ACE tremolo - Adventures with a Magnatone-inspired headphone amp

[samhay]

Having been seduced by Martin Simpson's recent use of a stereo Magnatone amp (https://www.youtube.com/watch?v=I-ecWcnz8Tk), I thought I would have another look at the Magnavibe. As with the amp, it sounds better in stereo with the 2 channels driven with LFOs that are 180 degrees out of phase - hence my experiments with anti-correlated LFOs.
The only stereo guitar rig I have is a pair of headphones, so I moved on to using op-amp phase shifters that can do double duty as headphone amplifiers (note the over-sized C11 and C12). NE5532s can quite happily drive most headphones and it doesn't seem to need to be anything more than unity gain. I figure this can sit at the end of my pedal board and do double duty as a mono effect and stereo headphone amp.
The Tremolo control is equivalent to a dry mix control and fully CCW you get a high pass filter with the corner frequency swept up through the audio band - a tremolo of sorts. With the tremolo control CW you get a 180 degree phase shift like the Magnavibe.
The LFO is the Magnavibe PSO with a couple of single-supply op-amps working as LED drivers and phase inverters. The depth control works like a buffered version of that from the Tremulus Lune and the op-amp must be able to tolerate input at ground. The LFO / LED driver works, but is not perfect and I think I will have a play with a PIC implementation before I commit to building it.

Schematic (view for larger version):




Sound clip (recorded direct, probably best with headphones):

https://dl.dropboxusercontent.com/u/11996927/ACE_tremolo.mp3

Dry (Depth at 0) then Tremolo CCW and phase shift off - i.e. LFOs in phase. This is followed by phase shift on (LFOs out of phase) and Tremolo incremented to CW. Depth and Speed changes should be fairly obvious.

[Kipper4]

Nice work Sam 

[samhay]

Thanks Rich.
I realised I hadn't decried the clip properly - the first part is with the LFOs in phase via the Phase Invert switch and I have updated the description in the post.
The option of playing with the the phase of the two LFOs is quite nice and I am planning on playing with a dual LFO PIC where you can vary the relative phase of the 2 LFOs. It's weird though, once you have been playing in stereo with the LFOs anti-phase for a while you stop hearing it. It comes back once you have rested for a while - amazing what ones brain can get used to.

[blackieNYC]

I won't be able to listen for a while, but isn't the right channel going to get a squarer waveform than the left?  Due to those 220k s?
Very cool.  Have you considered recombining to mono, perhaps with different filtering?  Or, I wonder what it would sound like now, in inverted L phase mode and mono?  Maybe not much, but it must do something a little interesting.
I have a (insert name of any commercial pedal here)Boss tremolo with stereo in and out that I love so much I want to replace it with something I built.  Like this.

[samhay]

IC2B has a gain of +1 or -1. It works like this:
http://www.geofex.com/article_folders/polarity_reverser/polarity_reverser.htm
The difference is that there is a BFC (C16) that allows the DC offset found on the non-inverting input to be maintained. The 10M resistor is a pull-down resistor to stop the BFC from popping and can be considered an open circuit.

I have thought about adding a mono/stereo switch that switches the IC1A input to the IC1B output. This will give you 2 stages of phasing if the LFOs are in phase, and something more complicated if they are out of phase. Will give it a try and see whether it's worth an extra switch.

This is mono-in, stereo-out, so won't quite replace what you have. Doesn't mean you shouldn't consider building something though.

[samhay]

Have you considered recombining to mono, perhaps with different filtering?  Or, I wonder what it would sound like now, in inverted L phase mode and mono?  Maybe not much, but it must do something a little interesting.

I have updated the schematic to show input switching to IC1A. I don't know what to call this switch, but it is a nice addition if you are predominantly going to use this a mono effect. The output from IC1A become a lot chewier - as would be expected for a 2 vs 1 stage phase shifter and it works with the LFO phase switch in either position. The stereo sounds ok in this mode, but the effect is obviously less symmetrical.

[deadastronaut]

nice work sam ....sounds cool, on headphones 

[samhay]

Thanks Rob. Does that mean it sounds like arse on anything else?
In any case, I will make a clip of the new Mono/2-stage option, which sounds quite acceptable in mono too.

[deadastronaut]

yes ha ha...

no it means mrs astro was watching the bloody tv...

[samhay]

Figured I should post an update as I had a question about the digital LFO, which is discussed here:
http://www.diystompboxes.com/smfforum/index.php?topic=109015.0

The project is still alive and I am in the process of building this:


(click for larger version).

I will post code and a vero layout once it it verified and tweaked to my liking.

The pic12F1612 is new and has 2 semi-independent PWM outputs (same frequency, different duty cycle) so it can drive 2 LFOs.
The LFO controls are:
mode 1: depth, speed and phase (arbitrary phase difference between 2 LFOs; 0 - 180o).
mode 2: depth, speed L, speed R (i.e. the speed of the 2 LFOs is independent, but they share a common depth setting).

[Kipper4]

Super stuff Sam.
This is going on my list.

[~arph]

Cool, I like the separate phase and speed settings, that is something I have not heard before. Looking forward to hear how that turns out

[samhay]

Was reminded of this tread today.
Links are all gone thanks to dropbox, so will repost the analogue LFO design. PIC design got put on the 'needs work' pile.

Schematic:
https://www.dropbox.com/s/gun6xwy6993xns1/ACE_tremolo_schematic.jpg

Audio clip:
https://www.dropbox.com/s/svk2ij1nlkjzds6/ACE_tremolo.mp3

[Kipper4]

Quickie Sam if you don't mind.

Did you try any different values for c7 c8

Say c7 47nf
      C8 68~82nf ?

[samhay]

I don't recall exactly, but as C7 and C8 are the C's to the LDR's R's, I must have auditioned them to some extent.
I think they were a compromise to get a decent sweep out of both the tremolo (actually a variable HPF) and phase shifter.

By all means, one should tweak these to suit the LDR used.
For reference, it would appear the equivalent cap in the Magnavibe is 100n:
http://revolutiondeux.blogspot.com/2012/01/bigfoot-fx-magnavibe.html

Edit - probably didn't try different values for the 2 stages as I was trying to make them equivalent. Could be interesting. Could be even more nauseating.

[Kipper4]

Obliged thank you.

[samhay]

If you give it/something like it a try, please let us know how it goes.
Might give me some impetus to get this working properly again.

[Kipper4]

Will do.
Cheers.