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E1DA Cosmos ADC

So, the idea works!!! Below is the proof and the proof of input capacitor distortion.

1) I have bypassed the input capacitors in the Cosmos ADC right channel. The left channel was unchanged, with coupling capacitors connected. THD vs. frequency was measured. 10V Cosmos input range. 1.97V input voltage went from Topping D10s to OPA1622 eval board buffer with balanced input and single-ended output. See below

buffer.jpg

2) the output from the buffer was first connected to Cosmos L channel (with capacitors) and the result is the red track.
Then the measurement was repeated in the R channel (bypassed capacitors) and the result is the green track.

input _cap_bypass.png

One can see that the LF distortion has completely disappeared. I hope this test should persuade the capacitor distortion deniers. 3-wire unbalanced-to-balanced RCA-XLR cable was always used.


Edit: REW sweep has limited resolution, so here are another plots from STEPS
Cosmos_inpcap_bypass_small.png
 
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Full disclosure: I didn't spend the time to reverse engineer the circuit diagram for my Cosmos ADC. So, these comments are based on more general ideas. (Is LKA's schematic shown in post 1556 accurate??)

From PMA's measurements and observations, I wonder if the issue isn't so much the cap distortion, per se, but the common mode distortion characteristics of the Cosmos input. I ask this because Pavel has shown that a sort of balanced input signal from a power amplifier made by using an external voltage divider that is balanced gives better results. He's also measured the CMRR for the Cosmos ADC input to be ~60 dB, which is what I measured on mine. That sounds like a lot, but might not be. (Much higher than most audio devices, though!)

Lots of authors have discussed the issue of common mode distortion in opamp like circuits. Here's a couple examples:

Douglas Self

Walt Jung

ADI Crew

The general idea is that in order to minimize distortions in an opamp circuit, you need symmetry and balance. That way the symmetry and balance of the opamp itself allows for cancellation of common mode signals of any kind. This means that you need for the opamp inputs to "see" identical impedances at both inputs. Otherwise you get mode conversion in the circuit as described by Whitlock and Floru:

Whitlock and Floru

This is why you need really tight precision resistors in an instrumentation amplifier system.

Nash

OK, enough of the links, most of which you guys already know about.

This really becomes an issue when you have blocking capacitors at the input of a differential amplifier where you hope to maintain really high CMRR. At high frequencies where the caps are pretty close to being shorts, you're usually limited by the resistors in the opamp "bridge". But, as you go lower in frequency, the caps add a significant impedance in series with the bridge. If the caps are identical in capacitance, that's not a problem. But, how often do you get large caps like electrolytics where they match to like .01%?

I think this might be what is being demonstrated here.

One solution is to DC couple. Duh. No caps means no imbalance as a function of frequency.

Another is to match the caps. Hard.

A third is to make the caps really large in value so that their impedance in the band of interest is as close to zero as possible. Matthias Carstens modified his AutoRanger with larger caps to get better low frequency distortion. Note that these are film caps in the AR, which should have close to zero distortion to begin with.

Carstens' AutoRanger

Yet another solution is to increase the impedance of the input circuit so that the difference in impedance caused by cap mismatch is relatively smaller compared to the "bridge." Of course, this adds noise that you observe when there's no source device at the input shorting out the divider resistor impedance with regard to noise. Plus, you can bring on bias issues for the opamp and so on.

Or, you can use a different circuit at the input that is less sensitive to impedance differences between the non-inverting and inverting inputs to the ADC. That's what PMA's divider does to a degree.

So, I'm NOT suggesting that electrolytic capacitor distortion isn't a real thing. There, larger values means that the voltage drop across the caps is lower, which reduces the distortion as Cyril Bateman and others here (Capacitor Distortion) have noted. Basic capacitor distortion could well be the problem here. But, PMA's use of a balanced divider to get better measurements kind of belies that.

Now here's my editorial comment: This is another example of why evaluations of various circuits need to be done in the proper context. Using an Audio Precision test system can mask some deficiencies. Not because the AP is bad in any way - it's because it's great in almost every way. (Except price - what a strange correlation...) The AP outputs are really balanced and are designed to have really high common mode rejection. Same for the inputs. The AC mains connection is highly isolated for signal currents. That means the AP has minimum impact on the device under test. Minimal Heisenberg effects, except as you add them in yourself. But, what other audio gear is so perfect in that regard? So, it's quite likely that various pieces of gear will behave very differently in actual use than they will when tested in an ideal test system. This is especially so when you add in various digitally based products like computers and DACs, which have common mode noise currents galore that propagate between boxes with the AC main systems as the very imperfect current return. Basic stuff that is usually dismissed. OK, end of my editorializing.
 
You know, the capacitors are a basic issue (and remember that even in simulation there must be a return path, thus R6, R7 for simulation purposes with balanced input. Microcap knows and would refuse to start analysis without the ground return path).

View attachment 385656

Big C1 and C2 may keep charge and their behaviour is unpredictable (long sweeps, starting at 2 Hz, previous initial conditions).

The design should be direct coupled (with a necessary DC shift) to get good LF parameters, or use much higher input impedance and polypropylene capacitors.

I think the problem is not because there is no return path in a physics sense. Instead it's a basic Spice thing. You can easily follow the current path in the schematic when you look at it. Spice can't.

Floating Nodes

If you used a battery powered oscillator for V4 in an actual system, there'd be no return path to ground per se - it'd be a floating signal source. Spice doesn't like that. Microcap and LTspice don't simulate when you have those. You can also get funny results if you aren't careful with this and the simulation actually runs. I often add 1G faux resistors in several spots for a simulation, just to be sure.
 
I already wrote IVX on diyaudio about input capacitor distortion. He claimed that my capacitors are faulty and I should replace them with new 220u ones.
 
Same preamp as a source, used as single-ended drive and balanced drive of Cosmos ADC. With balanced drive, H2 drops of about 12dB.

View attachment 385469
Hi pma, interesting input indeed but I see no problem with H2 if use AUX inputs and 13.2Vrms(AP shows the THD+N at that voltage -96db, hence H3 is not ADC's problem as well). This voltage is limited by AP SYS2522 unbalanced output, max of balanced is 13.2*2 = 26.4Vrms. Should I try ADC's XLR instead of AUX?
 

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Not only a higher capacitor value can help, but also the capacitor type and voltage rating.

SE measurements, THDvF, 96k 24b, 1.7Vrms range

Red trace - stock cap 220u/10v
Green trace - Jamicon 470u/10v

D10s -3dBFS- Cosmos 96k THDvF Rch 220u Lch 470u.png


Orange trace - Hitano EXR 220u/16v
Blue trace - Nichicon 1000u/10v

D10s -3dBFS- Cosmos 96k THDvF Rch Hitano 220u Lch Nichicon 1000u.png


cosmos-adc-input-caps.jpg


cosmos-adc-input-cap-replace.jpg
 
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I still do not like the electrolytic capacitors in the signal path of the measuring instrument.

I have switched Cosmos to 2.7V input range and again compared distortion with bypassed/unbypassed coupling capacitor. Yes bypassed capacitor lets some current to flow to the buffer output, but it makes absolutely no problem.

The buffer arrangement with bypassed caps is this (I know that Cosmos does not use OPA1632, but in principle it makes no difference):

Cosmos+OPA1622.png

Please note the 3-wire signal cables.

And a comparison of distortion with bypassed/unbypassed capacitors:

Cosmos_inpcap_bypass_2.7V_rew.png


I tend to say that the coupling capacitors as used are a design flaw.
 
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Ok, I see that with XLR at 20Hz, it affects H2 only and SE.
The effect depends on Cosmos input range = different resistor value in series with coupling capacitor. It also depends on signal source output impedance. In other words, the resulting LF distortion is quite unpredictable and the results are not reliable and transferable if we measure various signal sources. To me, this is more important than the ability to measure -140dB distortion at 1kHz and a specific level. Consistency first.

The only cure is to avoid electrolytic coupling capacitors, unfortunately.
 
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So, in principle, if you or I always used either a Cosmos Scaler or APU as the front end of a Cosmos ADC system, the caps could be bypassed with no bad results? No bias issues?
 
So, in principle, if you or I always used either a Cosmos Scaler or APU as the front end of a Cosmos ADC system, the caps could be bypassed with no bad results? No bias issues?
9822 needs about 2.5Vdc bias, hence you can't bypass the caps(by shorting them). Just increase it a few times, if there is any interest in reading 20Hz H2 from SE output.
 
I already wrote IVX on diyaudio about input capacitor distortion. He claimed that my capacitors are faulty and I should replace them with new 220u ones.
you told me about the THD difference, pma about the particular H2 harmonic, that makes a difference. The alcap-related distortions almost don't affect the Total_Harmonic_Distortions because H3 dominates, and H3 is the same with/out alcaps but H2 is noticeably higher.
 
9822 needs about 2.5Vdc bias, hence you can't bypass the caps(by shorting them). Just increase it a few times, if there is any interest in reading 20Hz H2 from SE output.
I haven't tracked down your circuit but it appeared to me it is 1612 dual in an FDA hookup, thus their outputs would maintain bias even with shorted DC blockers. With zero signal there would a "pullup current" at the XLR inputs and the input common mode voltage of the opamp pins would change. Like when we had a 500R:500R gain setup this would represent 1k connected to 2.25V (4.5V/2) pulling 2.5mA with inputs grounded (or very low-Z source). Opamp CM voltage would halve to 1.125V.

Obviously, pulling the XLR inputs to, say 20V common mode (like from single supply bridge amp) would exceed 1612's input CM range at high sensitivities whereas an AC-coupled input would allow things to work under the same conditions, so we have to keep an eye on that and also note that the allowed input CM range is slightly asymmetric.

So, as long as we keep opamp pin common mode input within reasonable limits and are aware of the bias currents, I see no reason why this wouldn't work well. But maybe I'm missing something...
OTOH, AC-coupled is a bit more robust vs CM voltage so that's a strong point to keep AC-coupling (with better spec'd caps, though) unless there is really an advantage with DC coupling in a specific case.
 
I haven't tracked down your circuit but it appeared to me it is 1612 dual in an FDA hookup, thus their outputs would maintain bias even with shorted DC blockers. With zero signal there would a "pullup current" at the XLR inputs and the input common mode voltage of the opamp pins would change. Like when we had a 500R:500R gain setup this would represent 1k connected to 2.25V (4.5V/2) pulling 2.5mA with inputs grounded (or very low-Z source). Opamp CM voltage would halve to 1.125V.

Obviously, pulling the XLR inputs to, say 20V common mode (like from single supply bridge amp) would exceed 1612's input CM range at high sensitivities whereas an AC-coupled input would allow things to work under the same conditions, so we have to keep an eye on that and also note that the allowed input CM range is slightly asymmetric.

So, as long as we keep opamp pin common mode input within reasonable limits and are aware of the bias currents, I see no reason why this wouldn't work well. But maybe I'm missing something...
OTOH, AC-coupled is a bit more robust vs CM voltage so that's a strong point to keep AC-coupling (with better spec'd caps, though) unless there is really an advantage with DC coupling in a specific case.
I recollected that for the original Cosmos ADC I have considered and tried to move the alcap from the FDA input side to the 9822 input. I noticed, that 9822 floating inputs are already biased to Vref/2, and the Zin claimed as high as 3k. However, I found that impedance drops at HF a lot, and later for Cosmos ADCiso I even reduced the recommended 36ohm for 20ohm to get 3db better 10kHz H3 performance vs the original Cosmos ADC. Furthermore, that strange 300k to GND makes some a tiny DC bias needed for a noiseshapers loop stability, that res would be over 1M if the Zin of 9822 is 3000ohm at DC. I guess, >1M res will make DC bias more dependent on environmental humidity + alcaps leakage dependence on temperature. Sure, not a big deal to cut the DC with the internal DSP but I like the idea of keeping it Off.
 
you told me about the THD difference, pma about the particular H2 harmonic, that makes a difference. The alcap-related distortions almost don't affect the Total_Harmonic_Distortions because H3 dominates, and H3 is the same with/out alcaps but H2 is noticeably higher.

IVX, don't make excuses ;), distortion is distortion, whatever harmonics are affected. On diya I showed a graph with bypassed caps vs stock, the THD 20Hz difference was 20dB. Cosmos is built for measurement, all the little things count.
 
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I recollected that for the original Cosmos ADC I have considered and tried to move the alcap from the FDA input side to the 9822 input. I noticed, that 9822 floating inputs are already biased to Vref/2, and the Zin claimed as high as 3k. However, I found that impedance drops at HF a lot, and later for Cosmos ADCiso I even reduced the recommended 36ohm for 20ohm to get 3db better 10kHz H3 performance vs the original Cosmos ADC. Furthermore, that strange 300k to GND makes some a tiny DC bias needed for a noiseshapers loop stability, [...]
That last thing is interesting, I had already suspected that ESS introduce deliberate DC offset to "fix something", so basically a hack. The ADC cannot seem to handle a true zero signal properly and that's sort of weird, I mean the offset introduced is so small it could easily be swamped by the driver's offset.

Anyway, this ADC, like any other D/S with switched capacitor input, wants to see very low source impedance and therefore be DC-coupled to the driver.
As for that driver, I've always wondered why feedback isn't used around the glitch capacitor (1nF in the ESS datasheet circuit), that way very low output impedance could be held throughout the whole audio range:
1723444407027.png

This -- or something very similar -- is what I'm going to build, in an ESS DAC+ADC combo. The filter frequencies are lowered as flat bandwidth doesn't need to be higher than 30kHz or so.
The application calls for DC coupled input and I hope it's not causing any troubles...
 
@LKA That’s it. Up to 20dB at 20Hz, depending on input range selected and level. Confirmed by 2 independent measurements on 2 different pieces at 2 places. There is no excuse for such behaviour. Electrolytic capacitors are unpredictable and there is no place for them in the measuring instrument signal path.
 
IVX, don't make excuses ;), distortion is distortion, whatever harmonics are affected. On diya I showed a graph with bypassed caps vs stock, the THD 20Hz difference was 20dB. Cosmos is built for measurement, all the little things count.
I honestly measured offered tests on diyaudio.com and didn't find the THD issue. Pma mentioned a particular H2 issue, I tested again and found the problem.
 
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