Tips for low voltage/ 5V audio circuits

[ElectricDruid]

Hi All,

I've been thinking about having a go at a programmable pedal using MCP42xxx series digipots. These run at 5V, and can only cope with signals up to 5V. So then I started thinking that it might make sense to use 5V op-amps (like the MCP6002 - am I starting to sound like a Microchip fanboy?!?) as well and just run the whole thing on 5V.

So...has anyone done anything like this? Experiences with digipots? Experience with 5V op-amps? Things I should know before I learn the hard way?

Any and all pointers and advice welcomed! Thanks!

[ElectricDruid]

I take it everyone is going for 18V voltage-boosted stuff and I'm the only one even crazy enough to attempt such thing?

Surely someone else has at least had a go with 5V digipots?

('Yes, I played with digipots, and don't call me Shirley"?)

[EBK]

At the risk of speaking over my own head, if you are going low voltage, consider current-amplifier-based stuff.
My favorite pedal that I've ever built has a VCA that runs off of 5V but has plenty of overhead

[Kipper4]

Digipots......
What is this witchcraft?

Sounds interesting though Shirley.

[Ripthorn]

I've been thinking much along the same lines. I want to create a generic digipot module that can have different value pots on it, but with common layout, interface, and MCU control and then have it tie in to the analog points of a signal. Are you sure those digipots only handle up to 5V signals? I was under the impression that they were essentially a switchable resistor array where the 5V was simply for the control section and then signals could exceed 5V. If I'm wrong, I would love to know now rather then when I start putting something together.

Regardless, you are likely to have 9V supply just due to how common it is, so you could do most of the stuff at low voltage and then put in a regular opamp run at 9V to boost the signal cleanly enough to get back to unity or better compared to input. I think the biggest issue will be if you can't meet or exceed unity relative to input, as most players will just avoid it.

I love the idea of digipots with an MCU so that things can be analog signal path with programmable presets, MIDI switchability, etc. I think it's an untapped area, especially in the DIY community. You usually only see it in things like Chase Bliss or other very expensive boutique pedals. I know a lot of DIY guys are hesitant when it comes to MCU's, coding, and digital in general, but if a generic module with plenty of explanation and walkthrough is provided, it shouldn't be too bad.

[ElectricDruid]

Are you sure those digipots only handle up to 5V signals? I was under the impression that they were essentially a switchable resistor array where the 5V was simply for the control section and then signals could exceed 5V. If I'm wrong, I would love to know now rather then when I start putting something together.

Yes, unfortunately, I am sure. I'd love it if you were right, but the datasheet is clear on this point:

http://ww1.microchip.com/downloads/en/DeviceDoc/11195c.pdf

Page 2, under the "Resistor Terminals" heading, it says: Voltage Range 0-Vdd. Note 4 says polarity isn't important, so it doesn't matter which end is +5V and which is 0V, but don't try going over Vdd.

There are some high voltage variants available too - The MCP45HVx1 and MCP41HVx1 series chips. Those have 10V to 36V analog handling capability. The bad news there is that they still won't work at 9V!! But bang the supply up to 10, 12V or more and they'd be happy.
They're good, but they're not available in a DIP package, and you only get one in a chip. I suppose that doesn't matter so much in SMD.

Regardless, you are likely to have 9V supply just due to how common it is, so you could do most of the stuff at low voltage and then put in a regular opamp run at 9V to boost the signal cleanly enough to get back to unity or better compared to input. I think the biggest issue will be if you can't meet or exceed unity relative to input, as most players will just avoid it.

Yes. Given that a typical guitar signal is maybe 1Vpp, it *shouldn't* be a problem. But "typical" and "guitar signal" aren't really terms that go together that well, and there are people out there with crazy hot-rodded pick-ups and active boosters and god only knows what else who think that blasting the next thing in the chain with a 15Vpp signal is perfectly reasonable!! And what's worse is...they're right, it is! Crunchy, but reasonable!

I love the idea of digipots with an MCU so that things can be analog signal path with programmable presets, MIDI switchability, etc. I think it's an untapped area, especially in the DIY community. You usually only see it in things like Chase Bliss or other very expensive boutique pedals. I know a lot of DIY guys are hesitant when it comes to MCU's, coding, and digital in general, but if a generic module with plenty of explanation and walkthrough is provided, it shouldn't be too bad.

Exactly! There's a whole area here to play with if we can work out a decent way to do it reasonably simply.

Tom

[Scruffie]

What about some CMOS in linear mode for its input @5V, sure you can use the spare inverters or whatever elsewhere in the circuit and then you can put 9V op amps up front if you want.

Depending on what you were doing with the rest of the 5V supply, not having the rest of the audio powered by it might make sense?

[wayfaerer]

I don't have much valuable input except to say I spent a heck of a lot of time a few years ago trying to get a circuit with those digipots in it to work the way I wanted. I ran into the same problem and posted about it here: https://electronics.stackexchange.com/questions/395772/ although I wasn't really seeking help, just trying to understand the problem better.

The idea was to have an analog pedal that was controlled by a microcontroller via the digipots. So many things went wrong along the way that I don't even remember what ended up being the last straw, but I scratched the project. Now, a few years later I am building an all digital pedal that's already basically working, with much less effort than I spent on my digipot/MCU pedal.

However, to answer your question with an idea you've probably already considered, you could simply attenuate the incoming signal by whatever factor is needed to get the signal under 5V, and then amplify it again at the end... assuming you're still using a 9V power supply.

Otherwise, in my personal and probably unpopular opinion, most pedals have no business exceeding 5V on their input or output, unless perhaps they are meant to go in an amp's effects loop where the voltages are higher. I mean, sure, if they can get that loud, then fine, but I would have absolutely zero problems buying a pedal knowing it required the input to be less than 5V, and couldn't put out more than 5V.

Good luck!

[Ben N]

I would think the 10 volt versions would make more sense, but yeah, the restrictions on digipots is quite frustrating. But Molten Voltage did manage to create a short-lived line of digital-control-for-analog-circuits products, so this must be doable.

[Ripthorn]

That's kind of a bummer. I wonder if there could be a limit in front that somehow feeds the gain of an amplifier at the end so that recovers whatever the limiter took out. Probably very tricky to implement reliably.

[Phoenix]

How about framing the question in a little more familiar manner?

I want to use a PT2399 delay chip in a standard 9V pedal circuit, but it is a 5V part and only has 3.5Vpk-pk input headroom.
This has not been a significant impediment to using the chip in many, many 9V pedals without issue, so we can apply similar techniques here.

It's rare that we'd want a pot (digipot) across the entire 9V rail, most active parts can't swing the entire rail on their output and we often deliberately clip them too, so AC coupling, voltage division and clamping when absolutely necessary makes these digipots quite usable in 9V circuits.

Sure, it'd be nice if their ratings were higher than our rails so we just didn't need to worry about them, but in many circuits they'll just be drop-in replacements or require minimum modification. Some pre-attenuation and post gain may be required sometimes but that's not too difficult.

[Ben N]

I have a board for a compressor that uses the THAT 4301 running on charge pump supplied +/-12v. It's the kind of thing that I would love to make the dynamic heart of a MIDI controlled pedalboard, but many of the controls are attached directly, or nearly directly, to the bipolar supply, and are therefore looking at as much as 24v across them. Unless I am missing something, I can't do that with a standard digipot. Higher voltage digipots exist, but not in all values or levels of increments, and they are pricey, plus they require higher voltages for control, as Tom noted, which makes interfacing with uCs trickier. So unless I am going to get involved in trying to engineer (i.e. blind trial and error) a complicated, expensive, space-hogging solution with, say, vactrols, which have their own issues with taper and consistency, I'm stuck. So I eagerly await developments by people who actually know what they are doing. Carry on, Tom.

[niektb]

I've been playing around with the idea of a distortion pedal with digipots and a midi interface but I believe an issue with digipots is that the tolerance is ridiculously high (around %20 or so) which makes it difficult to design a consistent pedal circuit

[ElectricDruid]

I've been playing around with the idea of a distortion pedal with digipots and a midi interface but I believe an issue with digipots is that the tolerance is ridiculously high (around %20 or so) which makes it difficult to design a consistent pedal circuit

There's nothing ridiculous about a tolerance of 20%. That's pretty typical for *normal* pots too. Just chalk it up as "analog character" and don't worry about it.

Phoenix' point about the PT2399 is a good one. FV-1 is another example, runs on 3.3V. And my own Flangelicious runs the BBD on the same 5V supply as the processor so that I can clock it with 5V signals. That reduces the headroom through the BBD to about 2V or so, I seem to remember. Works well enough.

Of course, there are plenty of examples of audio processing circuits that use even lower voltages (no-one is complaining about their iPhone's audio quality, for example) but the trouble is most of those are mainly digital. ADC in->digital stuff happens at low voltage->DAC output->bit of a boost on the output->Voila! There isn't much scope for noise in such a circuit. You can do digital processing at 1.8V and it's just as noise free as it would be at 15V. That's the great thing about digital.

What I'm thinking of is a fully analogue circuit running at 5V throughout. That's much more of a risk for noise, especially if it's got lots of gain and/or stages. For example, imagine a versatile clipping circuit with multi band parametric EQs before and after it. With some careful tuning, you'd be able to get pretty much whatever drive sound you wanted, but the complexity of such a beast would make programmability a real benefit (and a MIDI interface, like Niektb suggests). But then you run into the issues - all those stages running at 5V, with high gain in the middle? It sounds like noise might be an problem to me.

I guess I just have to try it and see. Does anyone know a good *low noise* audio op-amp that runs at 5V and ideally goes rail-to-rail? The only 5V op-amps I have around is the MCP6002, but that's not a very hi-spec device.

[niektb]

I'm very curious how a MIDI drive pedal would turn out! Would maybe also fun to include some sort of 'Preset' footswitch for those that don't have a MIDI switcher/looper
I often find that drive pedals (or at least the cheap ones haha) aren't super flexible so that I always use them in 'that one' sweet spot and never touch the knobs anymore... So I think it would benefit from a flexible EQ and multiple clipping (soft and hard f.e.) options.
Fun thing about digitally-controlled pedals is that you can actually change a lot of parameters simultaneously while the user is only touching one knob

In the past I've used a single-ended opamp on a 3V3 supply from Analog Devices: AD8616 (or 8615 or 8618) (in an active DAC output filter), they aren't the cheapest and not sure if you can get one easily but from what I can tell from the specs it easily outperforms your MCP opamp

[PRR]

If you need mind-expansion: Dr. Newton Leif (writing as Barrie Gilbert).

https://www.analog.com/en/analog-dialogue/articles/considering-multipliers-part-1.html
https://www.analog.com/en/analog-dialogue/articles/dr-leif-why-resistors-are-noisy.html
https://dl.acm.org/doi/10.1023/B:ALOG.0000011161.44537.da

Low voltage is not a hindrance. Hearing aids work on 1.4V, go down to 21dB SPL and up over 100.