Running guitar effects at 5 volts

[joakinrox]

Hi guys, i wanted the input of some more knowledgeable folks than myself about a project I have in mind.

How hard would it be to convert circuits for 5 volt operation While keeping the same sounds? I'm asking because i want to build digitally controlled effects and most digital pots are rated for 5v (I know there are several higher voltage ones, but they tend to be very expensive)

My end goal is to have a pedalboard and to power all effects on 5 volts but to keep their original sounds.,

Is it just a matter of adjusting the gain resistors of amplyfying stages in proportion the modified voltage levels? I dontknow if this wouldchange impedances or frequency responses of effects...

Or could it be as simple as reducing my guitar's input signal in proportion to the change in source voltage to achieve the same practical headroom? (But at lower output volumes)

I'm particilary binterested. In doing this with fet based effects.

I'm asking about this before building the circuits because i have all the parts in the shopping cart, but if this is not feasibleid have to change a lot of them

Anyways any thougths, suggestions or recomendations are welcome. Thanks!

[mth5044]

I think it might vary from circuit to circuit as to how much you have to change. I'd imagine it might do something to the noise floor too? There are circuits that take the voltage down and then back up, like FV1 based pedals as the chip runs on 5V, possibly stuff with pt2399's and the like.

One thing that will be affected will be purposeful  clipping. Diodes have clipping thresholds that you have to reach to get the distorted sound. If you reduce the voltage swing then try to run it through LED clipping... it would sort of be like rolling off the volume of your guitar I suspect, maybe without the tonal changes sometimes associated with it.

Could you set up buffers/gainstages around each pot to bring the voltage down and up? Not sure how the noise would be. If you use SMD components and make a dozen PCB's at once you could sprinkle them through the circuits without having to change the meat and potatoes?

I'm talking out my arse, see posts below for better ideas

[Mark Hammer]

Some things already DO run off 5V, and not just digital pedals.  For instance any time based effect that runs off an MN32xx-series BBD generally converts the 9V supply down to 5V to power the chips.  If you were lucky enough to score an SSM2166 dynamics control chip for a compressor, that also runs off 5V.

There is nothing wrong, either, with using a 9V supply and down-regulating part of the supply down to 5V for those pedals that can use it, and retaining the 9V capability for those that need it.

[joakinrox]

So for instance, in for example i feed my circuit on 5 volts (0.55 times 9 volts) and i attenuate my input signal to 0.55 times the original signal, could i leave the gain stages intact? And for  replacing leds for clipping, could i for example  in a red led (around 1.8-2 forward voltage) for something like a series silicon + schotty diode? (Around 0.9 to 1 volt i think?)

Another question, could you look a this datahseet? These are cheap digipots, and from what i understand from the datasheet the log is 5 volts, but across the terminals it can take +/- 5 volts, so i could power an effect it with a 9 volt baterry without damaging the pot??

X9C102, X9C103, X9C104, X9C503 Datasheet - Intersil
PDFhttps://www.intersil.com › documents

[Ruptor]

A general rule of thumb is if you change the voltage and/or the components you will change the circuit characteristics and the sound. I think 9V was chosen because of the 9V battery but I can't see why everything couldn't be done on 5V it is just that most circuits have been designed for 9V.

[Phoenix]

The most challenging aspect with your run-of-the-mill guitar effects is going to be how to manage any pots that are referenced to somewhere other than ground. Gain pots on opamps that are floating at Vref are one in particular that comes to mind.

Also as mentioned, the reduced headroom may complicate diode clipping thresholds, though should be manageable.

[joakinrox]

The most challenging aspect with your run-of-the-mill guitar effects is going to be how to manage any pots that are referenced to somewhere other than ground. Gain poots on opamps that are floating at Vref are one in particular that comes to mind.

Also as mentioned, the reduced headroom may complicate diode clipping thresholds, though should be manageable.

I don't understand what you mean by that. Could you please explain it  in more detail? Can't I just bias the wiper of the digipot at 1/2 vdd and put a large output cap after the wiper t avoid clipping?

[merlinb]

For most effects it will be pretty easy using rail-to-rail opamps (specially designed for 5V operation). But a few effects do rely on the extra headroom, or use opamp clipping which will alter the sound when squashed down to 5V levels.

[joakinrox]

another thing i was considering was replacing  the potentiometers with  led+LDR combos sealed in epoxy glue (1 for rheostats and 2 for voltage dividers). This method would eliminate all noise, bias and bandwidth related issued.


I was thinking of programming a microcontroler to vary the LED's brightness using pwm  (this idea would need a one time calibration procedure to determine the duty cycle of each individual LDR combo needed to reach the desired resistance range, say from 0 to 1Mohm, or 0 to 1k)

what do you think of this approach? I've seen around the forum that some people do this to control modulation based effects, has any of you tried to replace pots with this arrangement?


what material have you used to insulate the leds+LDRs from light besides heatshrink (I'm inclined to use black heatshrink + some epoxy outside to make a more stable mecanical conection)


Another issue might be that LDRs are very temperature dependant, but maybe a temperature sensor inside the pedal could help me account for this factor?

¿What do you guys think?

[Phoenix]

I don't understand what you mean by that. Could you please explain it  in more detail? Can't I just bias the wiper of the digipot at 1/2 vdd and put a large output cap after the wiper t avoid clipping?

The easiest possible option would be to simply drop digital pots in as replacements in a stock circuit, given that typical effects units DON'T feature rail-to-rail opamps, and as such have swing of only about 5Vpp even when operated from 9V (discrete transistor designs can be quite different of course, swinging more-or-less depending on the design). However, see the case below, where the digital pot is powered from the 5V regulator, but is biased at Vref, which is 4.5V. Even if the opamp only swings 5Vpp, that's 2V to 7V, so those 7V peaks will exceed the maximum 5V rail of the digital pot.
You can't always just lower the operating voltage of the entire circuit to 5V, because the circuit will not operate correctly.
Of course, you can try substituting rail-to-rail opamps like Merlin suggests, which may work for most opamp circuits.

You do also occasionally find digital pots which have one of their terminals connected to ground, but they're not too common.

The small range of values available is also probably a significant limiting factor.

[amz-fx]

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[R O Tiree]

One thing you will have to watch like a hawk is that an awful lot of these PIC-controlled digital pots have a max "wiper" current of only 1mA.  Exceed that rating and cue magic smoke.

[EBK]

Some devices work on input current rather than voltage, giving you more headroom than you may think.  The THAT4316, for example, runs on 5V.

[Ruptor]

I am confused about what you really want. In your first post you want digital pots for a microprocessor.

How hard would it be to convert circuits for 5 volt operation While keeping the same sounds? I'm asking because i want to build digitally controlled effects and most digital pots are rated for 5v

Yet in this post you come up with LDRs

another thing i was considering was replacing  the potentiometers with  led+LDR combos sealed in epoxy glue (1 for rheostats and 2 for voltage dividers). This method would eliminate all noise, bias and bandwidth related issued

If it just the elimination of crackle from pots you could use a FET as the variable resistor and a big cap/filter on the gate to block crackles.
Thinking about lower voltage operation I think it would be better to run off 3V from two batteries. It would give plenty of headroom for 0.7V diode drops and reduce the gain required of the original signal that has to leave the box at 1V anyway. 

[ElectricDruid]

How hard would it be to convert circuits for 5 volt operation While keeping the same sounds? I'm asking because i want to build digitally controlled effects and most digital pots are rated for 5v (I know there are several higher voltage ones, but they tend to be very expensive)

My end goal is to have a pedalboard and to power all effects on 5 volts but to keep their original sounds.

As others have said, some circuits will be harder to convert than others, and reducing clipping thresholds proportionally is an obvious snag. But nothing's impossible.

One possibility you haven't mentioned for controlling circuits at this kind of level would be a multiplying DAC. If you feed the analog signal to the Vref input, you can control how much of it passes with the digital input.
Also consider that the audio part of the circuit (or most of it) could still work at a standard 9V, and only a few elements within it might need to work at 5V. This is how I did the Flangelicious flanger, which uses a PIC for digital control and runs the BBD at 5V, but the rest of the circuit is standard stuff at 9V.

Tom

[amptramp]

Another vote for the multiplying DAC here.  The DAC0808 and DAC1208 are available just about everywhere.  The usual audio pot is a logarithmic item but if you have enough steps the control granularity for a linear DAC should not be a problem.

But if you need to run a circuit at a fixed voltage, if you bias the non-inverting input to an op amp to 4.5 volts as is done in a lot of effects, the circuit will drive the inverting input to within a few millivolts of that voltage as long as it remains in the linear range (not using op amp clipping).  You can place a digital pot on either the input or feedback side with the inverting input driven to 4.5 volts so you don't have to accommodate the full input swing but you may have to reference the digital pot control lines to the 4.5 volt level.

[joakinrox]

Another vote for the multiplying DAC here.  The DAC0808 and DAC1208 are available just about everywhere.  The usual audio pot is a logarithmic item but if you have enough steps the control granularity for a linear DAC should not be a problem.

But if you need to run a circuit at a fixed voltage, if you bias the non-inverting input to an op amp to 4.5 volts as is done in a lot of effects, the circuit will drive the inverting input to within a few millivolts of that voltage as long as it remains in the linear range (not using op amp clipping).  You can place a digital pot on either the input or feedback side with the inverting input driven to 4.5 volts so you don't have to accommodate the full input swing but you may have to reference the digital pot control lines to the 4.5 volt level.

I investigated about the multiplying dacs, and  from what I gather they seem to have a buffer in the "wiper" right?  I think this would  make them ideal to replace "output volume" pots found in most effects,

But for potentiometers on places like tonestacks, wouldn't the prescense of a "buffer" in the wiper change the frequency response of a tonestack circuit?

[bool]

Yes; in most if not all cases you would loose the pot-to-other-components interaction so the "tone curve" would be compromised.

[JustinFun]

On a slightly different but related note I was thinking about building a pedal power supply that runs off a 5v usb mobile charger, on the grounds that:

- they've become very common and cheap (and rated up to 2.1a in some cases)
- i saw a micro-usb breakout board on bitsbox and though it might be unteresting to try.

So I built a voltage doubler as per the attached. I was kind of hoping that the natural voltage drop in the circuit would bring me to 9v (hence the rather optimistic label on the schematic) but it's closer to 9.8v. I'm assuming there's not enough headroom for a 9v regulator, so would welcome any advice on my options to get to the 8.8-9.2v range without clobbering current availability?

[reddesert]

Use 1N4001 (or other silicon) diodes in the charge pump rather than 1N5818 and you'll get more voltage drop, bringing you closer to 9V.

Be aware that the charge pump is limited in output current - maybe 100 mA ? - so even if you have a higher amperage USB charger, the output of the 9V pump will be limited.