Mark Hammer Idea

[EBK]

Have you checked out Tom's actual VCA implementation?  It replaces that pot with a control voltage.
The pic you have is his single-gang pot cross-fader.
http://electricdruid.net/single-vca-crossfader/

[Kipper4]

Yer I did see that Eric but I didnt breadboard it since I assumed it was intended to be a fast acting crossfader. I did watch the video that they made of the module with it in though and it sounds good.

and ota's scare me. So I'll keep it analogue for now.
Rich

[mimmotronics]

>It feels to me like there must be a way to use one combined circuit rather than a separate phase-shifter and tremolo in parallel.

If you take an op-amp phase shift stage (Phase 45, 90, etc, etc.) and short the feedback resistor between output and (-) input, the dry signal is removed and you get a variable low pass filter. This can sound quite nice and might be suitably tremolo-y.

That's exactly the sort of thing I had in mind. Thanks.

That Fuzz/Tremolo actually does a hard switch from one to the other, if I'm reading it right. It'd be nice if you could get a softer fade between the two effects based on average level. So if you were to replace the feedback resistor in the op-amp phase shift stage with an OTA-acting-as-a-resistor, you'd be able to cross fade from phase shifter to lowpass.
I know the OTA-as-resistor thing works best if one end of the resistor you're replacing is grounded. Does it also work if that's a virtual earth not a real ground point?

Still, at this point, the circuit is perhaps getting complicated enough that there isn't much to be gained over a two-circuits-in-parallel approach.

Tom

I noticed the hard switch between the two as well, which I'm assuming would not be too pleasing. What I'm envisioning is a sort of VCA-based biasing being applied to a transistor operating as a linear amplifier, which is amplifying/attenuating the signal you want to fade in. Maybe OTAs are the way to do it?

[Kipper4]

Have you a link to something similar to what you're proposing.
I'm not to familiar with OTA's and vca based biasing being applied to a transistor operating as a linear amplifier.
Are we talking like in some compressors/ gates. If so s there a link to a schematic?
Welcome aboard the good ship Stompbox.

Rich

[mimmotronics]

Have you a link to something similar to what you're proposing.
I'm not to familiar with OTA's and vca based biasing being applied to a transistor operating as a linear amplifier.
Are we talking like in some compressors/ gates. If so s there a link to a schematic?
Welcome aboard the good ship Stompbox.

Rich

Good to be here! Just brainstorming, doing some research on a schematic to reference now..not as familiar with compressor/gate circuits as I'd like to be, but I can see the same concept being relevant here.

I will try and explain: think of a set-up similar to one of the JFETs in JFET switching circuits. Now think of taking that gate terminal and using it to attenuate or allow (not amplify) whatever signal you're passing through. That gate would be varied by a (narrowly-ranged) voltage generated by a VCA which is proportional to the attack of the raw guitar signal.

I could be crazy; I'll have to breadboard it and share the results/schematic with you.

[Kipper4]

I think I get where your coming from.
I'll  be interested to see how you get on.
Even if it doesn't work sometimes just sharing our ideas creates sparks of intrest enough to spur others on to ideas that might be appropriate.
Like electric druids earlier post and Stephen Giles subsequently sharing the fuzz trem.
It's all good.
Thanks

[Kipper4]

Have you checked out Tom's actual VCA implementation?  It replaces that pot with a control voltage.
The pic you have is his single-gang pot cross-fader.
http://electricdruid.net/single-vca-crossfader/

One thing I couldn't see very well is what pin the pnp transistors control voltage goes to.
Can someone enlighten me please.
Thanks
Rich

[EBK]

Pin 1

[Kipper4]

Thanks Eric
That would be the amp bias input then according to the datasheet.
Tolerance of upto 2ma. Scary.

http://www.ti.com/lit/ds/symlink/lm13700.pdf

I'm used to outputing upto 7volts from an envelope detector. Although I'm sure it can be padded down to within a range of voltage who's currant can fit. I guess a BAR or divider would help.
Not my field really so I could be barking up the wrong tree.

I note however that the cross fader (lm13700) is expecting a 0-5v control voltage.
I'm not quite sure if the lesser voltage will only partially cross fade the input signals.
Is that how that work?

Cheers

[EBK]

I believe that circuit coverts an input voltage to a control current.  You can do the same.  No need to worry.   

[ElectricDruid]

That's exactly what's going on - it converts an input voltage to an output current, which is what the 13700 expects at its control input.

I'd post a decent reference for that part of the circuit, but oddly I can't find one. Anyone? It's not like it's some amazing thing I came up with myself...it must have been around for 40 years at least (since the advent of op-amps).

Tom

[Kipper4]

I knew I'd seen this. I printed this off recently.

http://www.geofex.com/Article_Folders/VCA%20Applications.pdf

[PRR]

> it converts an input voltage to an output current
> It's not like it's some amazing thing I came up with myself...

The 5K input resistor actually does the V/F conversion (and the source must be able to deliver 1mA).

The opamp, PNP, and other 5K just bend that around to go "down", as needed for the OTA Iabc pin.

You can put 10V into the "0V-5V" input before the OTA smokes. It may survive a bit more (but you can't complain if it doesn't). Scale your input to never go much over 5V, or limit the input voltage.

[Kipper4]

Thanks Paul.
this may be a goer then with an ota crossfader. I'll breadboard it at some point with the envelope detector.

[StephenGiles]

Thanks Paul.
this may be a goer then with an ota crossfader. I'll breadboard it at some point with the envelope detector.

What are you using for the envelope detector, or have I missed something ?

[Kipper4]

Probably something of this ilk Stephen. At this time it's just a series of ideas in my head.

http://www.diystompboxes.com/smfforum/index.php?topic=117156.msg1087463#msg1087463

[StephenGiles]

Probably something of this ilk Stephen. At this time it's just a series of ideas in my head.

http://www.diystompboxes.com/smfforum/index.php?topic=117156.msg1087463#msg1087463

You may want to consider how the envelope detector is triggered - presumably by the leading edge of the note played?

[Kipper4]

Is there another way?

[ElectricDruid]

Can I suggest that you should convert the half-wave rectifier into a full wave rectifier? It'd reduce ripple in the envelope follower and give you a better result. Costs you another op-amp, I admit.

http://www.play-hookey.com/analog/feedback_circuits/full-wave_rectifier.html

T.

[Kipper4]

It's entirely possible I may even go a step further with the envelope. Thanks Tom.
I saw this here
http://www.deeptronic.com/electronic-circuit-design/envelope-detector-circuit-with-separate-attackrise-and-decayrelease-time-settings/

I got to thinking how malleable it was.

I haven't breadboarded this yet. I've just redrawn it for my envelope archive.
I'm curious to know if you guys think I should reference the caps, decay pot and other stuff to Ground rather than +V/2 (Vb my drawing).



What am I missing? Any improvements before I bread it? Of course all opinions are welcome.
Thanks guys
Rich