Help: Boss Oc-2 Octave Doubler Mod plan.

[thehallofshields]

Hey guys, I want to pack a discrete octave doubler circuit into the OC-2 simply using a pair of diodes and the 273-1380 transformer. Something simple like:



Since both this plan and the output of 1/4 octave of the pedal go to 100k pots (I'm assuming Logarithmic), I figured I'd just wire each output to a dpdt before going to the pot.

I have a weak understanding of Octave Dividers (or doublers) and especially the OC-2's 'flip-flop' circuitry, but I figured it would be sensible to try tapping the input pins of the dual-opamp and buffering it through a JFet to use as the input of the Octave Doubler (transformer).



Anyone want to help me out with this idea? Shoot it down? Call me a noob who's in over his head? Any feedback would be appreciated.

Thanks.

[Skrogh]

"You're a noob who's in over his head!"
All right just joking there...

I'm not entirely sure I understand what you wan't to hook up where, but I can tell you for sure: There is no way you're going to fit a transformer inside the original box
If I understand what you are saying correctly, the theory is right though. (but I'm note quite sure where you want to tap the input to the octave-up circuitry from)
Try pointing out what you want to go where on this schematic:
http://www.hobby-hour.com/electronics/s/oc2-octave.php

[thehallofshields]

Hey Skrogh, I read some of your comments over at talkbass. Basically, I'm not sure if there is any way to get at the opamps square-wave before it gets flip-flop divided.

If, say, one of the pins has it, but is simply not in use, I could make great use of a squared fundamental signal. I run it through -2Oct's path, send it straight into -2Oct's pot, or put into a small octave up circuit.

As far as the transformer for the octave up: the 273-1380 from radio shack don't look too big. I know it's crowded in there, but I figured I could stash the transformer in the battery compartment. -This is just a guess, and I probably shouldn't have filled my post with confusing speculation.

Again, what I'm really asking is: can we get at the squared fundamental Q anywhere in the Oc-2 circuit?

Thanks.

[Skrogh]

The "fudamental" sqarewave is easily accessible. There is one in phase at pin 13 of IC7 and one out of phase at pin 12. Both will be about 90degrees out of phase with the original input as far as i remember. And a bit less synthy and more fuzzy than the oct-1 sqarewave.
Now how to shift the fundamental up an octave and still have it sounding good, is the real issue here.

By the way, since you're on TalkBass too, I'm guessing you are using it on bass? In that case, are you going to hard-wire it to the oct-2 section or planing on having a switch to get back into "stock mode"?

Best,
Søren

[thehallofshields]

Thank you very much for the info Skrogh.This gives me a good starting point.

Now would it be worth it to try to send the out-of-phase, squared fundamental through a capacitor(s) to get it in phase with the direct? I can't quite imagine how a 90* offset square-wave would interact with a sine.

I think I'll ditch the octave up idea for now; I'll save that for a straight up fuzz. I know newbs are always 'flip-floppy' with with their questions, and I wish I could re-title this thread now. And no I don't play bass, but the guys over at talkbass had some great ideas, and all seem to agree that the -2Oct is a bit 'gimmicky'.

My plan as of now:

(1) Put Oct2 and the Squared Fundamental on a dpdt on/on/on switch. I plan to put the 1-meg resistor on the switch's out. (I'm just assuming that the voltages are similar.)

(2) Wire pots between the gates of the 2sk30a's of the -1Oct and the -2Oct paths to their corresponding volume pots. I believe this will give me the 'synth-mod'.
-I need to spend some time tinkering to figure out what size of pots to use (could use some advice), but this way I should be able to mix the filtered and square waves to taste.
- I do expect to run into some phase problems in doing all of this, and I'd appreciate any tips in that dept.

[Skrogh]

Just build the OC-2 in LT-spice (circuit simulator). The fundamental is almost in phase with the input, so no trouble there (as long as you tap it from pin 13). It is however a pulsewave and not a squarewave (and really loud!).

About the pots, depending on what version (Jap/Taiwan) you have, fitting them in there will be difficult.

I cracked a little smile from the 'flip-floppy' comment hehe... hey we were all newbs once, I consider myself a bit of a newb too.

[thehallofshields]

Okay I'm thinking a little bit more about this Octave-Up idea because I bought the 273-1380 Audio Transformer at Radio Shack for $3 today and it fits very neatly into the Boss battery compartment. But now I'm wondering what the result of Full-Wave Rectification of a Pulse-Wave would be? Would I just end up with a solid DC current? - That would suck!

[amptramp]

You don't end up with DC unless you add a filter capacitor.

There is another method of getting a double frequency output and that is to use the identity:

cos(2x) = 2cos²(x)-1 where x is an angle or 2*pi*frequency*time

You can use a multiplier IC like a Motorola 1496 to get a square of the signal and use capacitive coupling to eliminate the "-1" term which is a DC component.  If you use fullwave rectification, you get a lot of harmonics that may not be desirable as shown in:

http://www.falstad.com/fourier/e-fullrect.html

Just drag your cursor over the harmonic magnitude elements in the picture to get the exact magnitudes and over the phase elements to get the phase.  As you can see, Figure 30 in the first data sheet on the following site has already done it, but beware of overloading - analog multipliers like to operate at a few hundred millivolts:

http://www.datasheetcatalog.com/datasheets_pdf/M/C/1/4/MC1496.shtml

Fullwave rectification as shown has a threshold of two diode drops with a bridge rectifier.  A centre-tapped transformer would give you a better sound with only one diode drop.

[Skrogh]

Fullwave rectification of a pulse-wave would in an ideal world leave you with DC, but in the real world you'll probably end up with something like two very narrow pulses for each incomming pulse.

Amptram has some good points, but i think an important question to ask is what kind of octave-up sound you want?
Distortet and nasal? (Fullwave rectifing)
Distortet, but somewhat smoother? (squaring, like amptramp talked about)
Wild and fuzzed? (I think the shoctave includes a pulsewave oct-up, I'll look into that)
"Clean" (Clean as in EHX POG. This can't be done analogly, without a rack unit)

If you allready have the transformer and the parts needed for the titanboost, I'd breadboard that before putting it in the OC2. Find out if you like the tone of it

[thehallofshields]

Well, I did it... sort of.

I gave up on the Octave-Up idea (out of space), and instead:

On left dpdt: -1oct and -2 oct bypass the filtering and go straight to their volume pots. I'm pretty sure this is the same as the regular 'Synth Mod', but since I had an MIJ I had to make my own baby-board for the pots and Led so I could have space for switches.

On right dpdt: Squared (or Pulse) Direct Signal, and cuts the Clean. This adds a lot of fun to the sound. It's messier than the squared -1oct and sounds more like a punchy overdrive than you might expect. I gently soldered a 4.7 meg resistor to leg 3 of IC7, and with that huge resistance, it ends up being just below unity gain with the direct pot at max.

One problem. Any added capacitance to 13 or 3 of IC7 causes the pedal to leak a tiny bit of fuzz, even when the circuit is closed and the pedal is bypassed. I am doubtful that shielding the wires will fix this, so I can live with it for now.

[thehallofshields]

I'd like to upload an .mp3 clip. What hosting service do you guys typically use to embed audio?

[Skrogh]

I'd like to upload an .mp3 clip. What hosting service do you guys typically use to embed audio?

I use Sound Cloud. http://soundcloud.com/
Cool that you got it to work. If you have problems with noise in-between notes I have schematics for a gating "hack".

[thehallofshields]

If you have problems with noise in-between notes I have schematics for a gating "hack".

Oh no way. That would be awesome. As it is, it sounds like microphone rustling and popping between notes, especially with the Pulse-Wave. It's tolerable, but my digital reverb (RV-5) is registering the spikes as 'attacks' and it tries to add noise-artifacts to them (like a spring reverb 'boing'). -Digital Reverb gets too messy to use.

[Skrogh]

It's kind of hacky. I'm using the comparator, normally comparing a low-passed clean with the negative-envelope - but I'm comparing the lowpassed-clean with a fixed threshold. This makes tracking a tiny bit worse, but gates the notes really clean. Since the J-flip-flip triggers on a rising edge from the positive-envelope-comparator (made that word up) tone should be the same.  There is a better way of doing this, but it involves extra ICs.


Original from:Boss OC-2 Dual Octave Down Guitar Pedal Schematic Diagram


There are two mods here, my "bleed less"-synthmod and my "hard gate"-mod.

Explanation:
--Break connection between the red wires.
--Connect (1,1) in the first schematic to 1-1 in the second schematic. (1,2) to 1-2; (2,1) to 2-1 and (2,2) to 2-2
--Both switches are shown in "stock position".
--Vb is the bias-voltage, available between R45 and R25.
--This symbol: _|_ means "ground", connect it to the casing, or anywhere with the similar symbol in the b/w schematic.