X32 Preamp Design

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cmichnovicz

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Aug 5, 2018
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3
Hi all,

I am trying to diagnose some issues with a few channels on an X32 producer and have been really studying the schematic for the X32 preamps as a result. I think I understand what's happening from IC17-B onwards to the right, i.e. a balanced to unbalanced differential amplifier, then a voltage divider (with a switch to lower gain further), and an additional gain stage (with a switch to increase gain via a change in feedback). I am confused about the middle block (the two transistors and IC45A&B). How are these working and how is the gain setting being accomplished? I have some funny feeling that this is a beefed up long-tail pair where the op-amps are supplying current or something, but I can't really wrap my head around it. It seems like there is some fixed gain in this section that is then compensated for by the feedback network of DG411 switches, and by removing sections of the feedback network, the gain can be increased? I just don't really see what the feedback network is actually working on and where that gain is coming from.

Thank you!
 

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Hi all,

I am trying to diagnose some issues with a few channels on an X32 producer and have been really studying the schematic for the X32 preamps as a result. I think I understand what's happening from IC17-B onwards to the right, i.e. a balanced to unbalanced differential amplifier, then a voltage divider (with a switch to lower gain further), and an additional gain stage (with a switch to increase gain via a change in feedback). I am confused about the middle block (the two transistors and IC45A&B). How are these working and how is the gain setting being accomplished? I have some funny feeling that this is a beefed up long-tail pair where the op-amps are supplying current or something, but I can't really wrap my head around it. It seems like there is some fixed gain in this section that is then compensated for by the feedback network of DG411 switches, and by removing sections of the feedback network, the gain can be increased? I just don't really see what the feedback network is actually working on and where that gain is coming from.

Thank you!

IC45 A/B, IC17-B, together with T25, T17 form a three op-amp type Instrumentation Amplifier.
Gain of such a circuit is set by a resistance value between the GAIN1 and GAIN2 points shown.
 
I am confused about the middle block

Kind of a pet peeve of mine, that schematic is what I think of as the space saving layout, not the brain effort saving layout.
When I was first starting schematic work it was expected that the devices be oriented with more positive power supply connections to the top, more negative power supply connections toward the bottom, that that devices be spaced in a way to make logical signal flow easy to follow.

That topology was popularized by Graham Cohen via a 1984 Audio Engineering Society pre-print, so is often referred to as Cohen design, or Cohen double-balanced design.
This picture from Cohen's "history of microphone preamps" paper is (I think) easier to read and see what is going on:
1712857914152.png

The two transistors are in something very similar to a common-emitter configuration, providing voltage gain with the signal taken at the collectors.
Rather than having the emitters grounded, the op-amps provide current feedback through resistors to the emitters of the transistors, which both sets the DC bias current and provides wide bandwidth feedback.
The resistor between the emitters (which is located off-board in your schematic connected through the ch 1 gain1 and chan1 gain2 pins) reduces the amplitude of the feedback signal, which increases the closed-loop gain of the front end (so lower value resistor is higher gain). In most designs the resistor shown as "R6 GAIN SET" in the picture I copied in is actually a resistor along with a capacitor to reduce closed loop gain to 1 at DC so you do not have to worry about amplifying DC offset. That is capacitor C190 in the Midas schematic you posted.

Cohen passed away about 9 years ago, but a friend or acquaintance of his has kept some of his papers online here:
https://www.leonaudio.com.au/cohen.htm

This file "Double balanced amplifier notes" has a lot of good commentary on the design:
https://www.leonaudio.com.au/double.balanced.mic.amp.notes.pdf
 
Kind of a pet peeve of mine, that schematic is what I think of as the space saving layout, not the brain effort saving layout.
When I was first starting schematic work it was expected that the devices be oriented with more positive power supply connections to the top, more negative power supply connections toward the bottom, that that devices be spaced in a way to make logical signal flow easy to follow.

That topology was popularized by Graham Cohen via a 1984 Audio Engineering Society pre-print, so is often referred to as Cohen design, or Cohen double-balanced design.
This picture from Cohen's "history of microphone preamps" paper is (I think) easier to read and see what is going on:
View attachment 126721

The two transistors are in something very similar to a common-emitter configuration, providing voltage gain with the signal taken at the collectors.
Rather than having the emitters grounded, the op-amps provide current feedback through resistors to the emitters of the transistors, which both sets the DC bias current and provides wide bandwidth feedback.
The resistor between the emitters (which is located off-board in your schematic connected through the ch 1 gain1 and chan1 gain2 pins) reduces the amplitude of the feedback signal, which increases the closed-loop gain of the front end (so lower value resistor is higher gain). In most designs the resistor shown as "R6 GAIN SET" in the picture I copied in is actually a resistor along with a capacitor to reduce closed loop gain to 1 at DC so you do not have to worry about amplifying DC offset. That is capacitor C190 in the Midas schematic you posted.

Cohen passed away about 9 years ago, but a friend or acquaintance of his has kept some of his papers online here:
https://www.leonaudio.com.au/cohen.htm

This file "Double balanced amplifier notes" has a lot of good commentary on the design:
https://www.leonaudio.com.au/double.balanced.mic.amp.notes.pdf
Do you know where I could find the book mentioned on that website (Microphone Engineering Handbook by M. Gayford)? I am very curious about it but don't see it online anywhere. Only one chapter is on that website. Thanks
 

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