Another op-amp question.

[3nity]

Im working on a pair of Jensen Twin preamps and im using a dual reverse pot.
My question is why is so important to have a dual and not 2 singles independents rev logs?
Will the second op-amp go crazy oscillating if they dont rotate at the same time?
I was thinking that maybe the second will act as an attenuator??

Whats the thoughts??

 

[PRR]

Schematic?

> why is so important to have a dual

You want both stages working as good as possible.

In Jensen-think, "good" is as much NFB as possible, meaning closed-loop gain as low as possible. In *both* stages of a 2-stage chain.

And I assume both stages use the same opamp.

So the goal is to split the total gain desired, half in each stage. 20dB+20dB for 40dB, 45dB+45dB for 70dB.

Otherwise some user will set 1st stage at 10dB and second at 80dB to get 90dB. The 90dB stage has non-zero THD. And the small 1st stage gain lets 2nd-stage hiss add to the 1st-stage hiss.

And who really needs more knobs?

(Yes, I know a dual-REVerse-audio is a hens-tooth.)

 

[joaquins]

45+45=70? :S

Note that john hardy did a change in the schem of the twing from jensen, he use the pots as in his M series, not as a pot but variable resistor, in order to keep fixed the pole on the feedback between the cap and the feedback resistor, nothing to do here but good to know when building. Also may be more confortable to do the resistor trick like that, because is not affecting nothing, if using the resistor trick as a pot it would change it total resistance. Just tie a resistor of 1.2x the value of the final pot and use a linear pot of 5x the value of the final pot, this way it would behave as a rev log without much trouble getting dual rev log pots.

Also you could buy log pots and open them and change the tapers, the one in the bottom to the front and back, major surjery but it works. It needs to be this kind of pot to do that, not the best around but you could get some decent ones and they allow this that most other wont.

Using a switch could be other way, but 2 gang 24 positions to get nice resolution may be not so readily available neither, could be done with 12 wich may be easier.

JS