6DJ8/ECC88-SRPP Tube Preamplifier

[sfthurber]

You would probably be better of changing to a mu follower which has lower distortion and then add some Negative feedback to set the gain. Attached is a schematic of one I posted elsewhere in response to a similar question.

Cheers

Ian

interesting...
would the gain end up around 7-8 dB if omitting c2 in the schematic?
what would be the outcome of replacing c2 and the cathode R with the red LED as suggested earlier in the thread?

 

[ruffrecords]

interesting...
would the gain end up around 7-8 dB if omitting c2 in the schematic?
what would be the outcome of replacing c2 and the cathode R with the red LED as suggested earlier in the thread?

No, the gain is set by the negative feedback. You want as much open loop gain as possible so it is better to retain C2 in order to do this.

Replacing C2 with a RED LED should have little or no effect. it should maintain the open loop gain and, as I said, the colsed loop gain is set by the negative feedback.

Cheers

Ian

 

[sfthurber]

No, the gain is set by the negative feedback. You want as much open loop gain as possible so it is better to retain C2 in order to do this.

Replacing C2 with a RED LED should have little or no effect. it should maintain the open loop gain and, as I said, the colsed loop gain is set by the negative feedback.

Cheers

Ian

thanks for your quick and very cogent reply!

 

[ruffrecords]

thanks for your quick and very cogent reply!

My pleasure. The gain, in theory, is set by Rf/Rin, which, with the values shown should give 430/100 times = 12.7dB. However, theory assumes an infinite open loop gain. In this circuit the open loop gain is around 30 times and the closed loop gain is close to 10dB. I would guess that reducing the 430K to 390K would get you in the 7 to 8dB region.

Cheers

Ian

Edit: P.S. the input impedance is equal to Rin, in the circuit shown, 100K

 

[sfthurber]

ok then, those results (7-8dB 100k) should be good for my purposes. thanks!

 

[waltube]

my error for this thread