Pro Audio Design Forum

... Balanced Front End With Hans Polak's help I recently redesigned the Flat Moving Coil Preamp PC board. With highly-mismatched pairs the servo control point in the original design increased IM distortion because it varied collector current. By moving the servo injection point to the emitters ...

... The flat moving coil preamp doesn't require the thermal hat but I found that the servo activity at infrasonic frequencies is far less due to greater thermal mass and greater device coupling. If a nut seems like overkill gluing the ...

I've updated this design with a new PC board. Hans Polak convinced me that I should use emitter servo injection to reduce ZTX851 matching requirements. In the process we discovered that huge improvements in IMD2 distortion could be realized by matching resistors and C4/C5. ...

Updated January 25, 2021: A new PCB version, assembly instructions and Project Manager link are available. Improvements include a simpler servo, selected components to improve CCIF IMD2 distortion and an output pad for head amp applications.

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.

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 err...

Makes sense

... 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 ...

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 ...

... The OVC in Bill's backyard circa 2013: https://proaudiodesignforum.com/images/jpg/Bill_Sebastian_OVC_Backyard.jpg The OVC was a servo-controlled mirrored trough with a video camera at one end and a monitor at the other. The whole gizmo resembled a kaleidoscope. An array of incandescent ...