Author Topic: Fender brownface vibrato in a stomp...does it exist? What about with JFETS?  (Read 40333 times)

Steben

Actually, that comes pretty close to the EA Trem version, which is here:
http://geofex.com/FX_images/FakeFenderVib2.pdf

Differences are
- JFETs biased for bigger signals without distortion
- Modulation circuit shown; you'd need something like the LFO to make the A and B signals

sorry RG, haven't seen your version yet.
the LFO: yeah of course, I didn't draw it yet since LFOs seem a bit generic to me. one could also use the typical opamp LFO, or the phozer's/EA trem....

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- Modulation applied to source instead of gate, which seems to be less touchy

really? I believe you yet I wonder why. Do you mean the biasing stuff?

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A note about my schemo drafting conventions:
- One wire which meets another at a right angle connects.
- There should never be a four-wire crossover; if there is, it does not connect. There should be a crossover hump just to make this blindingly clear, but sometimes I miss them.

Solution: a dot where it connects. no discussion.
Actually I draw the humps as well to stress it a bit.
Rules apply only for those who are not allowed to break them

Steben

A little closer translation brings me to this:
http://aronnelson.com/gallery/albums/Stebens-fuzzy-pot/vibratrem.gif

That schematic is real purty. What software did you use?

Sean Costello

Autocad, my friend.  ;)  Pure vector style.
Rules apply only for those who are not allowed to break them

R.G.

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sorry RG, haven't seen your version yet.
Oh, no, no apologies are needed. Convergent evolution happens all the time.

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Solution: a dot where it connects. no discussion.
I know. I just hate going back and putting a dot at each intersection. I inevitably forget a dot here or there. It seems to me that my error rate is lower with the only-three-wires version. I forget crossover humps less than I forget dots.  %-)
R.G.

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rocket

- Modulation applied to source instead of gate, which seems to be less touchy

I don't quite understand your circuit, for me it looks like like the fets at the sources (Q5,6) are acting as parallel restors to the source resistor, not varying the bias of the fets (Q2,3).


- Modulation circuit shown; you'd need something like the LFO to make the A and B signals

What is the frequency range that you can achieve with such a phase shift oscillator.
I would like 1:20 (1Hz to just audible, for ring mod sounds, even though that's probably not suiting for the brownface trem)  but only could achieve maybe 1:3 or 1:4  both in reality and with ltspice.

rocket

Any news on the brownfacetrem?

markm

Just a thought but,
What about some mods to the original Phozer circuit from the fellas at ROG?
It use the LFO from the EA, perhaps it could become an EA with some slight phasing and maybe.....just maybe "fake" the BrownFace Trem sound reasonably well?


R.G.

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I don't quite understand your circuit, for me it looks like like the fets at the sources (Q5,6) are acting as parallel restors to the source resistor, not varying the bias of the fets (Q2,3).
Yes. That's what it does, and that's what I meant by modulation at the sources.

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What is the frequency range that you can achieve with such a phase shift oscillator.
I would like 1:20 (1Hz to just audible, for ring mod sounds, even though that's probably not suiting for the brownface trem)  but only could achieve maybe 1:3 or 1:4  both in reality and with ltspice.
By changing two of the resistors in parallel as I did, you can get more than 1:4, but probably not more than 1:10.

Yes, we would all like to get wide speed ranges, but it's very hard to do with such simple circuits. I have designed much wider range LFOs, but they are a completely different design, and do not produce sine waves like a phase shift oscillator does.

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Any news on the brownfacetrem?
Yes. I breadboarded it and I like it. A couple of other people have reported they like it here. I've posted two different designs which work. I'd say that's pretty good news.
R.G.

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sfx1999

Brownfacetrem? I thought this was about the brownface VIBRATO (I mean the TRUE vibrato, some brown Fenders had it).

Leo Fender has confused us all.

If you are actually doing a tremolo, is there any change of getting a Magnatone vibrato pedal?

markm

Brownfacetrem? I thought this was about the brownface VIBRATO (I mean the TRUE vibrato, some brown Fenders had it).

Yeah, that's the one.

R.G.

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Brownfacetrem? I thought this was about the brownface VIBRATO (I mean the TRUE vibrato, some brown Fenders had it).
Leo Fender has confused us all.
Yes, he has. It's the amplifier "vibrato" that does tremolo, not the guitar "tremolo" that does true vibrato.
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If you are actually doing a tremolo, is there any change of getting a Magnatone vibrato pedal?
There's a really good chance - the Magnatone vibrato is a true vibrato, done by making a phase shifter without adding the dry signal in. In that sense, every phaser with the dry signal disconnected is a vibrato in the style of a Magnatone vibrato.

A long time ago, maybe ten years, I built up a vacuum tube version of the Univibe along the Magnatone lines. Worked fine. The exact operation of the Magnatone amps can no longer be built, as the critical part, a softly varying voltage variable resistor (varistor) is no longer made and old stocks have been depleted. The newer varistors are specialized devices which have a sharp, snap action variable resistance for limiting transient voltages, and are not suitable for the smoothly varying resistance that a Magnatone styled varistor needs. But you can do the same thing with LDRs (which I did), JFETs, PWM resistors, and so on.
R.G.

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markm

I found this quite sometime ago on the net.....
I don't even know where but, maybe it could be useful?  ???
The description states that it produces "phasing".
Any thoughts RG?




markm

Oh yeah.....
Or this?


R.G.

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I found this quite sometime ago on the net.....
I don't even know where but, maybe it could be useful? 
The description states that it produces "phasing".
It doesn't. It's amplitude modulation - tremolo. It's interesting though. That's only the second time I've seen a current-mode phase shift oscillator, the first being the univibe.

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Oh yeah..... Or this?
Input fed to a highpass on one side, lowpass on the other, the two blended back together. Presumably A and B are places to insert a current (and hence gain) modulating LFO, possibly out of phase between A and B.

It looks like my first concept of the Fender Concert/brownface/Pro tremolo, a single pole filter on each side. It won't have the phase funnies that the Fender circuit does, but it might be useful. It will be harder to balance out the LFO; that's probably what the 25K pot is for.
R.G.

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Ben N

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Are the 10k pots just bias adjustment?

stm

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I found this quite sometime ago on the net.....
I don't even know where but, maybe it could be useful? 
The description states that it produces "phasing".
It doesn't. It's amplitude modulation - tremolo. It's interesting though. That's only the second time I've seen a current-mode phase shift oscillator, the first being the univibe.
I see there is at least a resistor missing on that schem, probably in series with the power supply or somewhere between the supply and Q1's collector.  As drawn it won't oscillate since there is no feedback path in the oscillator.  Also, the modulation control signal is taken directly from the supply voltage.
« Last Edit: May 17, 2007, 02:23:08 PM by stm »

SeanCostello

It looks like my first concept of the Fender Concert/brownface/Pro tremolo, a single pole filter on each side. It won't have the phase funnies that the Fender circuit does, but it might be useful. It will be harder to balance out the LFO; that's probably what the 25K pot is for.

What sort of phase funnies are in the Fender circuit? You had mentioned that it was closest to a twin-T filter. Are the filters 2nd order that it crossfades in between? Or is it one of those weird circuits like the Fender tone controls, that has to be explained by a fairly complicated equation in the s-plane?

I am assuming that the circuit can be simulated by a weighted balance between two time-invariant circuits, and that the input mix does not affect the cutoff frequencies of the lowpass and highpass filters. The crossfading will affect the notch frequency and phase shift of the output, but this is standard. It reminds me of the knob on the old Oberheim SEM-1 module, that crossfaded between the lowpass and highpass outputs of a state-variable filter. Or a Big Muff tone control - is this a close relative of this circuit?

Thanks,

Sean Costello (I decided to revisit the spring reverb/brown Fender vibrato recreation for the FV-1 that I mentioned earlier)

SeanCostello

On an older thread on a different forum, there is a link to results of a simulation of the harmonic vibrato in the Showman, but the link is no longer valid:

http://www.fenderforum.com/forum.html?topic_number=327608 - about 2/3rds of the way down the page.

Does anyone here have an analysis of the crossover network / frequency response that they would be willing to share?

Thanks,

Sean Costello

Ben N

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There was an email link over there, so I emailed the guy to ask if he would share.

Steben

It won't have the phase funnies that the Fender circuit does

Euhhhh...... why?  :-\
Rules apply only for those who are not allowed to break them

R.G.

The Fender vibrato is not a simple first-order crossover filter, so it has excess phase shift in both sections. That's another way of saying that the sections are not designed to be complementary to eliminate the phase changes around the crossover frequency. Phase linearity at the crossover frequency is a common issue in designing hifi speaker crossovers.
R.G.

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