DIY 140B Pickup Circuit

[RhodesWarrior]

Hi all... This has probably been discussed to death but....

I'm working on replacing the stock amp in my 140b with a more basic mic(ish) level output..  So far, I've had moderate success with using the stock bias circuitry to bias the reeds and taking a signal straight off of the reed bar (before the stock pre-amp).    My google skills and my enthusiasm far exceed my electrical engineering chops (which explains anything below that sounds like ill-informed nonsense) so I'm seeking a second informed opinion before I let the magic smoke out of anything.

The reed biassing schematic for the 140B looks fairly simple.  A small low (mA) current 1:1 AC isolation transformer and replicating the factory half wave rectifier should be doable it for under $20 (obviously with switches and fuses on the mains side).  A quick test says any DC bias over about 40V will generate a usable signal (I tried 5 9V batteries in series which works but barely). A higher biassing voltage would seemingly give a cleaner signal with more headroom.   Is there any benefit to increasing the bias voltage?  Stock, it's 180V on this piano (so rectifying 120VAC should get me in that ballpark).  Is there any advantage/disadvantage to increasing that?  I know the 200 series pianos run at over 200V.   

For the output, I'm concerned about high voltage AC (from the reeds getting shorted/buzzing) making it's way past the coupling capacitor and frying expensive vintage noise-making things which lack adequate input protection.  Wurlitzer didn't seem to care much about this (though their pre-amp seems more than able to survive the 100V+ AC pop) - or maybe there's something in the schematic that protects against this - but I can't see it.  One of the 200 schematics I looked at looks like it has a diode to protect against shorts, but the rest don't.   I've mocked up a circuit  using the stock .1uf Cap, followed by a 22K resistor with resulting signal grounded via a 3.3V zener diode to send any large AC transients to ground.  That output then goes to a passive DI and then to an external pre-amp.

This configuration seems to generate a relatively clean signal with no AC transients in excess of the diode breakdown voltage when I purposefully short the reed bar, though there's still a *lot* of hum from the reeds without the lid on the piano (which isn't much different from the amplified output on the stock amp though).  Is there anything I'm missing here that could:
1. Make this safer
2. Make this sound cleaner

I considered volume and tone controls on the signal, but I'd like to leave things as "clean" as possible to the pre-amp.

The stock amp will get repaired in due time (I have a nice set of audio-grade nichicon caps to clean it up with), but I'd like the option of using the piano with more modern amplification and bypass the internal amp completely.   99.998% of the time, this going to be getting played through an external amp and pedal board...

[cinnanon]

Borish electronics has just designed a preamp that runs off a 9V battery only and outputs line level. Supposedly the reedbar is only at a few volts potential.

[RhodesWarrior]

So on further investigation (for anybody who runs across this), you really need some kind of pre-amp to correct the impedance.  A transformer/DI ~sort of do it, but it's noisy and not correct.  I have a couple of different op-amps in the mail to see if I can come up with a configuration that mimics the stock tone and perhaps allows me to up the gain and lower the rail voltage so I can use a standard 12 or 24V AC wall wart for power.   Should make for a fun winter project.

After replacing the filter caps on the power supply, re-routing all the AC lines, getting rid of the AC to the cheek block, redoing the mess of ground connections, and cleaning up all the grounds, the amp is acceptably quiet...  Noise is also deceiving on the 140 when you're tinkering since the lid has to be installed to properly shield the pickup.   The stock power amp seems to have lots its magic smoke in the process and now makes even less (good) noise but taking the signal at the volume pot (which on this piano is inserted directly between the pre and power amps) provides an excellent quality clean output for the time being and is easily reversible.