MK-1 Preamp Anatomy (INPUT STAGE )

INPUT STAGE

The 2 signals coming from the 2 pickup are processed entirely by the leftmost IC1, used as non inverting HIGH impedance dual Operational Amplifier. 

Impedance seen by each pickup is 470K Ohms.  

Each pickup is respectively independently amplified by a factor of 2 (6db).

GAIN = 1 + (3900Ohm : 3900Ohm)  =  2

The 2 amplified signals are then MIXED externally by mean of the BALANCER, a  dual Audio tape <-> reverse Audio tape potentiometer organized in a way that any rotate mix position will imply same load and  no changes in the overall volume.
I think this is very well designed.

The mixed signal out of the potentiometer is then passed to the TOGGLE SWITCH:

• In the PASSIVE POSITION  the mixed signal is routed directly to the VOLUME potentiometer and the entire Equalizer is then BYPASSED. BE AWARE there is no REAL passive configuration here ...........as the IC1 will remain active for mixing and amplifying  the 2 pickups.

• In the ACTIVE POSITION  the mixed signal is routed to the Equalizer input branch, processed , received back from the Equalizer output branch and finally sent to the VOLUME potentiometer.

In the active position half of the IC2 is used as non inverting gain \ impedance adapter and decoupler just before entering in the equalization section. 

The factory gain here is 1 but  this  can be incremented using a proper resistor to be placed in the circled post We can see in the picture:

• FACTORY GAIN = 1 + 3900 : Rx( when no present ->∞ )   =  1

• CUSTOM GAIN = 1 + 3900 : Rx  = .....

There is no reason to change the gain unless finding differences in the Volume perceived in ACTIVE\PASSIVE mode. 

In the next post we'll start to describe the Equalizer section!

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