Magnavibe. Chorus? And more volume from op-amp inverter buffer

[nonost]

Hi. Can the Magnavibe do chorus? I'm blending the output with the dry signal and it gets more chorus-ish but I was wondering if it can get any closer. Schematic here:



I'm splitting the signal so I can blend the output wet with the dry signal. The Magnavibe looks like it inverts the phase so I had to use an inverter buffer for the dry signal. I'm using this little circuit from effectslayouts.blogspot.com:


http://effectslayouts.blogspot.com/2019/02/inverting-phase-buffer.html

The circuit works great but the output is a bit under unity. Can I get a bit more of volume from this?

Cheers!

[antonis]

Can't trace layout decently but, as far as I can see, non-inverting out (in phase) is taken by a short between pins 1 & 2 (no room for gain)

Inverting out (inverted phase) utilizes a 10k/10k NFB so you may get some gain (maybe volume or maybe distortion - it depends on signal amplitude) by raising 10k between pins 6 & 7 value..

[Mark Hammer]

"Chorus" - the sense of multiple musicians playing the same thing - will require combining a pitch-modulated version of the signal with a clean unmodulated one.  Tat CAN be done, but there are a few considerations.

One's gut instinct would be to reduce the value of the emitter resistance in the Magnavibe circuit to increase the gain of the transistor.  HOWEVER, its functioning as a phase splitter (equal and opposite outputs from the collector and emitter) requires that those two 7k5 resistors remain the same value as each other.

Having said that, however, there is nothing wrong with using a dual op-amp to provide a wee bit of gain as an input stage feeding the Magna-Vibe circuit, and a mixing stage to combine the pitch-modulated signal with the unmodulated.

Having said THAT, the pitch modulation of the circuit is pretty gentle in my experience, so I'm not sure just how rich and obvious a chorus effect it would provide.

Instead, I will suggest making a dual Magna-Vibe.  So, op-amp input stage feeds two copies of the MV circuit, and their outputs are blended into an op-amp mixer stage.  I made something very similar a few years back and was struck by how rich and immersive it was.  I used a dual-ganged pot for speed (20-25k) and another for depth, such that overall speed and depth could be adjusted simultaneously.

The key thing to remember here is that the individual LFOs are independent and unsynchronized.  So even if you turn them both faster or slower at once, there is no guarantee that the one circuit will be sharp or flat at the exact same time as the other one.  Occasionally they will be close, and occasionally opposite, but you can't predict when.  It is that aperiodicity that makes the audible result so pleasing.  And because the pitch contrast between them is both modest and fleeting there is none of the perceived pitch wobble one normally gets with a BBD-based chorus.

I'm a big fan of the Boss DC-2 - what I consider the King of Chorus pedals.  A big part of what makes it sound so good is the use of two counterswept delay chips.  When one chip is clocked to shorten the delay, the other is clocked to produce a longer delay.  The result is never consistently flat OR sharp, but always a bit of each.  I think what I have suggested here aims for a very similar result.  Perhaps even better since the "oppositeness" comes and goes.

My own "double Magna" is sitting in a state of disrepair after taking it apart to rehouse.  I need to get back to it and make a demo of some kind.  Incidentally, it sounds A-MA-ZING in stereo.

[nonost]

Thank you for taking your time Mark.

The MV is subtle, yes. I know that I can't expect a real chorus. The tremolo it's quite good and easy to get BTW.

Can the mixing stage be just a blend pot? I'm afraid I don't have room in this box for anything else. Right now the output it's slightly under unity, but it might be my ears.

I'm using the dual op-amp "schematic" posted above. IC1a buffers the input signal and I send it to the Magnavibe. IC2b inverts the input signal(dry) and I send it to the blend pot. The output of the Magnavibe(wet) goes to the blend pot

So, I can lower the emitter resistors or increase the feedback loop resistor that Antonis mentioned. Right? I'm increasing the 10k resistor since it's faster. Thanks Antonis!

Yeah, the Boss is really really good. But quite complex!

I came across your post about the stereo MV time ago and I'm sure I will end up doing it.

[bluebunny]

[aside]

Yeah, the Boss is really really good. But quite complex!

Search out the Dimension-P (on this forum).  Does a similar thing with LFO counter-sweeping two PT2399s.  Sounds pretty good.

[/aside]

[Mark Hammer]

Kind of makes one wonder what other sorts of effects might yield interesting and satisfying sounds via countersweeping.  For example, where a pedal normally uses a pair of bandpass filters to respond to the envelope, what if they were counterswept, or even LFO-modulated independently?

Part of what makes the Dim-C so "complex" is all the additional circuitry needed to manage noise.  The PT2399 chip also has to contend with noise but has the lowpass filtering incorporated.  Those little ceramic 100nf caps aren't there for nothing!

[nonost]

I put more gain into the MV, but you don't get more modulation. Although it helped to get unity at not full vibrato settings.

I get some chorus-ish sounds with the blend, but it also get some tremolo? Kind of weird.

[nonost]

I've always felt the MV as having a bit of volume modulation. With this mod is more obvious. When you mix dry and wet signals you can hear it. The more dry signal the more you hear it. With such a simple design I'm afraid there is not much more room for improvement but, is there any way to reduce this volume modulation?

Cheers!