Hello all diyAudioers!
Years ago I built a EL34 SE amp with global feedback that I just couldn't drive hard enough, so i built a version of the 4S pre with a 12AU7 and it did wonders for the system. That system also used a rotary switch for input selection, and I got tremendous crosstalk if two sources were active. Now, years later, I'm finally getting around to designing a full-featured preamp, based on the humble 4S and 12AU7. The design goals are:
1. Multiple inputs with relay based source switching
2. Tone stack
3. Tape loop
4. MM Phono stage using EF86 (I have a stack of Phillips TS45 tubes which are nearly identical)
5. Signal chain 100% analog
5. DC filaments
After a few months of research, I have a starting design that I'm happy with, however I do still need to run the curves on all the tubes, design the power supply, and prototype the sections. In the meantime, I got a bug to design a board to hold the rear panels RCA's and all the relay switching. This will feed the amplifier stages and be controlled likely by an Arduino Nano or Adafruit ItsyBitsy using simple rotary selectors to feed the necessary signals to the MCU for the switching. That will be housed on a separate board with the 5V power supply, et. al.
One of the things that I've been struggling with is the grounding scheme. What I've come up with is an isolated audio ground that can either be tied to earth (Chassis) or lifted using a parallel resistor and capacitor in what appears to be a fairly common arrangement. There will be a separate binding post on the rear, connected to earth, for the turntable ground if needed.
I would welcome comments on the circuit design, the grounding scheme, etc., as well as the amplifier circuit itself. Please understand that the circuit topology is essentially fixed, so please don't say that i should be using your favorite topology. More eyes the better...
Finally, while I am an engineer, I'm not an electrical engineer. So please cut me a little slack.
Best,
Chris
Years ago I built a EL34 SE amp with global feedback that I just couldn't drive hard enough, so i built a version of the 4S pre with a 12AU7 and it did wonders for the system. That system also used a rotary switch for input selection, and I got tremendous crosstalk if two sources were active. Now, years later, I'm finally getting around to designing a full-featured preamp, based on the humble 4S and 12AU7. The design goals are:
1. Multiple inputs with relay based source switching
2. Tone stack
3. Tape loop
4. MM Phono stage using EF86 (I have a stack of Phillips TS45 tubes which are nearly identical)
5. Signal chain 100% analog
5. DC filaments
After a few months of research, I have a starting design that I'm happy with, however I do still need to run the curves on all the tubes, design the power supply, and prototype the sections. In the meantime, I got a bug to design a board to hold the rear panels RCA's and all the relay switching. This will feed the amplifier stages and be controlled likely by an Arduino Nano or Adafruit ItsyBitsy using simple rotary selectors to feed the necessary signals to the MCU for the switching. That will be housed on a separate board with the 5V power supply, et. al.
One of the things that I've been struggling with is the grounding scheme. What I've come up with is an isolated audio ground that can either be tied to earth (Chassis) or lifted using a parallel resistor and capacitor in what appears to be a fairly common arrangement. There will be a separate binding post on the rear, connected to earth, for the turntable ground if needed.
I would welcome comments on the circuit design, the grounding scheme, etc., as well as the amplifier circuit itself. Please understand that the circuit topology is essentially fixed, so please don't say that i should be using your favorite topology. More eyes the better...
Finally, while I am an engineer, I'm not an electrical engineer. So please cut me a little slack.
Best,
Chris