King of Tone / Paragon - High/Low Gain calculation

Rpschultz13

Rpschultz13

Well-known member
I've been doing some digging into the different gain options for the King of Tone, Prince of Tone, PPCB Paragon, AionFX Pauper/Theseus, etc. I'll use the Aion Theseus schematic because it's easier to understand, easier to correlate to a non-inverting op amp schematic.
  • Aion Theseus has the gain switch (SW1B) that KoT/Pot don't have.
  • PoT has the gain set standard to High, no Low gain option. Effectively where SW1B is open so R2 = 100k, R3 is ignored.
  • KoT has the option on build, but with Low gain (or SW1B closed), it's effectively 1k into the drive pot (1k || 100k = .999k)
1789775346903.png
This is a non-inverting op amp. Gain is 1 + Rf/R2.
1789775578942.png
  • Ignoring C3, Theseus R4 and R5 are in parallel, effectively 15k to GND, which is R2 in the non-inverting schematic above.
  • Theseus R2+DriveA is then Rf above.

  • Low Gain, Pot FULL CW, Gain is 1 + (100k + 1k)/15k = 7.7
  • Low Gain, Pot off CWW, Gain is 1 + (0 + 1k)/15k = 1.07
  • High Gain, Pot FULL CW, Gain is 1 + (100k + 100k)/15k = 14.3
  • High Gain, Pot off CWW, Gain is 1 + (0 + 100k)/15k = 7.7
Am I doing this right? It seems to me that there is NO overlap between the High and Low gain setting. And it's interesting that PoT doesn't give you the Low gain option. I seem to want to run my rig right in the middle. High gain with Gain off is too much, but Low gain I have to practically max it out to get where I want it... so that seems to track with my math of no overlap.

Middle ground? 47k?
  • MIDDLE Gain, Pot FULL CW, Gain is 1 + (100k + 47k)/15k = 10.8
  • MIDDLE Gain, Pot off CWW, Gain is 1 + (0 + 47k)/15k = 4.1
Thoughts?
 
Another idea would be to keep the low gain 1k, and change R4 (27k) to 15k. That in parallel with 33k is about 10k. Would give a max gain of 11, and still a min gain of 1. I’m gonna try this tomorrow.

Note: I hesitate to mess with R5, I think C3+R5 is a filter
 
jwin615 said:
Are R4/C4 also not a filter?
Click to expand...
Yes you're right. I did a spice analysis. If I change R4 to 15k:
  • Gain increases from 7.3 to 9.8
  • Center frequency of that gain stage shifts from 4.6k to 5.3k (with pot maxed.
However it's important to note that the center frequency shifts anyway with the 100k pot. See below. Graph says volts, but input is 1 Volt so the volts are equal to the gain.
  • Magenta is stock, 27k, pot maxed. LOW Gain 7.3 center frequency 4.6k
  • Light Blue is 15k, pot maxed. EXTENDED Gain 9.9 center frequency 5.3k
  • Dark Blue is 15k, pot 50%, Gain 5.7 center frequency 7.2k
  • Red is stock 27k, pot 50%, Gain 4.3 center frequency 6.6k
1789830922731.png

I messed around with C3 and C4... that's a no go, extreme changes to bode plot. Those need to remain as is and live with the small frequency shift.
 
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