Clean boost pedal for Rhodes

[Adam91294]

Hi I was wondering what some of your guys thoughts on clean boost pedals for the fender Rhodes, I have been looking at the mooer pure boost and joyo ironman let me know what you think 
I'm not looking to spend over a hundred on a clean boost in less there is one you guys absolutely recommend
Thanks,
Adam

[pnoboy]

Any decently designed boost pedal should be fine.  This type of pedal is the simplest of all pedals, and if you spend $100 or more on one you are being ripped off, IMO.  I'd start by looking on ebay or craigslist.

[The Real MC]

if you spend $100 or more on one you are being ripped off, IMO.

You'd be wrong, as the cheap boost pedals are not high impedance and will suck tone.  To get optimum tone you want input impedance of AT LEAST five megohm.   

My Countryman Type 10 gets a nice hot signal and the high input impedance really brings out the Rhodes tone.  My other boost devices just sucks the tone.

You may only want to spend $100 but this is not at all like a guitar.

[pnoboy]

 Well, I did say decently designed--a low-impedance clean boost is not decently designed, and not suitable for guitar or Rhodes.  However, the requirement for 5 megohms is excessive.  Many classic tube amplifiers, such as those from Fender and Marshall, use 1 megohm input resistors.  I might add that the Janus Rhodes preamp had a 10k input impedance, and the older Peterson preamp had about a 30k input impedance, and many people think they sound good.

A quick look around suggests that most boost pedals have an input impedance of 1 meg, with at least one as low as 300k, which should be fine for a Rhodes.  If you had bad luck with a boost pedal it may well have been for reasons other than input impedance.

[pnoboy]

I should add that the stage piano uses a 10k volume pot, so unless you bypass that circuit, the use of a clean boost with an input impedance of any reasonable value will not change the tone of the piano.  It's clear that Fender did not consider high impedance to be important.

[Tim W]

Pnoboy is correct in his reasoning. The math doesn't lie. The 10k volume pot already sets the load on the harp. Anything an order of magnitude higher (100k) will have a negligible effect on a stage pianos level or frequency response. Unless, of course, you have an extremely long cable run or high capacitance cable, which is unlikely.  Even standard grade guitar cable (rapco, etc) is perfectly fine and won't introduce excessive capacitance in this application.

One could build a booster circuit with the correct components for $10 or less and an hour of time.  if done properly it would sound great and be low noise.

Sorry, but I am one of those people who believes everything in audio can be measured. Our test equipment is more accurate and sensitive than our ears. Our hearing perception can easily change if we are tired, hungry, etc. or even if we want to believe we are hearing something we want to hear.

A lot in the world of audio is specsmanship. But one must fully understand the conditions under which those specs were generated... And if they are really meaningful for your application.

[The Real MC]

I'm an EE and you folks are neglecting a few things.

Parallel impedances.  Just because you see a 1M resistor on the amp does not imply a 1M impedance.  You are neglecting the input impedance of the input tube.  Because the impedances are in parallel, the effective impedance is LOWER than the 1M resistor.

Voltage divider law.  The 10K volume pot in the passive stage piano circuit on top of a 100K impedance results in a loss of 1/10, worse with increasing frequency.  That is not "negligible".

Feedback network and circuit design in the preamp circuit.  Do not be deceived that the Peterson/Janus preamps sound good because you see 33K or 10K resistors in the schematic.  In the Peterson circuit you have neglected the feedback network in the preamp circuit that is a pre-emphasis system designed to make up the lost treble from low input impedance.     That will be an important element missing from a booster.  While the pre-emphasis does its intended purpose, it is done at the expense of other frequencies and is an unavoidable consequence of the approach. 

As with the 10K in the Janus circuit, that resistor not the input impedance.  That 10K resistor is in series with the opamp inverting input therefore input resistance is much higher taking into account the feedback network and the input impedance of the opamp itself.

That 10K volume pot on stage pianos loads down the pickups and sucks the tone.  Try the signal right off the RCA jack on the harp and you will hear the difference.

Don't knock 5M input impedance as "excessive" until you have tried it.  I tried boosters with 1M input impedance and they don't compare.  I know this from experience.  EQs and pre-emphasis networks will not restore harmonics that are lost from input impedances.  You cannot boost what isn't there.  With the 5M input impedance there is far better signal integrity.

Sorry not everything in audio can be measured on test equipment.  Our ears have psychoacoustical processing that test equipment does not measure.  A good example is resultant tones.  If you combine two sine waves one a fundamental and the other a fifth above, your ears hear a tone an octave below the fundamental which is a resultant.  It's an old pipe organ trick.  If you performed a spectral analysis on the resultant, the analysis will show the two original independent harmonics but not the harmonic an octave below.

[pnoboy]

Wow--there are so many things in your post to respond to.  Your post is in italics, and my responses are in regular type.

Parallel impedances.  Just because you see a 1M resistor on the amp does not imply a 1M impedance.  You are neglecting the input impedance of the input tube.  Because the impedances are in parallel, the effective impedance is LOWER than the 1M resistor.

Correct--however, the input impedance could not be higher than 1 meg, which was the important point.  Also, tubes have a very high input impedance.  I quick internet search suggests that a 12AX7, commonly used by Fender, has an input impedance of about 5 megs, so the real input impedance of the stage would be about 833k ohms.  Of course, if one wanted to be rigorous, one would have to consider the complex input impedance and take into account the Miller capacitance, but that seemed well out of the scope of the discussion.

Voltage divider law.  The 10K volume pot in the passive stage piano circuit on top of a 100K impedance results in a loss of 1/10, worse with increasing frequency.  That is not "negligible".

I never claimed that the 10k volume pot had a negligible effect on the piano's tone, only that unless one removed it from the circuit, the input impedance of a buffer that was, let's say, 100k or higher would have a negligible effect on the impedance seen by the harp.  Additionally, what 100k impedance are you talking about?  Are you claiming that the harp has an output impedance of 100k?  If so, at what frequency does it's output impedance equal 100k? One of these days, I will measure the complex impedance of the harp, but not today.

Feedback network and circuit design in the preamp circuit.  Do not be deceived that the Peterson/Janus preamps sound good because you see 33K or 10K resistors in the schematic.  In the Peterson circuit you have neglected the feedback network in the preamp circuit that is a pre-emphasis system designed to make up the lost treble from low input impedance. 

I never claimed that the 33k input impedance made the piano sound good, only that it apparently did not destroy the tone.  If the 33k input impedance decreased the treble as you claimed, it would have been far cheaper for the Fender company to have used a higher input impedance and gotten rid of the pre-emphasis circuitry.

As with the 10K in the Janus circuit, that resistor not the input impedance.  That 10K resistor is in series with the opamp inverting input therefore input resistance is much higher taking into account the feedback network and the input impedance of the opamp itself.

Sorry to be blunt, but you're dead wrong about this.  Check this schematic, http://www.fenderrhodes.com/org/ch11/fig11-1.jpg.  You'll see that the input stage is a simple op-amp inverter.  As such the 10k resistor goes to a virtual ground, which is the summing junction at the op amp's inverting input.  Therefore, the input impedance of this circuit is exactly the impedance of the resistor as long as the op amp stage has sufficient excess gain, which one would hope the designer assured.

That 10K volume pot on stage pianos loads down the pickups and sucks the tone.  Try the signal right off the RCA jack on the harp and you will hear the difference.

That may be true--I haven't tried it, but it doesn't change the fact, that unless one removes the factory volume control, super high input impedance of a boost circuit will not change things.

Don't knock 5M input impedance as "excessive" until you have tried it.  I tried boosters with 1M input impedance and they don't compare.  I know this from experience.  EQs and pre-emphasis networks will not restore harmonics that are lost from input impedances.  You cannot boost what isn't there.  With the 5M input impedance there is far better signal integrity.

Again, if you specifically specify removing the stage piano's controls, then we would have to consider your premise.  However, I am very skeptical that a 5 meg impedance would sound different than a 1 meg impedance signal booster.  Consider the capacitance of a typical shielded cable that might connect from the harp to the signal booster.  A 6-foot length of typical coax would have a capacitance of about 180 pF.  At 5000 Hz, that capacitance would have an impedance of 177k.  Why would that capacitance not destroy the tone, but a 1 meg resistor would?

[The Real MC]

As with the 10K in the Janus circuit, that resistor not the input impedance.  That 10K resistor is in series with the opamp inverting input therefore input resistance is much higher taking into account the feedback network and the input impedance of the opamp itself.

Sorry to be blunt, but you're dead wrong about this.  Check this schematic, http://www.fenderrhodes.com/org/ch11/fig11-1.jpg.  You'll see that the input stage is a simple op-amp inverter.  As such the 10k resistor goes to a virtual ground, which is the summing junction at the op amp's inverting input.  Therefore, the input impedance of this circuit is exactly the impedance of the resistor as long as the op amp stage has sufficient excess gain, which one would hope the designer assured.

Sorry to be blunt but you have a complete misunderstanding of virtual ground.  By your argument, if the 10K resistor is indeed the impedance then virtual ground is a very low impedance path to ground into the opamp inputs.  This conflicts with the high impedance properties of the opamp inputs.  Virtual ground is the potential at the differential inputs as a result of the balancing action of the opamp and is not necessarily a path to power/signal ground.

[Tim W]

Pnoboy responded before me but I agree with him on all counts.

I too would like to eventually measure the impedance characteristics and frequency response of a harp.

As he correctly stated, going direct off the harp is different than going to the stage tone and volume controls. I stated that with the 10k volume pot in circuit going out to an amp or booster with a 100k input would be negligible. This is because the 10k pot is dominant in loading the harp. The extra 100k would not change things considerably.

Pnoboy is also absolutely correct on the Janus input impedance. To be picky the series cap is also part of it- and causes a high pass response with a cutoff freq determined by the 10k resistor and harp output impedance- but it is probably not important at audio frequencies of interest. Because of virtual ground due to the opamp, the input impedance is 10k. The input current goes around the feedback loop- not into the opamp inputs.

Best regards,

Tim

[pnoboy]

Read it and weep, Real MC, https://en.wikibooks.org/wiki/Electronics/Electronics_Formulas/Op_Amp_Configurations  I am an expert at analog design with op amps, and have 30 years experience doing so.  I have had dozens of successful products released into the marketplace, and am the holder of about a dozen patents in the field of electronics.  I don't know everything, but I do know an awful lot about op amps and analog circuit design.  It's my area of expertise.

[Tim W]

For the record, I am also an EE with 15+ years experience doing custom analog and mixed signal IC (ASIC) design. I have designed many opamps, DACs, ADCs, sensor interfaces, and other analog circuits in bipolar, CMOS, and BiCMOS processes. I also have a lot of chips out there in the medical and automotive markets for companies like GE and Panasonic that are still in production. I know a fair amount about this subject- but I still learn something new every day- and hope to continue doing so.

[pnoboy]

I agree about learning something new every day--electronics is a huge, rapidly developing field, and no one knows it all.

[The Real MC]

Read it and weep, Real MC, https://en.wikibooks.org/wiki/Electronics/Electronics_Formulas/Op_Amp_Configurations  I am an expert at analog design with op amps, and have 30 years experience doing so.  I have had dozens of successful products released into the marketplace, and am the holder of about a dozen patents in the field of electronics.  I don't know everything, but I do know an awful lot about op amps and analog circuit design.  It's my area of expertise.

You will not see me cry because I do not sing the blues.

I have been in the electronics field since 1984 and have obtained a Bachelors degree in Electronic Engineering from RIT in 1988.  I frankly have found too many internet sources of electronics to be in error therefore I must demand better resources that are not online. 

Shall we open my college books?

From "Operational Amplifiers with Linear Integrated Circuits" by William Stanley 1984 printing page 39 it states for the inverting amplifier (identical to the configuration in the Janus) with the non-inverting input tied directly to ground "the inverting terminal is forced to be at ground potential, that is VinvertingInput = 0.  This condition is referred to as a virtual ground, meaning that the inverting terminal is not really connected to ground"

I concede on the input impedance from "Opamps and Linear Integrated Circuit Technology" by Ramakant A. Gayakwad 1983 printing pages 128-129. 

But do not be deceived by virtual ground - not every opamp closed-loop circuit results in virtual ground at the differential inputs.

And I would appreciate it if you would not so quickly dismiss my suggestion of 5M input impedance booster for the Rhodes given that you have not even tried it, much less compared the sonic results to preamps of lower input impedance.  I have other devices that I tried on the Rhodes, I did the comparisons, and my ears do not lie to me.

[pnoboy]

I did not dismiss your claim about high input impedance buffers out of hand, but qualified my response by referring to the 10k volume pot used with the stage piano.  If the pot is in the circuit, then it defies all logic that a 5 meg vs. 1 meg input-impedance buffer could make a difference.  However, I also mentioned that if you had specified removing the pot, then one could entertain the possibility of ultra-high input impedance making a difference.  I do remain skeptical and strongly suspect that some other parameter of your gain booster caused you to prefer its sound.  There may be some subtle difference in its frequency response or its levels of harmonic or intermodulation distortion.  I would ask you not to dismiss these ideas out of hand.

[Chris Carroll]

Wow, a lot of smart guys on this post for real. For the rest of us dummies who are looking for the real deal tube clean boost, I can only suggest the best Clean boost available. Whether it be for the Rhodes (Tine Bomb) installed in the piano  or guitar ( Tube Bomb) in a stomp box  you will not find a better suited clean boost. To this day, I have never met anyone who did not rave about it.  It started out as Rhodes boost pedal but is quickly turning guitar players on all over the world.  We sell this at a very fair price, for those who do not wish to DIY with transistors caps and resistors-

http://www.vintagevibe.com/products/fender-rhodes-stage-tine-bomb-pre-amp?variant=899477483

[David Aubke]

Chris, I noticed on the product page "This product will not work for a Mark II without modification to your piano."
I'm curious what modification is needed.

[Student Rhodes]

This is all great stuff guys.  Perhaps one of you can design a quiet, direct replacement for the Peterson preamp?  Every single one of mine has noise issues that seem impossible to sort out, and I presume are just part of the design.   
I'd pay a couple hundred bucks for one.

[Adam91294]

Hi guys I went with a Mooer boost pedal $50 it works well for my needs thanks all!

[pnoboy]

This is all great stuff guys.  Perhaps one of you can design a quiet, direct replacement for the Peterson preamp?  Every single one of mine has noise issues that seem impossible to sort out, and I presume are just part of the design.   
I'd pay a couple hundred bucks for one.

The design would be easy, but one would need layout a PCB, then order some blank PCBs, which are expensive in small quantities, then one would need to get the metal enclosure fabricated, and stuff the PCBs.  All this is expensive in small quantities--way more than any one person would or should pay.  Before all of that, describe the noise issues.  Is it hum?  Hissing? Something else?

[8675309]

Lots of valid information here!
Guys and gals- with the amount of boost pedals available today its cheap enough to buy and try- figure out what works best for you - call it a day! Live and be well, go make some music!

for what its worth those MXR 10 Band EQ pedals are great for boosting/eq and dish out unique tone shaping.
Beware there is a point where excess boost creates unwanted microphonics- I've found this to occur with any pedal that adds gain/distortion.

Wow, a lot of smart guys on this post for real. For the rest of us dummies who are looking for the real deal tube clean boost, I can only suggest the best Clean boost available. Whether it be for the Rhodes (Tine Bomb) installed in the piano  or guitar ( Tube Bomb) in a stomp box  you will not find a better suited clean boost. To this day, I have never met anyone who did not rave about it.  It started out as Rhodes boost pedal but is quickly turning guitar players on all over the world.  We sell this at a very fair price, for those who do not wish to DIY with transistors caps and resistors-

http://www.vintagevibe.com/products/fender-rhodes-stage-tine-bomb-pre-amp?variant=899477483

^^^^As for this guy^^^^
With the amount of pedals on the market- you nor anyone else can claim your product is the "best" There is no best its about personal preference, debate it all you like, there is no victor when personal preference is concerned.   For the record I looked at this "Tube Bomb" and Frankly sir its a "Time Bomb" as there is no ventilation for heat to escape!
At what point of operation does the tube or other electronics inside your sealed box suffer from heat or create noise?
Electronics operate best when cooled, cool or tepid at best, not sealed and baked.

For the record, Every tube product I've seen in the last 20+ years may it be a tube amp or gizmo has some sort of ventilation or cooling fan.

Good luck!