Octave up/full-wave rectification

[Openknow]

Hello, I'm new to the site.

I have a very basic question about octave-up effects: why can't we just use a bridge rectifier to create the octave sound? Is it something to do with a high forward voltage due to four diodes being used? And won't this problem be solved with higher gain?

[mac]

Welcome to the forum.

IIRC there is an octave up circuit out there that uses an opamp driving the primary of a small transformer, and a bridge rectifier connected to the secondary. Sorry but I dont remember where I saw it.

mac

[CynicalMan]

The problem is that you have to isolate either the input or output of the rectifier from ground. Otherwise you'll short out one of the diodes and you'll only get half wave rectification. The easiest way to isolate it from ground is with a transformer on the input, but you might as well take advantage of the centre tap and save two parts, as most octave ups do.

John Hollis did use a bridge rectifier with a transformer in his Titan Octave.

Edit: I think that's what mac's talking about.
http://www.hollis.co.uk/john/titan_boost.jpg
http://www.hollis.co.uk/john/titan_octave.jpg

[egasimus]

Why don't you use a phase splitter instead of a center-tapped transformer? I don't know if that'll help, but here's a circuit I was working on a while ago. It lacks input and output buffers, but does double full-wave rectification, at least in theory.

[merlinb]

Hello, I'm new to the site.

I have a very basic question about octave-up effects: why can't we just use a bridge rectifier to create the octave sound? Is it something to do with a high forward voltage due to four diodes being used? And won't this problem be solved with higher gain?

Yes the forward voltage is a problem. Even with higher gain, the output signal will suddenly disappear when the note decays to less than ~0.6Vpk.

Transformers are medieval technology. It is much easier to build a precision rectifier using an opamp. This can use a couple of diodes or even a bridge rectifier followed by a balanced amplifier (to get round the grounding problem). For example, you can see the precision bridge (U3A and D1-D4) and balanced amplifier (U3B) in this circuit (in this case it's actually part of a compressor, but the theory is exactly the same):

[R.G.]

There are a number of reasons, many of which have been listed here. The single biggest one is the small size of a guitar signal compared to a diode drop, or to the drop of two diodes in a full wave bridge.

You can, in theory, solve this with more gain, as you thought. It's just that you have to use so much gain that you run out of power supply voltage to do the gain. You have to make the signal so big that 1.4V of diode drop is negligible compared to the signal being rectified even after it's decayed by a lot. Otherwise you get both odder distortion than FWB gives by itself and the gating effect. The amount of total signal (not just gain!) you have to have before the FWB drop begins to be something that can be ignored for audible effects is quite large if you do it well - multiple tens of volts. The ear is good at picking out some things.

There are ways around this. One is to use a transformer to eliminate dependence on the pedal power supply. The first "Octavias" did this. The center tapped transformer is more important for cutting down on the forward drop you're trying to make negligible than for lowering parts count. Another is to use diodes already biased nearly to conduction. This concept usually requires a solid state phase splitter; it's a technique used in the Fender Blender, Armstrong Green Ringer, and Fox Tone Machine. The Univox Super Fuzz and its half-a-dozen brothers and cousins uses a phase splitter and a forward biased transistor base-emitter junction to do the same thing.

You can use a full wave bridge by converting the signal voltage to a signal current and then sensing the current and reconverting this to a signal voltage; that's what's going on in the EH-5950; using a part as the feedback of an opamp forces a current through the part, and the opamp makes up any voltage differences to make the proper current flow. I hadn't looked at the 5950, but now that I do, that's what's going on. It's funny - they could have achieved full wave rectification with the same number of opamps (2) and fewer diodes (2) and fewer resistors by using the classical "precision rectifier" circuit with two opamps. I can only guess that they were doing something else with the signals that made them want to use this funny floating FWR.

The classical two-opamp FWR has not received much use in pedals. I dimly remember Anderton using one, and John Hollis used one in his omnidrive. That's strange, because the two-opamp precision FWR does the job you want, down to millivolts, and doesn't have other oddities about it.

[azrael]

The problem is that you have to isolate either the input or output of the rectifier from ground. Otherwise you'll short out one of the diodes and you'll only get half wave rectification. The easiest way to isolate it from ground is with a transformer on the input, but you might as well take advantage of the centre tap and save two parts, as most octave ups do.

John Hollis did use a bridge rectifier with a transformer in his Titan Octave.

Edit: I think that's what mac's talking about.
http://www.hollis.co.uk/john/titan_boost.jpg
http://www.hollis.co.uk/john/titan_octave.jpg

whoa, 30 volts peak to peak?? is that safe for guitar? haha