Reversing LFO Phase

[El_Chupanibre]

Hello! First post on this lovely forum. I've had some experience with MadBean's PCB's, but never came to the forum (as they were so well made I never needed any help.)

Anyway - I'm curious about flipping the phase of an LFO to get both an in phase and out of phase swing. I thought it would be simple enough to take the output of the LFO and split it between an inverting and non-inverting opamp stage set for gains of -1 and 1 respectively. I am getting some wiggle out of the non-inverting output, but the inverting output only varies ~0.3v around 8v (whereas the non-inverting output is giving me 2.6-5.9V depending no the position of the controls.

I think my problem might be with how I'm biasing the inverting opamp. The non-inverting is simple enough with the LFO feeding into the + input and the gain set to 1 with two 10k resistors. The inverting opamp is also set up with 10k resistors for a gain of -1, but the bias isn't set to ground. But first the LFO.

The LFO in question is the tremulus lune's LFO based on the schematic from fuzz central (http://fuzzcentral.ssguitar.com/tremulus.php). The only modification I've made is getting rid of the 'fine' pot.

My thinking (which is most likely wrong) was that the 'Spacing' control changes the bias of the LFO depending on the position of the potentiometer. When the spacing is roughly in the middle, the bias is around 4.5V, but change it in either direction and the bias changes as well. So if in the inverting opamp I were to bias it back at 4.5V, the spacing would be nullified or at least not be an out of phase replicate of the original signal. I biased the inverting opamp to the the juntion of lug 2 of the spacing pot, the + input of the 4558 and all them resistors. As I type that out, I realize that that cannot be the right thing to do... but I'm not exactly sure why.

In summary: I'm trying to get in and out of phase outputs from the tremulus lune's LFO. If someone see's where I went wrong or has a better idea, I'd love to hear it!

Thanks a lot and I look forward to working with the forum.

[madbean]

Have you tried running the inverted op amp sequentially after the non-inverted, or are you running it in parallel?

Sorry, TL;DR all of your post so I'm just throwing that out there.

[El_Chupanibre]

No, I have not. But that leaves the problem of how to bias the inverting opamp.

[midwayfair]

There's another way you can do it: you can feed the output of the op amp to a complementary pair of transistors (an NPN and PNP). I used this method recently to convert a CV to out of phase LED drivers and I found it to be simpler than trying to do it with an op amp, but then I'm not very good at tinkering with op amps.

I'll see if I can remember an example of it with the Lune's LFO.

Essentially, you'd hook up the CV output (that's pin 7 of the op amp) to the bases of the complementary pair, put a small resistor (~470R-1K should be fine) between the transistor and a power rail (I think doesn't really matter which), and the LED between the transistor and the other power rail.

       9v
        |
        LED
        |
        C
CV > B
        E
        |
       1K
        |
        G

Note that you will get better performance if you can actually match the transistors and use 1% metal film resistors for the CLR.

[El_Chupanibre]

Wow, that's pretty clever. Thank you both for the very quick responses by the way.

I'll try out the transistor method, but why does that invert the phase? If using a pair of 2N3904/6, don't they have the same turn on voltages, so they'd open up the C-E juntion at the same time?

Another question - what exactly 'turns on' the transistor? The input voltage or the current? Can you parellel multiple strings of transistor's this way / what is the limiting factor here?

Finally, what about the depth control? is it possible to tap the CV out of the lug2 of the depth pot?

Thank you.

[midwayfair]

I'll try out the transistor method, but why does that invert the phase? If using a pair of 2N3904/6, don't they have the same turn on voltages, so they'd open up the C-E juntion at the same time?

Another question - what exactly 'turns on' the transistor? The input voltage or the current? Can you parellel multiple strings of transistor's this way / what is the limiting factor here?

Finally, what about the depth control? is it possible to tap the CV out of the lug2 of the depth pot?

The Lune's LFO controls an LED. If you're inverting the phase of it, it's because you need the control voltage to turn on two LEDs in opposition. That's what the transistors will do. The potential of the LFO's CV will determine which "direction" the transistors will pull current when the voltage swings in a certain direction.

I'd breadboard this first, obviously. For one thing I'm not sure if you need to decouple the LFO output.

The limiting factor is the drive of the OP amp, I'm pretty sure. There's an upper limit to how many things you could drive, and there will be some loss with each transistor, I think. (But the transistors also help prevent ticking in the unlikely situation it appears, so there's that.)