Help me understand how the Superfuzz Octave section works please

[Derringer]

I understand that the first Q shown here operates as a phase splitter --- to create two signals that are equal in amplitude but 180 degrees out of phase

But I can't figure out why the next part of the circuit creates an octave and doesn't just have the opposite signals canceling each other out.


I'm sure that it has something to do with the collectors being tied together and sharing a resistor and the emitters being tied together and sharing a resistor and bypass cap ... but I don't know enough to understand why this configuration does what it does.

in this thread : http://www.diystompboxes.com/smfforum/index.php?topic=75007.0

R.G. said with regards to octave acheiving "The only difficulty with diodes, and the reason for the plethora of octaving circuits, is that we want to rectify very small signals with them, smaller than the necessary forward drop to turn on a silicon diode. The Green Ringer and the Fox Tone Machine (and its clone/copies) use two ordinary diodes, but bias them with a bare trickle of current so they are just at the edge of conduction and only a tiny signal is needed to turn them on to pass their appointed phase. The Blender is similar. The Univox Super Fuzz uses the base-emitter of bipolar transistors in a similar manner, but adds the quirk that the rectified signal is amplified at the same time.[/b]

can someone elaborate on that for me please?

thanks!

[R.G.]

But I can't figure out why the next part of the circuit creates an octave and doesn't just have the opposite signals canceling each other out.
I'm sure that it has something to do with the collectors being tied together and sharing a resistor and the emitters being tied together and sharing a resistor and bypass cap ... but I don't know enough to understand why this configuration does what it does.

Your suspicion is correct. All you're missing is that the transistors are active devices, and in this case switching from active to off operation.

First, let's mentally think of the two transistors as being truly identical, a perfect match, and all the resistors as being perfectly precise at their stated values. That means that the two bases are sitting at the same voltage in addition to the obvious one of the emitters and collectors, which are tied together. This will force each transistor to be conducting exactly half the current flowing through the collector and emitter resistors, so the two are exactly balanced.

When we apply a phase-split signal to the bases, one is forced up and the other is forced down. The one that is forced up pulls more of the combined current, the one which is forced down pulls less. The capacitor on the emitters lets the two transistors temporarily pull more or less current, so the currents are not perfectly fixed, as they are in a differential amplifier connection. The one that is forced up causes the collectors to dip.

The one that is forced down... turns off. It is biased into active conduction, but it's on the low end if its signal current. Just like in a multiplier cell, the less current it pulls, the more its hfe drops, so the effect is more than simply it pulls less current. And with a few tens of mv lower on the base, it turns off almost completely. The other transistor is now amplifying and inverting the positive-going half cycle for the rest of the half cycle, until they come into balance again.

When the signal reverses, the previously-off transistor becomes the active one, and makes a similar waveform on the collector. So each half cycle is formed into the same signal direction at the collectors by the action of the down-going transistor turning off and not participating. You get a downward-going full wave rectified waveform at the collectors.

There is only a tiny bit of handwaving in here. The action is not truly switching, but it's very close.

[Derringer]

going to take a few more slow reads ... but I think that I have the gist of it


Thank you !

[earthtonesaudio]

How different is this from the same sort of configuration with the BJTs replaced with JFETs or MOSFETs?

[Derringer]

still works with FETs apparently

www.geocities.com/tpe123/folkurban/fuzz/snippets.html

scroll to the bottom for the "octup!"

[R.G.]

still works with FETs apparently

But differently. See the articles at GEO on the JFET doubler, MOS Doubler, and Mu-doubler.

For FETs, the gate-source voltage relationship is such that the FETs don't come so close to turning off. Rather, both devices continue to amplify, but the amplified result does cancel at the drains. However, the second order distortion reinforces, and there is rather a lot of that, given that FETs are square law devices. In this case, the result is not a full wave rectified waveform, but rather much more like a smooth squaring of the input waveform. The amplitude is much smaller as well, given that the amplified part of the waves cancels, leaving only the much smaller distortion products to reinforce.

[Derringer]

"It's a peculiarity of distortion that even order distortion is single directional; this means that Q2 and Q3 generate second order distortion that zigs the SAME way instead of opposite, so the second order distortion products reinforce at the drains instead of cancelling. "

Now that' interesting.

The link to the MOS Doubler seems to be dead. Any chance you can fix that up?


all this reminds me that I need to build a mu-amp and start experimenting with it

Thanks as always for all the info RG!

[R.G.]

I'll see if I can find it. However, the MOS doubler suffered from needing a post-amplifier to get a good output level. That's the real reason for the Mu-Doubler.

MOS devices are even more linear than JFETs, usually. That should be good, but in this case the distortion is what we're looking for. So it needs a whalloping gain to get the tiny distortion back up to level. The Mu-Doubler is actually a better place to start.