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Old 12th February 2013, 07:10 AM   #11
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Thanks for the info on the I2S

My first post suggested converting the USB data to optical, regardless of the fact that USB specifies copper for the physical layer physical layers can be changed. Just like we convert ethernet to optical to get data off HV decks the rest of the network stack has no idea what physical layer is being used

If you want really good CMRR building an instrumentation amplifier out of high performance OP amps, is another option the problems with a single op amps CMRR is the impedance is not equal on both inputs, it might be possible to swamp the imbalance with resistors low impedance inputs are less noise prone anyway.
A low pass filter on the input is not a bad idea with noise being proportional to sqrt Hz it pays to only amplify the bandwidth the application requires.
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Old 12th February 2013, 07:30 AM   #12
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Quote:
Originally Posted by twest820 View Post
So I see a lot of elaborate isolation solutions without any measurements indicating the solutions are actually necessary
I've yet to find a measurement which reflects the SQ problems caused by it well enough. But I do hear it so its definitely an issue - ears are the ultimate arbiter. I did have an intriguing issue in relation to isolation recently because I was using a class 1 amp (with mains earth) and lifting the 0V to chassis link improved the soundstage significantly. I didn't bother trying to measure this difference. I played around though and managed to preserve the soundstage with a custom wound inductor which maintained the 0V to mains earth link only at LF. But then on connecting my (class 2) DAC to this, all was fine so long as the DAC wasn't fed (over USB-S/PDIF adapter) from a mains fed laptop. Laptop on battery was fine, but with the mains adapter plugged in the noise was relatively massive (I documented this on another thread - obviously measurable). Now I'm using an amp powered from a laptop SMPSU (class 2) there's no issue at all.
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Old 13th February 2013, 03:15 AM   #13
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Yup, no measurements. I rest my case, your honor. Somwhat more seriously, to the extent I've modeled such networks trafo leakage inductance often seems to be the dominant source of ground bounce. Leakage is rarely well characterized, though.

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Originally Posted by metalsculptor View Post
My first post suggested converting the USB data to optical, regardless of the fact that USB specifies copper for the physical layer physical layers can be changed.
Sure, but it's kind of a hassle as USB mixes differential and single ended signalling. Since USB offers power it's usually trivial to supply the USB chipset from the bus, moving one more noise to the digital side of the isolator and making things that much quieter for the DAC.

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Originally Posted by metalsculptor View Post
If you want really good CMRR building an instrumentation amplifier out of high performance OP amps is another option
Not as simple as it might seem at first glance; CMRR in an instrumentation amplifier is proportional to gain. In the unity gain case the amplifier's gain resistor becomes an open circuit, the input pair of op amps become buffers, and the CMRR ends up being controlled by the resistor matching of the third op amp's feedback network. This is the same as a single op amp with a differential feedback network, so the net result is more circuit for less performance. To the extent home audio systems need gain (and a good chunk of them don't) the optimum location for it tends to be in the DAC output buffers as this maximizes SNR and minimizes nonlinear distortion in the rest of the analog signal path. Nominally this means the rest of the signal path operates at unity gain.

That said, if one's building a power amplifier in a system with a gain structure such that a gain above unity is required for the power amp using a instrumentation amp type front end allows a bit more performance to be gotten out of the circuit if one doesn't wish to go so far as to build a composite amplifier. This happens, for example, when the power amp needs to swing beyond +-28V or so (which is pretty darn loud unless you're trying to use a resistor as a speaker). Hypex does this in their UcD and nCore amps, for example, presumably because maintaining stability of an op amp feedback network wrapped around a class D modulator tends to be a bit tricky.
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Old 13th February 2013, 03:25 AM   #14
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Quote:
Originally Posted by twest820 View Post
Not as simple as it might seem at first glance; CMRR in an instrumentation amplifier is proportional to gain. In the unity gain case the amplifier's gain resistor becomes an open circuit, the input pair of op amps become buffers, and the CMRR ends up being controlled by the resistor matching of the third op amp's feedback network.
Yeah I try to avoid using that architecture in a unity gain configuration, its way better suited to gains of 100 or up. CMRR also degrades if there's any source impedance mis-match too. Have you come across AD830 though? An entirely different beast, not based on the traditional opamp architecture, but something infinitely more interesting. I recently found NXP has some parts based on the same Barrie Gilbert architecture. The AD830 still manages 100dB or so of CMRR at unity gain (and impressively beyond audio freqs too) but the source impedance still matters and is likely dominant in the real-world performance.
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Old 13th February 2013, 03:46 AM   #15
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Linearity's an issue with the AD830 too, though it is kind of a cool part. I've never found an application to use it in, however.

Do you happen to have part numbers for NXP's Barrie Gilbert implementations? Not finding anything on a quick search.
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Old 13th February 2013, 04:02 AM   #16
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Yeah linearity is an issue on paper, yet in digital audio (subjectively) it works better than parts that on paper have better linearity. Just goes to show how measurements can be deceptive

I suspect the parts I referred to are obsolete, datasheets are available and they do show up on Taobao. TDA8575, TDA8578/9 come to mind. Their chipamps for car radio also appear to be based on the same architecture - they do sound extremely good.
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Old 13th February 2013, 04:10 AM   #17
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But I do hear it so its definitely an issue - ears are the ultimate arbiter.
Yes, well, it'll definitely be an issue if you're hoping to sell one to abraxalito.

The real issue could be, why are you designing and building this DAC?

If you intend to sell them, or hope that people will buy boards and build them, then you'll have to pander to all the audiophile phantasmagoria under the sun, or at the very least provide the objectivist buyers well-founded specs, probably supported by test results generated using professional quality test equipment. Speaking of which, have you seen the nwavguy's ODAC?

NwAvGuy

If you just want to listen to it yourself, you'll probably be satisfied to design it to meet a reasonable specification, but if you want other people to want it, then you might be better to design it to meet as unreasonably demanding a standard as you can possibly imagine, since there appears to be no level of technical achievement or measured performance guaranteed to meet with the approval of the golden eared.
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Old 13th February 2013, 04:19 AM   #18
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The real issue could be, why are you designing and building this DAC?
Wow, as they say 'speak of the devil'. I just mentioned you over on another thread. Simple answer to this question - 'for fun'.

Quote:
If you intend to sell them, or hope that people will buy boards and build them, then you'll have to pander to all the audiophile phantasmagoria under the sun, or at the very least provide the objectivist buyers well-founded specs, probably supported by test results generated using professional quality test equipment.
I've no aversion to measurements but no I won't be providing 'proper' measurements where NwAvGuy gets to define what 'proper' means. Measurements made with my portable Sony recorder though and shown on Audacity suit me. If anyone needs more 'professional quality' then I can do without them as 'customer'. I don't intend to sell them myself, I prefer the ARM business model.

Quote:
Speaking of which, have you seen the nwavguy's ODAC?
Yes, studied that and his O2 amp too. Which incidentally was the inspiration for the name of my DAC which is 'Ozone' (O3, geddit?). He makes unsubstantiated claims for its transparency though, and I took him to task for this before his 'disappearance'.

Quote:
If you just want to listen to it yourself, you'll probably be satisfied to design it to meet a reasonable specification, but if you want other people to want it, then you might be better to design it to meet as unreasonably demanding a standard as you can possibly imagine, since there appears to be no level of technical achievement or measured performance guaranteed to meet with the approval of the golden eared.
The golden eared buy on SQ though. They're my prime target customers.
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Old 14th February 2013, 04:49 AM   #19
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Originally Posted by abraxalito View Post
TDA8575, TDA8578/9 come to mind.
TDA8579 is still in production. It'd be nice if the datasheet showed more of the feedback implementation but, eh, back burner project to figure out the innards I guess.

Difference amplifiers like the AD8270/8271, LT1995, and INA134/137/154/2134/2137 are worth a look. Quite a bit more linear than the TDA8579, generally higher CMRR, reasonable noise levels, and thermal tracking that's quite a bit more expensive to achieve with discrete resistors (put a few volts RMS across a 1-2k 0805 or 1206 0.01% 5ppm and the accuracy is likely to halve). For cases where one wants CMRR beyond what 0.5 to 0.1% resistors can deliver I would probably default to the INA154 despite the INA family's low +PSRR requiring a good reagulator.
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Old 14th February 2013, 05:04 AM   #20
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Had a quick squint at AD8270 - don't buy the 145dB distortion claim, also I reckon it won't have linearity where it matters for audio - which is low level IMD in the presence of OOB signals. LT1995 though looks more interesting and is reasonably priced but I reckon LTC6552 holds more promise for audio and is slightly cheaper. I'm going to avoid INA154 as the only clue to it being linear is measured at 10VRMS output - hardly audio territory
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