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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 11th May 2018, 01:50 PM   #21
Hans Polak is online now Hans Polak  Netherlands
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Hi Bill,

The 0.01% for the Aurak was absolutely correct, because it uses a different topology, being current driven and not voltage driven, not at all having a CM gain of 0db like U8/U9 in posting#10.
A high CMRR can only be achieved in that case by carefully matching resistors.

Hans
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Old 11th May 2018, 01:58 PM   #22
billshurv is offline billshurv  United Kingdom
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Designing a universal diff-in/diff-out Head Amp
Yeah, that was the bit that didn't click until this morning.
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Old 12th May 2018, 04:04 PM   #23
Hans Polak is online now Hans Polak  Netherlands
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Originally Posted by scott wurcer View Post
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.
Scott,
I have looked at the 2i2, but I can’t find the input impedances of both inputs, they are probably different, and I can’t find the amount of current that can be supplied.
With 6k81 resistors it will be only a few mA.
Do you have any additional info?
Hans
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Old 12th May 2018, 05:07 PM   #24
scott wurcer is offline scott wurcer  United States
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Standard phantom power setup 6K81 on each input for 0V left at 14mA, I have some nice diagrams in my Linear Audio article. In practice mics rarely draw more than 3 or 4mA but with cap coupling there are tricks to reuse the current. You need to take care to keep the draw balanced on each input.

The input impedance is in the service manual, typically it's fairly low from XLR and high on TRS in line-in mode. You need to be careful to note how the combo XLR/TRS is used. I've seen devices that don't let you use a stereo TRS as a low level differential mic input and apply phantom power.
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Old 12th May 2018, 06:09 PM   #25
scott wurcer is offline scott wurcer  United States
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Hans here's a couple of ideas for a simple buffer for MM/MI into a phantom powered mic input. Differential and single ended. You may want to increase that 47u cap and of course you can use zeners or IC shunt regulators.
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Last edited by scott wurcer; 12th May 2018 at 06:14 PM.
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Old 12th May 2018, 09:01 PM   #26
Hans Polak is online now Hans Polak  Netherlands
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Originally Posted by scott wurcer View Post
Hans here's a couple of ideas for a simple buffer for MM/MI into a phantom powered mic input. Differential and single ended. You may want to increase that 47u cap and of course you can use zeners or IC shunt regulators.
I've put the Balanced version in LTspice. Noise from 20Hz to 20Khz is 455nV after Riaa and A-Weighting, corresponding to 3.2nV/rtHz.
So when you put the output in a Mic input of some 2i2 device with a high enough input impedance, you have your gear complete.


Hans
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Old 12th May 2018, 10:13 PM   #27
scott wurcer is offline scott wurcer  United States
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That looks good, perfect (for MM/MI) in front of many of the field recorders too. I have one more version that might be of interest but it will take a couple of days to draw up. As drawn I think you could give each FET more current if you need to drive a little more load, but I think the levels are so low it does not matter.

The single ended version above is less interesting and you don't need the second FET. It was originally meant to be a jumper programmed SE/Diff circuit.

I have also built versions using the floating battery idea (9V) for completeness. It would be nice to put all the versions together in a library so everyone can choose what works best for them.
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Last edited by scott wurcer; 12th May 2018 at 10:16 PM.
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Old 13th May 2018, 01:42 AM   #28
billshurv is offline billshurv  United Kingdom
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Designing a universal diff-in/diff-out Head Amp
Driven 300 miles this evening so brain mangled. Did start looking at 2i2 manual earlier but reading on phone was a pain. But this I must build.
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Old 13th May 2018, 01:55 AM   #29
scott wurcer is offline scott wurcer  United States
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Driven 300 miles this evening
And you stayed in the UK! This is an afternoon project with the junk bin, if you have any problems I'm here.
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Last edited by scott wurcer; 13th May 2018 at 01:57 AM.
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Old 13th May 2018, 02:02 AM   #30
billshurv is offline billshurv  United Kingdom
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Designing a universal diff-in/diff-out Head Amp
I am also conflicted as just realised that the Tascam field recorder is the same price as the 2i2.

Was taking mother home to wilds of norfolk. So only 150 each way.
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