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USB-audio headphone-amp component selection

gamma3, gamma24 (plug-in for the γ3)

USB-audio headphone-amp component selection

Postby Shorty » February 10th, 2017, 3:16 pm

Hi to everyone!

I'm usually listening to music while working at my PC and this year I thought that I would gift myself with a high-end solution.
This setup does not need to contain any other signal sources besides USB-Audio since I only use headphones in conjunction with my PC/MAC.
I was reading this forum for quite some time the last few days, so that I came up with something along the line: USB -> ζ1 -> γ24 -> β22 -> Sennheiser HD800S (of course fully differential).

Now to my basic questions:
1.) As I understand, I will need a LCDuino module for the γ24 and I should include two ε12 protection boards, too. Is that correct?
2.) I will need two σ11: one for the γ24 and one for ζ1 and LCDuino, correct?
3.) For best performance I should use two independent σ22 set at 30V for the β22, correct?
4.) Since I am using digital volume-attenuation, could I replace the potentiometer for the β22 boards with a fixed resistor?

And I got another more complex one:
5.) Since both α24 and β22 are (or better: can be configured to be) fully-differential line-amps, I had the idea that a substitution could be possible - allowing for a "shorther" audio-path without unnecessary op-amps.
Would it be it possible to substitute the two α24 modules directly with the 2x2 β22 boards? Or what changes would be required?

Some questions might sound silly, but I just want to be sure.

Best regards
Thorsten

PS: I'm not sure that this question belongs into this subforum since I'm talking about a cross-over project between the Y3 and the B22 headphone amplifier - if it's better suited to the B22 forum, please move this thread there...
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Re: USB-audio headphone-amp component selection

Postby amb » February 10th, 2017, 11:39 pm

Sounds like you're building a "not quite γ3" and a balanced β22.

Answers:

1. Yes, the SRC4392 and WM8741s on the γ24 run in software controlled mode (via I2C). The LCDuino-1 provides such control, with the γ3 firmware. Note that the firmware also assumes the presence of the γ3 backplane board and its components, some of which also require software control. If you're not using the backplane, then you will need to make some modifications to the firmware.

2. You could use only one σ11 for everything, but it won't give you the benefits of multiple supplies, and you would have to forego the remote control power on/off capability, because that requires one σ11 to be constantly-on (for the LCDuino-1) whike the other one is turned on and off by remote via a solid state relay. On the real γ3, separate σ11s are required for the galvanic isolation to work.

3. Yes, it also drops the current draw on each σ22 by half, reducing heat dissipation on each of its MOSFETs.

4. That resistor already exists as R1 on the β22 boards. No additional resistors are required.

5. Here things get a little more complicated. The analog output lines (L+, L-, R+, R-) from the γ24 each has about 2.5V DC offset referenced to ground. In the γ3, the α24 drops that DC offset to near zero when the OPA1632 differential opamp's Vocm pin is connected to ground (it sets the common mode reference level). The balanced β22 has no such feature, so you will still have a high DC offset at the outputs (in fact, multiplied by the gain of the amp). If you insist on going this route, then you need DC blocking capacitors at the inputs of each β22 board. I suggest not skipping the α24. It is there for a reason in the γ3 design.

Another comment: If you've come this far, why not just build a real γ3? The galvanic isolation feature is good to have. It eliminates the possibility of ground loop through the USB cable. You'll also avoid having to change the firmware.
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Re: USB-audio headphone-amp component selection

Postby Shorty » February 11th, 2017, 4:50 pm

Thanks for your comments and yes, you could probably call it that way :)

@1: Since I have a single input and don't need IR-remote control or a display, I wanted to modify the LCDuino firmware anyway, so that wouldn't pose a problem.
@2: galvanic isolation was on my list, since I really like that feature.
@5: That leaves me in a bad spot for my initial idea, but got me thinking and stirred up some more questions.
Since I come from software design, adding large and cumbersome libraries where only one small function is needed sometimes leads to a lot of unwanted side-effects, so I generally always think along the lines of removing everything from a design that's not absolutely needed - that's why I thought of removing the α24.
A build of the "real" γ3 is not that much of an option for me since I lack other high-quality sources that I could connect, I certainly have no other high-quality DACs I'd like to output to and I don't even have high-quality speakers so that the connection to another amp would make sense - most of the features would simply be unused and "waste" a lot of space. On the other hand the use of the pre-designed board would free me from designing my own and prevent any errors in the process.

That brings me to my new questions:
1.) As I understand the documentation, one of the reasons to use two different power supplies for the β22 and to have an independent analog voltage regulator for each of the Wolfson DACs on the γ24 is to prevent any channel-crosstalk. Staying true to that theme, shouldn't I then use two separate +-10V σ22 for the each of the α24?
2.) If so that would increase build cost quite considerably - and taking your info into consideration that the α24 only lowers the DC to "near zero" (which I interpret as: it doesn't completely remove it), is a coupling capacitor really that bad if I spend the money on a good one?
Judging from your phrase "if you insist on going that route" it seems to be, but for me as a complete newby to high-end audio (I only use an ASUS XONAR STX-II with Senheiser HD600 at the moment), I'm in a toss-up. Some posts I read on the net claim that op-amps are pure evil - praising either tubes or discrete designs as well as silver/oil capacitors, while others insist on "no caps in the audio path". Tubes are a no-go for me (something that purposely degrades the signal to sound "warmer" is not the right way for me), but I hope someone can enlighten me on the issue of α24 vs capacitor in my case.
3.) Does anybody have a good link on balanced audio? I always thought that it is somehow similar to LVDS (low-voltage-differential-signalling) where one of the main goals is to remove (common mode) noise. So what is the problem with common mode DC on the signal?

PS:
I'm not too familiar with analog electronics - I hated those topics during university and unfortunately forget most of the things I had in my short-term memory for the exams. I just expect my ICs to have nicely designed inputs and do all my work in the micro-controller - more complex designs are for our analog guru. Therefore it's not my intention to step on someone's toes - I don't belong to any particular camp, but just want to learn. I know that audiophile discussions can get out of hand quickly, so please bare with me and if you have some good links for me to read, it is really appreciated. As an example I really liked the following read on 24bit/192kHz: https://people.xiph.org/~xiphmont/demo/neil-young.html
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USB-audio headphone-amp component selection

Postby amb » February 12th, 2017, 12:08 pm

Shorty wrote:@1: Since I have a single input and don't need IR-remote control or a display, I wanted to modify the LCDuino firmware anyway, so that wouldn't pose a problem.

The default "first time" power-up input selection is USB with the current firmware. If you don't have have a remote control then it will stay there. Note that without a remote control or display, you'll be missing many features and capabilities (see the LCDuino-1 website "User guide" section for details), as most of them are software controlled via the remote.

A build of the "real" γ3 is not that much of an option for me since I lack other high-quality sources that I could connect, I certainly have no other high-quality DACs I'd like to output to and I don't even have high-quality speakers so that the connection to another amp would make sense - most of the features would simply be unused and "waste" a lot of space. On the other hand the use of the pre-designed board would free me from designing my own and prevent any errors in the process.

If that's the case why even bother with balanced interconnection? You did say you want to build a high end system in your original post. If the "reference" γ3 has extra inputs and outputs that you won't need, then you don't have to populate them.

1.) As I understand the documentation, one of the reasons to use two different power supplies for the β22 and to have an independent analog voltage regulator for each of the Wolfson DACs on the γ24 is to prevent any channel-crosstalk. Staying true to that theme, shouldn't I then use two separate +-10V σ22 for the each of the α24?

No, because α24 draw very little current compared to the β22. Interchannel crosstalk via the power supply is a non-issue on the α24. Of course if you want to go crazy and use two power supplies for the α24s anyway, then there is nothing to stop you (except the γ3 backplane isn't designed for two analog supplies, should you choose to use it).

2.) If so that would increase build cost quite considerably - and taking your info into consideration that the α24 only lowers the DC to "near zero" (which I interpret as: it doesn't completely remove it), is a coupling capacitor really that bad if I spend the money on a good one?
Judging from your phrase "if you insist on going that route" it seems to be, but for me as a complete newby to high-end audio (I only use an ASUS XONAR STX-II with Senheiser HD600 at the moment), I'm in a toss-up. Some posts I read on the net claim that op-amps are pure evil - praising either tubes or discrete designs as well as silver/oil capacitors, while others insist on "no caps in the audio path". Tubes are a no-go for me (something that purposely degrades the signal to sound "warmer" is not the right way for me), but I hope someone can enlighten me on the issue of α24 vs capacitor in my case.

If every OPA1632D opamp is "perfect" then the DC offset would be zero. But it's the real world and there are parts tolerances, so the typical DC offset will be a few millivolts.

Coupling capacitors are best avoided, they are directly in the audio path and introduce nonlinearities, no matter how good the active circuit is, the effect of the capacitor will be there. How bad it is depends on the capacitor used, non-audio grade electrolytic capacitors are especially bad, and polypropylene film capacitors are good, but the latter is much larger in physical size per unit of capacitance than the former.

IC opamps used to be evil a long time ago (in the days of the 741 or 301). Things are different today. There are many excellent opamps that rival or surpass discrete circuits (in both measured specifications and sound quality), and provide superior thermal behavior due to the small size. Those who believe that opamps are still evil either still live in the past or have not done any honest comparisons. All the opamps used on the α24 are excellent modern opamps with superb sonics.

One thing I neglected to mention in my previous post: If the α24 is not there then you also lose the 100KHz 3rd order low pass filter which removes ultrasonic junk from the DAC.

3.) Does anybody have a good link on balanced audio? I always thought that it is somehow similar to LVDS (low-voltage-differential-signalling) where one of the main goals is to remove (common mode) noise. So what is the problem with common mode DC on the signal?

Yes it is like LVDS when it comes to the concept of common mode rejection, except we're talking about analog audio signals rather than digital pulses. A common mode DC isn't a problem per se when "seen" across the hot and cold sides, but the output voltage swing will be severely limited if there is a significant common mode DC offset. If your "zero reference" level is several volts higher than actual zero, the top of the signal swing may clip (depending on the analog circuit's power supply voltage). Also, if you connect either output to an unbalanced device which references actual ground, then the DC offset becomes real and could cause speaker/headphone damage.

PS:
...
I really liked the following read on 24bit/192kHz: https://people.xiph.org/~xiphmont/demo/neil-young.html

This is also the same Neal Young who came up with the Pono Player which supports high resolution, high sample rate up to 24/192. .. :)
But anyway, I do agree with most of what he said. In practical high end audio, 96KHz sample rate is quite sufficient and anything higher just makes for more data to store and to move around.
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