DC Servo question...

babyhead · Jul 7, 2005

I am in the final stages of my Fred Forssell on a Fabio board mic amp:

http://www.forsselltech.com/schematics/JE16 Mike Preamp.PDF

Should there be any DC current on the pin2 servo output?

I am so close to having this together! My baby...

-BH

PRR · Jul 7, 2005

> Should there be any DC current on the pin2 servo output?

Do you mean the main XLR output pin 2?

That should be less than 1 milliVolt DC.

The servo output is U2A pin 4. That could be several milliVolts, whatever is needed to correct the input error of U1A, the JFET990 opamp (or whatever you have there; does not matter for DC conditions).

If you have huge DC at XLR output pin 2, remove the servo chip U2. For most opamps, the output DC should now be a few milliVolts. For the slightly-odd JFET990, it might be as much as a Volt. Anything else means a wiring error.

babyhead · Jul 7, 2005

OK... So what if the DC voltage is oscillating? :shock:

I pulled out the OPA604 and had a stable 27.8 mV...

Put it back in and it dropped from 70 mV to -80 mV...

And the JFET 992 is getting pretty hot.

The difference between Fabio's servo and Mr Forssell's is that Fred's has reservoir caps 0.1/50v C5 and C6 and Fabio's doesn't. Should I rig these onto the board?

Hmmm... or just put my a-22 in there!

Flatpicker · Jul 8, 2005

I used the exact servo circuit that Fred published and it works great. I made my own pc board layout utilizing a ground plane. I also keep the in's and out's separated. There should be no oscillations and the 992 should not run hot. Check the layout/wiring and try adding the caps (although Fabio's board seems to work fine according to everyone who uses it).

Another hint: Make an op amp with the same "990" format footprint using a 5534, a 8 pin dip socket, a 1" square piece of proto-board, and some thick resistor/cap leads. Plug that into your circuit in place of the 992 until it is functioning properly. That way you don't run the risk of smoking a $65 992! :shock:

DrFrankencopter · Jul 8, 2005

The circuit looks fine to me....for more on servos check out my SSL paper http://www.cankar.net/sheme/ssl_mod.pdf . Be forewarned though that I'm using an inverting configuration in my paper...not a big difference, but slight value shifts are necessary.

Check you soldering/wiring. If the servo is oscillating check your grounding to near the inverting side of the servo. Double check the power rails at the op-amp...is it getting power on both rails? Double check the value of the cap in the feedback loop, if you have too small a value in there, it will try to cancel signal from the 992 rather than DC.

Cheers,

Kris