Determining imput impedence of a preamp line stage

[trombone]

This may seem like a dumb question. I know that you can get close to the input impedence of a power amp by taking the value of the load resistor on the input jack. But how do you determine the impedence of a line stage in a preamp? In my case I'm talking about an MFA Luminescence which has a phono stage and line stage. How do I determine the input impedence of that line stage when using the phono? I'm asking so as to get an idea of what the output cap of the phono stage should be. Is it the impedence of the volume control (50K)?

 

[Eli Duttman]

Is it the impedence of the volume control (50K)?

That's likely the case, but posting a schematic will allow members to provide firm opinions.

 

[kevinkr]

My recollection is that the input impedance of the line stage is 50K if we are talking at the volume control as opposed to the gain controls. (It has been a while since I worked on this particular unit.)

 

[trombone]

Hi, Kevinkr

It's been a while. The gain controls are 50K and they are the only ones in play when using the phono stage.

The volume control is also 50K. When using that control, the gain controls are all the way up.

That answers my question. Thanks.

 

[kevinkr]

If the gain controls are still in circuit and they are 50K as I seem to recall then for the phono stage mode the line stage input impedance is 25K...

 

[HollowState]

Schematic I drew up years ago.
PM me if you want the PS. It's MFA's and I don't want to be scolded for posting it.