Svf control voltage

[Kipper4]

Thanks guys.
The current mirror is on the cards R.G.
Thanks Paul.

I've spotted an ommision on the love pump drawing above.
The Gnd is missing from pin4.

I'm in the process of moving everything to a bigger breadboard. More room, more chips, more transistors.

Bedtime now.

Zzzzzz...

[Kipper4]

Everythings working, just a few more tests.

I've had a swap around and the scope is on the Desk, about time.....
A corner is now dedicated to test rig.

Then It's probably time to go expotential.

I'm sticking with the LM13700 SVF for now.
LM3080 VCA
CD4093 oscillators. I must scope those to see how they're wanging the power supply rails.
LM358 Attack Decay.

[diffeq]

Might as well use CD40106 instead of CD4093. Same price, but two extra oscillators per package 

[Kipper4]

I did some trials with the cd40106 and it was ok. I thought it a bit shy on voltage output but I guess if it was buffered it would be ok.

I've made a few small tweeks on the ota filter (reply 11)
What I want to know is. Is there a way of cutting out some of the high freqauncy gash.
Like putting a small (pf) value cap in a NFB loop as is done in distortions (eg tubescreamer)

Oh yer and How do I go about working out the Fc (cut off) for an filter like this please.
I can see the caps but am unsure which resistors they're working with to form the Fc.

Thanks
Rich

[Kipper4]

Qoute from R.G in a previous post.
with regard to lm3080
I'm using both lm3080 and lm13700...

link to post
http://www.diystompboxes.com/smfforum/index.php?topic=117220.0

The qoute
"it's hard for me to get to my drawing machine right now, so let me try one more version with words.

Imagine two NPNs, both with their emitters tied to ground. The bases are tied in parallel, and one NPN's collector is also tied to the parallel bases. The remaining collector is pulled up to some V+ voltage.

The parallel bases and one collector is the input. The other collector is the output.

When you let a little current flow into the input to ground, the input transistor lets some of the current through its base-emitter junction, and hfe times as much current through the collector-emitter junction. The whole thing acts like a simple forward biased diode, following the diode equation for the current being an exponential function of the base voltage. But thSo ere's that second NPN there. It's base emitter is paralleled to the first NPN's base emitter, so its base-emitter voltage must be equal to the input NPN's base emitter voltage. If the two transistors are close to identical, then for identical base-emitter voltages, they must have identical collector emitter voltages.

In practice, they do, very closely, and there are ways to make them come even closer to "perfect.

So we have two NPNs, both emitters grounded, one base/collector as an input, and one collector only as an output.

We can allow, say, 100uA, to flow in the input side to ground and then measure how much current flows to ground through the output collector (which was pulled up to some V+, remember, so there is current available for it). We'll find that 100ua +/- some percentage of accuracy flows. Generally we can consider them to be identical.

A current drizzled from a positive voltage source into the input side is "mirrored" by an identical current sucked down into the output side.

This is very close to what is happening on the Iabc pin of the LM3080. It's Iabc pin ***is*** the input to an NPN current mirror.

This works great if you only want to drop current into Iabc from a more positive DC voltage. It is a PITA if you are worried about DC accuracy down around your V- power supply because of that one-diode-drop offset.

Enter the PNP current mirror. Make a PNP circuit with both emitters tied to V+, both bases and one collector tied in parallel, and one collector pullled down to V-. Works same as the NPN mirror, but now any current you pull down from V+ through the mirror input is mirrored as a current flowing down from the PNP mirror output.

My obvious next step is to tie the output of the PNP mirror to the input of the Iabc NPN mirror. Now I can pull down on the PNP mirror's input, and the same current flows into the NPN mirror's input.

This probably seems like a silly waste of more parts, but it turns out that with today's ICs, it's easier to pull a controlled current from a higher voltage than to generate a controlled current into V+. And the insertion of those two PNP transistors lets me save more parts and circuit complexity than they cost me."


So here's what I've drawn up



Do I need more?
Like an op amp buffer before?

Assuming I'm look at matching the Hfe of the pairs as much as is reasonably practical from the parts bin.
Or should I be looking to try to match the be junctions? Not quite sure how this is done unless its done with the germ tester.

DO my values look ball park?

Cheers
Rich

I'mo learn this shit if it kills me or makes my head implode.....
Which ever comes first.

[idiot savant]

So here's what I've drawn up



Do I need more?
Like an op amp buffer before?

Assuming I'm look at matching the Hfe of the pairs as much as is reasonably practical from the parts bin.
Or should I be looking to try to match the be junctions? Not quite sure how this is done unless its done with the germ tester.

DO my values look ball park?

Cheers
Rich

I'mo learn this shit if it kills me or makes my head implode.....
Which ever comes first.

You don't need to be that complicated. Take a look at the EFM wildcat. Especially VCF3 and the VCAs.

http://www.modular.fonik.de/files/EFMforum/orgEFMfiles/wildcat_001.pdf


Also check out the old "synthy filters for guitars" thread here:

http://www.diystompboxes.com/smfforum/index.php?topic=104178.msg937481#msg937481


Eric Archer has a decent filter on his site as well. The current source he uses is a bit nicer, but the simple ones work fine for filters. For a VCO, maybe not accurate enough.

http://www.ericarcher.net/wp-content/uploads/2014/07/9v-analog-diy-lpf.pdf


Good luck! I've been having a blast with my MS20 and Polivoks filters!

[Kipper4]

Thanks I'll take a look.
I've seen plenty of simpler arrangements but went with r.g. keen method.
Polyvoks  that's a new one on me.

[Kipper4]

Would this be ok?





Also Ray Wilson did this
Except he used a tl084