couple ways to do it, depends on your needs and how much you might want to modify the thing.
Easiest is to eliminate the control. Can either remove it from the circuit and wire around it, or just turn it all the way up and not touch it. Electrically they end up in the same place so far as the amp is concerned.
If some amount of attenuation is needed, a voltage divider with a small cap across the "upper" resistor in the chain to compensate for the HF loss works quite nicely. In theory you could do that with a pot, but the cap value would be correct at only one position. There is probably some mathy way to come up with this cap value, I just measure stuff and pick things based on measurements. Its probably going to be some small-ish number of pf, a variable cap out of an AM radio chassis should provide suitable range and a square wave display will tell you when its right. Too small and the leading edge is rounded over. Too big and its got overshoot. Goldilocks will have a nice sharp square corner.
A less ideal but possibly feasible solution is a lower value volume control, but that assumes the source has low enough source impedance to drive it. A 10k pot is harder to drive, but has far less effect on frequency than say a 500k pot.
A better way to retain a control with a high value that allows for easy drive without the roll-off problem is to stick a buffer stage in there. Miller capacitence is grid to plate capacity * gain + 1. A buffer has a gain of 1, so the Miller capacitence ends up low enough to be insignificant with anything but the most absurd volume control values. Buffer can be whatever you want. Cathode follower for the purists, or you could do a FET source follower or op amp stage, depending what you prefer and what space and power supply is available for the job.
Easiest is to eliminate the control. Can either remove it from the circuit and wire around it, or just turn it all the way up and not touch it. Electrically they end up in the same place so far as the amp is concerned.
If some amount of attenuation is needed, a voltage divider with a small cap across the "upper" resistor in the chain to compensate for the HF loss works quite nicely. In theory you could do that with a pot, but the cap value would be correct at only one position. There is probably some mathy way to come up with this cap value, I just measure stuff and pick things based on measurements. Its probably going to be some small-ish number of pf, a variable cap out of an AM radio chassis should provide suitable range and a square wave display will tell you when its right. Too small and the leading edge is rounded over. Too big and its got overshoot. Goldilocks will have a nice sharp square corner.
A less ideal but possibly feasible solution is a lower value volume control, but that assumes the source has low enough source impedance to drive it. A 10k pot is harder to drive, but has far less effect on frequency than say a 500k pot.
A better way to retain a control with a high value that allows for easy drive without the roll-off problem is to stick a buffer stage in there. Miller capacitence is grid to plate capacity * gain + 1. A buffer has a gain of 1, so the Miller capacitence ends up low enough to be insignificant with anything but the most absurd volume control values. Buffer can be whatever you want. Cathode follower for the purists, or you could do a FET source follower or op amp stage, depending what you prefer and what space and power supply is available for the job.
