Author Topic: Brand new still sealed ''The Rhodie'' preamp  (Read 7456 times)

Offline sean

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Re: Brand new still sealed ''The Rhodie'' preamp
« Reply #30 on: April 21, 2017, 04:03:04 PM »
DC Rex -

Well, we can't yet confirm the component values in the original Rhodie preamp with 100% certainty.  However, we have some great data from Janne.  Janne measured the caps in her Rhodie with a multimeter, and recorded C3 =.002uF, and C4=.005uF.  These values may be 100% the same as Gresco designed 37 years ago.  However, none of the literature for the Rhodie indicates an EQ scoop, nor any removal of mud around 200Hz - just bass and treble +/-15dB EQ. 

Using non-equal values for C3 and C4 has a drastic effect on the frequency response plots, and makes it impossible to get flat EQ response.  HOWEVER, I have not personally built the two circuits to compare them yet.  Since I have not heard the difference with my own ears yet, I can't make a decision nor provide advice yet.  Pnoboy may be 100% right.

My circuit has been slightly modified from what Janne provided in that I chose equal values for C3 and C4.  I chose to use 4.7nf capacitors (an easy-to-find standard value nowadays) after monkeying around with values between 2 and 5 nf (.002uf and .005uf) in a circuit simulator.

Using either Janne's or my own choice for values of the bass EQ capacitors, the Rhodie appears to have the bass and treble EQ regions significantly overlapping.  The Rhodie treble control can affect audio frequencies starting below 200Hz; and the Rhodie bass control affects low frequencies but still influences frequencies well above 200Hz.  This may be very musically useful with a Rhodes piano.

The Janus I preamp (as shown in figure 11-1 in the service manual) has a lot more gain than the Rhodie, and looks to have a 3dB/octave roll off above 4KHz.  The Janus I treble and bass controls don't seem to interact as much, and the EQ regions seem well-separated.  The Janus I bass control is effective below 400Hz; and the Janus I Treble control is effective above 400Hz.  Well, the 3dB point for the bass control seems to be 240Hz, and the 3dB point for the treble control looks to be 580Hz.  You can indeed get flat EQ response with both knobs set to their center point on the Janus I.

Notice that the Janus I and 100Watt-Suitcase preamp (figure 11-1), the Peterson Preamp (figure 11-8), and the Jordan preamp (figure 11-17) all have symmetrical bass and treble EQ with no midrange scoop.  This is why I prejudicially assumed that the Rhodie preamp would not have the very wide 4dB dip in response centered around 200Hz but extending all the way up above 1KHz (as caused by using C3=2nf, and C4=5nf). 

All the common Fender Guitar amplifiers have a very aggressive midrange scoop.  Even if you crank the midrange to 10, you still get midrange scoop unless you turn bass and treble to zero.  I guess we also have to remember that room acoustics and amplifier volume settings greatly change the need for EQ.  I don't feel much need for EQ in my headphones, but I can't live without EQ in the basement with the amp cranked up.

Give me a few more weeks, and I will build a few EQ circuits to compare.


« Last Edit: April 21, 2017, 04:09:01 PM by sean »

Offline pnoboy

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Re: Brand new still sealed ''The Rhodie'' preamp
« Reply #31 on: April 22, 2017, 08:27:08 AM »
I happen to have a Rhodie preamp as well that I plan to have installed in my '78 suitcase Rhodes at some point as the primary EQ circuit. Have never been very satisfied with the Janus EQ. Does the attached schematic have mods done to the original Rhodie preamp and is this frequency response the one of the modified Rhodie? Or is this an original Rhodie with no mods? I'm a little confused.

By changing a few caps and resistors in your Janus preamp, you can make the tone controls work like the ones in the Rhodie preamp.  Have you done a side-by-side comparison of the Rhodie to the Janus?  I wouldn't expect much difference in the sound.