Twisted Pear Audio Opus DAC - Page 8

If that's what is being said, I don't agree. Whether a DAC is balanced or not is only one part of the equation. If a mediocre DAC could be made great just by making it balanced, wouldn't they all be that way? Sure, a DAC may be generally improved by adding a balanced output, but it also depends on how it is implemented. Simply because a DAC is balanced does not make it great. And vice-versa, of course. You must understand the entire DAC topology, and not just focus on one aspect like balanced or unbalanced. If the Opus is a good DAC (and I can't make that judgement not having heard it), then it is not just because it is balanced. It is the overall design. Making the balanced-to-single-ended conversion should not make an otherwise great DAC poor or mediocre (maybe just a teenie weenie less great). IMHO, YMMV.

double post, sorry.

I agree in priciple with what you've said - but I was wondering the same thing as Bearmann. If you study the Ballsie, you'll see that the Wolfson DAC's quad outputs are funneled through two sets of opamps in series - two LM4562's and an LME4974. Using a USB input card is even further extending things - they employ a second PCM2707 DAC and state that the USB card for the OPUS can be used as a "single-ended, mid-level, standalone DAC" by itself. So, if you're wanting a USB-input, stereo source DAC similar to an Alien - only "high end," you'd have to add three opamps and another DAC into the signal path of the OPUS.

There may not be anything bad with any of that, but it seems we're taught to avoid opamps in order to be king of the hill these days, and the PCM 2707 seems to really confuse things.

So, not trying to cause trouble, but I think Bearmann's question was a good one, perhaps.

I thought that the USB input is handled by the PCM2707 and then I2S is sent to the WM8740 DAC. The "standalone DAC" is referring to using the DAC inside the PCM2707 and getting line level analog output from there, this would not be a "preferred intended" use of the Opus, in fact, it bypasses it. Under normal circumstances, that PCM2707 is just a USB receiver, nothing more.

As for the Ballsie, I have no idea how it's done or why it's done that way. But then again, it's not surprising since I don't know balanced to single ended conversion architecture.

I only referenced the PCM2707 stand alone because they imply "mid-level". Maybe that would have some effect on degrading the USB processing since it is in the path (lower level subroutines)? Again, it may just not be optimum to take an OPUS and turn it into a single ended, but that's the reason I thought it was a good question. I added the USB question because that's how I'd like to use it.

I hardly know any of the architecture, period.

That's the PCB mount transformer... the standard toroid (non-PCB mount) is the TE62063-ND at $24.98 each. http://dkc3.digikey.com/PDF/T073/P2145.pdf

Greetings,
at first to state this clear: I've no idea about the whole dac-architecture in detail. My knowledge ends somewhere between the fact, that you need a receiver for the spdif-stream through usb and an (integrated) I/V-stage at the DAC-chip itself.
I've probably the same problem as tomb...
All my rig is single ended and the Opus DAC seems to be a really nice and reasonable priced diy-dac. BUT is it still worth the ~$300 (dac, usb, ballsie, psu) if you will use it "only" single ended?!
I know that wether a ballsie nor a zapfilter will decrease the soundquality of the Opus in that way, that'll be on AlienDAC level ...
And I know that the PCM2707 is only used to transform the spdif-stream through usb to I²S and in that fact it's not acting like a PCM2707-based DAC (like AlienDAC and so on) ...
I simply don't have the technical background to judge if it's intelligent to build a balanced DAC (with all the effort) and use it then just single ended?! This question stays ... or is it redundant?!

best regards,
bearmann

Balanced-to-single-ended conversion can be performed with a single op amp per channel. Just take a look at my ezDAC design! That's how I did it. Like you guys, I too am somewhat scared by the prospect of several op amps in a row. Seems like bad things can happen. However, there are plenty of real-world designs out there using several op amps per channel, and at the same time, loved by audiophiles. These folks may also point out that using a single op amp, as in my design, also has its pitfalls. Indeed, even using a single resistor for I/V (again as I have) may be the simplest and seemingly most "pure" design, but it has flaws as well. There is no perfect design. For example, you may increase dynamic range at the expense of linearity. Or you may increase noise at the expense of frequency response. These are totally random examples, and may not actually be the case, but hopefully, you see my point.

Couple small points:

You can use the Opus single-ended without the Ballsie, by just using the +OUT and GND from each channel. It will be -6dB (half of a balanced signal), but no op-amps, if that's what you are after. You just need a bit more gain afterwards.

The Ballsie uses one op-amp for Bal->SE conversion (LM4562). The quad-opamp (dual 4562) is used as a low-pass filter. The caps for the filter are through-hole and shipped un-mounted, so the filtering is optional. You can feed the Ballsie a balanced signal, and take balanced out after the filter, without using the Bal-SE converter, or use both. You can also feed the Ballsie from a dual-mono balanced setup.

If you do not need the Ballsie for SE, why did we make it? Options. Balanced or SE Output Buffer, optional Output LP filter (500kHz).

As for the PCM2707, we called it "mid-range" because the WM8740 is a better DAC, so it's relative. We were both suprised at how good it is though. I use a USB board as my PC headphone amp now at work. Bus powered makes it simple (not ideal, but simple). Sounds fantastic compared to the sound card in my work machine, which is not hard to do.

It's all about options for us, and not dictating how the modules will be used.

I didn't know that you could get SE without the ballsie, but then I haven't studied the shematic that well yet. This certainly is good news since it removes complexity(read price).

Using only one output will decrease the signal by 3 dB, not 6 dB. The main thing however is that it will also decrease the dynamic headroom and signal to noise ratio by 3 dB.

3dB in power, 6dB in voltage - voltage really being the more appropriate.

As far as I know you can always use a balanced source single ended provided that you're willing to live with the lower output levels. As Brian said, that's really just a function of how much gain you need/have downstream. Me, using my balanced DAC single-ended hasn't presented an issue at all and I'd forgo using the ballasie unless you really need the maximum output level from the DAC.

That and the common mode rejection of noise.

Or an output buffer (SE or balanced) and/or want to LP filter.