Author Topic: Rhodes Preamp Bypass  (Read 119 times)

Offline 5077

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Rhodes Preamp Bypass
« on: February 10, 2018, 05:30:09 PM »
Hey EPers,

I have a Rhodes Mk I Stage 73 (ca. late '77/early '78) and I have to say that the passive electronics are...not great.  I've always preferred the sound I get coming directly from the harp via the RCA and as such usually use an RCA-to-1/4" cable from the harp to my amp or pedals.  This makes the lid a bit crooked, and not really usable to stack other keyboards on.  It also kind of looks shitty. 

I'd like to just bypass the electronics, and while I'm at it, the RCA altogether--if possible.  The RCA has two wires connected to it; the 1/4" input has two wires connected to it--can I resolder the wires from the pickup rail and connect them directly to the 1/4" input?

Of course, this would mean I can't use the volume and bass boost knobs, which is fine--I can EQ elsewhere in my setup, and I go directly into a volume pedal.  I also realize that I won't be able to easily get back to the "origianl tone"--I'm also ok with this.  I've thought about it and I cannot imagine a situation where I would want/need the tone of the passive electronics.  Directly from the pickup rail is superior in all ways in my opinion.

Would it work?  Should I do it?

Online pnoboy

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Re: Rhodes Preamp Bypass
« Reply #1 on: February 11, 2018, 06:12:07 PM »
I wouldn't be hard, but if you send the signal to a volume pedal, it would need to have a buffered input, or you'll run into the same problem.  I think all you have to do is to remove the ground wire from the volume pot, and turn the volume and bass knobs all the way up.  The rest of the wiring can remain in place, and you can undo your mod very easily.

Offline sean

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Re: Rhodes Preamp Bypass
« Reply #2 on: February 13, 2018, 12:19:33 PM »


Why not just make an 18" cable with an RCA plug on one end, and a switchcraft 1/4" jack on the other? 

Pull the Rhodes RCA cable out of the harp connector, and flop the cable down beside the harp support.  Unscrew the 1/4" jack from the namerail, and push it out of the way (let it hang there behind the name rail).

Plug your new cable into the harp, and put the 1/4" jack through the hole in the namerail.  The Rhodes still looks original (the knobs don't work), and you have your direct out.

If you want to change it back to original, it will take you two minutes.

Sean

Offline 5077

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Re: Rhodes Preamp Bypass
« Reply #3 on: February 14, 2018, 08:05:25 PM »
Ok, yeah, I can see how sean's idea could work, and although my electrical savvy isn't sophisticated enough to know how (or rather, why) pnoboy's idea would work, I still have one question:

Part (but only part) of the reason for disconnecting the RCA and going direct from the pickups to the 1/4" jack is to eliminate the RCA which is based on my vague sense that the RCA connection degrades the sound somehow--is that true?  Or is there no difference between the fidelity of an RCA and a 1/4"?  And if there is a difference, how big of a difference?  Am I splitting atoms by insisting there be no RCA?

Offline sean

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Re: Rhodes Preamp Bypass
« Reply #4 on: February 15, 2018, 02:27:09 AM »

I don't believe you are splitting atoms.  You are inventing unicorn leprachaun atoms.

The RCA connector has been providing hi-fidelity audio connections for more than seventy years.


In situations where you don't have to worry about a cable getting yanked out, or needing strain relief, or care that the hot gets connected before the ground, or are not too picky about corrosion resistance, a plain-old RCA jack is great.  Most folks that take their signal straight from the harp still use the factory-original RCA jack.  Some folks get fancy and pay for a gold-plated RCA jack.  Some folks mount a 1/4" jack on the harp.  None of these are going to change your quality of life in any significant way.

However, the reason that some people choose to take their signal direct from the harp (and avoid the bass boost and volume pot on the name rail) is because their choice of amplifier (or preamplifier) doesn't perform well with the Rhodes 10KΩ volume pot across the input.

Many of the old-fashioned tube amps have high-value resistors across the inputs before the first tube.

e.g., the Fender Twin circuit (schematic URL AWOL) shows that if you plug your Rhodes into Jack 1, the first component it hits is R1 - a 1-MegΩ resistor.  If we plug our stage Rhodes into this jack, then we have put the 10KΩ volume control in parallel with this 1-MegΩ resistor.  Even if we have the volume set to max on the namerail, this 10KΩ in parallel with the 1-MegΩ resistor brings the combined value down to 9,901 Ohms.  This greatly reduces the (already low) signal strength of the Rhodes before it hits the first tube.

This is the reason for the notorious tone suckage of the Stage Rhodes into some tube amps.  This is also the reason that bypassing the namerail circuitry fixes the problem.  If you come straight off the RCA jack at the back left of the Rhodes harp, then you have no 10K Ohm potentiometer to be in parallel with the input resistor on the amp.

Sean


I will have to find the online fender schematics and update the old post at https://ep-forum.com/smf/index.php?topic=6868.msg34468#msg34468

Online pnoboy

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Re: Rhodes Preamp Bypass
« Reply #5 on: February 15, 2018, 12:26:16 PM »

I don't believe you are splitting atoms.  You are inventing unicorn leprachaun atoms.

The RCA connector has been providing hi-fidelity audio connections for more than seventy years.


In situations where you don't have to worry about a cable getting yanked out, or needing strain relief, or care that the hot gets connected before the ground, or are not too picky about corrosion resistance, a plain-old RCA jack is great.  Most folks that take their signal straight from the harp still use the factory-original RCA jack.  Some folks get fancy and pay for a gold-plated RCA jack.  Some folks mount a 1/4" jack on the harp.  None of these are going to change your quality of life in any significant way.

However, the reason that some people choose to take their signal direct from the harp (and avoid the bass boost and volume pot on the name rail) is because their choice of amplifier (or preamplifier) doesn't perform well with the Rhodes 10KΩ volume pot across the input.

Many of the old-fashioned tube amps have high-value resistors across the inputs before the first tube.

e.g., the Fender Twin circuit (schematic URL AWOL) shows that if you plug your Rhodes into Jack 1, the first component it hits is R1 - a 1-MegΩ resistor.  If we plug our stage Rhodes into this jack, then we have put the 10KΩ volume control in parallel with this 1-MegΩ resistor.  Even if we have the volume set to max on the namerail, this 10KΩ in parallel with the 1-MegΩ resistor brings the combined value down to 9,901 Ohms.  This greatly reduces the (already low) signal strength of the Rhodes before it hits the first tube.

This is the reason for the notorious tone suckage of the Stage Rhodes into some tube amps.  This is also the reason that bypassing the namerail circuitry fixes the problem.  If you come straight off the RCA jack at the back left of the Rhodes harp, then you have no 10K Ohm potentiometer to be in parallel with the input resistor on the amp.

Sean


I will have to find the online fender schematics and update the old post at https://ep-forum.com/smf/index.php?topic=6868.msg34468#msg34468

A 1meg input impedance does not suck tone or volume.  The effect of the 1meg input resistor is miniscule by comparison to the 10k resistor of the stage piano's volume control--a 1 meg resistor is 100 times bigger than a 10k.  It's the 10k volume-control pot that is the source of the volume loss and tone change--the input impedance of almost any guitar amp is much higher than 10k and will not affect the tone and/or volume of the piano.

Offline sean

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Re: Rhodes Preamp Bypass
« Reply #6 on: February 15, 2018, 06:20:33 PM »


Yeah.  Correct.  The 10KΩ pot is the culprit.  I guess it was unclear without the schematic.

See http://ampwares.com/schematics/Twin_Pro_Tube_Amp_SchE45.pdf

The Rhodes volume pot in parallel with the 1MΩ R1 is the unhappy circumstance.

Sean