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RE: Tre's sketch is also correct

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Tre'

In Reply to: RE: Tre's sketch is also correct posted by Triode_Kingdom on January 08, 2025 at 14:17:13

I simply redrew the schematic from the preamp that is the subject of this thread. That resistor is 150 ohms and the output cap is too small at .47uf
I left out the 330 ohm "plate resistor" for the top triode. Without it there would be no signal for the bottom triode. Oops.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

Edits: 01/08/25 01/08/25

Follow Ups:

RE: Tre's sketch is also correct - Triode_Kingdom 20:34:28 01/08/25 (4)
With those values, I'm confident this qualifies as a White cathode follower. I don't believe the addition of a 150Ω resistor in series with the upper cathode has a significant impact on the circuit's operation. Easy enough to confirm in SPICE, but I don't think it's necessary.
- I do find myself wondering - Ralph 10:27:50 01/09/25 (2)
  Why on earth that coupling cap is such a low value!
  
  I think if I were tinkering with something like that I'd have tried to use a common cathode resistor or a CCS, just to get as much differential effect out of the output as I could.
  - Yes, it does seem small... - Triode_Kingdom 16:10:02 01/09/25 (1)
    but it's not clear to me from the print exactly what impedance it's feeding. There's a 15KΩ, then a stepped attenuator and finally a transformer. :/
    - RE: Yes, it does seem small... - Tre' 16:27:48 01/09/25 (0)
      If you leave the transformer out (it's only there for a unbalanced output) then you're left with the 15k in series with (2.7k//whatever the input impedance of the amp is). If we call that 2.7k then, with the attenuator turned all the way up, the .47 cap is playing into 17.7k. 2nd edit, Ralph already said this.
      That gives a -3db point of 19.13Hz. That would still be 1db down at 38.26Hz and there would be phase shift all the way up to 191Hz. Turning the attenuator down would make it better but having a high pass filter, in the audio band, that changes with the volume control setting is bad form.
      I mis-spoke. That's not the volume control, it's the trim. So it's not as bad as I thought.
      Tre'
      Have Fun and Enjoy the Music
      "Still Working the Problem"
      
      Edits: 01/09/25 01/09/25
- I agree. nt - Tre' 07:16:01 01/09/25 (0)
  .
  Have Fun and Enjoy the Music
  "Still Working the Problem"

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