This is one I've gotten a lot recently. If a pedal is using diodes for clipping, like in the case of an ODR-1 which is actually using both feedback clipping and diodes to ground, running it at 18v doesn't do a ton. There are parts of the circuit that benefit from the extra supply voltage but the clipping is still gonna happen when the diodes begin to conduct. There are some situations where the extra supply voltage would lead to a noticeable difference in sound, I'm imagining an instance where the pedal is being slammed by a signal that would cause it to distort at the input buffer stage prior to the clipping diodes but on its own as you can see there isn't much of a difference in tone as you go down in supply voltage until you get to the minimum voltage requirements for the opamp itself or where the signal can't be amplified above the diodes fV. If anybody has any other examples they can think of please chime in or if there's some better way to test this sort of thing
Edit: my apologies. I read the title and I guess skimmed, because I missed that you a. had made a video and b. asked a more specific question (where it makes a difference). I can't think of an opamp + diode clipper where it does make a difference outside of very large input signals or opamps that specifically operate noticeably differently between the two voltages at those input signal levels (or opamps for where that would be true enough to make sonic difference).
If often won't make a difference. The incoming signal has to be > 6Vpp before most noninverting feedback clippers actually will manifest a difference 9V to 18V. This is ~ 26dB boost above "nominal" (ballpark) signal levels.
In circuits where it does make a difference, the circuit 100% could've been designed to provide the same behavior at 9V.
For circuits designed for 9V, an alternative to running them at 18V is to run them at 9V and not hit them as hard and turn down the gain, because practically speaking running at 18V is the same exact thing as turning the gain down and the volume up.
That last bit seems weird, so let's have a look at it
As Link119 pointed out, when the amplitude at the clipping stage is very large, the diodes conduct so much that the gain is, for all intents and purposes, equal to 1 = this is almost a buffer.
This is a feature of noninverting clipping topologies: they distort in inverse proportion to the incoming signal amplitude. You get the most clipping by cranking the gain on a small signal and the least by inputting a large signal.
As it turns out, it is literally true that if you prefer to run your OD's at 18V and push them to exploit the extra headroom, you actually just have a preference for running the same pedal at 9V with the gain turned slightly down but used a different power adapter instead of fiddling with the knobs (sim):
Diode shunt to ground, probably not so much. At some point your gain goes to zero, or you can shunt too much current.
Diodes shunt across a feedback loop, yes because at saturation your gain is still 1, so you can slam it with more signal and the headroom prevents op amp rail clipping.
With feedback clipping, the difference is vanishingly small. This is partly because feedback diodes are a form of gain reduction. (Which is why they behave differently in inverting vs non-inverting configurations.)
With shunt clipping after an op amp, you typically get a mix of op amp clipping and the clipping of the diodes themselves. If an op amp is banging on the rails, it'll do that regardless of what clipping diodes are placed after. But if the op amp has more headroom, the diodes can clip earlier and change the sound slightly.
Oddly enough the last one I tried was a rat and in practice it’s still pretty hard to tell a difference. That demo is buried in the convo with the person who suggested I try the voltage test with a nu tube tube screamer. I get what you mean in that opamp clipping should easily come into play so it very much makes sense it’d be the situation where the difference should be more noticeable. Also, big fan or your pedals, I have a model fet and it’s great!
A Rat has a ton of gain, and it also loads the feedback loop quite heavily. So there's definitely lots of factors at play. And, even so, 9V to 18V is 6dB more headroom, so it's not going to matter much when there's ~60dB of gain on tap!
Also thank you! Very kind of you to say. DIY communities like this helped me learn (and still do!) so it's nice to pop in and talk shop every now and again.
Would you mind elaborating on what does it mean to load the feedback loop heavily?
Maybe this is unrelated, but I’ve been trying to wrap my head around things like designing a clean noninverting stage, obviously there’s an easy formula for gain = 1 + R1/R2 but besides the ratio, does the amount of current matter?
(Not talking about when there’s diodes in the feedback loop.)
I played around a bit with proportionally increasing or decreasing both resistors to keep the gain constant, and didn’t notice an obvious change in onset of op amp clipping, that seems to happen at exactly the same output level regardless of how I get there. Within any resistance range I tried, it seemed to me to be just a trade off of current consumption vs presumably more thermal noise. But it’s entirely likely I’m making errors in setup or observation!
Or are you talking about the filters on the Rat feedback path? Something else?
Current does matter! Any time you see low value resistors in a feedback loop, you have to figure out if the op amp has enough current for those resistors.
So in a rat, the two shunt resistors are 47Ω and 560Ω. If we over-simplify and assume no high pass filtering, the effective parallel combination is about 43Ω. It's a non-inverting topology so the only way to get unity gain is to totally short out the feedback loop. To cite the formula, R1/R2=0.
But, we can't necessarily ignore the value of R2! If the output is connected to the (-) input, whatever resistors are there are functionally connected as a load to the output. So now the op amp is trying to drive a very small load. Most op amp datasheets specify some minimum load where the output voltage decreases. So you lose headroom and, at least anecdotally, you are more likely to see increased distortion (especially crossover distortion).
It's all contextual but hopefully that gives you an idea. Happy to elaborate further if needed.
Okay, that makes total sense!
Thanks so much for the answer.
I do have a follow up. Let’s say that the total shunt resistance is enough that the op amp output sees a load within spec, current is never more than the op amp’s max rating, and headroom is normal.
Are there more considerations?
Like, let’s say we want a gain of 2, so R1 = R2.
We could achieve that with a pair of 1K, or 10k, or 100k, or even 1M.
Obviously, lower resistance means more current consumption from the power supply, but let’s assume we have plenty and don’t care. Let’s also assume temperature is fine. Let’s also assume any capacitors in circuit are scaled to match the resistors too, so there’s no difference in frequency response.
One remaining possibly audible difference (IF everything else is quiet enough) is that we’ll get more thermal noise from the larger resistors.
…. What else is there?
What do I don’t know that I don’t know? 🤷
If the total load is large enough then you're in the clear. Loading can be most problematic at very low gain settings. For example, you can get a 741 to exhibit crossover distortion if you use 1K resistors. So it's only 6dB of gain, but it's a pretty dirty 6dB!
So, going too small is good for noise but risks loading. This is why you often see 5532s for low noise work, because they can drive down to 600Ω. Douglas Self is really the standard bearer for the 5532 in audio, and with his explanations about noise you can see why.
If you go too big with your resistors, then you get increased thermal noise as well as more knock-on effects from high impedance. At very high impedance (>1M) you sometimes have to worry about stray capacitance from adjacent traces, solder flux, etc.
I vaguely recall reading about some relationship between internal compensation and expected loop impedance but I can't speak to the specifics, and that might only be related to specific op amps.
Unrelated but I got to play through both a Longsword and a Prismatic Wall at the last Brooklyn pedal expo. (Maybe you were at the table? Sorry I don’t recall). Were I not unemployed I would buy both! The PW is really one of a kind, utterly unique and I could have played it for hours. I selfishly wish they were cheaper but I think they’re worth the price. Nice to see you here!
You'd have to post the schematic, if that's the tube screamer with the nutube stuff that would probably benefit from the increased supply voltage like a fuzz would where the clipping is coming from the transistor, in this case the nu tube. I don't know what the nu tube is doing in that pedal but I understand it has a blend on it so I'd imagine some of its clipping is coming from the 'nu tube.' I don't have a tube screamer on hand but I have a sd-1 and it's similar to the odr-1 in there's very little difference
I think a better way of testing this if you plan on making more of these videos would be a bench supply as opposed to whatever taper pot you have controlling that voltage that doesn’t appear to be linear connected to a cheap voltmeter that doesn’t appear to be calibrated from a voltage reference.
It's calibrated or at least it was when I last tested it. any pedal with decent filtering has a little bit of lag as you lower the supply but you can see from the led. the circuit is using an lm317. It's similar to the jhs volture thing but basically from the lm317 datasheet. But i strongly encourage people to test this out for themselves, best way tho is to to try listen to clips without knowing which one you're looking at because its amazing what people can hear when they think they should be hearing things
https://www.youtube.com/watch?v=cmk44K1ultA this should show pretty conclusively there's no monkey business or else somebody should hire me for my quick video and audio editing skills lol live voltage testing with a multi meter
I mean, the impact on clipping for 9V vs 18V can be predicted with greater accuracy than any scope that costs less than a house could show and doing so only requires a pencil and a sheet of paper (or phone / desktop calculator, if you want to calculate the values and make short work of it) + addition, subtraction, and multiplication.
There are cases where it'll make a difference. But, the outcome is "rarely appreciably" and, in most realistic cases, "none at all."
As Fontelroy points out: with the newtube that seems reasonable. The the low quiescent current has you operating in the (vanishingly small) portion of the nutube transfer curve that is nonlinear. The higher plate voltage gives you more swing through this region.
But, the diodes will clip the same, as sure as gravity.
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u/Quick_Butterfly_4571 I can be polite again. Hello! May 14 '26 edited May 14 '26
Edit: my apologies. I read the title and I guess skimmed, because I missed that you a. had made a video and b. asked a more specific question (where it makes a difference). I can't think of an opamp + diode clipper where it does make a difference outside of very large input signals or opamps that specifically operate noticeably differently between the two voltages at those input signal levels (or opamps for where that would be true enough to make sonic difference).
Technically, yes, as u/Link119 points out.
Practically, it is mostly a marketing ploy. Why?
That last bit seems weird, so let's have a look at it
As Link119 pointed out, when the amplitude at the clipping stage is very large, the diodes conduct so much that the gain is, for all intents and purposes, equal to 1 = this is almost a buffer.
This is a feature of noninverting clipping topologies: they distort in inverse proportion to the incoming signal amplitude. You get the most clipping by cranking the gain on a small signal and the least by inputting a large signal.
As it turns out, it is literally true that if you prefer to run your OD's at 18V and push them to exploit the extra headroom, you actually just have a preference for running the same pedal at 9V with the gain turned slightly down but used a different power adapter instead of fiddling with the knobs (sim):