Author Topic: Switchable octave up  (Read 281 times)

Breadbored

Switchable octave up
« on: February 22, 2021, 03:23:54 AM »
Hi! I'm planning an octave up fuzz build similar to an Octavia with a transformer and two diodes for the octave up stage. I'm thinking that it might be nice to toggle the octave on/off and I'm thinking that I can just bypass the diodes with a DPDT, however it dawned on me that it'll short out the transformer secondary and drop the reflected impedance on the primary. Can it be done this way or will it cause problems?

anotherjim

Re: Switchable octave up
« Reply #1 on: February 22, 2021, 04:34:35 AM »
It survives. An odd side effect when the input signal is distorted and asymmetric (guitar usually will be) is that the rectifier doesn't cancel 100% when shorting and you get a spikey waveform that sounds very brassy or "Bronx Cheer". However, the amplitude is reduced obviously compared to the non-shorted state. An interesting side-effect is that reflection to the primary creates the same wave character on the driven side.

You can also get there by mixing the input with the full rectifier in a blend, since one of the rectified half waves will cancel the input and there is only the normal reflection back to the primary. This is only mildly brassy (more trumpet-like). Without the shorting, the transformer output wave is more like the input so the cancellation is fuller and you don't have the spikes.

I think it's more usual to simply disconnect a diode with the switch, not short it. Shorting is more interesting though.



Croeso i Diystompboxes.

iainpunk

Re: Switchable octave up
« Reply #2 on: February 22, 2021, 09:15:51 AM »
the follwing schematic describes how i would do it:

ignore that its a humbucker, and immagine it being a transformer.
the downside is that the non-octave side is more than twice as loud, so clipping to limit the volume is a welcome addition after the octave stage.

cheers
half man - half snail - 6 feet to scale - Snail man's - not frail - He's been - to jail - snail man is fuccing real
『snailpilled』

Breadbored

Re: Switchable octave up
« Reply #3 on: February 24, 2021, 04:59:57 AM »
Thanks guys! I'll just have to breadboard and try it out   :icon_biggrin:

anotherjim

Re: Switchable octave up
« Reply #4 on: February 24, 2021, 11:04:20 AM »
Iains' scheme is a rectifier bypass, it doesn't provide half-wave itself.
I think there are many options with the transformer Octaver. It would take a multipole rotary switch to get all of them in a single pedal design.
I've just boxed up my own transformer design. I'll be updating the thread for it soon.
Croeso i Diystompboxes.