Question everything: Why 9V in the first place?

[rezzonics]

Very interesting comments, I am glad to have brought up this discussion here. But there is something I have not fully understood regarding the use of 9V or even 18V to have more headroom. Is there any real utility in having lots of headroom? What's the point of having 9V, 18V or 27V of headroom when 1V is already "loud", does it have to do with overdrive and breaking up the amp input? I am thinking here on the famous Klon Centaur Boost / Overdrive pedal. It has a first dual opamp section powered at 9V, but the second dual opamp section is powered between -9V and 18V. I guess that the input voltage into the guitar amp can be huge. Is that the goal of an overdrive pedal? Breaking up the guitar amp? Is it better if the overdrive pedal saturates the preamp instead of providing an already saturated signal?
Wouldn't it be similar to provide to the amp a clipped or saturated 3V signal instead of a non saturated 20V signal and let the saturation happen in the guitar amp?
More than that, each guitar amp may have different biasing and different saturating level at its input, so wouldn't actually be better to control the saturation level inside the pedal instead of relying on guitar amp break-up?
I thought that the difference between an overdrive and a distortion pedal was the level of clipping, soft vs hard, and that distortion starts clipping at lower levels, but looking at the Klon Centaur it seems that there is more here than I thought.
Could a Klon Centaur "3.3V clone" sound as overdriven as an original Klon Centaur? Thanks to all for this rich discussion.

[Fender3D]

...there aren't many 2.1mm jacks where the contact to be dislodged to switch between internal and external power can be moved by the inside of the plug.  .... become the manufacturer's choice to have outside-pos or outside-neg.

Well, they could have just switched the negative pole...

[thermionix]

Is that the goal of an overdrive pedal? Breaking up the guitar amp?

Originally, yes, I think that's how they were intended to be used, although many people don't do it that way, some do.  I guess people use clean boosts as "overdrivers" too.  Back when I used to gig with (only) an old TS-9, I would set the gain to minimum, and the volume to unity.  I would pick up a little dirt from the pedal, but mostly I was using it as an EQ and buffer, to remove the lowest frequencies so I could crank the amp and not have it "flub out" (for lack of a better term).  Maybe that's a third type of application.

[R.G.]

Well, I would say yes and no.
If we were operating in a world that eschewed batteries entirely, I would say yes and agree with you on your assessment of Boss/Roland's dead weight.

But there aren't many 2.1mm jacks where the contact to be dislodged to switch between internal and external power can be moved by the inside of the plug.  It could certainly be engineered, but the traditional design of such plugs is that the shaft of whatever is inserted bumps a leaf contact out of the way to provide power from the outside rather than the battery on the inside.  Where the power always comes from the outside and nobody has to disable any battery, it does become the manufacturer's choice to have outside-pos or outside-neg.  But in this instance, though it was certainly Roland's choice to use that style of jack, it was not necessarily their choice to have the normalized/disabling contact be the one touching the shaft of the plug.

Good point. But it illustrates the point in another way - even big Boss didn't go have custom-made switching jacks made. They went with the commercially available earlier parts.

We deal with it, of course, but having the positive supply on the outside barrel of a negative-ground power connector causes some issues, notably the death of internal regulators if your daisy chain spare plugs contact a grounded metal surface and the regulators didn't have protection for that situation designed in. I suspect that whomever did the design for this at Boss didn't think the power supply would supply more than one pedal at a time.

I often speculate how the pedal world would be different if the early adopters of vacuum tube amps had duotriodes cheaply and readily available to make a front end differential pair instead of a single ended input.

[Tony Forestiere]

> In the computer world...
My father worked for another 3-letter company (one of the 7 dwarfs).

What? Your father worked for "DOC"? 

[rezzonics]

short answers:

9V used to be called transistor battery. you're on to something there, plenty of info online about that. at this point, 9V has been standard for so long, and so many people use DC adaptors plugged into the wall, that to make a (commercial, mass-fabbed) pedal that takes a different voltage is bad for business.

lower voltage doesn't *necessarily* mean longer life, life in batteries is about current supply

yes, you can get away with a lot regarding ripple frequency, but you'll find, sometimes, there be dragons...

you certainly can use less than 9V, in fact some famous pedals as well as celebrated DIY projects do just that. check out Tim Escobedo's "circuit snippets"

rarely, can you simply drop voltage down, especially from say, 18 to 3.3, and have everything turn out the same.

others can reply in greater detail, i'll have more time later if you're curious. best thing to do? breadboard and experiment!!!

I have made LTSpice simulations on frequency and time domains with the Klone Centaur pedal. Results can be seen here. I made simulations with gain varying from 0 to 10 and treble set to 10 (maximum). I then made simulations with gain set to max and treble control varying from 0 to 10.
Small signal gain can vary from 47db to 75dB (with treble at 10) with gain peaks in the area of 670 - 810Hz. With gain set to 10, treble can vary the gain from 52 dB to 75 dB with peaks around 650 Hz.
On the time domain I used 4 different 300mVpp sinewaves at different frequencies: 82Hz, 440Hz, 2kHz and 4kHz, the signal amplitude saturates the output op-amp powered between -9V and 18V (27V in total) with output signals of up to 24Vpp at high gains and volume at maximum.

I then tried to implement a similar circuit powered at 3.3V with rail-to-rail opamps. I modified the passive values (R and C) and tried to keep a similar frequency response with maximum peak at mids, but reducing the gain to get a similar level of saturation in the time domain with a signal that cannot exceed 3.3Vpp. The small signal gains where reduced from a maximum of 75dB to a maximum of less than 40dB.

I think I will have to try this circuit to see how it sounds. I doubt that the result could be identical. I really didn't expect such a huge signal at the output of the Klon Centaur. 24Vpp!!!

[R.G.]

You also need to not lose sight of the fact that almost all circuits have unseen, and often forgotten components - the thing or circuit that drives its input, and the thing or circuit that loads its output.

In the case of modelling distortion pedals, it's often good to remember that a guitar pickup looks like a signal voltage running through an inductor of 0.5 to 2H (yes, the decimal point is right, Henries) and 4K to 18K of wire resistance and then the guitar controls. This has profound effect on how it can drive an input.

You also need to realize that what is receiving the signal may be a guitar amp input which is a 12AX7 grid driven through 68K (many times) and that the grid simply can't move more than about 1.5V positive.

How an effect sounds depends on what drives it and what it drives.

[rezzonics]

I finally made the Klon 3v3 clone pedal, it sounds quite good to me considering I cannot compare with the original, but it has lots of output.
Assembly and electrical test can be found here
Schematics and layout can be found here
I tried etching and I am not quite satisfied with the result, too much overetching, I need to improve the technique:

[vigilante397]

I'm sitting in the back of a van on the way to a gig out of town and this thread has been hands down the most interesting passer of time for me. Thanks everyone

[rezzonics]

Can a JFET based overdrive circuit be powered at 3.3V? I took the Catalinbread Dirty Little Secret overdrive pedal that emulates a Marshall super-lead / super bass amp and tried to see if I could clone it using 3.3V instead of 9V.
I have to say that it's a bit of a challenge, JFETs in general don't work very well at low voltages, it's difficult to bias them and it's difficult to make them work in the active region.
But I think I found the "dirty little secret" to make them work.
The answer can be found on this blog post I published:
Dirty Little Secret 3v3 clone: LTSpice analysis
Spoiler: Pinch-off voltage

[R.G.]

Yep, learn how the lil' beasties act, and then design accordingly. Anyone who works with JFETs sooner or later comes to that.

The J201 is unique as far as I know in having such a small pinchoff voltage, and lower current JFETs are useful at lower voltages as long as you can pinch them off some.

Here's another path for you to look at: MOSFETs may be depletion (i.e., like tubes and JFETs, needing a bias to turn them off) enhancement, needing a bias to turn them on, or *zero* biased: they pass some current with Vgs = 0, less with Vgs negative (for N-channel devices) and more with Vgs positive.

So sure, JFETs can be biased for overdrive at 3.3V. MOSFETs and BJTs can too. You just have to work at it. There are some early germanium designs for hearing aids that gave linear amplification when working from a single battery cell in the 0.8 - 1.6V range for the whole mess.

[PRR]

> early germanium designs for hearing aids that gave linear amplification when working from a single battery cell

I have the impression that today's hearing aids get the electret preamp, a _DSP_, and a many-mW power amp all under a single cell.

I could be wrong. The available academic papers are all talk and the guys actually doing working product don't publish. But obviously 3V is possible for all this.

The JFET in a 99-cent electret capsule will reliably be <0.4V Vp. It has been decades since I hacked these and I do not know if you can extract the JFET from the current crop.

[amptramp]

It is quite possible to use A CD4007 unbuffered version and ignore the upper PMOS device.  Just use pins 6, 7 and 8 as gate, source and drain for one n-channel MOSFET and pins 3, 4 and 5 as gate, source and drain of an isolated n-channel MOSFET and pins 10, 9 and 7 as gate, source and drain of another n-channel MOSFET with a common source to the first one.  These devices operate down to 3 volts and you can use the p-channel MOSFETs as loads to make a linear inverter that approximates an op amp.

[Fender3D]

I'm sitting in the back of a van ...

Hey Nathan
I thought you were whistling a new Otis Redding tune.... 

[rezzonics]

Spice simulations of a version of Dirty Little Secret with MOSFETs instead of JFETs and at 3.3V
Dirty Little Secret MOSFET 3.3V version: LTSpice analysis.

Schematics:


I think it's not a DLS anymore but does it sound similar?

Here are some guitar samples sound tests made with the simulations output form the original effect circuit, the JFET 3.3V version and the 3.3V MOSFET version:

Dirty Little Secret LTSpice simulations sound test

[R.G.]

> early germanium designs for hearing aids that gave linear amplification when working from a single battery cell

I have the impression that today's hearing aids get the electret preamp, a _DSP_, and a many-mW power amp all under a single cell.

Yes, both those are true. Early hearing aids were a simple amplifier running from a single cell. Today's CMOS logic can do better.
Much better.

The drop in logic voltages from early CMOS's 15-18V max to 5V, to 3.3V, and down to 1.2V for much of the compute world's RAM and CPUs is a reflection of how MOSFETs scale down when you make them smaller, faster, and lower power.