Author Topic: A different sort of preamp  (Read 235 times)

Offline pnoboy

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A different sort of preamp
« on: April 08, 2018, 10:52:31 AM »
I’ve been a fan of the stage piano for a long time, having owned both a suitcase piano and 2 stage pianos.  I think that Fender made some unfortunate choices in the suitcase amp that caused it to be much heavier than it needed to be as well as to have a tone quality that was not to my liking.  The great advantage of the stage piano are the wide variety of amps and speakers that are available, allowing a much greater range of tone, volume range and size.   

Because of the variety of available amps and pedals, I’ve been a bit surprised at how much interest there is in preamps for the stage piano.   Why build a special preamp that may require irreversible modifications to one’s piano when one can pick an amp and pedal to get the desired tone?

The Dyno-My-Piano preamp is a good example.  It creates a large hole in the frequency response centered around 500Hz.  That’s fine if you like that sound, but for less money and fuss, one can by an equalizer that can not only create that tone quality, but many others as well.  Tube preamps also seem to enjoy some cache.  But, why not just buy a tube amplifier if one finds tubes to be advantageous?

People often think the soft clipping provided by tubes will be a boon to the Rhodes’ sound, but that idea is largely a siren song.  If one is playing moderate to complex chords, any audible distortion will sound really awful.  If one wants to play single-note leads and power chords like guitarists do when using distortion, then full-blown distortion such as provided by distortion pedals will work well.  Also, tubes, specifically triodes, provide soft clipping over a relatively narrow range of volume that doesn’t work well if one is playing using both single notes and chords together.

After thinking about all of us, I decided that there was one type of preamp that would make sense.  The output of the Rhodes harp is pretty low—lower than most guitars.  Also, the sound of the harp changes when loaded by the control in the stage piano.  So, I thought if one could design a preamp that would deal with these issues, and, at the same time, require no modifications to the piano and no external power supply, it would be worthwhile.  So, that’s what I did.

My preamp is very small and mounts by its ¼” phone jack, using the preexisting hole in the name rail.  A phono input jack connects between the harp and the preamp, and the output of the preamp connects to a 2nd phono jack using the preexisting phone plug for the piano’s name rail. The preamp is turned off unless a guitar cord is plugged into the piano.  Given the preamp’s tiny load current, a 9-volt battery will last more than 400 hours—that’s a lot of playing..  So, if you play your piano 10 hours a week, the battery will last at least 40 weeks—pretty close to a year.   If the preamp or its battery should fail, in about 2 minutes one can unplug the preamp from the harp, and plug the front-panel controls back into the harp.  Here are the specs, which were derived from direct measurements or SPICE simulation.  BTW, the preamp is a totally discrete design, with no ICs.  With its high impedance, and the short cord between the harp and preamp, none of the harp’s signal is lost or suppressed.

All specs were measured with a 2V P-P input signal and an output load of 10 kohms
Distortion              approx. 0.25%, largely 2nd harmonic
Input impedance   2 megohm
Output impedance   500 ohms—set by series resistor
Freq response      6 Hz to 110 kHz 3db down, flat in between
Max input              3 V P-P
Slew rate              0.3 v/usec
Gain                    5.5 dB
Output noise      2.8 uV (50 Hz to 10 kHz)
Power supply      9V battery
Current drain      1 mA

This 1st pic shows the preamp itself.  If I relayed it out, I could make it much smaller—I purposely left lots of room between the components for this initial trial in case I needed to make some modifications.

The 2nd pic shows how the preamp is mounted on the name rail.  The ¼ phone jack is removed, and the preamp installed in its place.  The green and yellow wires were previously soldered to the removed phone jack.  The little battery box is attached to the name rail by double-stick foam tape.  This stuff holds ferociously, but can be easily removed by slicing through the foam with a single-edge razor blade.

The final pic shows what the installation looks like with the name rail in place.

Offline jimbobothy

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Re: A different sort of preamp
« Reply #1 on: April 09, 2018, 10:27:23 AM »
.... makes sense to me  8)