Vibra-matic (from GEO)

[vdm]

Hey y'all (back to my texan roots),

I've been playing with the EA trem i built a while ago and tried out RG's Vibra-matic.
Its essentially an envelope detector that effectively increases the depth of the trem as notes decay.
It is working quite well in that I have found some useable settings, only problem is i can't make the envelope stay open.

What i mean is, ill strike a chord and the trem will fade in but i have to stop playing and wait for the blinking led to go back to fully on before I can use it again.
I was hoping that I could have it set that if I continue to play notes/chords i will get straight signal until i let a chord ring.

After looking at some modular synth designs, I had the idea that I'd like to use the dynamic envelope to control an EF. Any ideas on where to tap the signal to send to the next pedal?

trent

[R.G.]

The way a phase shift oscillator works is that it has just barely enough gain to start oscillating. If you stop it by clamping down on the feedback that keeps it oscillating all the time, it stops and just sits there. When you release the clamp, it fades up with tiny oscillations at first, growing to full strength. I detected the note with a somewhat sloppy envelope detector, then released a clamp on the LFO whenever there was a note.

From your description, the clamp is not reasserting itself. That means that the second opamp's output is not flipping high at the end of a note, so it could be that either you're getting a little DC bleed from the first opamp, or that the second opamp is not getting the right voltages at its input, or not swinging high enough to turn on the clamp. Does diddling the decay and threshold controls not change things?

I can help you with that if you'll read me the voltages on the opamps and transistor both before you hit a note and right after.

The envelope detector is a sloppy one. It's just enough for my purposes in gating the oscillator on and off. If you tapped it off, you'd tap it at the 10uF capacitor. But there are better envelope detectors.

[vdm]

Thanks RG,
I'll write down some voltages today and see whats going on.
About the envelope thing- i meant id like to have essentially the dynamic envelope of the whole pedal
ie. normal attack but on the decay the tremelo starts kicking in.
i could do this by feeding the straight trem signal into an envelope filter, but i have this grand idea of having 3 pedals so i can use distortion/compression at the beginning of the line without it affecting the shape of the envelope.
i never liked Envelope Filters when run with a distortion in front, so i thought I could buffer it and send one output to standard chain, and the other to create a CV for the Filter

The decay knob and threshold knobs seem to work ok, but there is very little rotation that produces the right sound. Turning the threshold too far either results in an inability to trigger the EF (constant trem) or it stops the oscillator functioning altogether with no trem with or without a note.

I havent been working with envelope detectors long, but is this circuit simply an envelope detector with its output tied to the trem circuit? i could try hooking my dr quacks EF to see if that helps...

cheers

[vdm]

Ok, here's some voltages with interesting results... i guess it makes you think about how the thing actually works...

Battery - 8.6V

IC - At rest (no input) LM358
1 - 2.05
2 - 1.19 (by touching this pin, oscillation began to occur)
3 - 1.08
4 - 0
5 - 0.25
6 - 0.17
7 - 7.30
8 - 8.59

IC - After attack
1 - 3.4
2 - 1.19
3 - 1.08
4 - 0
5 - 0.25
6 - 0.93
7 - 0
8 - 8.59

Transistor (Unknown NPN with gain ~350) - At Rest
C - 0.009
B - 0.69
E - 0

Transistor - After Attack
C - Gradually lowers to ~0 and then starts to oscillate as the trem takes over
B - 0.06 - stays here whilst there is signal at the input but returns to 0.69 after it is removed or decays
E - 0


This is all making me think that my transistor is not functioning correctly. Perhaps I should try a different one - shame i didnt socket.. i even had the sockets ready to go! bah!

anyway.. any musings are welcomed.. and i take it someone has got this thing to function properly
mind you i do like what it does atm.. just want the normal function (too)

thanks again,
trent

[R.G.]

Actually, those voltages look like it's working as designed.

The way it works is this. With no signal, The first section of the opamp has no signal, and so it puts no current into the 10uF cap, so the voltage on the cap decays to 0V through the decay and threshold pots. The second section has its positive input tied to a sslightly positive voltage, but its negative input is tied to the threshold pot wiper. With the cap at 0V, the negative input is more negative than the positive, so the output goes high. This turns the base of the NPN on, and the collector clamps the feedback network of the LFO.

When a signal starts, the first opamp pumps charge into the 10uF cap through the rectifier diodes. This raises the voltage on the cap and at the wiper of the threshold pot. When the minus input of the second opamp is pulled higher than the positive input, the output goes low, pulling the base of the NPN transistor to ground, which turns it off and releases the collector's clamp on the LFO.

The trick is to set the threshold so that it releases the LFO, and the LFO builds up to oscillation over a note, but that when the note ends, the LFO oscillation quits. Since you were saying that it keeps oscillating, it may be that the threshold is quite delicate to set on yours. What pot taper is the threshold pot? Should be linear.

I've had a couple of folks email with reports of working units. Not a lot, but some. My breadboard version worked OK while it lived.

[vdm]

hrmm.. im not sure i understood correctly what the effect was meant to do in the first place...

When you play a note the tremelo effect is gradually brought in over the sustain of a note (working perfectly)
At the end of a notes sustain or with no signal the tremelo is turned off (working perfectly)

but heres the thing:

I thought that if i played many notes in quick succession the envelope would stay high, and oscillation would not start until the sustain of the last note
But after reading your explanation it makes sense that after any note is played the transistor unclamps and remains that way (allowing tremolo) until silence occurs.

I guess I misinterpretted the aim of the effect... it could still prove useful... but not quite what i was hoping for.
I must say though - it does its job fairly well, and at least now i can look at the schematic and understand *how* it works, which is much better than before i built it..

thanks RG
youve been a big help and im very appreciative!
Trent

"what are we going to tonite brain?"
"the same thing we do everynite pinky.. TRY TO TAKE OVER THE (effects) WORLD!!!"

[R.G.]

Yep, you have the right idea in mind now. You were thinking it was a lot more sophisticated than it is.

However, now that you tell me what you want, I can see how to make it do that. Right now I have it set up as just a simple envelope detector and comparator. When the sound is higher than the threshold setting, it lets the tremolo start. One could easily use a second comparator so that when the sound is higher than a second threshold, the clamp was still held on, so that the LFO was only turned loose after the signal had decayed down a bit and was still not yet silent.

In fact, a number of the young tigers reading this right now are thinking "Hey, I can just use an LM3914 and have ten different levels of things to do as the signal decays!" Yes, you can, but that starts slicing the cheese very thinly.

There are some obvious problems with the two-level sensing. The envelope detection works on loudness only, so soft notes would not trip the "too high to trem" but would start the tremolo like the unit works now. It would make setting both thresholds delicate. Not unworkable, but delicate.

[johngreene]

There are some obvious problems with the two-level sensing. The envelope detection works on loudness only, so soft notes would not trip the "too high to trem" but would start the tremolo like the unit works now. It would make setting both thresholds delicate. Not unworkable, but delicate.

Why not just make the threshold a slower time constant version of the input? Make it such that it can never increase as much as the input. That way as the note decays, the input crosses the threshold and the threshold follows behind like a shadow until the input increases again. Maybe add a control for hysteresis....

Just a thought.

--john

[johngreene]

Like this:
http://www.greene-pedals.com/vibra2_follow.pdf

--john

[Processaurus]

I've been meaning to try the Geo Vibramatic, its a really cool idea, like keyboards that vibrato the trails of notes. 

heres a quick thought, for what vdm is asking for, I'm wondering if you can get that just by reversing the two signals to the comparator section with a DPDT switch, to invert the output of the comparator, and have the tremolo be on for quieter notes, and off as long as you're playing loud.  You could even use a DPDT on-off-on toggle switch, so that in the middle position, its a regular tremolo pedal.  I think you could do that if the -input of the comparator is held higher than the + input, (by a permanent 1M resistor to Vb, the reference voltage. and a 1M permanently stuck between the +input and ground). 

An LED with a 5K resistor in series connected between the comparator output and ground might be nice to have for seeing instantly how the envelope detector is working.



I don't know if the trem would stay on for the attack of the first note after a silent period, but it would be easy to try.  If it does, and is a problem, then the two comparator idea would be the way to go.  In that case I don't think the threshold for when you aren't playing would be hard to set, you'd just set it just above your noise floor. 

John, that totally went over my head, but sounds innaresting

[johngreene]

The circuit I described and posted does exactly that. The 'delayed' threshold trails behind the input signal. So, when you play loud, the input is always higher than the threshold. However, when the note decays, the input falls faster than the threshold therefore enabling the tremelo as long as the note decays. As soon as you play again, the input passes the threshold and the tremelo is disabled. The negative side of the comparator is connected to the positive side through a 1K resistor. The cap on the negative side takes time to charge up so the positive side remains higher. The 100K to ground forms a voltage divider to insure the negative side never equals the positive side at maximum input. When the Postive side drops, the negative side can't drop as fast because of the charged capacitor so it's voltage remains higher than the positive side tripping the comparator.

The 500k feedback resistor adds hysteresis to keep it from chattering if the envelope is noisey.

--john

[R.G.]

Good thoughts, guys.

The problem with getting too fancy is that the tremolo LFO does not start up or fade in instantly. It takes a noticeable fraction of a second for the LFO to start and fade in once it's let go. That's one reason the thing works reasonably well with the single threshold - there's a delay as the LFO comes up to speed. Get too sudden with on/off cycles and it'll never come fully up. Turning it on all the time as an option is perfectly reasonable.

All of these ideas need breadboarding and ear-testing to see how much adaption of playing technique is needed.

[johngreene]

All of these ideas need breadboarding and ear-testing to see how much adaption of playing technique is needed.

heh, but isn't that what it's all about here?

--john

[Paul Perry (Frostwave)]

What about having a LFO permanently on, and feeding it to the trem stage according to the logic of envelope states? Maybe with a bit of a depth ramp up and down?

[gez]

Nice idea Paul!

[R.G.]

heh, but isn't that what it's all about here?

It absolutely is - I inserted that for the benefit of the beginners, not the experienced crew. I get a lot of beginners that will simply go build something without knowing that, though. I have to deal with a varied audience.

What about having a LFO permanently on, and feeding it to the trem stage according to the logic of envelope states? Maybe with a bit of a depth ramp up and down?

Yeah, if I were redesigning it from the ground up, I'd probably use a continuous LFO and run that through an OTA to ramp the LFO in and out. That gives you ramp control from on/off to slow fades. One might be able to use one section of a 13700 for the LFO, the other for the VCA, and get by with a simple pot/capacitor/switch for the ramp control. I'd probably use another LM13700 for the basic tremolo stage as well.

When I did this I was looking for the minimum I could add to the EA trem, which is quite popular with the minimalists.

[Processaurus]

Super idea, Envelope controlled depth.  Much better than envelope controlled speed, like line 6 does.  That would really kick that boring tremolo sound.

When I did this I was looking for the minimum I could add to the EA trem, which is quite popular with the minimalists.

I got that, its kind of weird when the mod has 10 times more cicuitry than the original unit.  If I were to go to the trouble of making a controller as fancy as what you guys are talking about, I'd sure use something more sophisticated than the EA as a base.

[soggybag]

I've been working on adding an envelope follower to the Frobnicator for a while now. If it were'nt for work and my lack of electronic genius I would have finished by now. I think I will have something worth sharing sometime soon. Though I figure if most of the group could whip something like this up with a little thought.

I thought as an added bonus this might make the ring mod mode of the frobnicator more interesting. The frobnicator uses an OTA as suggested.

I've worked out my envelope follower finally. The current problem is how to best splice it into the frobnicator. My envelope drives an LED/LDR.

[cpm]

sorry for refloating such an old topic 

Testing the vibramatic circuit, it works fine. The only issue is when the transistor kills the lfo (most of the times), there is an audible "pop" noise, like a sudden change in level from the low part of the oscillation to the full sound.

Im sure there is a simple solution for this, maybe an inductance placed at the collector of the vibramatic NPN?

[R.G.]

You have an unwanted DC connection in there somewhere. Check for shorts and wiring again. Mine didn't pop.