Q&D VCF

[joer0952]

Hey, I'm new at making my own stompboxes so if these questions are stupid Im sorry in advance, but I was looking at the following layout on a post on this forum :

I also found the following schematic that it was designed off of on folkurban.com:

I have a question I assume the layout is correct, but I don't understand based on the schematic why on the layout pin #7 on the ic is connected to the 9v?  Also, what does cv mean, and what do I hook up to the cv in?

[Taylor]

I have a question I assume the layout is correct, but I don't understand based on the schematic why on the layout pin #7 on the ic is connected to the 9v?  Also, what does cv mean, and what do I hook up to the cv in?

Notice that the IC is a 741. This is a single opamp, and pin 7 is the power pin. It might look weird because we much more often use dual opamps, where pin 8 is power. When you see an IC or transistor, and you're wondering how to hook it up, the first place to check is the datasheet. You can usually just google, for example, "741", and the first PDF you see will be it:

http://www.national.com/ds/LM/LM741.pdf

That shows you the pin configuration. On to your second question:

CV is "control voltage". In electronics it's often very handy to have something where the important parameter (in this case filter cutoff frequency) is controlled by a voltage. Then you can control it all kinds of weird and fun ways. In this case, say you had built an LFO. That oscillator puts out a control voltage, and you can plug this CV into the point on that schematic that labeled CV, and you can have your filter repeating "wowowowowowowowow". You can also use an envelope follower circuit to make a CV that is controlled by your playing volume. Louder volume=more positive voltage (or more negative if you invert it). Then you have an auto-wah.

The shorter answer is that, if you don't already have a CV source (LFO, envelope follower, etc.) then you don't have anything to plug into that spot. But that's ok, it will work without that. Just use the pot (or expression pedal) to control the filter cutoff.

You might just build your box with a 1/4" jack connected to the "CV" spot. Then, someday down the road, you can build a CV source and make some more unusual sounds.

[joer0952]

Thank you that makes alot more sense now.  I just don't want to follow what it says, I want to understand you know.

[PRR]

> why on the layout pin #7 on the ic is connected to the 9v?

AND why is pin 4 connected to ground ? ? ?

Certain things, you are "assumed to know".

If some detail is "self evident", the schematic may leave it off, reduce clutter.

The instructions on a box of macaroni say to boil it; they do not tell you to use a pot. They assume that if you know how to boil water, you know to use a pot.

The 741 chip needs power. The schematic drawer was focusing on the complicated audio and filter details, "assumed" that builders would know to connect power to the 741.

> questions are stupid

Nobody is BORN knowing this stuff. I can actually remember my first 741. I already knew it needed power, but the connections were not clear to me. And this was when the 741 was NEW and COSTLY. So no, it's NOT a "stupid question".

[joer0952]

ok, does anyone know a good LFO layout that I can plug into the cv in?  Can I use the PWM's LFO, i built that already?  Another thing I was wondering is there anyway to just hook up a 1/4 in jack to the cv in on the q&d vcf and use my pwm lfo as a cv while still being able to use it independently w/o having to switch patch cords?  I hope that makes sense lol.  I don't even know if I understand what im saying.

[joer0952]

Ok I made the Q&D VCF on veroboard, but when I plug it in there is a very load high pitch feedback.  Here is the layout I used:

And here is the LFO I used from www.beavisaudio.com/

When I turn my amp volume all the way down it is still there but lower.  When I adjust the pots it remains but the pots do have an effect on the freq of the squeal etc.  If I remove the 40106 chip the feedback squeal remains, but if I remove the 741 it stops.  Here are my voltages:
battery = 9.53v
red lead = 9.26v
black lead = 0
IC1 (741)
pin 1 = 0        pin 8 = 0
pin 2 = 4.55v  pin 7 = 8.77v
pin 3 = 4.11v  pin 6 = 4.34v
pin 4 = 0        pin 5 = 0
IC2 (40106)
pin 1 = 4.03v     pin 14 = 8.86v
pin 2 = 4.03v     pin 13 = 0
pin 3 = 0           pin 12 = 8.86v
pin 4 = 8.86v     pin 11 = 0
pin 5 = 0           pin 10 = 8.86v
pin 6 = 8.86v     pin 9 = 0
pin 7 = 0           pin 8 = 8.86v
D1 (Led)
(+) 8.85v   (-) 0
D2 (1N4148)
K = 4      A = 3.30

both electrolytic caps have ~4v on the (+) end and 0v on the (-) end

[joer0952]

I'm using a signal probe and i can trace the signal to pin 3 and its fine, but when i put the probe up to pin 2 i get the signal with feedback so i guess the signal coming into pin 2 is causing the problem, but when i remove the 40106 trigger it doesn't stop so i don't think its the lfo cv.  When I hook the probe up to pin 6 I get the signal with ungodly feedback, I guess that makes sense since this is an opamp.  Also, when i probe over the 1M resistor I don't get a signal, I dont know if that is suppost to happen.  I also changed the 741IC just to see if that was the problem with no change.  Im new at this and dont have alot of knowledge about circuit boards so any help would be great.  Oh one more thing when the probe is only hooked up to the circuit ground I also get a little feedback.

[joer0952]

I connected the output of the above lfo to the cv in of the Q&D VCF, is that ok should it work?