-=> Mike Ross said to Greg Mayman
 -=> about "BAG OF CHIPS" on 02-13-04  11:42.....

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

I don't know about the ceramic having a lower impedance but they
certainly have a lower output voltage.
 
 GM> Interestingly, adding a capacitor across the low load resistor
 GM> can extend the bass response.

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

One of the electronic magazines in the early 1960s published a
circuit for a transistor buffer for crystal pickups, one resistor
and two germanium transistors....

.              c -----+-------- -3v
. in --||-----b       |
.     .01uf    e      c      2 x small signal PNP
.              |-----b            transistors
.                     e    -  +
.                     |-----||-- out
.                 R1 10k   2uf
.                     |
.                    gnd

The first transistor received its bias solely from the collector
to base leakage. It was supposed to have an imput impedance of
several megohms. I built it up at onbe stage but I must have used
extra low leakage types as it wouldn't work for me.

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

Darn right there! In the early transistor amps, each stage was
designed as an independent item, and coupled to the next stage
either by capacitors or transformers.

Once they woke up to the fact that they could direct couple the
stages, the field really moved ahead.

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

Yeah, I guess it was.

   ,-./\
  /     \ From Greg Mayman, in beautiful Adelaide, South Australia
  \_,-*_/    "Queen City of The South"    34:55 S  138:36 E
       v

... Let he who takes the plunge remember to return it by Tuesday.
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