JLM - jensen twin servo project

THe classic Jensen twin servo uses a 1k pot and a 1k21 feedback resistor along with a 43.2 ohm gain limit resistor. The JLM and SC versions use much higher vlaues 22k feedback and 10 gain. Plus the feedback caps are also an ordedr of magnitude different.

The question is what are the tradeoffs in each set of values?

--Ethan

The question is what are the tradeoffs in each set of values?

Click to expand...

The lower resistor values can drop the noise slightly but that depends on the opamps open loop gain and noise figure. Also the higher values can give higher DC offset at the output of the opamps depending on opamp input type but with DC servo this is not relevant. So in practice it makes little difference at all.

Also remember that FB and Load in the Jensen circuit are both driven by the opamp output so the impedance here needs to not go lower than what the opamp can drive. So if you are already driving a 600ohm load and 1k pot the opamp will see a load of 1/(1/600 + 1/(1k2 + 1k + 43r2)) = 473R which is to low for any normal DIP IC's. So a JLM99v, 990C, 992, M99 etc would be needed.

So if you wanted to build the twin servo and keep the same gain and bandwidth but use say our dual 10k rev log pots because you couldn't get 1k ones for example.

R5a and R5b would go up in value 10 times from 1k to 10k
R4 and R11 would go up in value 10 times from 1k2 to 12k
R6 and R12 would go up in value 10 times from 43r2 to 430R
C5 and C6 would go down in value 10 times from 470pF to 47pF

No other parts need to change

THanks Joe,

Those are exactly the answers that I was looking for. When I read about monolythic opamps I went "Oh Duh". The values were selected to allow for them.

My plan is to take your dual board and implement a twin servo on it using the Mnats 990 and some of the circuit changes gleaned from comments onthe list. It may involve some cutting or traces....

You are correct about the 10k pot selection over the 1k that I cannot find.

I am eagerly waiting the arrival of the boards and 10k pots I ordered from you.

thanks, Ethan