Brand new still sealed ''The Rhodie'' preamp

[Tehu]

This weed-end, i've been lucky to find out a still sealed, brand new ''The Rhodie'' preanmp for sale in Montreal. I bought it and paid 50$ for it.
Since i'm planing to use it, I unpack it very carefully.
Here are picture of the packaging and of the unit unpacked.

[Tehu]

Since I have a suitcase with Peterson Preamp, i don't find it very usefull for me on my Rhodes, but it give a good boost on the sound of my passive Elepian.

Also, the design is hided, everything colored in black, plastic plate on the other side.

[Ben Bove]

That's a cool find!

[laberge]

This weed-end

hehehe...

but really there's probably some people in this thread: http://ep-forum.com/smf/index.php?topic=4291.0 that would really love to see more pics/hear more on that elepian piano you have there. Those are like Mark III Rhodes rare...

[Tehu]

I started a thread about my elepian, with pictures, sounds demo.. tips for adjusting not long time ago, it didn't get much enthusiasm!

[Tehu]

The Elepian thru The Rhodie preamp:

https://www.youtube.com/watch?v=2juT0DX4dtU&feature=youtu.be

[JanneI]

Hi all,

Has anyone back engineered this "rhodie preamp"? Or are there schematics somewhere? All the components are painted black. I found one in a mk2 stage 73 (plastic keys, 2482), first one that I've come across here in Finland. It's not working and since I've never heard one I'd really like to repair it! All help appriciated!



Cheers,
Janne

[The Real MC]

You can start by replacing that rusted 9-volt assembly, telltale signs of acid leakage render them useless.

[JanneI]

Of course..but I was hoping to get a veroboard diy version of this quite simple preamp. Seems like a couple of tantalums, diodes, 1 ic and a bunch of resistors..

[Tehu]

Hi,

The design is hidden, and even in the original packaging, no information!
Maybe try to carefully remove the black paint.

[JanneI]

Thanks! I was hoping that someone somewhere would have back engineered this. Seems like a IC, resistors, two tantalums, couple of diodes. My version is opened from the bottom side, so all the pcb traces can be seen. Maybe someone was trying to fix it before.

Does anyone have a soundclip of this preamp somewhere? I'd like to hear it in action before ripping it apart for "inspection"

[pnoboy]

You really don't need to back engineer it if you have the equipment  to measure frequency response.  If you can, measure its frequency response with the bass and treble controls set to various values.  Then measure its input impedance.  At that point, you'll know everything of importance.

[Tehu]

I'll try to do a demo with my Rhodes to show the sound

[JanneI]

That would be great! And please record same straight from the harp rca for reference!

[Max Brink]

Very cool to find one in the original packaging!!!

Has anyone back engineered this "rhodie preamp"? Or are there schematics somewhere? All the components are painted black.

I'm not an electrical engineer, so I can't explain that is going on at a theoretical level, but my understanding of the circuit is that it is a very basic IC preamp. I'm pretty sure with a little reverse engineering the whole thing could be "photocopied" for under $10-15 of parts...

[JanneI]

Hi all!

Does anyone know where to find those old Bourns potentiometers? The volume pot is broken and the legs are little peculiarly shaped on these pots. Since the pcb is mounted with the pots to the name rail, I wouldn't want to use different type of pots... if there's no exact replacement pot available, then there's no option.

All help appreciated, thanks!

Cheers,
Janne

[sean]

http://www.potentiometers.com/select_Bourns.cfm

http://www.mouser.com/Bourns/

Are there any markings on the pots installed on the preamp?

R.A. Gresco is probably still alive.  Anybody in California know what he is up to now? 

Sean

[Tim Hodges]

http://www.potentiometers.com/select_Bourns.cfm

http://www.mouser.com/Bourns/

Are there any markings on the pots installed on the preamp?

R.A. Gresco is probably still alive.  Anybody in California know what he is up to now? 

You could probably get in touch with him through their Facebook page:

RA Gresco Guitars

It's run by Roger and his nephew Kyle.

[Jenzz]

Hi .-)

I had a Mk1 witrh the Gresco installed some years ago...

The pots are Bourns 81/82 series pots... Since there is a 'RoHS compliant' marking on the datasheet, these seem to be still available...

See attachment...

Jens

[JanneI]

Thanks! I turned out that the pot wasn't broken, problem was elsewhere, now solved. I did some backengineering on that rhodie preamp and found out that it's really a basic active eq with one ic. I lifted and measured the resistors and capacitors, and drew a basic schematic.

[JanneI]

Here's my unverified veroboard layout. I figured that since I don't have much time I could just share this if someone likes to try it out. A wise move would be draw a pcb with 9mm alpha pots with same mounting holes as in mk1 panel.. Maybe I'll do it someday

[sean]

Very nice job, Janne!

It appears to be based on the tone control circuit designed by P.J. Baxandall and published in 1952 (originally with tubes).

See http://www.learnabout-electronics.org/Downloads/NegativeFeedbackTone.pdf if you want to start with the original history. 
TI has a good application note showing how to build it with their op amps http://www.ti.com/lit/an/sloa042/sloa042.pdf.
There is also a good writeup about using the circuit in a passive network with no amplification at http://makearadio.com/tech/tone.htm.

I will build it on a breadboard, and see how I like the sound and the crossover point.

Your schematic shows really small values for C7 and C2, but your board layout drawing shows very reasonable 10uF value for C2.  Is that just a typo on the schematic? 

Also, the 50K volume control should be an audio-taper potentiometer.  (The bass and treble pots in the feedback network should be linear.)

Sean

[pnoboy]

Very nice job, Janne!

It appears to be based on the tone control circuit designed by P.J. Baxandall and published in 1952 (originally with tubes).

See http://www.learnabout-electronics.org/Downloads/NegativeFeedbackTone.pdf if you want to start with the original history. 
TI has a good application note showing how to build it with their op amps http://www.ti.com/lit/an/sloa042/sloa042.pdf.
There is also a good writeup about using the circuit in a passive network with no amplification at http://makearadio.com/tech/tone.htm.

You lose 20dB of signal passing through the Baxandall network, so you need the op amps to make up that signal loss.

I will build it on a breadboard, and see how I like the sound and the crossover point.

Your schematic shows really small values for C7 and C2, but your board layout drawing shows very reasonable 10uF value for C2.  Is that just a typo on the schematic? 

Also, the 50K volume control should be an audio-taper potentiometer.  (The bass and treble pots in the feedback network can be linear.)

Sean

There is about a 20dB loss using the"James" tone control circuit, which is like the Baxandall circuit, except the components are placed at the input of the following tube/transistor/op amp.  The version shown in the schematic is placed in the feedback loop of the op amp and has no loss at all.   It's a very standard topology--nothing special at all.

[sean]

I built the circuit in a circuit simulator (I used TINA-TI, based on Designsoft's Spice-like kit, free download from Texas Instruments).

There is the TL;DR summary:
1.  The EQ caps should match, or you get asymmetrical response curves and no way to set it for flat response.
2.  The isolation caps in Janne's unit are kaput.  They should be replaced with new 10uf caps.

Here is my revised schematic (same topology as Janne's drawing, just drawn differently):

[sean]

Below is a plot showing the expected frequency response for the circuit I just posted.  With the component values shown in the schematic, you get a lot of EQ within the range of fundamental frequencies of the notes on a 73-key Rhodes.  A few piano notes are shown on the graph for reference.

[sean]

Two more diagrams as attachments.

First one shows why you should not use Logarithmic Audio-Taper Pots for the bass and treble controls in a Baxandall circuit.

Second diagram shows the curious behavior when the capacitors are mis-matched in the Baxandall circuit.

[pnoboy]

One thing I do like about this preamp is that the designer selected the values so the frequency around which the controls works is at 200 Hz, which is often a place where a Rhodes can sound a bit muddy, and where one would like a dip in the frequency response.  With the circuit set up as it is, turning both the treble and bass controls up the same amount effectively creates a dip in the response right at 200 Hz.

[DynoCzarus Rex]

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. 

[JanneI]

Nice job Sean! I'm no expert by any means, so I'm happy to see someone more qualified to continuing this.

I measured the caps with a semi-cheap DMM, so the cap measurements are "guidance only" -values (and the C2 obviously wrong in the schematic: NOT 11nF, 11uF). My idea was to breadboard this and try different components, but since you already did this and even showed plots that shows how it clears the 200Hz "muddy-area", I feel that there's no need for more prototyping. Thanks a lot!

One thing that I didn't remember to measure was the dimension between the the pots, does anyone have this? I think it's the same dimension as the mounting holes in MK1 name rail? Would be fun to draw and order a prototype pcb, even though it's just a basic eq... but it might be hard to find a concentric 1M pots? I think Bourns have them on Mouser.

Best,
Janne

[JanneI]

I found this on my cellphone, here's the original note from where I made the schematic. I think those caps were 10uF tantalum caps.

[sean]

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.

Sean

[pnoboy]

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.

[gacki]

(resurrecting a very old thread...)

Well, we can't yet confirm the component values in the original Rhodie preamp with 100% certainty.

Actually, now we can.
I'm currently working on a Rhodes with such a Rhodie preamp; and an evening with a bottle of acetone yielded the following results:

C3: 4.7nF
C4: 2.2nF
C5: 2.2nF
C6: 2.2nF

(all also confirmed with measurement)

C1 and C2 are of course 10uF/10V.

[sean]

gacki -

Awesome!  Thanks.

Are you numbering the caps the same as in the link below?
https://ep-forum.com/smf/index.php?topic=8992.msg51947#msg51947

I ask, because your values for C3 and C4 are opposite of what Janne measured.

Sean

[gacki]

I'll recheck that tomorrow against my own drawing; but the 4.7n cap is the one Janne identified as 5n in the drawing of the placement of the parts. I'll probably upload an annotated picture of the bottom traces.

[gacki]

I've re-checked it and it seems to me that Janne's original schematic has swapped the values for C3 and C4.

The values align well with https://ep-forum.com/smf/index.php?topic=8992.msg52107#msg52107; on my unit the cap next to the volume potentiometer is the 4.7n cap (and on Janne's component drawing it's 5n). So the position checks out. However as we can see from the traces on the bottom this cap is connected to R6 and not R11.

[drpepper]

Here's my unverified veroboard layout. I figured that since I don't have much time I could just share this if someone likes to try it out. A wise move would be draw a pcb with 9mm alpha pots with same mounting holes as in mk1 panel.. Maybe I'll do it someday

Thank you for sharing this Janne. I built this circuit and it works well. I did spot a little error that the C2 negative leg needs to go to pin 7 of the opamp.
I made a couple of tweaks adding a switch to hear the difference between some of the cap values posted here. 

I have posted what I modified, though I have just butchered the previous version in paint.

I also posted a little video of it working here:
https://youtu.be/3hgq8O1YwS4