Jensen Twin Servo 990 - Design Ideas

[mrvision]

I tried to describe with characters the circuit I am using right now in a previous post:

Forgot to mention... Adding a DOA footprint for those that want to use DOA's as servos will also require Input Bias compensation circuits.

There are a few options out there but which one do you guys think will work better to correct Ibias in this particular case?

Right now I am using the following input bias circuit on the (-) input of the DOA's and it works well to correct DC offset:

 +15V
    |
    |
  10K trim --- 100K---270K --  (-) 990
    |                     |
    |                1 uF cap
    |                     |
 -15V                 GND

It works perfectly with the 990 but the other Fet DOA does not like it.

 

[ppa]

you should make a try decreasing the 270k resistor to 100k.

 

[ppa]

This might be a dumb question... but the last time working on the the circuit last week I was a little confused about the method of setting up the Input Bias Null circuit. I recall reading somehwere that the best way was taking measure of DC present at the output of the DOA at minimun Gain and then disconnect the servos and turn the gain all the way up and adjust the trimpot until the DC at the output of the doa was + - 1mv of the DC voltage at minimun gain. It ceirtanly worked for the first stage but with the Second Stage when I turn the gain up the trimpot reaches its limit and there is still a lot of -mV left to bring the DC close to the reading at minimun gain.

to set the trimmer the steps are the following:
1- disconnect the servo
2- give max gain to the pre
3- set the trimmer for 1mV DC out
4- wait 10 minutes with the pre on so the circuit becames at the right temperature, with no signal in the input.
5- re-set the trimmer for the min. offset voltage value
6-  at this point connect the servo
7- mesure if all is ok with the offset voltage.

 

[Kingston]

Any new developments on this?

A DOA servo version of a twin servo preamp somehow seems an exciting prospect to me.

 

[mrvision]

Still working in the design, I have added the footprint for DOA Servos to the PCB's and I am also working in a relay stage switching circuit. Last week I changed the FET DOA with another 990 and now Offset voltages are lower and more easy to control. It seems that some FET DOA's will not like the Twin-Servo unless you make some modifications to the original circuit so I am sticking more to BJT Op Amps just for now...

 

[mitsos]

just having a beer and thought I'd bump this...

any news?