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[pipelineaudio]

Where can I learn about how to pull these off? I see some cool info on Jensen's webiste, but Im still a little confused.

[Sahib]

pipelineaudio wrote on Wed, 01 December 2004 01:44

Where can I learn about how to pull these off? I see some cool info on Jensen's webiste, but Im still a little confused.

I am kind o hesitant to ask the nature of your question but excuse my ignorance.

Your header says DC servos and you are talking about pulling them of but then again you mention the info on Jensen's site. Are you meaning how to pull DC servomotors off the faders of your desk or servo pre-amp topology?

Cemal

[pipelineaudio]

Ahh I mean like DC servos to keep a balanced in or output balanced. I think. I am pretty confused

[amorris]

I think you are referring to the jensen twin servo dc offset servo correction circuit. this refers to having an opamp track the dc offset of the output of another op amp and having the servo self correct with a superlowpassed millivolt swinging output voltage. the SSL 9k preamp has this type of thing. it is an advanced circuit design concept that would be hard to "try out" without some serious homework.
http://www.comtran.com/as/as083.pdf

[Sahib]

amorris wrote on Thu, 02 December 2004 21:07

I think you are referring to the jensen twin servo dc offset servo correction circuit. this refers to having an opamp track the dc offset of the output of another op amp and having the servo self correct with a superlowpassed millivolt swinging output voltage. the SSL 9k preamp has this type of thing. it is an advanced circuit design concept that would be hard to "try out" without some serious homework. http://www.comtran.com/as/as083.pdf

You nailed it and I agree.

[dpaton]

amorris wrote on Thu, 02 December 2004 15:07

it is an advanced circuit design concept that would be hard to "try out" without some serious homework.

I wouldn't go that far. At the lowest level, a DC servo is nothing mroe than an integrator in the positive feedback loop of an amplifier. The caps around the NFB of the servo op-amps are sized in relation to the input and output resistors as I recall. I honestly don't remember the formulaic relationship, but I'm sure someone here can dig it up before I can paw through my old notes for it. From what I can recall, you compensate the noninverting input of the servo withthe same RC combination you use for the input and feedback resistor around the inverting input (like the 2nd stage in the Jenses Twin Servo ckt). IIRC, the values are calculated for the desired Fo value (1/2piRC, ~0.18Hz in this case). The output resistance relationship escapes me however, as does a 100% confidence in the Fo calculation. I'm sure someone will fix it if it's wrong...

-dave

[BuzAllen]

The following doc's have some info/examples of DC servos:

Datasheets for the INA103, OP176

There a more than a couple configurations available for this, search the app notes and datasheets at Analog Devices, Linear Technology and TI/BB and you'll find some info.  I also recall there being examples in Walt Jungs Opamp book, out of print but I have seen copys used online.

To get a feel for how they work you can use a spice simulator.  If you don't have one Linear Technologies has a free one availble (LTspice).  Implementing them is more complicated than simply simulating them but you can get an idea of how different time constants in the servo loop respond and look at freq domain plots and play with inv and non-inv configurations etc...


Have fun,
Brian Allen