Repairs, Maintenance & Upgrades > Preamps, Modifications & Upgrades

An alternate Volume/Tone control circuit for Mk1 Stage EPs

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sookwinder:
I have had my 1973 Mk1 Stage Rhodes for just shy of a year now. Love it.
What I don't love is the crappy volume/tone controls. I know in Leo's day, everything was done to reduce production costs, for example design attributes in valves amps that we would just take for granted these days (including safety features that lower the risk of destroying the valves) were just not even considered back in the day.

Most likely that mind set was continued once Leo retired and obviously the Rhodes production was not immune.
But seriously the volume and tone controls circuit fitted to my Mk1 stage is amateurish at best.
The tea lady (or coffee lady) would have had a better chance of designing volume and tone controls for the Stage Rhodes.

There are many issues that arise.
The circuit is noisy.
Most of the signal is lost to ground because of ridiculous component (size) choices.
The bass boost circuit really doesn't do what its name suggests or at least does it partially but at the same time has a detrimental affect on the volume.
The circuit disobeys fundamental design rules as far as output/input impedance when connecting the Rhodes to a standard guitar amp, which causes reduction in frequency response  (ie. make it sound like there is a wet blanket over the speaker).

In short it actually causes the player to rely on the control on the amplifier to fix up the short comings of the signal coming out of the Stage Rhodes.

So given we have a long weekend here in Oz, I decided to investigate.

Initially I just set up a cable connected directly off the harp RCA through to the Phono jack on the name rail, bypassing all of the circuit.  Yes this did seem to improve the frequency response, but it also seemed to add a continuous static, not (white) noise but a static noise. Grabbed another cable and the results were the same. Encouraging, but not a solution.

Next idea was to change the value of the volume pot in the circuit.

One of the members here (Sean) had shown in another thread the following circuit for the Mk1 Stage:



The circuit in my 1973 Mk1 is different again.
I checked each of the pots and they both are 1973 production. 
Thanks to Max from CEPCo for the hand modelling services.


So I built up a wiring loom exactly the same as what appears to have been shipped with the EP back in 1973, except I changed the volume pot to 1M.  Given that many guitars utilise 1M volume pots, this should not have caused any issues.  (Typically Strats with single coil pickups have 250k pots, Les Pauls etc with humbuckers have 500k and sometimes 1M volume pots. Jaguars and Jazzmasters use 1M pots)

While I still felt the frequency response of the output was under par, there was a distinctly noticeable volume increase.

After some deliberation I decided to approach this issue as if I was designing a section of an amplifier.

The design would need to take into account that it needed to be compatible with amps that have high input impedance. (ie the classic valve amps for the 60s and 70s that we all love).  The correct choice of components, in particular the potentiometer for volume control, should allow the vast majority of signal to get to the amp and not be lost to ground.
 
The Bass boost function is a complete waste of time.  I have never known of anyone, be it in a studio or live, stop and say "my Rhodes stage EP does not have enough bass, damn! I need more bass boost".  Every player I know is always reducing the bass controls on the attached amplifier in the vain hope of dialling in some brightness and chime.

So I thought about how one would go about designing a bass roll-off control. It is better to try and get the signal coming out of the Rhodes as close as possible to what you want, rather than relying on the amp to control it.

I went for the classic High Pass Filter route.

For those who were asleep in classes, this is a high pass filter.  As the name suggests it lets through the high frequencies and blocks the lower frequencies.  There is a frequency called the cut off frequency, this is where the signal had dropped 3dB.  It doesn't mean everything below that frequency suddenly vanishes, but rather at that point that is the start of the roll off of all lower frequencies.



I wanted a design that allowed the cut off frequency to be adjustable.
If you wanted everything, that should be available.
If you wanted a little attenuation on the bass notes, likewise this should be able to be dialled up.
And lastly if you wanted the bass way down this should also be achievable.

Half a day of experiments and some calculations led me to the following design:

 , where the relationship between R1 : R2 is about 1:4.5 or 1:5. 
I had chosen the Volume pot to be 1M and the Bass roll off pot to be 250K, which meant that (using the standardised fixed resistor values) R1 could be either 47K or 56K.
The relationship between the combined R1+R2  (remembering that R2 is variable) and the capacitor then gives the range of the possible cut off frequencies.


I also added an extra function, using a CTS 1M pot that has an inbuilt switch. The switch is a SPST, and is independent of the functionality of the rest of the pot.  I.e. it does not turn the pot on or off, but rather the switch is activated by pulling the shaft/knob out and deactivated by pushing it in and allows 'something else" to be activated by this SPST switch.  I used it to have an additional capacitor added or removed from the circuit.  The circuit has two possible ranges of cut off frequencies (that overlap slightly).



The table below shows the cut off frequency relative to the percentage of Bass Roll-off control being selected. (and for each of the two possible - useable - values of R1 (47K or 56K)


Here is the layout drawing and some installation photos.
Note 1: there is just enough room between the harp and the name rail, at the far left, to be able to carefully locate the larger CTC 1M pot (with switch) without the need to remove the name rail
Note 2: you will need to cut off (with a hack saw) about 5mm of the shaft for the CTS 1M pot. Take about 10 minutes from go to whoa.  For what ever reason these CTS 1M pots with the switch feature have extended shafts.






So how does it sound... well great .. just what I hoped for.
The signal level is much louder, there is less (white) noise, no static noise and the bass cut off function is very easy to operate.
Fully counter clockwise the is no bass roll off (the starting frequency at those position is 26Hz, below the piano, below your ears).
Then just rotate the knob clockwise to gradually reduce the bass notes relative to the higher notes.
What I particularly liked is that I can set it up so I can clearly hear in the signal the physical striking of the tines on the upper register keys. This has often been overwhelmed by the lower notes.

The circuit is in the Rhodes and is staying put.
Enough said.

Time for dinner

David






jimbobothy:
Nice, if I ever get a Rhodes I’ll be doing this mod  8) Thanks for the detailed post.

Tim Hodges:
Funilly enough I was going to put up the schematic for the older stage preamp with the 1UF capacitor last week but forgot to.

Excellent write up for the alternative, will give it a go.

d-rock:
Excellent!

I think we all struggle with the low volume and noise coming out of our Rhodes. I've been boosting mine with a preamp pedal (Tech21 Liverpool) and have gotten good results but there is still a good bit of hiss that I just live with. This is really nice.

pnoboy:
I'm going to express a contrarian viewpoint, and I hope I can do so respectfully.  The OP presented 2 Fender stage-piano schematics--one hand drawn and one printed.  The printed one represents what Fender used for most of the life of the stage piano, and, except for the component values, I think that it is quite a good circuit, and better than the one designed by the OP.  Before I explain my opinion, let me mention my experience with my Rhodes stage pianos.  I have owned 2--the second of which I currently own.  In both cases, I found that the pianos could sound tubby if not properly equalized, and the tubbiness originated in the 100 Hz to 200 Hz frequency range.  Lower down the bass end of the piano, less equalization was required.  I think this results largely from the fact that most guitar speakers severely roll off bass as one goes below 80 to 100 Hz. 

Therefore, my experience was that a bit of a reduction in the 100-200 Hz range without too much reduction further down in the bass was just the ticket for good sound.  Of course, the amps, speakers, and speaker cabinet have lots to do with the sound of the piano, so my experience can't be considered universal.  I gigged with the 1st piano, and played it through 2 12" speakers in an open-back cabinet.  I think that speaker arrangement is pretty typical for many Rhodes players.

The Fender tone control provides what is often called a shelving high-pass filter.  The capacitor and volume-control resistance makes the high pass, and the resistor across the capacitor creates the shelf.  BTW, Fender calls this control a “bass boost,” but that’s just some marketing nonsense—it’s actually a bass-cut control.  In fact, it's kind of a 1/2 bass control.  Whereas most bass controls go from cut at one extreme, to flat in the middle, to boost at the other extreme, the Fender control goes from cut at one extreme to flat at the other.  Consider the image below .  It shows the frequency response with the bass control set for full cut and partial cut.  At the partial cut shown, it reaches a shelf of -6dB, where it flattens out.  At full cut, it's heading for a shelf, but doesn't quite get there at 20 Hz.
I find this type of control very useful.  It can cut the mid to upper bass frequencies without completely suppressing the lowest frequencies.  The OP's circuit cannot do this--as a simple high-pass filter its, bass rolloff never stops.  I have found that I never need that type of frequency shaping with my Rhodes pianos. 

As to the selection of component values.  It seems that Fender likes to suppress the sound of the hammers hitting the tines--I think of it as a ping or ting.  I say this, because, in general, Fender has used low input impedances in their pianos.  The Peterson preamp has a 33k input impedance, the stage pianos has a 10k input impedance, the early Janus preamps have a 10k impedance, whereas the late MkII preamps had a 47k impedance.  If one wants a higher impedance to let through more of the ping, the components used in the Fender schematic can be scaled as long as the RC product remains the same.  For example, one can multiple the potentiometer values by 10 and divide the capacitor's value by 10--hence, a 100k volume pot, and 500k tone post, and a 4.7nF capacitor.
The OP also mentions static or hiss.  I have never encountered this with my stage pianos--they have all been very quiet.  I wonder if the OP's problem relates more to his amp than piano?  The 10k pot does reduce amplitude, but if increased to 50k or more, that problem is resolved.  In short, I have no real criticisms of the Fender circuit--it does what the manufacturer wanted it to do in an efficient way.  In that sense, I think it's rather an elegant design.  If you want more ting in your sound, scale the pots up and the capacitor down. 

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