Stereo Magnavibe - sweet

[Mark Hammer]

Wanted to add some spice to a little lunchbox amp I'm making, so I spent the afternoon and evening perfing a pair of unsynced Magnavibes.  They have individually adjustable speeds, but I used a dual-ganged pot to adjust the intensity of both at once.  I need to figure out how to get the LFOs to sweep slower than they do, but at the moment it is simply luscious.  Used one at a time, Magnavibes add a pleasant jiggle.  But used two at a time, in stereo, it's immersive, like a Dimension C chorus, though not quite as thick.  I described it somewhere else as if your guitar turned into a snowglobe.

Recommended.

[duck_arse]

more info please, Mark. stereo two outputs? stereo split and mixed to single output? 2 identical magnas, or different Lyman caps each?

I did some messing w/ 2 stage magnavibes a while back, but single osc. and I did some range switching kerfuffling recently, gives hi-mid-lo ranges [but w/ 4 cap pso]. how slow do you want your oscs to run, 1/2Hz? 1Hz? 3Hz?

[Mark Hammer]

Good morning from chilly Canada.

Mono in. Dual identical outs.  I used this schematic.  Not having 4m7 resistors on hand, I used a pair of 2m2 in series.  The pots were 20k, so I used 47k in parallel with the speed pot to compensate.  The dual-ganged 20k adjusts intensity for both at once.  Red LEDs and 500k dark LDRs from Tayda.  I still want to make it sweep at a lower rate.  One on its own probably wouldn't sound particularly thrilling at a slower rate, but in stereo, you still get movement.

[anotherjim]

Have you tried increasing the x3 1uF in the PSO? Maybe 2.2uF? Maybe it will still work if you only increase one of them & be enough? I've never really tried seeing how different the 3 caps can be -  we always pick 3 the same.

[Mark Hammer]

In fact, using 2u2 instead of 1uf was my thought, initially, when I saw that I couldn't make the LED flash especially slowly.  I may just do exactly that in future.  For now, I simply want to box it up and hear what it does in the context of the rest of the amp.  I've also got too many things to tend to before we motor off to the in-laws for a week of hostag...er, festivities.

I have a small stereo Fender Sidekick amp with a stereo effects loop.  I may just try and kluge an improved version into that amp.  I've already experimented with running a phase-shifted signal through one of the channels (clean in the other) and it sounds nice.  I imagine a pair of unsynced Magnavibes would sound even nicer.  The stereo chorus sunds pretty decent, but isn't quite as animated, because you've got one of the channels run dry, rather than modulated.  Modulating both will be more animated.

[Marcos - Munky]

Interesting. Do you think the stereo outputs mixed to a single mono output would give a similar result?

For the lfo, it's probably worth to use a different one, maybe the tremulus lune lfo.

[Mark Hammer]

I doubt that mixing to mono would provide the same richness.  The effect of one stage is fairly subtle.  Mixing two of them to mono would simply provide aperiodic modulation of a subtle effect.  My sense is that the stereo arrangement is required to have something this subtle seem animated.  On the other hand, if each unit was two stages (a little more pronounced effect) and used different value collector-output capacitors, such that the two Magnavibe outputs were audibly different from each other, then perhaps a mono mix would be pleasing.

I guess what I'm trying to say is that the spatial element helps to make a subtle effect seem more obvious.

[thermionix]

Pardon what's probably a dumb question, but isn't that a tremolo rather than a vibe?

[Eb7+9]

time varying mixing of phase/anti-phase signals is a common way to produce phase-shift offset in the Bode responses ...
the circuit is plotted here with the resistance swept logarithmically between 100r and 100k:
(see Wilder, US patent 3,418,418)



the Univibe variant of the idea doesn't have the treble drop component in its response as this one does ...
one could say this earlier approach has a two-in-one thing going for it

[Mark Hammer]

As the graphs show, however (and thanks for that JC), the impact of the phase shift on the audible portion of the guitar signal, is not especially severe for the upper frequency content of the guitar.  One needs to keep in mind that one sort of runs out of frequency content from a guitar above 5-6khz.  So, while there IS treble loss, it has to stand at the back of the line, relative to pitch change.  But the slight modulated treble content doesn't hurt.

[ljudsystem]

Cool thread !

I've been thinking of building something like this too. I tried the magnavibe blended with a clean signal into an amp but that just resulted in a rather dull tremolo effect (I guess mixing two singnals out of phase cancels each other out...?) spliting the signal and sending it to two amps is probably a different matter. If you place the amps far apart you might get enough time delay for a chorus type effect...


Have you thought about splitting the signal with a crossover filter like in Briggs dual band vibrato?

[Mark Hammer]

If I was aiming for a deluxe effect pedal, I might.  This was just intended to make a lunchbox-sized battery-amp sound a little more interesting, with as little fuss as possible.

But it's a good idea.

[Mark Hammer]

Preliminary status report:  I'm still having a tricky time getting a clean stable sound from the Hammond Meteor front end (probably a question of the interfacing between preamp, effect, and power amp stages), but even with a pair of TBA820 amp chips driving a pair of 3" full-range speakers 4" apart, it sounds magical, even from a few feet away.  I may just follow up on some of the suggestions for slowing the LFO rate down, but in general I'm pleased with the psychoacoustic outcome.

If you're planning on a stereo amp or rig, this is a really nice addition.  Not overbearing or disorienting like the Ibanez Flying Pan.  Subtle enough to not be disruptive or distracting, but enough to capture one's attention.

[anotherjim]

There are a few effect types that only work properly when 2 or more signals are mixed "in the air", so if electronically produced, you need separate amplification. For some reason, you cannot mix them down to mono and get quite the same sound.
There's an effect I know that was once commonly heard - and I don't know its proper name. The way pipe organs could make a sort of vibrato/chorus was to have the separate rank of detuned pipes. Each was about 8Hz flat or sharp or the normally pitched pipes. The result when the detuned and normal are heard together is a beating intermodulation frequency of about 8Hz. When electro-mechanical and electronic organs came along, they had to use separate tone generators and amplification. Personally, I always find that sound annoyingly bombastic, but I've only really heard it on recordings. Maybe it's another thing altogether if you're in the same room? Well, anyway, that's one effect you are never going to hear on guitar. That detuning needs to be on every note by the same number of Hz. You can't even do it with a synth. Only a sample-based instrument can reproduce the effect as far as I know.

Does anyone know any articles on "mix in air" versus electronic mixing?

[Mark Hammer]

At least once a year, the general topic of rotating speakers, and pedals that emulate them, comes up.  And the consensus of those who own or use both is that using an emulator pedal in mono captures only a modest amount of the magic found in a true rotating speaker that distributes the effect around in space.

So yes, jim, there are certainly some "stereo" effects that cancel out in air; mostly pedals that use a common LFO and provide sum and difference signals out of the two outputs.  But some effects really only acquire or express their liveliness by being distributed in space.  While digital emulation can be pretty decent, standing in a true physical reverberant space yields different reflections reaching each ear.  It may not sound like "stereo", in the sense of clearly distinct things reaching (or rather, directed at) each ear.  But I'll wager that a binaural recording of, say, a parking garage, or cathedral, would be easily differentiable from a mono digital emulation as a "true" physical space.

That said, it is always tricky to balance off the right amount of effect and movement that creates listener interest, against the distraction of too much movement and having to rapidly divide one's attention.  What I like about this stereo Magnavibe is that the effect IS subtle, and strikes the right balance.  If it were, say a pair of unsynced phasers, it would be unlistenable after 30 seconds, because the attentional demands of two sound sources so different would be fatiguing.

[Eb7+9]

... frequency content from a guitar above 5-6khz. 

look carefully Mark, there's already a couple of db's shaved off at 1k ...

[Mark Hammer]

Understood.  For me, it's more a question of what sticks out more and commands the attention of the listener..

Some effects have multiple parameters modulated, with some parameters sticking out more than others.  For instance, Uni-Vibes have an impact on the overall signal ampitude, but what we focus on is the moving EQ dip.  Rotating speakers also modulate the overall amplitude and bandwidth, with a bit less volume and treble as the rotor points away from the listener.  But that's not what we pay attention to most.  It's not unimportant to the overall effect, and would certainly leave the effect wanting if we could somehow remove it.  It's just not how we experience the effect when our limited attention is at the heart of the experience.

When Uni-vibes and phasers are compared, a big part of their audible difference is how each directs the listener's attention.  Vibes ARE fundamentally phasers, but since their impact is to create wide and shallow dips, rather than tighter notches and peaks, the listener's attention is directed in a different way.  With a more obvious notch and peak, the listener is more compelled to follow where those notches and peaks are located at each moment; something that doesn't happen with vibes because the location is much harder to pin down.  It's also why the slowest LFO speed on a vibe is noticeably faster than the slowest speed on a phaser.  With a clear focus-of-attention, phasers have the freedom to sweep very slowly, which vibes can't have with such a diffuse impact.

Effects like flangers and chorus also change what we attend to as the speed changes.  At faster speeds, what sticks out is the pitch change, while at slower speeds, the comb-filtering stands out more.

I think there is much to be gained by applying a cognitive/attentional analysis to effects.  It's not the ONLY way to look at them, but nicely supplements other approaches.

[Mark Hammer]

I was so swept up in the sound of the circuit that I plum forgot that the Magnatone amps have this built in.  At least some models do.  You can hear it illustrated at the 4:00 mark here:  https://www.youtube.com/watch?v=9pJcjVzIPZQ  Pretty dang sweet.  I don't know if the amp uses dual unsynced LFOs or one LFO that countersweeps.

[PRR]

> I don't know if the amp uses dual unsynced LFOs or one LFO that countersweeps.

One LFO and phase splitter.

https://www.magnatoneamps.com/schematics/magnatone_m9.jpg

[Mark Hammer]

I can see the phase splitter (and thanks for that), but that's a mono amp from what I can see.  The video I linked to is a stereo amp.  There may well still be a phase-splitter in the Panorama Stereo, to send counter=swept versions to each channel.  Whether unsynced independent LFOs or a single counterswept one, you gotta admit, it's a lovely sound.