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Thanks, Burnedfingers. I want to build something I have not built before, take it as a challenge. I am thinking of a preamp device specifically for harp with a balanced and unbalanced input and a few other features to help the harp player on stage. I have some ideas I'd like to explore. The possibilities are endless.

Don

Fast forward to today, I built a variable frequency notch filter based on the schematic I got here. It worked! Notched out the feedback frequency nicely, which turned out to be right around 4kHz. It did not do the whole job alas as the harp player has an amp with 3 different size speakers. Once the 4k feedback was notched out there turned out to be a second frequency around G below A 440 so I built a 2 frequency notch filter. I am going to his house tonite to see if it works. Wish me luck. I will repost the schematic of the original parametric eq as it works nicely and is a simple circuit. Next: build the same thing using 12AX7s!

Don

So what do you think about building this with 12AX7s?

Don

If you have to build using tubes pick something better than 12AX7.

Indeed!

Low mu, high current tubes, like the 12AU7, are the ones used in geetah EQ's that need reasonably high "Q".

Cheers!

I took the Notchback II to my friend's house last night and it worked beautifully. He tried 3 different microphones which all had different feedback characteristics and the Notchback was able to kill 90% of the feedback making it possible to play at far louder volumes than otherwise. I had to pry it out of his hands to get it back to make the next series of modifications. One of the disappointing things about it was that it is noisy, it makes quite a pronounced hiss. I will try quieter op amps but I have never faced this problem with solid state before.

Building the circuit with 12AU7s makes a lot of sense since the gain is low in the circuit.

Don