Introducing the Flexidrive

[Mark Hammer]

An idea that came to me yesterday, while listening to senior management make speeches.  Thank goodness for free pens and notepads at those events, eh?  I hope the name isn't already taken.  The document explains everything about the circuit so I won't go into it here except to say that it is in the combined spirit of the Roseyray and the Quadrafuzz.  I'm 85% done wiring up the first one.  I'll see if I can post some samples this weekend so you can hear what it sounds like.

http://img.photobucket.com/albums/v474/mhammer/Flexidrive.png

[Marcos - Munky]

It looks interesting, but the image is very small. I can't read it (well, actually I can, but I need to get my eyes very close to the monitor).

[Mark Hammer]

Okay, then let's try this:

[puretube]

whole lotta filt`rin` goin`on! 

[Mark Hammer]

Ya got that right, buddy.   Here is the text that apparently the PNG file makes hard to read.


The Flexidrive

I forget what I was doing, but it involved thinking abut something where the wiper of a single pot was being used as the ground connection for two points in a circuit in a complementary fashion.  That started me thinking about using such a pot to adjust the gain of two noninverting op-amp stages.  But what could I do with such a control.  In past I had toyed with the idea of a scaled back version of something like the Anderton/PAiA Quadrafuzz.  It dawned on me that I could use such a control to adjust the gain of two distinct clipping stages, each dealing with its own portion of the frequency spectrum: one high, and one low.

The balancing of two amps was also something I had tinkered with in past, and also something I recalled Jeff Beck doing with a Princeton and larger Fender amp run simultaneously.  The smaller speaker and power of the Princeton would require pushing it hard in order to compete volume-wise.  The combination of a clipped mid and treble range and a cleaner bass range presented interesting possibilities.  It got me to thinking about how I might be able to easily combine different degrees of clipping in the high and low band and different amounts or blends of each signal half.  And so, the Flexidrive was born.

Here's how the circuit works.  IC1a provides a bit of boost (x4, approximately).  This gets us started and, because gain is multiplicative, also reduces the design demands for generating enough gain in subsequent stages to produce sufficient drive for clipping.

The output of IC1a is split and passes through a simple highpass filter formed from C11 (.01uf) and R13 (18k), with a rolloff below about 880hz, and a simple lowpass filter comprised of C5 (.01uf) and R5 (18k) with a rolloff above 880hz.  Neither rolloff is that steep, but will be complemented in other parts of the circuit.

IC2a is the gain stage for the high end.  You will notice a great similarity between this and the MXR Distortion+ and DOD250.  The feedback resistor (R15) and the feedback cap have a high end rolloff around 7.2khz.  That leaves lots of sizzle, but tames a lot of the potential hiss. C14, R16, and one leg of the pot VR1, form the ground leg of this gain stage.  With a minimum resistance of 3k9, and a maximum resistance of 103k9 along this leg, the potential gain with a 1M feedback resistor is 10.6-283 (44-1063 when muliplied by 4).  That upper end is pretty intense gain, but since this stage will be dealing with the generally lower-amplitude high end, we need that potential gain.  We also need that feedback cap to keep things stable at such a high gain. The combination of the pot, R16, and C14 (.027uf) gives a lowend rolloff of around 46hz at lowest gain and a rolloff around 1.2khz at the highest gain.  That's actually quite a spread, but also increases the diversity of sounds one would be able to get.  Since we are using a divide-and-recombine approach, the net result is actually more of a potential for heavily scooped sound at highest gains, which is pretty much what players want.

IC2b handles the low end.  In combination with the lowpass filter on the front of this stage, feedback capacitor C7, with feedback resistor R , provides a second high-end rolloff around 720hz, very similar to a Tube Screamer.  Unlike a Tube Screamer, however, the cap selected for the ground leg (C8) is much larger in value (.22uf).  R8  is also much larger in value, because the generally higher amplitude bass end doesn't need quite as much gain.  With this resistor at 6k8, the gain range for this stage goes from to 10-163.  That may not seem like a huge gain range, but again, remember it is multiplied by 4 from the input stage, so that actually produces a net gain of about 41-673, which is still pretty extreme, even if it isn't as high as the treble side.  At maximum gain, the low end rolloff starts around 106hz, which is low enough for guitar.  At lower gain settings, this rolloff point drops much lower to retain all possible bass content.

When the drive-balance control is set to its mid-point, assuming a perfectly linear 100k pot, the treble side sees a 53.9k resistance to ground (for a gain of 19.5 and a cumulative gain of 81), and the bass side sees a resistance of 56.8k and a combined gain of 77.  So relatively even gain for top and bottom.  This won't result in a serious fuzz but will produce some dirt from each signal half.  I have not attempted to be able to get a "clean" sound with this unit because that's not what it's about. 

Rotating the drive-balance pot in one direction increases the bass drive while it decreases treble drive, and rotating it the other way does the opposite.  Some users may find that they would like a slightly different drive-balance and are encourage to experiment with fixed resistors in parallel with one or both legs of the drive-balcne pot to achieve different tapers or balances.  Alternatively, it might not be a bad idea to experiment with one or more values of R  on the input stage to adjust the multiplying factor upwards or downwards for both channels.

The outputs of IC2a and IC2b each go to a DC-blocking cap, a medium-value fixed resistor, and a back-to-back clipping diode pair.  I've used standard silicon resistors because they produce more output, and because we are capable of driving each signal half hard enough to produce serious sizzle if we want.  R10, R12, R18, R20, and pot VR2 provide a channel balance or mix control that selectively bleeds off the one side or the other to ground.  C10 and C16 provide a little more treble filtering to tame hiss and maintain tonal difference between the channels.  Because the  amount of drive in each channel can be adjusted, this balance/mix control allows you to adjust the level of each side, independent of drive.  This makes it possible to have a hard-driven teble side mixed back with cleaner low end dominating, or a heavily distorted low end with most of the treble attenuated.  It is this ability to adjust both drive and mixing that earns the circuit its name - Flexidrive.

The two channels are mixed together in IC1b, with a tiny bit of gain to compensate for what you lose through the balance/attenuator circuit.  The output goes to the volume control, and there you have it.

Again, what I like about this is not that it achieves some sort of signature fuzz sound, but that it can be dialed to achieve a very wide array of tonal signatures from gritty and well-balanced to extreme variations with just 3 knobs.  That makes it the sort of circuit that encourages experimentation, and that is my favourite kind of circuit.

[Dragonfly]

interesting schemo...i did "something" like this quite a while back with a fuzz pedal, basically using a pot to pan between two different fuzz stages....however, yours looks like a WAY more advanced idea than MINE

AC

[pyrop]

Hi Mark.
The flexidrive looks very interesting & I think this sort of circuit would be great in a wah shell.
I've been thinking on how one could incorporate two different dist circuits & blending them through a wah pot & this looks like it!
One query I have with your design is the MIX / BLEND area.
When you have the pot maxed one way, a fair bit of signal from the opposite side also gets bled to ground. In this case there is about 17K between the signal & ground.
I presume this still ok, just wondering.

pyrop

[Mark Hammer]

The Six-shooter looks interesting too, although it pans *to* two different stages, rather than *from* them.  Not better or worse, just a different approach, and also worth exploring.

Pyrop's comment about passive bleed is true.  I'm thinking that maybe I ought to tweak some component values there; perhaps using a 250k blend pot or else scaling back R10/R18.  I have the circuit up and "working" though it isn't working quite right just yet so no samples folks....for now.  There is a broad range of sounds but the output level is not stunning just yet.  There may also be some soldering issues (it's a perfboard).

If anyone notices some errors in the drawing/design, let me know.
More later

[R.G.]

Did you consider making the panner be an inverting panner? Just curious.

[Mark Hammer]

Did you consider making the panner be an inverting panner? Just curious.

Inverting in what sense?  I *did* start wondering if maybe I ought to make the mixer/output stage inverting, and may try it out tomorrow if that seems like a good idea.  I usually like to try and keep input and output phase coherent, but it's not an absolute.

[Meanderthal]

 I like it! Paralell tuned distortions... Might make one HELL of a bass distortion. Reminds me a little of the Hog Grinder, but better thought out/more efficient and flexible.

[moosapotamus]

Might make one HELL of a bass distortion.

Oooh, yeah... & adding a clean blend might give some extra coolness combined with RG's inverting panner idea, too.

~ Charlie

[Mark Hammer]

Might make one HELL of a bass distortion.

Oooh, yeah... & adding a clean blend might give some extra coolness combined with RG's inverting panner idea, too.

~ Charlie

I'm still not sure exactly what was implied by inverting panner, but one of the intents here is that when the drive-balance control is rotated fully the signal half with the least gain applied would be relatively close to "clean".  I realize a gain of 40-ish and a pair of silicon diodes is unlikely to be completely clean, but it should come reasonably close.  One can always adjust the gain-setting resistors or tinker with the diodes such that lowest gain will assure more cleanliness.

Now, if I could get mine behaving properly, then I could post a sample or two and we could make some progress.  I should probably have heeded my own advice and used a bloody socket for the op-amps!

[puretube]

Non-lossy.

you can design some (inverting), (active)  panner with an inverting basic opamp-circuit.
(dunno if it`s covered by R.G.`s: pan-for-fun,
or: polarity-inverter...).

but it`s similar to those tone-circuits, that you can add/subtract from the dry signal,
to get a boost or a cut or a linear response.

[Meanderthal]

when the drive-balance control is rotated fully the signal half with the least gain applied would be relatively close to "clean".

Yep, that's the beauty of this idea(for bass anyway). Since the desired end result is in fact distortion, I've always thought a well designed and tweaked distortion sounds more intense than the old "just mix in some clean" cop out. The whole thing about distortion on bass is the loss of perceived bottom end caused by creating all those harmonics. This is something that can usually be tweaked to taste by playing around with voicing caps. And, believe me, distorted bottom end does indeed sound good if handled properly!

With this circuit, you get to decide just how distorted the lows are compared to the highs as well as the relative volumes, not just how much clean signal gets through.  Excellent idea!

[Processaurus]

That drive balance looks like a neat knob to have... I agree, this has potential to be a real winner for bass overdrive, I've always liked paralled distortion sounds where one is making the low end and the other the treble, it always seems to sound huge.

[pyrop]

Sticking with the passive theme, How about using a dual gang pot?


Wire it up as above so when turned one way the resistance decreases to ground and the resistance to the output increases on one side of the signal and vise verser when turned the other way.
I would say use a 100K for the pot still but dual?

pyrop

[Meanderthal]

 Doh! Sorry, my mistake...

[JHS]

Looks interesting.
I'm tinkering a bit about the connection of R23 to ground on the outputstage, no decoupling cap or a connection to VREF.

JHS

[Mark Hammer]

Sticking with the passive theme, How about using a dual gang pot?
(image omitted)
Wire it up as above so when turned one way the resistance decreases to ground and the resistance to the output increases on one side of the signal and vise verser when turned the other way.
I would say use a 100K for the pot still but dual?
pyrop

A good idea and reasonable to implement.  I set design constraints for myself that would involve components that I was sure people could at a Radio Shack on a Saturday afternoon.  Hence the single pot approach.  I *am* a big fan of using dual-ganged though.

Looks interesting.
I'm tinkering a bit about the connection of R23 to ground on the outputstage, no decoupling cap or a connection to VREF.
JHS

Yeah, wondering about that myself.