Vibrato using MOV's

[Guest]

I have been interested in doing a vibrato circuit based on the ones used in Magnatone guitar amplifiers. I finally got around to breadboarding it and thought I would share the design with the forum. I poster the schematic here.

I ended up buying a sample of most of the varistors I could find on sale and just tryed them in pairs until I found some that worked reasonably well. There is a pretty narrow window of values that work on my circuit. Below a 20VRMS rating not much happens, above 30VRMS rating the signal gets killed. I settled for 25VRMS ones from Littlefuse (V39ZA1).

The audio signal is slightly frequency modulated to make a nice smooth vibrato, and it also modulates the amplitude for tremolo effect. All in all I am pretty pleased with this little circuit.

 

[Gus]

Thanks for the post I have wondered what that circuit was like.

 

[SSLtech]

Hmmm...looks to me as though it won't affect the frequency... just the amplitude. (at least I'm not seeing how it can affect frequency)

The MOV's idea is interesting... it looks like a higher level variation on the diode bridge idea, or perhaps the other way round depending on "which came first, the chicken or the egg"...

Look slike U2 is configured as two inverters, and that the thing might rely on applying the same control system simultaneously to an inverted and a 'right-side up' version of the signal, in order to benefit from some self-cancelation of any control voltage feed-through... smart enough.

U1b produces the modulation waveform, U1b provides an inversion. Any inversion gain inaccuracy round U1b will produce more distortion the deeper the tremolo cuts in, since the opposing outputs are needed to attack opposite poles of the signal waveform equally... not actually 'wrong' if you like more 'character' in the unit though!

Keith

 

[adamasd]

This is great, I have been wanting to build one of these for awhile but never had a clue as to what MOVs to use. Time to order parts.

You have Va listed as 300 Volts, is Vb also 300?

adam

 

[adamasd]

never mind, its pretty much self explanatory when you actualy pay attention to the whole schematic

 

[NewYorkDave]

FWIW, I recall that Magnatones used copper oxide varistors.

 

[Svart]

so you pretty much have a preamp with a few MOV's and a DC bias that pulls up everything to the clamping level of the MOV's?

 

[gyraf]

Excuse me for my ignorance, but what is a "MOV"?

Jakob E.

 

[Svart]

metal oxide varistor. related to a bidirectional diode, once the voltage across it reaches a predetermined point, it starts to conduct, shorting voltage across it in a linear relationship to the voltage.

guess that's an easy way to describe it. :thumb:

EDIT: i forgot to explain it's use.. usually it's used to protect against transients/spikes and acts much faster than zeners and silicon diodes.

 

[Guest]

Here is how I think the frequency (really phase) shift occurs;

The audio signal is split in U2a but is blocked in the lower half by the MOVs with no control voltage applied. Signal passes through the small capacitor with it's associated phase shift and happily onward.

With control voltage applied the MOVs turn on and allow signal to pass with additional phase shift. Signals are summed and pass onward.

U1 is a fairly common oscillator that splits the signal for MOV control with the second half.

I recall NYD telling me some time back about copper oxide varistors, but was unable to find any.

I have always thought this idea clever and am glad I finally got the gumption to mock it up.

 

[gyraf]

Thanks, Svart..

 

[Svart]

no problem :thumb: just glad to be able to pay ya back for all the questions I've asked you!

 

[NewYorkDave]

Wanna see the patent?

775kB PDF

 

[Guest]

Thanks a lot for that patent. Reading it clears up some of my assumptions.

 

[BYacey]

A MOV is basically two zener diodes connected "back to back".

 

[BYacey]

A MOV operates basically like two zener diodes connected "back to back".