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Discrete Unity Gain Differential Buffer?
Discrete Unity Gain Differential Buffer?
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Old 12th April 2019, 11:25 PM   #11
mchambin is offline mchambin  France
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Quote:
inputs need to be high impedance
Why ?
How much ?
I have never seen a 0dB amp followed by a 60dB amp.
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Old 18th April 2019, 12:34 PM   #12
exclamationmark is offline exclamationmark
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I'm building a digital snake box/recording interface with digitally controlled potentiometers for preamp gain control. I'm using combo XLR/TRS jacks. The TRS jacks are intended for line + instrument input (8k+ output impedance) and anything up to +22dBu. The XLR inputs have an input impedance of roughly 3k from the phantom power supply.

Basically, using digital potentiometers provides a very unique design challenge (noise, distortion and voltage ratings). The easiest way to insert them into an op-amp stage with minimal effects, is to use them in a non-inverting configuration as the feedback resistor. I'm OK with a small increase in noise - my gain stage is close to 1nv/hz in noise (in theory...), which is much likely smaller than the balanced to single ended stage. The challenge is very much finding a low-noise balanced to singled ended stage to make this topology work.

As another poster pointed out, maybe I'm solving the wrong problem or trying to solve too many at one time. I'm feeding anything from a few mV to something as large as +22dbU into the same input. I was planning to just amplify everything up to the rails and then have a static gain dump to interface to the ADC, as the noise impact of doing this seems quite minimal. I'm not even sure 60dB gain will be enough to hit the rails with a typical mic input.

A lot of interfaces I've seen have a separate mic/line/instrument switch to pad the input, so maybe I will start to look in that direction. This is only one piece of the puzzle!
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Old 22nd April 2019, 02:00 AM   #13
Mark Tillotson is offline Mark Tillotson
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Paralleling 5532's will drop the noise, its easy to parallel unity followers via 10 ohm
resistors. Two 5532's give you 4 opamps in parallel, nearly 6dB less noise which will

put you on 2nV/rtHz or so. Double this up for balanced input and drive a low impedance
differential amp gain stage.


So yes, 0dB followed by gain is exactly what you need for differential to single ended
conversion - without the high impedance inputs you can lose lots of CMRR as a differential
stage has different impedances on each input.
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Old 22nd April 2019, 08:40 PM   #14
MarcelvdG is offline MarcelvdG  Netherlands
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Quote:
Originally Posted by exclamationmark View Post
As another poster pointed out, maybe I'm solving the wrong problem or trying to solve too many at one time. I'm feeding anything from a few mV to something as large as +22dbU into the same input. I was planning to just amplify everything up to the rails and then have a static gain dump to interface to the ADC, as the noise impact of doing this seems quite minimal. I'm not even sure 60dB gain will be enough to hit the rails with a typical mic input.
Between 1997 and 2000 I built a recording device for a local radio station (Haarlem 105). The gain of its microphone preamplifier could be switched between 23 dB and 63 dB in 5 dB steps, the nominal level at the microphone amplifier's output was -10 dBV. That is, a -10 dBV RMS (so -7 dBV peak) sine wave would just turn on the first red LED of the level indicator, at -5 dBV (-2 dBV peak) a limiter kicked in and at 0 dBV (+3 dBV peak) the ADC was just driven into clipping.

In practice the reporters typically used it with a Sennheiser MD21 dynamic microphone, spoke into the microphone from about 30 cm (1 foot) distance, always used the 43 dB gain setting and usually hit the nominal level in the peaks. That is, someone speaking into a Sennheiser MD21 from 30 cm distance typically results in a peak voltage of -7 dBV - 43 dB = -50 dBV, or just above 3 mV.

Small-diaphragm condenser microphones are typically 10 dB more sensitive than a Sennheiser MD21, large-diaphragm condenser microphones are typically 20 dB more sensitive than a Sennheiser MD21. Besides, pop singers usually sing from much smaller distances than 30 cm, and maybe you also want to use the microphones for loud instruments like trumpets or drums. All in all, the microphone level can be just about anything.
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Old 22nd April 2019, 08:45 PM   #15
MarcelvdG is offline MarcelvdG  Netherlands
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By the way, do you intend to use the thing you are building for far miking with dynamic microphones? If so, it would make sense to at least try and keep the noise level at maximum gain below the thermal noise of the microphone. That's about 1.8 nV/sqrt(Hz) for a 200 ohm mike. With discrete bipolar transistors, that means using a relatively high bias current and devices with lowish base spreading resistance, with JFETs using high transconductance JFETs with low 1/f and G-R noise.

Last edited by MarcelvdG; 22nd April 2019 at 08:48 PM.
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Old 23rd April 2019, 05:16 AM   #16
exclamationmark is offline exclamationmark
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I've kind of given up on the unity gain differential buffer idea, and am essentially going to steal the THAT 1580/1583 architecture, but in discrete form. Their way of dealing with the digipot problem is to vary the feedback resistors with 2x digipots, instead of the Rg (gain) resistor. I thought this might reduce CMRR due to poor digipot matching... However, a useful property (that I completely forgout about) of the typical 3 op-amp instrumentation amplifier is that identical input signals are always reduced to unity gain from the gain stage, no matter the feedback resistor ratio. This means that the following difference amplifier is able to reject it at its full CMRR.

The THAT 1580/1583 architecture is also kind of unique, since it doesn't use a differential input stage. It instead uses a single input transistor with an inverting gain stage and current feedback. This necessitates the use of a blocking cap + DC servo, but I think the trade-offs for manufacturability + noise are worth it. You no longer need a matched input pair, plus the noise for a single transistor is 3dB lower than a differential pair (plus I already prefer a blocking cap to remove external DC offsets anyway).

I think I'm going to use a JFET as the input stage, since with instrument inputs the noise current from BJTs will make the noise figure pretty horrible. I do wonder if the open loop gain will be reduced/distortion will be increased in place of a BJT though. I'm thinking of using something like a 2SK880 or equivalent JFET - the noise should be under 2 nV/sqrt(Hz). I also want to test a bunch of other JFETs as well that don't have published noise figures. Ideally, I want to plug line/instrument/microphone/whatever into this thing and just have it work as a universal front end with a low noise figure.

In any case, it's annoying that one of the better (and commercial) solutions to this problem is "just use more digipots".

Last edited by exclamationmark; 23rd April 2019 at 05:26 AM.
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Old 23rd April 2019, 08:44 PM   #17
MarcelvdG is offline MarcelvdG  Netherlands
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Do you need the input to survive a 48 V phantom supply for condenser microphones? That can also be a reason to use blocking capacitors and clamping devices of some sort.
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Old 24th April 2019, 03:21 AM   #18
PRR is offline PRR  United States
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Quote:
Originally Posted by exclamationmark View Post
...The THAT 1580/1583 architecture is also kind of unique, since it doesn't use a differential input stage. It instead uses a single input transistor with an inverting gain stage and current feedback....
That's not how I see it. Input pair with differential NFB to both emitters. This was normally followed by a 4-R diff-amp; I see THAT proposes another path using their 5171 part.

You know TI has a whole mike/line preamp with digital control gain on a single chip? Made to drop into large consoles wholesale, reduce the ugly old analog to a minimum, while allowing a maximum of controls on the touch-screen.
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Old 28th April 2019, 12:34 AM   #19
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by mchambin View Post
Why ?
How much ?
I have never seen a 0dB amp followed by a 60dB amp.
You have to analyze the differential amplifier circuit for input impedance - its not symmetrical. Driving it with low impedance signals isolates the inputs from these effects.


For instance a differential amp with 10k resistors everywhere the common mode impedances are both 20k, but the differential impedances are 20k and 6.7k for non-inv and inv inputs respectively. And driven from a transformer (floating) sees 10k on each leg. Input buffering means the input impedances are uniformly high and the differential amp's asymmetry doesn't interact with the source.


If your differential amp is going to have variable input impedance, its even more of a good idea to pre-buffer the inputs.
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