An new twist on a LDR based envelope filter

[rring]

yes after D3 you should a DC voltage that goes up and down (with the sensitivity all the way) up when you strum. If you don't see that ... there is a problem with the side chain. If that works, then measure the R of your LDR's and make sure that is changing with the voltage described above ....etc. What op amp are you using for the filter?

[Kipper4]

There's the hitch I've yet to put the vactrols in.
I'll do it now. There's another socetted 3k3 to put in too.
I'm using the ba15218 for the filter op amp
Ill check out what you said rring too.

[Kipper4]

the vactrols are in and im getting a change in resistance on the one i just put the dmm on.
im getting signal on the output but no wah
i guess this is the time i need to experiment with the 4k7 and 3k3?
by the way i use a diffused blue 5mm led for vactrol 3 as stated by the builder with the same op amp as i used.
i can easily build another green one though.
i'll check the resistances on the vactrols mean time too and post those up

[Kipper4]

got it.
it was a missing trace on vactrol 1 ldr to the 470k
impressive thanks dude
i'll have some more dumb ass questions when ive had a play.

[Kipper4]

I guess the reason it doesnt tick is the 10k /10uf to ground on pin5 of 1c2?
I'm also guessing that c6 is the cap that charges and discharges for the sweep? from Maartens comment.
I tried a 10k in the place of the 4k7 off the intensity pot and it works well also.

[rring]

"I guess the reason it doesnt tick is the 10k /10uf to ground on pin5 of 1c2?"... not sure what you mean here.

C6 and C9 are a matched pair and both must be shifted as required.

[Kipper4]

I wasn't trying to cause offence or suggest it ticks.
It's just a lot of times envelope filters with homemade vactrols tick.
This one doesn't so I was just trying to work out what does what in the circuit.
Never having used the type of filter before. It's a bit of a mystery.
It sounds great. I have no issues with the design or sound.
Good job well done.
Nice work mate.
Thanks
One more question
What effect would a transistor with greater hfe have on the rest of the circuit ?

[rring]

Oh no  I just didn't understand what you meant by ticking. Now I get it. I have not done a whole lot LDR's before so I that issue is not something I had ever come across. The cap and the  resistor just  provide the 1/2 supply bias from the other stage and feedback to provide the loop gain for the filter. The design only works correctly when that divider is at half. The cap ensures that it is only AC referenced to ground.

The transistor stage has a gain of  hfe/(1+hfe)  so as the hfe goes towards infinity the gain approaches unity. The voltage at the emitter will also increase toward the supply rail( which would be bad). Normally 2n2222a's and 3904's have a hfe between 100 and 200 and this will put you between 4 and 5 volts. You can always add a resistor to ground to make a divider to limit the max dc voltage set point.   

[Kipper4]

Thanks for that. I understand a little better now.
IIRC my emmitter voltage is around 5.5v..
It's a really cool easy to make low parts envelope filter and as you say yourself it uses everyday stock parts which is always a bonus.

[rring]

Not that it matters, but you can always lower your emitter resistor to 4.7 k or so to get the voltage at 1/2 supply. I am glad you got it working  though.

[rring]

 The NJM2068D op amp works great in this circuit. You can get it for 60 cents from Mouser and Tayda.

[Kipper4]

Great news thanks

[Kipper4]

I'm wondering if H11F1M could be used as octos

[duck_arse]

some reading, rich. not sure if it's exactly the answer, but all the big names are there.

http://www.diystompboxes.com/smfforum/index.php?topic=103749.0
http://www.diystompboxes.com/smfforum/index.php?topic=99516.0

[rring]

Couple problems.

The optofet is much faster then the LDRs so this will require a slower decay and slower attack to avoid the growl of modulation of the signal ripple getting through the control voltage. The LDRs are slow and smooth this out. You would have to change the the filter some on the side chan.

The bigger problem is that the optofets work well as linear resistors only over a small range of voltage across them. If the signal is over 100mV  or so it will start to distort. I have designed a number of popcorn compressors with the H11F1M and you have to use attenuation or shunt feedback to reduce the voltage across the device from the signal source.

I also used this part as a product mixer in a ring modulator device - it works great but I had to limit the voltage across it as described above.

In this filter design, I think the signal voltage across them will  and be to high and distort.

[Kipper4]

Thanks R and Duck Arse.
It was worth a punt.
I didnt look at the spec sheets however when I looked at the links I already built the optofet and I like it too. which I believe you had a hand in the development RR. So thanks for that too

[Kipper4]

This sounds awesome with my active bass too.

[Kipper4]

build no2 with a differant oppy
this time an opa2604 for ic2 and it sounds as good as the first and it's a cheaper chip

[rring]

The opa2604 looks like a really nice part but @ 9V it is at the lower specified limit of  supply voltage(+-4.5V).....notice any issues with this?

[Hatredman]

Seem like a good candidate for a charge pump