Brownface Vibrato Project

[markm]

Here's a sneak-peak at R.G.'s Vibrato.
I began work on this one the same day the forum went down.
Needless to say, it has occupied me for a little while lately!
Have a look;

[StephenGiles]

Looks interesting!

[markm]

Looks interesting!

Thank you.
It still has some bugs to de-bug but, if all goes well, it may be a complete project real soon!
Some volume issues and some LFO issues to address still.

[QSQCaito]

lo.lo.lo. love it

As I told you mark, any help needed just ask me.

Have you got an idea of PCB's actual size?

[markm]

lo.lo.lo. love it
As I told you mark, any help needed just ask me.

Well, fortunately for me, R.G. has been kind enough to lend me his time to try and sort it out.
Not that I don't want you to be involved, R.G. has been incredibly kind to say the least and hell, I've got THE Best working some of the bugs out with me so, I'm in good hands right now.
I will keep you updated.

Have you got an idea of PCB's actual size?

PCB will measure about 3 inches wide X 2 inches.
It's a biggin' but, It'll still fit into a 1590BB! 

[R.G.]

Mark, check your email. R26 needs a minor change, then I think you're good to go.

[ulysses]

nice work guys!!

let me know when you have a verified pcb and i'll build one too!

maybe you can do some audio demos mark? 

cheers
ulysses

[markm]

R.G.,
You have mail my friend!
Volume is no longer an issue and there is plenty of wobble and phasing........
One last issue I'd like to address but just as a teaser, this thing is VERY Cool!! 

nice work guys!!

let me know when you have a verified pcb and i'll build one too!

maybe you can do some audio demos mark? 

cheers
ulysses

You will know shortly ulysses, stay tuned!! 

[Processaurus]

Hi, looking this EA trem based version, I was wondering if it will have the same depth and headroom issues as the EA tremolo?  Imagining what will happen in the circuit, when the JFETs that change the gain, Q5 and Q6,  are off, there is still a minimum gain set by the 2.2K resistors from the source to ground on Q2 and Q3.  Also will it give the volume boost the EA does?

The 1.4v Vsrc is interesting to me, I'm curious what advantage tying the sources of the gain changing JFETs (Q5 and 6) to that rather than ground will give?  Is it so you don't have to be as picky about finding JFETs with a low Vgs off? 

[markm]

Ben,
This has nothing to do with your post here but, could you repost the Mods you had posted before?
I think they may have been the victim of the server incident. 

[tjcombs]

Looks great! But......What is the un-used hole to the right of the input for?

[audioguy]

cool, cant wait!!

[Processaurus]

About the depth thing, it would still sound quite nice probably, without being able to get each complementary (the treble and bass) side completely off, but it would deprive you of the full brownness/phasiness of the effect.  Transmogrifox's mod to the EA that substitutes a transistor constant current source, for the source resistor that I mentioned, would work great.

Ben,
This has nothing to do with your post here but, could you repost the Mods you had posted before?
I think they may have been the victim of the server incident. 

Sure,

Here's a thought, on using a pot to blend in the brownness, brownface vibrato on one end, trem on the other.  Start with a high value pot, with the two outer lugs hooked to the two antiphase LFOs.  Send the wiper to the JFET that is controlling the amplitude of the lowpass, bass side if the filter.  What it does is when the pot is turned to the antiphase LFO, it turns the bass up and down opposite the treble, brownface style, and when you turn it the other way, the bass is in sync with the treble, making it a tremolo (with a slight mid notch as the only quirk).  In the middle of the pot the LFOs cancel, leaving the bass right in the middle of the road, and the treble swerving back and forth to each side

[markm]

Thank you very much Ben, much appreciated!   

[R.G.]

Hi, looking this EA trem based version, I was wondering if it will have the same depth and headroom issues as the EA tremolo? 

Pretty much. After all, it IS two EA trems run in anti-parallel.

Imagining what will happen in the circuit, when the JFETs that change the gain, Q5 and Q6,  are off, there is still a minimum gain set by the 2.2K resistors from the source to ground on Q2 and Q3.  Also will it give the volume boost the EA does?

If the JFET is up to it, there's still a minimum gain of about 1.5.

Of course, reducing signal is easy. It's getting gain that's hard.

The 1.4v Vsrc is interesting to me, I'm curious what advantage tying the sources of the gain changing JFETs (Q5 and 6) to that rather than ground will give?  Is it so you don't have to be as picky about finding JFETs with a low Vgs off?

It actually corrects an oddity about the EA trem that I'd always wondered about. JFETs have a limited range of Vgs that they act as a variable resistor over. This extends from Vgsoff where the JFET looks like several microns of glass (that is, high resistance indeed) to Vgs=0, where the channel resistance is as low as it's going to get. If you drive the gate more positive, you start conducting from gate to channel.

The EA trem uses a grounded source and drives the gate from an AC source. That is, half the LFO cycle goes negative, and half goes positive. The positive half is above Vgs=0. I haven't dug into that a lot, but it can't be very linear there. So I pulled the sources up to a positive voltage and fed the gate with the same ground-referenced LFO.

If you did this perfectly, the gain cell would have a fixed gain at zero depth and modulate both to higher and lower gains. This should produce a smoother tremolo.

It's a theory. We'll see how well it works. I may want to dink with that a bit when we get some listening tests.

Ideally, you'd use JFETs with VGSoff = -3V or so. Hence the spec of the usual phaser suspects: 2N5292, 2N5485, 2SK30A.

[Rodgre]

Looks great! But......What is the un-used hole to the right of the input for?

Check out the schematic on R.G.'s site. You'll see that it goes to the ring of the input jack.

Roger