Changing gain on an old Shure M61 pre.

[Walrus]

I thought I'd have a play with an old Shure Germanium Phono/Mic/Tape pre.
In the attached schematic, I've deleted the components that gave the equalisation for Phono and Tape, which leaves an amp with
63dB of gain and input Z of about 20k (according to the data sheet).
I want to use an OEP A262A3E wired 1:6.45 on the input so I can add phantom easily, but this adds even more gain!

Is it possible to make one of the resistors variable, to act as a gain control?

 

[dmp]

The portion you deleted is lowering the gain with negative feedback. If you make that negative feedback broadband it will lower the gain. The stock cap values give the equalization curves, so the 5nF should be larger. And you could adjust the amount of negative feedback with a pot or switched resistors for some control. If you simulate it in Circuitlab (online) you could figure it out pretty quickly.
Alternatively you could put a pot on the output for adjustment. Are you going to use a output transformer?

 

[Walrus]

Just tried Circuitlab, but by the time I'd constructed the circuit, it timed me out and wanted money to continue.
So lowering the value of the 1.2M resistor should reduce my gain, by increasing feedback?
I'll have a quick play on the bench.....

 

[dmp]

Just tried Circuitlab, but by the time I'd constructed the circuit, it timed me out and wanted money to continue.
So lowering the value of the 1.2M resistor should reduce my gain, by increasing feedback?
I'll have a quick play on the bench.....

That's really annoying - I thought they let you use it as long as you wanted, but couldn't save unless you paid. I will not recommend it anymore then.

I wouldn't change the 1.2M since it is DC coupled and will change the Q1 operating point. I was thinking of putting in a pot for the 10k, connecting orange to lavender permanently, and increasing the 5 nF so it is broadband. 5nF and 3k9 give a knee at 8kHz.
I have a subscription and will try it quickly.

 

[dmp]

Stock circuit, about 70 dB of gain. This is some amount off because I used the default silicon transistors.

 

[dmp]

Added the 10k feedback circuit with a trial and error on the cap. Arrived at 2.2uF and this brings the gain down to 30 dB

 

[Walrus]

Thanks for doing that for me, really helps.
I did quickly try putting a 100K pot over the 1.2M, only gave 12 dB of adjustment.
Just tried your suggestion and yes, it does bring the gain right down. Unfortunately it also lowers the potential max output before clipping
as you raise the input level.
Stock I get 10dbu output before clipping, with the 10K pot at max it's about 4dBu.
On that basis, it may be best to use a variable pad on the input and just run the pre flat out.

I'll play a bit more tomorrow, time to go home now.