garp wrote:Please excuse my post in your thread Oldfogey. I have just one question for Amandarae.
Amandarae,
Great explanation. Thanks. I just want to know this: Does a preamp's input impedance change when you manipulate the volume control? Take the Tono preamp for example which has a 100K pot. So is it really fixed at 100K input impedance or does it depend on the position of the pot? If its the case, could this be an explanation why some systems sound best at high volume (not specifically referring to the Tono preamp here).
Thanks for the help.
garp,
Sorry bro'! I did not catch your post earlier. My apologies!
But to answer your inquiry based on my ability:
A preamps input impedance is a different beast as compared to its output impedance. When we say input, it is basically the impedance seen by the signal going in with regards to the impedance presented by the first stage of pre amplification. Most of the time, it is the value of a pull down resistor (conventionally large say, 100K) just before the volume control or it can also be straight through to an active device.
With that scenario, input impedance "will" change if the volume control is manipulated (increase in our case) but only slightly because all the impedance after the pull down resistor is directly parallel to it.
The volume control is not really a "gain" control! In fact, it is nothing but an attenuator. It is common but a wrong notion that as we increased the volume (CW) we think that we are increasing the gain. No! What we did was to decrease the attenuation (open the valve as an analogy) so that the incoming signal to the active devices (amplification) increases.
Fortunately, I believe most manufacturers specify the input impedance of a preamp when the volume control is fully open. Typically, it is the value of the pull down resistor before the potentiometer (volume ) just ahead of the amplification/phase reversal circuit. Remember that a potentiometer is conventionally use as voltage divider in this case.
Okay, that is all I can say. I could be wrong but again, this is how I understand it.