Rhodes Stage Piano Pre-amplifier (and Dedicated MarkII Name Plate)

If you are a Rhodes Stage Piano player you have already faced the problem: how to rise the output level of my electric piano to a mixer level?

In this instructables I will show you a possible solution to pre-amplify your Rhodes MKII. I will also share with you Gerber files to have the pre-amplifier PCB and name plate manufactured by professionals at low cost.

Printed Circuit Boards

1x Custom Aluminum Faceplate

1x Preamplifier PCB      

Resistors and potentimeters                 

4x 100K ohm                                  

2x 10K ohm

2x 2.2M ohm

1x 2K ohm

1x 27K ohm

1x 680 ohm

2x 1M ohm single turn, linear potentiometer

1x 50K ohm single turn, linear potentiometer

(3x potentiometers knobs of your choice... the shortest, the better)                   

Capacitors                                   

3x 10uF electrolitic

2x 3.3nF non polarized                                 

2x 4.7nF non polarized                                      

Diodes, LEDs and op-amps

1x 1N4004 diode 

1x 3mm LED (with LED holder)

1x TL072 op-amp (or any other pin compatible preamp of your choice)

Connectors

1x RCA female PCB mount connector        

1x 1/4" audio connector

1x 5.5/2.1 power supply connector                                 

... a soldering station, some gram of solder, some wire ... the usual things in short! :)

Since it's introduction in 1970 the Fender Rhodes Stage piano has seen various attemps to obtain the best amplification/pre-amplification of the tiny, magnet-induced signal generated by pickups.

One of the most common (and rewarding) approach is the adoption of a "monolitic" amplifier like the Fender Twin Reverb or Roland Jazz Chorus, but also more studio-oriented outboard preamps are at our disposal. Dedicated Rhodes Stage Piano onboard preamplifiers are also available, but the choice is limited nowadays.

Whatever your favourite choice is to shape the Rhodes sound, there's one thing that everyone agrees quite well: the factory passive "bass boost" and volume controls are not top notch. They are often bypassed by getting the output signal directly from the harp RCA connector and in the past onboard modifications where developed to replace the passive electronics that constitute the aforementioned bass and volume controls (e.g. Chuck Monte's "Dyno-My-Piano" and R.A. Gresco's "The Rhodie").

Recently a nice post on Electric-Piano Forum opened by user Tehu (credits due for the NOS, still blistered, preamp picture attached) cought my attention. In that post a NOS copy of the mod called "The Rhodie" has been sourced and became the quid for a very interesting reverse-engineering operation of the circuit and disamina of it's main features.

EP forum users (in particlular "Jannel" and "sean") made a great work on this regard, and schematics are now available for everybody.

It turned out that the circuit is a classic, active Baxandall equalizer: a "simple" (hindsight warning!) but natural sounding equalizer patented in 1952 by Peter Baxandall.

The Baxandall tone control circuit is commonly referred to as a "shelving" control because of the shape created by the amplitude versus frequency response when boosting or cutting the low and high end frequencies. The shelving tone controls cause amplification (boost) or attenuation (cut) at a substantially constant slope or rate and then level off to a flat response.

Baxandall EQ is an active equalizer that use feedback to get symmetrical boost and cut. The Baxandall tone control requires an inverting amplifier stage with low output impedance to drive the filter circuits.

The Rhodie mod was intended to be installed in place of the passive bass and volume control on Mark I and II Rhodes. It actually was battery powered and used double potentiometer with dual concentric shaft to give you the additional treble control without the need to drill a hole in your valuable piano.

Being that:

• I am not a fan of battery powered circuits
• Those dual concentric potentiometers are expensive (notice that they are not simple stereo potentiometers)
• Aluminum faceplates can be realized by PCB manufacturers (I have adopted this in various of my previous projects)
• Instruments, being they vintage or not, deserves all the possible respect: mods must be 100% reversible!

... the most natural approach for me was the design of a brand new name plate with all the necessary holes to host (classic single turn, non-dual concentric) potentiometers, jacks and connectors, and a PCB to host the Baxandall equalizer.

This let me keep the original faceplate and bass boost/volume circuit assembled as it was intended by the factory, so in any moment I can turn my Rhodes back to 100% original (full reversible modification).

The new faceplate hosts:

• three potentiometers (bass, treble, volume)
• a power connector (12V DC, center positive)
• a power indication LED
• the 1/4 inch output connector
• a DPDT switch

... and a pretty (unobtrusive) "MARK II" underpaint text :)

Using the direct harp output is necessary in some situations (e.g. if your signal is going to an external preamplifier or full guitar ampli), so I placed a DPDT switch to completely bypass the preamp circuit.

The switch allows the user to select between a completely "clean" harp tone, or a preamplified one. This makes possible to connect another preamplifier (or connect directly to a guitar amp) in a blink.

The switch completely cuts the preamp circuit out of the audio signal path, so purists are welcome :)

Please notice that this "ad-hoc name plate" will only fit Mark II Stage Pianos!

This is because while the MKII factory faceplate leave a rectangular free space when removed (see attached picture), MKI faceplate has not a support function and holes are directly drilled on the main aluminum bar.

Potentiometers are, anyway, spaced by 2 inches (the "standard" dimension between knobs and "input" jack in both MKI and MKII Rhodes). This means that you can accomodate this project PCB, have the three potentiometers in front of you and drive the output e.g to the back of the piano.

I adopted only THT components, because any component in the audio path can affect your sound as a function of its composition (polyester caps? Polypropilene caps? You name it) and characteristics (TL072? NE5532? OPA2134?).

In addition to this, there are various operational amplifiers with the same pinout, so, with the help of a cheap IC socket, it is possible to swap amps and target the preferred sound.

Panel mount components have dedicated pads for soldering on the PCB.

With respect to the original drawing, power supply has been increased to 12V DC, center positive. This will likely alter the circuit frequency response with respect to the one published on EP forum, but made it possible to include in the circuit a inverse polarization protection diode without the risk to underpower the operational amplifier.

Even if there are no physical constrains, all components where placed as far as possible from the pickups (as close as possible to the name plate then) in order to avoid interferences.

All the unused PCB area is covered with copper and grounded (ground pour) to shield against electromagnetic interference (ok, this is best practice, but it was worth mentioning :) ).

The preamplifier assembly and installation is very easy.

The first thing to do is the preamplifier PCB assembly:

• Start by populating the preamplifier PCB, from the lower in height components to the tallest
• Lock the populated preamplifier PCB behind the faceplate
• Lock the other connectors in their places behind the faceplate
• Wire the LED, output connector, power supply connector and switch (Do not lock the audio output connector yet because it needs the bracket from the genuine faceplate in order to be correctly assembled, see later).

Then install it in your piano:

• Remove the Stage Piano top cover
• Unplug the audio cable coming from the harp
• Lift the arp to gain access to the face plate area
• Unscrew the bolt used to firmly keep the output connector on the genuine face plate
• Remove the whole face plate by unscrewing the 4 screws that keep it in place
• Put the new, assembled faceplate/preamplifier in place
• Place the metal frame that was previously holding the original 1/4" output connector back in place in order to adeguately sustain the new output connector
• Plug the audio out cable to the dedicated RCA connector on the preamp circuit
• Lower the arp
• Put the top cover back in place

... harder said than done: the whole operation will take no more than half an hour!

Components values are silk screened on the PCB, so there's no need for a components map.

Gerber files for both the faceplate and preamplifier PCBs are hosted >>HERE<< (Github)

My projects are free and for everybody. You are anyway welcome if you want to donate some change to help me cover components costs and encourage the development of new projects.

>>HERE<< is my paypal donation page, just in case you want to push the development of new projects ;)

PCB manufacturing of both preamplifier and aluminum faceplate object of this article was sponsored by JLCPCB, a high-tech manufacturer specialized in the production of high-reliable and cost-effective PCBs.

Their customer service is fast and helpful and PCBs a great value for your money!

By registering at JLCPCB via THIS LINK (affiliated link) you will receive a set of coupons for your orders. Registering costs nothing, so it could be a nice way to give their service a due try ;)