is better. It seems I should be able to post larger still - let me know if these aren't sufficient.

Cheers

Nigel

[jkeny]

Originally Posted by njepitt

I definitely like this SACD enhancer *much* better than the cap output. Nigel

In what ways can you say it's better - detail, presence, dynamics, etc?

[njepitt]

More dynamic range, yes, and more detail. I'm not quite sure what you mean by "presence", but let me guess that you mean a heightened sense that the sound could conceivably be real (or "live") rather than a recording. (Perhaps we could say that we can "suspend disbelief" more easily.) If that's what you mean then yes, that is the principal difference.

Actually, this is what we're all looking for, isn't it? I care much more about this than noise levels and so forth, although I am willing to believe that these more "quantifiable" factors contribute greatly to this "suspension of disbelief", and I wouldn't be surprised if people with better measuring equipment and more experience than I have could show this to be the case.

As things are, however, subjective opinions are all I have...

Cheers

Nigel