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Old 4th February 2013, 08:14 PM   #1
JanErik is offline JanErik  Finland
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Default Powering a high-end USB DAC

I am planning to build a high-end USB DAC with PCM2707 and PCM1794. 5V is of course available and 3.3V is easily done. But it will also need +-12V for output opamps.

Is there any sanity in powering this over USB and including an SMPS for the opamps?

Another possibility is to make it internal in the PC, and get 12 V from there, and then just use a voltage inverter circuit for the negative part.

I would really not want to include a mains transformer for this.

Last edited by JanErik; 4th February 2013 at 08:17 PM.
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Old 6th February 2013, 07:07 PM   #2
JanErik is offline JanErik  Finland
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No opinions?
One more alternative would be to just invert 5 V for example with the 555 charge pump circuit.
Would that be enough headroom with an LME49740, which is not r2r?
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Old 6th February 2013, 08:03 PM   #3
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Output swing (and thereby output impedance) is specified in the 49740 datasheet. You can also look at the output curves in the 49710 datasheet. Is that enough? Well, it depends on your system's gain budget and interconnect SnR requirements. Keep in mind -132dBFS DnR is 1uV of noise on a 4V swing. Like most Burr-Brown/TI DACs the PCM1794A is not specified for PSRR and does not have an eval board so the workback from 1uV to the required supply filtering is not exactly straightforward. For 5V USB in to 5V to the DAC I would start by looking at a CLCLC filter and seeing how low the cutoffs can be pulled---Cirrus uses about 4kHz on their eval boards but just because Cirrus parts can tolerate this doesn't mean TI's are similarly class A in the upper audio bandwidth. You might try TDK's MLZ series as the "dropout" would be around 30mV max---difficult to compete against with a regulator as LDOs like the TPS73101 or TPS7A4901 don't perform well at the low dropout corner.

Holding a 1uV noise floor across an interconnect probably implies a differential receiver with 0.1% resistors to get 60dB CMRR so that up to 1mV of ground bounce across the interconnect can be tolerated without throwing away too much of the DAC's DnR. So you might also look at the LME49724.

For -5V I'd consider something like an MAX1673 entry level for this application. Given the limited headroom here post regulation with a negative LDO like the TPS7A3001 isn't particularly attractive. A more desirable (and simpler and lower cost) solution is to hold the switching frequency far enough above the audio band an LC supply filter can provide good ripple rejection. If you can guarantee a sufficiently high load impedance the IV/output buffer operates class A the requirement relaxes as the LC cutoff can be lowered into the audio band.

If you go +-12 or do boost conversion things are easier as there's more headroom to work with. But I'd give +-5V and LC pi filtering a close look first.

Last edited by twest820; 6th February 2013 at 08:05 PM.
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Old 9th February 2013, 07:32 AM   #4
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You can power it however you want, the OP amps and DAC have their requirements, one thing I would try to break is the tie to the PC ground. If USB is used then find a modular DC DC converter with low parasitic capacitance between input and output. The high conversion frequency they use can be filtered with a compact LC filter, I would include a common mode inductor right at the output and before the first filter capacitor. Keep the corner frequency below 10 kHz if using a linear post regulator most do not have good rejection of higher frequencies. This will allow the use of any voltage even +/- 15V as long as the USB can supply the power.
As the previous poster mentioned the LDO regulators like a bit of headroom, read the respective data sheets to see the optimum value for line and load regulation.

Another vote for differential output, OTOH why not convert the USB signal to an optical signal and put the DAC in the power amplifier case? AFAIR USB requires 2 signal wires and optical transciever pairs are common as are multimode fibre pairs to suit. Anyway some food for thought, I set up instrumentation in noisy environments and always try to digitize the signal as close to the source as possible then send the signal out over an isolated link such as ethernet or better still ethernet over fibre.
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Old 9th February 2013, 06:59 PM   #5
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metalsculptor, do you happen to have measurements indicating the point at which isolation becomes necessary? For example, the CS4398 USB DAC I'm listening to whilst typing this uses CLCLC supply filtering and nonisolated grounds and measures at -100dB THD+N and 108 dBA DNR. This is a 4uV noise floor and just about best case datasheet THD peformance on the MAX4477 RRO op amps used in the output buffers. There are comparable RROs but the 4477's are the best overall tradeoff I'm aware of on the market so, given the 5V single supply and unbalanced output implementation choices the designers made for this headphone DAC, it's hard to argue with the parts choices or design quality. A differential output implementation should default to 114 dB DNR due to the swing doubling and I don't think it'd be hard to reduce the output noise by 1+uV by removing the explicit ground reference and switching to more capable op amps for the output buffer. That would result in 120+ dB DNR, which is pretty darn decent.
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Old 10th February 2013, 08:41 AM   #6
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Quote:
Originally Posted by twest820 View Post
metalsculptor, do you happen to have measurements indicating the point at which isolation becomes necessary?
No that is the whole problem with ground noise, every application can be different, Your current set up may work fine but there is no guarantee that the next system modification will not change all that. While I am all for measurement, ground noise is so ad hoc and people here appear to spend lots of time either tracking it down or blaming it for something, why not just design it out and forget about it when the signal is digital there is little reason not to isolate, analogue isolation is more problematic. From my experience with industrial signals, isolated systems are so much less trouble than non isolated.
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Old 10th February 2013, 05:14 PM   #7
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Hmm, I'll differ somewhat. Ground behavior is deterministic---it obeys KCL and KVL, after all---but is a different kind of circuit from what most folks work with in that most of the impedances one cares about are parasitics and much of the loading is from things which are usually ignored. Low impedances mean low voltage levels and measuring things therefore requires care, which means acquiring the data to deterministically troubleshoot a problem is beyond the measurement tools most DIYers have and their circuit skills. For example on another thread I replied on earlier this week someone asserted they wouldn't have ground loops if power supply connections were made with twisted pair. Sure, if ya twist the wires hard enough they'll break and open circuit the ground, removing the loop, but at that point the supply doesn't work so well.

So I see a lot of elaborate isolation solutions without any measurements indicating the solutions are actually necessary---ESS gets 135 dBA DNR out of a non-isolated eval board, for example---with most of the problems seeming to be the result of use of unbalanced interconnects and power amps with excessive gain. A cheap balanced implementation with 1% resistors and a power amp set for unity gain (enough for about 85% of folks given a preamp on +-15V) has about 65dB more immunity to ground effects than default home audio implementations. It's not difficult to push that higher if there's need but since ground voltage swings in DC coupled consumer electronics are typically a few millivolts or hundreds of microvolts I've yet to hit a case where measurements have shown more rejection's needed. In comparison, isolating grounds tends to introduce bounce around half the mains supply if the free space capacitances are left to float---which is why plugging or unplugging throws sparks and why my experience is isolation tends to introduce more problems than it solves---and most folks won't reason through the circuit topology to understand which ground lift, interwinding, free space, etc. capacitances are charging from where and what the resulting bounces will be like.

Industrial applications can and do develop significantly larger ground offsets and I agree different solutions are called for in such cases. But the scenario for this thread is a tower/desktop/laptop with USB to a bus powered DAC which might or might not have a line out (the OP doesn't say but USB DACs are often used with headphones, so it's possible the only DC ground connection would be back up the USB cable). If we assume there is a line out to a pre or power amp that hardware's likely on the same circuit in the house wiring or maybe an adjacent one (and it's all single phase with relatively low currents). The ground topology in this configuration is the same as with a non-computer, DAC containing source like a CD player so the ground loops are also the same. A computer is a different load than a CD player so the currents in the grounds are different. However, they're not necessarily much different from having a non-audio computer plugged into the same mains circuit---if the audio grounds lie in the return path of the computer's draw they'll be in parallel with the wall wiring, will carry some of the current if they can, and will therefore bounce with the computer's load.

Isolating the computer-DAC link attempts to insert an all stop filter on that segment of the ground loops. Nominally, this requires the highpass corner from the capacitance across the isolation gap lie above the lowpass is the cabling RLCG with the amount of rejection provided increasing as the high and lowpass corners move farther apart. I've never seen an isolator intended for audio be characterized so I don't know how deep the resulting stop band is. Generally with these sorts of things it's not too hard to get 40dB but 60+dB usually takes pretty good filter design. But a 60+dB ground rejection improvement is pretty trivial to hit with balanced interconnects and some amp gain tuning.

Switching from an unbalanced to a balanced receive is a matter of adding two resistors to a layout. This costs about 25 cents in DIY quantities. Amp gain selection usually involves choosing different values of resistor and capacitors that would be in the feedback network anyway and is a zero cost change. In comparison, building an isolator for 25 cents would be a pretty neat trick and it still wouldn't offer the improved noise rejection of balanced or the SNR/DNR improvements of an appropriately chosen gain structure.

Don't get me wrong; I'm not saying not to use isolation. Just that it's probably more desirable to implement other ground management solutions first and then add isolation if performance requirements still aren't getting met.
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Old 10th February 2013, 08:18 PM   #8
sandyK is offline sandyK  Australia
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metalsculptor
I don't know about other USB devices , but USB memory sticks have the 0 volts (black wire) internally connected to the screen wire.This is a potential earth loop .
Improved results can be obtained by fitting a 47 ohms .5W resistor in series with this black wire when used with an external +5V supply.. This has given improved results with USB memory sticks, and a member of another forum also reported improved SQ with his DACiT. In the case of USB memory sticks, even better results when used for .wav file storage and playback can be had with a low noise external +5V PSU, and the red and black wires disconnected at the PC end of the USB-A plug.The removed wires are replaced by a resistor , e.g. 220 ohms 1/2W across the terminals of the plug.Just be careful that it is insulated from the metal case of the plug.
DIY Audio member Erin from Melbourne suggested this modification and it works very well.
Regards
Alex
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Old 11th February 2013, 02:51 AM   #9
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Quote:
Originally Posted by twest820 View Post
Isolating the computer-DAC link attempts to insert an all stop filter on that segment of the ground loops. Nominally, this requires the highpass corner from the capacitance across the isolation gap lie above the lowpass is the cabling RLCG with the amount of rejection provided increasing as the high and lowpass corners move farther apart. I've never seen an isolator intended for audio be characterized so I don't know how deep the resulting stop band is. Generally with these sorts of things it's not too hard to get 40dB but 60+dB usually takes pretty good filter design. But a 60+dB ground rejection improvement is pretty trivial to hit with balanced interconnects and some amp gain tuning.

Switching from an unbalanced to a balanced receive is a matter of adding two resistors to a layout. This costs about 25 cents in DIY quantities. Amp gain selection usually involves choosing different values of resistor and capacitors that would be in the feedback network anyway and is a zero cost change. In comparison, building an isolator for 25 cents would be a pretty neat trick and it still wouldn't offer the improved noise rejection of balanced or the SNR/DNR improvements of an appropriately chosen gain structure.
I agree that ground noise is manageable, I spend quite a bit of time doing just that, usually nasty sub uSec stuff with amplitudes well over a volt and earth paths an appreciable fraction of a wavelength at the noise frequency. I also agree that analogue isolation systems are expensive, complex and might not meet the performance required, OTOH the signal here is digital which is simple to isolate and imposes no performance deficits on the analogue side.
A few data points, one popular analogue isolation amplifier has 6 pf of capacitance across the isolation barrier, poor PCB layout could increase this considerably. This amplifier has a specified CMRR of 120dB, I forgot at what frequency.
Digital isolation does not have these issues, think optic fibre. To use an extreme example I could take a signal from a pulsed radar modulator deck (40Kv 0.5us rise time) and pass it out of its RF enclosure via a 4mm hole 160dB attenuation there then pass the fibre into another RF enclosure some distance away with a similar 160 dB attenuation.
Using differential (balanced)signalling helps, it sure would fix many of the hum and noise problems in a domestic environment and the silly thing is many power amplifiers have differential front ends which are converted to single ended. The chief disadvantage of differential signalling is that most differential amplifiers CMRR falls off rapidly with frequency.

@SandyK, that was the loop I was suggesting to break, putting 47 Ohm in that line would probably improve things provided the noise is not of sufficient amplitude to mess up the digital signal.
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Old 12th February 2013, 03:46 AM   #10
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Originally Posted by metalsculptor View Post
Digital isolation does not have these issues, think optic fibre.
Sure, but this thread is specific to USB DACs (not SPDIF over Toslink) and USB 1.1, 2.0, and 3.0 are all copper interconnect standards. Optoisolated USB solutions, typically intended for medical applications, have been available for 15 years but audio oriented USB solutions are generally DC coupled. (As a historical aside, I was evaluating moderate cost, DC coupled USB audio interfaces in 1998 that were comfortably above 100dB SNR.) So optoisolation's typically done on the I2S link between the USB chipset and the DAC. If you have a look over in DIYA's digital line forum you'll find implementations of this, some fairly elaborate.

I don’t do much that’s discrete but a decent audio op amp exceeds 60dB CMRR below a several hundred kHz (see, for example, the typical performance graphs in the LME49710 and OPA1652 datasheets). Depending on layout and how much one chooses to spend on matching resistors this likely means CMRR performance of a balanced receiver is feedback network limited up to somewhere around a MHz. In an audio application that’s plenty of dead band to hand off to a lowpass before the op amp becomes the limitation. Most good designs will lowpass, either for RF rejection or to maintain stability in a composite power amplifier where the op amp’s providing a control loop for a chipamp or some power transistors. I would imagine this level of CMRR is more difficult to achieve in the input diff pair of a discrete amplifier as matching becomes rather involved. There’s limited need to do so in DIY, though; a fully differential op amp (such as the LME49724 or OPA1632) can swing for 50W RMS meaning amp drivers like the LME498xx family aren’t usually needed.

I’d not previously surveyed CMRR so hadn’t quite realized the advantage an op amp in the front end offers. I’d assumed 1% resistors (40dB CMRR) in my previous post so the numbers still hold, but not with as much margin as I’d thought. An LM3886 drops below 40dB CMRR at 50kHz typ, for example, and an LME49811’s 60dB CMRR point is probably around 20kHz
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