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In Reply to: RE: Thanks Ralph! posted by Triode_Kingdom on August 10, 2017 at 10:29:07
You might consider that you can do everything you want here except the protection relay with a single opamp like a TLO82, which is cheap and not really high performance, and yet still be effective and entirely inaudible.
First, I think you will find that you don't need all that much gain to do the job so you won't get into offset or stability issues with the opamp; a gain of 100 will be **plenty**! One opamp can sort out your correction bias voltage, the 2nd can be used to drive the LED.
I don't filter the LED signal as much; just a simple 6 db slope. In this way, at higher volumes the LED flashes with the music; it gives me an indication that I'm driving the preamp at a higher level. I use bi-directional LEDs.
You should be able to have no more offset then about 0.001 volts or so, which won't harm any amp made. It might even be lower than most cap-coupled preamps can do (unless they don't use feedback).
If you pull this all off, I think you will find this to be one of the most transparent preamp circuits you've ever heard. Its a big deal getting rid of that output coupling cap!
Follow Ups:
"Its a big deal getting rid of that output coupling cap!"
We're definitely on the same page on that point. I'm expecting this to provide an output Z of a few hundred ohms, and there won't be any phase shift or coloration from a large series cap.
About the opamp, I already have a few OPA445s on hand for this project. This device is rated to +/-45V, and that's what I need to handle the bias voltage directly. The followers will be 5686s in pseudo-triode. With this tube, the DCR of the cathode choke (~2,200 ohms) dictates a negative supply of roughly 24-28V at the bottom of the choke. With the cathode at zero, that will produce about ~12mA current into the cathode. To keep the cathode at zero, I'll need about -18V to -20V on the grid. So, I believe the simplest plan will be a +/-28V supply using a LM317/LM337 pair. That will provide the negative voltage for the tube, as well as powering all the active components and the output relays. Now, if I can just find time to start on the chassis...
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