Anderton's Octave Up?

[Dave_B]

Yesterday, for the first time in years, I re-read the article for Craig Andertons' Roctaver.  

At the end of the article he talks about being on the verge of a breakthrough for an octave-up circuit.  Does anyone know if he ever published anything on it?

[Joe Viau]

Both the Rocktave and the project that you allude to, the Octave Doubling Fuzz, are in Craig's book "Do-It-Yourself Projects for Guitarists."  It's a good read.

Check out this link:

http://diystompboxes.com/sboxforum/viewtopic.php?t=20937&start=0&postdays=0&postorder=asc&highlight=octave+doubling+fuzz

[Mark Hammer]

If he was on the verge of a breakthrough, the Octave-Doubling Fuzz wasn't it.

It's an okay octave-up unit, I suppose, but my feeling is it needed some refinements, and probably some hand-picking of components.  That being said, it is a very flexible unit.  Note that the fuzz and mixing stages are inverting, but the full-wave rectifier stage (that comes after the fuzz, for the octave up) is inverting too.  What that means is that the octave signal is opposite phase to the fuzz signal when they both reach the mixing stage.  The octave level control doesn't simply add "more" octave.  In fact, you'll get more octaving at less than max octave signal, because of that.

In some respects, an idealized version of this circuit would have had a simple transistor input buffer (the circuit uses one op-amp from a quad or dual as a standard non-inverting buffer).  The freed-up op-amp could then have been used for an inverting stage between the full-wave rectifier section and mixer.  Rather than a simple octave level control, the unit would now have a panning control (linear pot needed here).

Here is a redraw of the original ODF:
I've added a terminating resistor on the front and omitted the switching.  You can see that U1 is a unity buffer.  U2 is a clipper with a fixed gain of 100 and a full/reduced-treble switch.  U3 is the fullwave rectifier, and U4 is the mixer stage that blends fuzz and octave.  SW2 shunts the fuzz channel to ground for an octave only signal.

I've tinkered with the design a bit here:.
I've added a standard transistor input buffer, stolen from any number of Boss pedals.  Rather than using an attenuator between the buffer and fixed gain stage, I send the buffer directly to the gain stage and stick a 1M gain pot in the feedback loop.  One of the advantages of this is that at lower gain settings, there is minimal treble rolloff, and at full gain, treble is rolled off around 4800hz, so we can scrap the original tone switch.

U2 does what it did before, but its output goes to a unity gain inverting buffer.  The pot after that selects between inverted and noninverted versions of the octave, with the middle being cancellation of the octave signal.  Rather than a simple fuzz-cancel switch, I stuck in a fuzz level control, since I found the on/off action of the original a little limiting.

The gain of the output stage is set at 10, with a rolloff around 10khz.  The original had no "hiss protection" on the output stage, so this treble rolloff helps a wee bit.

This modded version ought to work a little better and more flexibly than the original.  It's still not likely to compare in wildness with the Tone Machine, Scrambler, Blender, Octavia, or any of Tim Escobedo's inventions, but it will provide a pleasingly broad range of textures.

[Joe Viau]

Thanks for the great info, it's always good to revisit old topics every now and then.

[Mark Hammer]

Thanks.  I realized there was an error on the second drawing/schem.  There should be a DC-blocking cap between the FWR "octave" stage and the mixer/output stage.  You can see a 2u2 cap in the original, but I forgot it in the second one.  You can probably stick it between the blend pot and the 10k resistor after it.  I'm sure no one is in a particularly big hurry to amplify whatever DC offset there might be in U2 and U3 by a factor of 10.

[jmusser]

I thought I was getting de-ja vu, and I was! http://diystompboxes.com/sboxforum/viewtopic.php?=28271&highlight=octave+doubling+fuzz

[jmusser]

OOPS!, left out the "t" after the php? try http://diystompboxes.com/sboxforum/viewtpic.php?t=28271&highlight=octave+doubling+fuzz   If I screwed up yet again, it's under the heading of " Craig Anderton Octave Doubling Fuzz Problem" by "fourtwenty" .