Dear all,
This is a question aimed more at increasing my learning/understanding of the Tubelab SSE design (or other circuits for that matter) - George recommends a 50K volume potentiometer for the Tubelab SSE. Why 50K and not higher/lower?
I have read about possible general explanations like being able to match input impedance to that of the source and not lose to much of signal, but it would be great to understand the science behind it in more detail.
p.s. found some explanation from George here:
https://www.diyaudio.com/forums/tubelab/176406-simple-se-volume-pot-value.html#post2350573
Anything additional is most welcome.
This is a question aimed more at increasing my learning/understanding of the Tubelab SSE design (or other circuits for that matter) - George recommends a 50K volume potentiometer for the Tubelab SSE. Why 50K and not higher/lower?
I have read about possible general explanations like being able to match input impedance to that of the source and not lose to much of signal, but it would be great to understand the science behind it in more detail.
p.s. found some explanation from George here:
https://www.diyaudio.com/forums/tubelab/176406-simple-se-volume-pot-value.html#post2350573
Anything additional is most welcome.
Last edited:
You can go lower without bound on the amp, it's up to what your source can drive. 10K and lower is no problem for most solid state sources like a CD player. Some tube preamps / buffers / phono stages don't like a low impedance load.
The impedance presented to the grid of the input tube makes a pole (low pass filter) with the Miller capacitance of the tube. This pole gets lower as the pot value is increased.
The TSE has a high gain tube with a very high load impedance. A rolloff in the 20 - 25 KHz range can be seen with a 100K pot. The SSE will do OK with 100K, but I chose 50K as a common denominator for both amps.
The impedance presented to the grid of the input tube makes a pole (low pass filter) with the Miller capacitance of the tube. This pole gets lower as the pot value is increased.
The TSE has a high gain tube with a very high load impedance. A rolloff in the 20 - 25 KHz range can be seen with a 100K pot. The SSE will do OK with 100K, but I chose 50K as a common denominator for both amps.
If there IS a pot, then the input impedance varies somewhat with where the volume control is set.
At minimum value, the pot resistance IS the input impedance, but there is no sound.
As the pot is turned up the resistor from the wiper to ground + the input capacitance of the 5842 tube is placed in parallel with the lower portion of the volume pot (wiper to ground portion).
At full volume the input impedance of the amp is the value of the pot, the resistor from the wiper to ground, and the input capacitance of the 5842, all in parallel.
With no pot, the input impedance is always the value of the resistor from the 5842 grid to ground in parallel with the input capacitance of the 5842.
The input capacitance of the 5842 is the grid to cathode capacitance, which is NOT specified, plus the Miller capacitance of the tube is the actual circuit gain multiplied by the grid to plate capacitance of the tube (1.5 to 1.8 pF). Experimental measurements show the total capacitance to be in the 70 to 90 pF range depending on the tube used. WE's have lower gain than Raytheons, so the capacitance is slightly lower. This is enough capacitance to kill some high frequencies if the source has a high impedance.
If needed the resistor from grid to ground can be raised to 300K maximum.
At minimum value, the pot resistance IS the input impedance, but there is no sound.
As the pot is turned up the resistor from the wiper to ground + the input capacitance of the 5842 tube is placed in parallel with the lower portion of the volume pot (wiper to ground portion).
At full volume the input impedance of the amp is the value of the pot, the resistor from the wiper to ground, and the input capacitance of the 5842, all in parallel.
With no pot, the input impedance is always the value of the resistor from the 5842 grid to ground in parallel with the input capacitance of the 5842.
The input capacitance of the 5842 is the grid to cathode capacitance, which is NOT specified, plus the Miller capacitance of the tube is the actual circuit gain multiplied by the grid to plate capacitance of the tube (1.5 to 1.8 pF). Experimental measurements show the total capacitance to be in the 70 to 90 pF range depending on the tube used. WE's have lower gain than Raytheons, so the capacitance is slightly lower. This is enough capacitance to kill some high frequencies if the source has a high impedance.
If needed the resistor from grid to ground can be raised to 300K maximum.
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