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Designing a universal diff-in/diff-out Head Amp
Designing a universal diff-in/diff-out Head Amp
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Old 10th May 2018, 08:18 PM   #11
Hans Polak is online now Hans Polak  Netherlands
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To show that the above CMRR is not just some fancy result of a calculation, I include the measured CMRR from my diff-in/diff-out MC preamp based on the AD797. CMRR is limited by the AD797's own CMRR.


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Old 10th May 2018, 08:37 PM   #12
scott wurcer is offline scott wurcer  United States
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Hans have you considered phantom power? Let's say for LP transfer using a popular external device like the Focusrite 2i2 which has phantom power for mics and decent low noise preamps. My results have been very encouraging with software RIAA. There are a lot of folks that want a simple solution to transfer LP's, and I like the idea of no batteries no external supply. etc.

I've been a little hesitant to promote this heavily because I actually think there is a nice cottage product in there somewhere.
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Old 10th May 2018, 09:35 PM   #13
Hans Polak is online now Hans Polak  Netherlands
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Scott, I will have a look into a USB driven supply.
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Old 10th May 2018, 10:16 PM   #14
billshurv is offline billshurv  United Kingdom
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Designing a universal diff-in/diff-out Head Amp
I run all low level off silentswitcher, but as I'm considering a 2i2 would be interested in if that is workable. The thing that annoys me a bit about the 2i2 is tracking of the gain pots but I am sure it could be POOGEd.

Hans: Your last diagram shows what I would consider as impossibly high CMRR. 90dB is normally considered the best that solid state can do and transformers are up to 115dB at low frequency. It doesn't feel right, but I can't immediately work out what it wrong.
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Old 10th May 2018, 10:59 PM   #15
Hans Polak is online now Hans Polak  Netherlands
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Hi Bill,
I don’t know where this wisdom comes from, but such a general statement cannot be made. In case of my measured CMRR it was the CMRR of the AD797 that was limiting this figure. Look in the specs of this opamp a see how close it follows my measurements. Had this been higher, I would without a doubt have measured higher figures.
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Old 10th May 2018, 11:28 PM   #16
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by billshurv View Post
Hans: Your last diagram shows what I would consider as impossibly high CMRR. 90dB is normally considered the best that solid state can do and transformers are up to 115dB at low frequency. It doesn't feel right, but I can't immediately work out what it wrong.
Bill you are probably thinking CMRR of a whole system, cable runs, source imbalance, etc. The raw CMRR directly at the pins of a bi-polar op-amp can easily be 120dB or more.
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Old 10th May 2018, 11:50 PM   #17
billshurv is offline billshurv  United Kingdom
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Designing a universal diff-in/diff-out Head Amp
I was, but because Hans was also claiming 100dB CMRR with 1% resistors by dint of cascading CMRR rather than another technique. I remain unconvinced till I have my head around it, esp in a balanced configuration as my (mis?)understanding was based around CMRR being normally measured balanced to single ended, not balanced to balanced. I'll get there.
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Old 11th May 2018, 10:30 AM   #18
Hans Polak is online now Hans Polak  Netherlands
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Bill,
Let me try to explain a bit in more detail. That may help to get more insight into the matter of CMRR in this case.

Please go back to the 20dB gain MM preamp that I presented in #10. This Amp starts Diff and ends SE, that's how I measured, just according to your expectation, but how is this signal composed?

The first stage with U8/U9 has a diff gain of 100, but has a CM gain of 1. The CM signal is still there but with the same amplitude as offered at the input. This CM signal can and will be further attenuated by the second stage with U6.

There is however a second mechanism at work in the first stage. When offering a CM signal to U8/U9, the + and - inputs of both OpAmp’s, in this case the OPA1652, will only see the same signal in an ideal situation. However in reality the CMRR of this OpAmp goes from ca. 125dB at 100Hz to ca. 90dB at 20Khz, see image below, but the two OpAmp’s will always be a bit different in their CMRR. This means that between their outputs, a conversion takes place from CM into DM which will be added to the CM mode signal. This DM mode part can never be removed by later stages and will be simply processed as every other DM signal.

So to put things in practice, suppose we use a 100mV input signal and suppose one opa1652 has a CMRR of 100dB at 1Khz and for the other 110dB. Now in CM, will this result in two 100mV CM signals coming from U8/U9, but with a slight difference of +/- 1uV at output U8 and +/- 0.3uV at output U9, both having the same polarity, resulting in a 0.7uV DM signal.

So in effect we now have a 100mV CM signal with a 0.7uV DM signal on top. Now entering U6, the DM to CM convertor, having one resistor 1% off the correct value, CMRR will therefore be 40dB and DM gain is -20dB. So the CM signal from U8/U9 will be attenuated by 40dB from 100mV into 1mV SE. The 0.7uV DM signal will be attenuated by U6 by 20dB into 0.07uV SE, and will completely vanish into the other 1mV. CMRR in total is now 20*log((100mV*10)/1mV) = 60db, just as presented in #10.

However when further fine-tuning CMRR of U6 to a vanishing low value, there comes apoint where the 0.7uV starts to become a border that cannot be passed. In that case CMRR will be 20*log(100mV*10)/0.07uV)= 143dB@1Khz.

Would I have used 20dB gain for U8/U9 and 0dB gain for U6, CMRR with the 1% off resistor would have been 40dB instead of 60dB and the absolute limit would have been at 123dB@1Khz.

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Old 11th May 2018, 01:11 PM   #19
Hans Polak is online now Hans Polak  Netherlands
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In the above posting, I mentioned a 1% resistor, but this had to be a 10% resistor like in posting #10.


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Old 11th May 2018, 01:20 PM   #20
billshurv is offline billshurv  United Kingdom
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Designing a universal diff-in/diff-out Head Amp
Hi Hans,

I'd been processing this as a background task whilst dealing with the matrix this morning (GUI feh, real men use VT100) and the penny had dropped as to what I had been missing. I think the pain of matching everything as close as I could for the balanced Aurak build had warped me a bit* and I wasn't thinking straight and I realised that it's the diff to SE conversion in the first stage that causes the problem in pro audio setups.

Has also given me a pile of other stuff to question over CM impedance stuff that I clearly haven't got my head fully around yet*.

It's a shame but never could find the words to explain to AndrewT properly about why a diff input has a CM gain of 1 and it's too late now...

*This warping of course I blame you for as you stated 0.01% matching. I did however find interesting numbers in the distributions of components. Experience is never wasted.
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