Mark Hammer Idea

[Kipper4]

True that Jim

Its on the breadboard having some treatment right now

[anotherjim]

That looks right. One thing is you can get more pump action if the amp driving the diodes was an 072, because it can swing closer to +supply than the 358 can. It's probably only 0.5v better, and may not make much practical difference, but it's something if you do need a bit more envelope range.

[Kipper4]

Thanks for the tip Jim
I'm getting a 5~7v pp swing at the led.

I've made a few changes, no doubt there will be more to come.
I've changed the gain of Ic2B. With the test instrument i can get the led only flashing when strummed hard.

Next up is that Attack pot, not sure how effective it is, maybe i'll try something 5x or 10x bigger.

[Kipper4]

Heres a mock up.
If you spot anything that wont work please chip in.



Will the phase of the effects affect the outcome?
Will the 2N5087 pnp work? Does the circuit care as long as it's PNP?
Bare in mind both Tremolo and Phaser will be run by the same LFO.
Is it unwise to use the other half of the lm13700 as a tremolo? As in RG's OTA applications?
link.
http://www.geofex.com/Article_Folders/VCA%20Applications.pdf

Thanks

[duck_arse]

with IC1A DC coupled to IC2B, R7 becomes redundant. I'm not sure about your D3 direction when it has ground hanging off the anode.

however - jim - if using an lm358, the output can swing to 0V, so if the opamp sinks the led current instead, won't the available range/swing/brights be increased?

[Kipper4]

Welcome aboard Stephen.
Good spot on R7.
You're correct D3 is reversed and goes to +9v as in reply 60
I got so many drawings in the folder it's easy to pluck the wrong one out. My apologies.

Otherwise no one sees any issues?

[Kipper4]

I also think D3 will be redundant later on. At the moment it's talking to me about what the envelope is doing.

Yes led's talk..... in my breadboard world......

[EBK]

They also listen, by the way. 

http://www.analog.com/en/analog-dialogue/raqs/raq-issue-108.html

[anotherjim]

however - jim - if using an lm358, the output can swing to 0V, so if the opamp sinks the led current instead, won't the available range/swing/brights be increased?

I mean the op-amp driving the diode pump. That one doesn't need to swing completely to 0v, but does need a symmetrical swing  so it works the pump capacitor (C2 in most recent scheme) efficiently. After the diodes, I would definitely prefer a 358 in order to follow the envelope voltage all the way down to zero.

[Kipper4]

Looking over some phaser schematics for the project I'm beginning to wonder if a 2 stage phaser is going to cut it especially as its the effect that kicks in when strumming gets harder.
What do you guys think?
Obviously a two stage OTA based would be a smaller pcb footprint than a 4 stage.
Here's a previous project by a forumite
http://www.diystompboxes.com/smfforum/index.php?topic=77241.
Also not forgetting the causality 4
http://www.diystompboxes.com/smfforum/index.php?topic=80456.0

Bare in mind
So far there's an envelope detector to control the mix of the two effects
The tremolo
The phaser (a single lfo will control speed of both effects.)
The OTA based single vca cross fader.
Some buffers.
The power supply

I Have another drawing for a tremolo based on RG's VCA application notes. I'll post it up later.

Thanks
Rich

[Kipper4]

Goddam wasted my day breadboarding the lm13700 vca with no sucess.

Rant over..

heres my drawing im working from.

[Kipper4]

I've managed to get the op amp crossfader version working and responding to the envelope.
Theres still some tweeks and breadboard nonsense to sort but if I cant get the ota version working at least I know I can go old school.

Yip yip......

[anotherjim]

In the OTA, why Q3? I don't know origin of this. Maybe it adds some log/exponential kink to the sweep, but should there still be some resistor into the OTA control pin to set a current limit?

[EBK]

Q3 is a current source.  Pin 1 of the OTA expects a control current.
To be more accurate, it is part of a current source.

[anotherjim]

Well yes, but so is the op-amp with a series resistor, which is the usual arrangement in OTA control. Will Q3 just cut off when control on R15 moves below Vb?

[EBK]

Well yes, but so is the op-amp with a series resistor, which is the usual arrangement in OTA control.

Would you mind sketching that up or providing a link to an example?

[anotherjim]

OP-amp (often just a buffer) > Resistor (somewhere about 10k per control pin) > OTA control pin.
Opamp input voltage change = OTA control pin current change.
Dod FX-25 already mentioned uses envelope cap direct to feed resistor into the control which doubles as the cap bleed off.
One Rich refers to...


I'd love to know where the current transistor idea comes from. A bi-polar supply synth circuit with exponential CV control?
A simplified version of this?

[Kipper4]

"I'd love to know where the current transistor idea comes from. A bi-polar supply synth circuit with exponential CV control?"

yes +12v -12v
Synth yes.

[Kipper4]

The op amp version works. Albiet with just clean guitar (no tremolo) and a plugged in phaser to the second differential input.
See the Electric druid site for details of the cross fader.
Using a version of the DOD440 envelope detector.
More on this later.

[Kipper4]

Some of the values in the detector have changed but essentially this is whats on the breadboard and in the video.
Except D4 the vactrol led is reversed and goes to gnd via the 2k2 CLR. So that it lights up when strummed harder.
As drawn It will go dark.