"Greg Mayman" bravely wrote to "Mike Ross" (11 Feb 04  08:52:00)
 --- on the heady topic of "BAG OF CHIPS"

 MR> The load resistance is often in the megohm range but I've seen
 MR> Philips using a 100K load but with added correction.

 GM> The pickup looks like a voltage source in series with a
 GM> capacitor, 0.002 uF is a typical figure.

One other thing comes to mind concerning crystal pickups, that is, some
of them are not really a crystal but rather ceramic instead. The ceramic
type seemed to have a lower impedance than the former type. A lot of
different materials, which exhibit piezoelectric properties, were tried
as transducers. Some specific types were highly regarded.


 GM> Interestingly, adding a capacitor across the low load resistor
 GM> can extend the bass response.

Yes, that's what I think was often done to fix the loading problem
especially when transistors came about. Tubes had very high grid
impedance and often were only biased with grid leak with many megohms,
just right for crystal pickups. By contrast, transistors had only 10's
of K ohm input impedance not Meg ohms.

A boostrap input bias could have been used to achieve megohm input
impedances but that took a lot of components. Anyways in the early
transistor days electrical engineers didn't really understand them
(majority/minority carriers, holes, conventional current etc) and would
take well proven tube designs and try to adapt them but it really did
require a different approach to the various new problems.


 GM> Some high quality ceramic pickups were as low as about 10mV
 GM> output. The ones in cheap record players had a MUCH higher
 GM> output. I heard of one that gave nearly a volt.

I think you have touched on the difference between the different types
either ceramic or crystal with many materials and combinations being
tried. One could have a great pickup but low level or an average pickup
with much higher output emf. I guess it was a trade off.

 Mike
 ****

... I worked hard to attach the electrodes to it.
--- Blue Wave/DOS v2.30