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RE: Output Impedance

Posted by Tre' on February 15, 2025 at 11:54:11:
"Earlier you questioned the value of the 100k resistor to ground"

No, I never mentioned the 100k resistor. My concern is the 330k series resistance added to the output impedance of the stage.

"...I thought that the resistance in series (330k in this case) only affects the calculation of the -3db rolloff point"

The -3db rolloff point is, more of less, the only concern when it comes to the output impedance of a circuit (a preamp in this case).

"Whatever portion of the 100k that's in series with the grid of the amp's first tube would be added to the output impedance of the preamp??"

If the pot is mounted in the power amplifier, then the output impedance of the preamp is driving the cable capacitance.

The pot's output impedance is the output impedance of the preamp plus the series resistance of the pot with that value in parallel with the portion of the pot that shunts to ground from the wiper. That is the impedance value that is driving the Miller capacitance of the first stage of the power amp.

I was under the impression that mwhouston has placed the 330k and the 100k resistors in the preamp.

"If that were the case it seems like the use of a voltage divider, or an adjustable voltage divider aka a volume pot, would technically create an impedance mismatch in many cases. Perhaps most cases."

I tend to agree.

The stage needs to be able to properly drive the shunt capacitance. When there is no pot at the amplifier, the shunt capacitance seen by the preamp's final stage is the cable capacitance and Miller capacitance added together. Miller capacitance is the capacitance between the grid and the plate of the input tube of the power amp multiplied by the gain of that stage and then the capacitance between the grid and the cathode is added. The -3db point of the low pass filter created by the output impedance of the driving stage and the total shunt capacitance is easy to calculate. If we want 20kHZ to be unaffected by that filter, then the -3db point of that filter needs to be at least 200kHz.

A capacitor is a reactive device, It's reactance (impedance) goes down as the frequencies go up. At the highest of frequencies the impedance of the capacitance reactance will be very low. The device driving that capacitance (the preamp's final stage) has to be able to deliver the current that the reactance of the shunt capacitance will require otherwise the preamp stage will distort.

So a stage has to have a low enough output impedance so that the low pass filter created between it and the value of the shunt capacitance is well out of the audio band and that stage has to be able to deliver the current needed to drive the shunt capacitance's low impedance at high frequencies.

A 330k ohm resistor in series with the output impedance of the preamp will make these requirements hard (if not impossible) to meet.

Tre'