Wurlitzer 200A preamp

[diko2]

Hi to all,

Newbie question here:

I'm still scratching my head over the  wurlitzer 200A preamp schematic.  I'm wondering why, if the reed pickup is acting like a condenser capsule, did they use a bipolar transistor in front of it instead of high impedance Fet transistor like i see in almost all mic preamp?

Won't it be better?

I've checked the reed pickup capacitance and i've found 240pF.

http://www.flickr.com/photos/43245460@N06/5807261057/lightbox/


Thanks

Diko

 

[BradAvenson]

If you look at the input impedance (formed mainly by R1 1Meg and R3 470k) it is pretty low by the 1gig microphone input standards.  The output level of the reed bar is far higher than a microphone capsule.  If you want to try an experiment, raise the reed bar polarization impedance and you will hear why they kept it lower.  As it goes up, so does the microphonics of the reedbar and the thumps and bangs of the player and keyboard. 

 

[abbey road d enfer]

The output from the tines is not constant; the amplitude of the bass notes is much higher than the treble, so the high-pass effect (6dB/octave) compensates.
I believe the Wurlitzer designers may have been concerned with the risks of putting an FET at the input; when you tune a Wurly, you may casually short out the pick-up about 100 timees. This produces 150V peaks that the bipolar stage seems to handle well; I would worry about an FET there.

 

[PRR]

Aside from the other comments, it may be "too old" for routine use of an FET. At least at a conservative shop like Wurlie. They probably use the same 1 or 2 NPNs for everything; unwilling to stock another part-number if it can be avoided.

As Abbey says, this mechanism does NOT want "flat".

270pFd against 380K is a bass-cut at 1,750Hz. Basically the whole keyboard. Lowest note 20db-30db lower than the top note.

The amplitude of the reeds/tines is probably semi-proportional to inverse pitch. The low notes make big swings. Now in the Rhodes, the pickups are coils, output proportional to velocity, not amplitude, so the output level is fairly constant across the keyboard. Replace the wound pickups with capacitance sensors, output proportional to amplitude, the big-swing bass tines will be much louder than the microscopic-swing top notes. It "wants" bass-cut all across the keyboard. It "happens" to work out that the practical pickup capacitance and practical BJT impedance gives a just-right bass-cut.

> like a condenser capsule

But it isn't JUST "like" a microphone. Small mike catches big bass waves poorly. OTOH, a small low-tuned tine smacked with a BFH (big felt hammer) catches the energy very well.

 

[diko2]

Thank you all for your replies and comments.

As always, i look at a small part of the schematic without thinking about the whole interacting parts.


Thanks