A Differential Deboo Integrator to Servo Mic and Low-Level Preamps

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mediatechnology
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Re: A Differential Deboo Integrator to Servo Mic and Low-Level Preamps

Post by mediatechnology »

I recently had reason to look for Douglas Self's "1C" servo and couldn't find the screen capture I had of it.

I seemed to recall that it was in "Audio Power Amplifier Design Handbook" but I couldn't find it in my 3rd edition copy.
It turns out that I did find the screen capture and also learned that it was introduced in the 4th edition.

Self's "1C" servo is a Deboo and is one of the few audio citations for it other than myself and cocolog-nifty's Differential Deboo.

cocolognifty's Differential Deboo: http://designideas.cocolog-nifty.com/bl ... erent.html

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"1C" Deboo Servo, ""Audio Power Amplifier Design Handbook," Douglas Self, 4th ed, pg 390-393: https://proaudiodesignforum.com/images/ ... dition.pdf

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mediatechnology
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Re: A Differential Deboo Integrator to Servo Mic and Low-Level Preamps

Post by mediatechnology »

terkio wrote: Thu Aug 08, 2019 9:21 am There is a catch in the Deboo integrator, differential or single.
It is sensitive to resistor mismatch.
This can be important or not, depending of the application.
When in need for near perfect integrators, one needs capacitors with very high insulation resistors. Using a Deboo configuration, the unavoidable resistor mismatch results in an additional parasitic resistor across the capacitor of a positive or negative high resistance.
I've been looking at servo performance in a preamp that has to correct large offset voltages and the more I experiment with the differential DeBoo the more I agree with terkio.

With large offsets to correct there is also a gain error due to resistor mis-match.
I was able to minimize it with resistor selection but the differential DeBoo requires ratio-matching 4 resistors to each other in addition to matching another pair of feedback resistors for 6 total.

In the situation where I've used the differential DeBoo it had small offsets to correct and worked well with 1% resistors.

For the input-capacitorless mic preamp, which may have large offsets to correct, I need to use a floating conventional integrator as a servo.
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Re: A Differential Deboo Integrator to Servo Mic and Low-Level Preamps

Post by emrr »

Makes sense
Best,

Doug Williams
Electromagnetic Radiation Recorders
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terkio
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Re: A Differential Deboo Integrator to Servo Mic and Low-Level Preamps

Post by terkio »

In the conventional configuration, you do not need any matching.
The important point, then, is the leakage resistor of the capacitor.
This resistor gives the DC gain that comes in the residual error. ( A perfect integrator gives a zero error, because it has infinite gain ).
One can calculate the error, replacing the capacitor by its leakage resistor in order to calculate the DC gain.
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mediatechnology
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Re: A Differential Deboo Integrator to Servo Mic and Low-Level Preamps

Post by mediatechnology »

What I found by measurement was that as the offset needing correction grew, servo authority diminished.
The gain error was in addition to static offset errors in the servo.

In the example I gave earlier for the headphone amp a trim was inserted in Rg.
Not a good solution however.
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