Need an Opinion on Tube Preamp

153

00:43:29

I've designed a 12AX7 (ECC83) preamp circuit and I'm looking for a second opinion on it. It's supposed to have very low THD hence the bootstrap in the plate chain and the LED at the second triode's cathode, acting as a constant voltage source. Some negative feedback i also found essential. So here's my circuit, what do you think? http://s2.postimg.org/6k8njmxk9/Custom_Image1.jpg

published 9 years ago

hurz

9 years ago

Isn't that positive feedback, and more a high pass boost?

thebugger

9 years ago

It's a negative feedback i think. First inversion would be positive, second inversion makes it a negative, i think? The feedback should work all the way down to 7Hz. I'm just thinking transistor-wise as they are not much different.

hurz

9 years ago

Two stages, each is an inverter, so it must be positive feedback and must be tuned carefully or this amp will be an oscillator. Negative feedback you can make for each stage itself. I would give NF a try at least at out stage to lower out impedance, cuz you have plenty of gain.

thebugger

9 years ago

Okay but isn't this a negative feedback. I mean in a 2 stage transistor preamplifier a negative feedback is developed when you return some of the signal from the second collector to the first emitter. Similarly from the second plate to the the first cathode should develop some negative feedback. The roll off frequency of the negative feedback is 7Hz in this case. Or am i mistaken?

hurz

9 years ago

Yes you right, i thought its going to the input gitter, but its comparable to a normal emitter point were it ends. Have you build this Amp already?

thebugger

9 years ago

No, i just designed it the other night. I don't have a few components yet, I'll build it when i acquire them. How much gain do you think it'll have. One part of the 12AX7 has a gain of 100. Both of them should have a really high gain, even higher than i want. I may just swap the tube with a lower gain one like ECC88.

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