Wah filter (R.G. Keen)

[fuzzo]

Hi guys,

I would like to do a wah-wah with AOP (less expensive than inductor wah). Useable like a fixed wah with an expression pedal to have a regular wah.

I found RG keen schematic . Actually the schematic I redrew is from a DIY project called "Zechen - weh" (it's a sequenced wah) and it's the same that RG keen shows on his webpage (technnology of wah).

http://www.geofex.com/Article_Folders/wahpedl/opampwah1.gif



I've a couple of questions:

What does the "sharp" control ? a boost control in inscreasing the AOP gain ?
I guess I can add in // to 4n7 cap another caps to change the wah range ?
I can add another control as" Q" or something else ?
Need a volume control at the output ?



i did a  veroboard, see you mistakes ?


thanks

[~arph]

I think sharp is the Q control. Change the 4n7 caps to whatever you like to change the frequency of the wah.. don't deviate too much from these values though.

[Mark Hammer]

The "sharp" control is simply a variable feedback resistance that will change the gain of the filter stage.  As the gain is increased, several things happen.  First, it gets louder, of course.  Second, the filter tends to emphasize the center frequency more (what is often referred to as resonance or Q).  Third, the center frequency shifts over a bit, with higher resistance settings (more gain/Q) moving the center frequency downward.  If you look at the schematic for the Seamoon Funk Machine - one of the earliest envelope-controlled filters or autowahs - you will see a filter stage that is very similar: http://www.geofex.com/FX_images/seamoon.gif  The Funk Machine substitutes a transistor used as a variable resistance instead of the 500k frequency pot shown in your drawing.  Getting hold of 5M pots may be tricky.  personally, I have never really found much use for increasing the total feedback resistance above 2M.  You could probably get away with replacing the 470k+5M combination with a 390k+1M combination to go from subtler to "sharper" filter sounds.  If you really want to explore the higher Q zone, then simply install a 1M resistor in series with the 390k, and wire up a SPST toggle to bypass that added resistance.  This will let you tack on an additional 1M feedback resistance (total of 390K+1M+1M) when you want it.  Trust me, you won't hear an awful lot of change between 2.39M and 5M.

To answer your other question, yes you can increase OR decrease the value of the two 4n7 filter-range caps to change the range of the filter.  Smaller values will shift the range upwards, and larger values will shift it downwards.

I would suggest the following mod.  Place a .01uf and .0068uf (10n and 6n8) cap in series in place of each 4n7 cap.  Now, wire up a DPDT on-off-on toggle to them as follows:
- the centre lug goes to the junction of each cap pair
- one of the side lugs goes to the other side of the 10n cap (same side lug for each half of the switch, of course)
- the other side lug goes to the opposite end of the 6n8 cap

When the 10n caps are shunted, the effective filter cap value will be 6n8.  When the 6n8s are shunted, the value will be 10n.  When neither cap is shunted and they are in series, the effective capacitance value will be 4n.  This will give you a low, high, and medium range for the filter, without any nasty switch popping as you change ranges.

Note that the center-frequency-changing aspect of the "sharp" control means that you can set the caps to "high" but tune the filter lower via the sharp control.  Not as much as with the frequency pot, but enough that both the high and low ranges can be easily made to overlap with the middle range.

Since increasing the gain via the "sharp" pot will alter the volume, consider replacing the 100k output resistor with a 100k log pot to achieve effect/bypass balance (OR boost if that's what you want).

[R.G.]

What does the "sharp" control ? a boost control in inscreasing the AOP gain ?

No, it's a Q control. High Q = sharper resonance.

I guess I can add in // to 4n7 cap another caps to change the wah range ?

Yes, as long as you change all three caps, the 4n7 and the 1n at the same time. You must keep the ratio of 1:2 for the 4n7/4n7/1n. One good way to do this is with a CD4053 to switch between the standard values and another set. Makes wiring the panel switch easy.

I can add another control as" Q" or something else ?

Blind Urge to Mod Syndrome?  

Need a volume control at the output ?

Not unless you don't have a volume control elsewhere.

[StephenGiles]

You can probably voltage control the Q - look up Jeurgen Haible who is an expert on that sort of thing.

[fuzzo]

thanks a lot Mark Hammer for your long and good explanantion.

You could probably get away with replacing the 470k+5M combination with a 390k+1M combination to go from subtler to "sharper" filter sounds.  If you really want to explore the higher Q zone, then simply install a 1M resistor in series with the 390k, and wire up a SPST toggle to bypass that added resistance.  This will let you tack on an additional 1M feedback resistance (total of 390K+1M+1M) when you want it.  Trust me, you won't hear an awful lot of change between 2.39M and 5M.

Any way, I think the 5M pots don't exist . I'll do like you said . Actually I need at least 2 controls for the wah (the circuit will put in a dead Boss DS2).

Yes, as long as you change all three caps, the 4n7 and the 1n at the same time. You must keep the ratio of 1:2 for the 4n7/4n7/1n. One good way to do this is with a CD4053 to switch between the standard values and another set. Makes wiring the panel switch easy.

Too complex for me, I think. I'll just use a DPDT to change the range.

o answer your other question, yes you can increase OR decrease the value of the two 4n7 filter-range caps to change the range of the filter.  Smaller values will shift the range upwards, and larger values will shift it downwards.

I would suggest the following mod.  Place a .01uf and .0068uf (10n and 6n8) cap in series in place of each 4n7 cap.  Now, wire up a DPDT on-off-on toggle to them as follows:
- the centre lug goes to the junction of each cap pair
- one of the side lugs goes to the other side of the 10n cap (same side lug for each half of the switch, of course)
- the other side lug goes to the opposite end of the 6n8 cap

When the 10n caps are shunted, the effective filter cap value will be 6n8.  When the 6n8s are shunted, the value will be 10n.  When neither cap is shunted and they are in series, the effective capacitance value will be 4n.  This will give you a low, high, and medium range for the filter, without any nasty switch popping as you change ranges.

That can be great !

Next step : draw the schématic with all this great new imformations and post it here.

It's decided I'll do this one !

thanks.

[fuzzo]

HI, guys

like I said, the new schematic :



I've a doubt about the wiring of caps in feedback loop with the on/off/on switch.

[fuzzo]

bump

[Mark Hammer]

Votres doutes sont bien placés.

The common of the toggle should go to the point where the two caps meet, so that it bridges one cap when pushed to one side, and the other cap when pushed to the other side.  In the middle it bridges neither.

[fuzzo]

 I don't manage to find how do the good wiring of on/off/on switch, can you post a drawing or schematic with the good one please ? 

[fuzzo]

little bump

[slacker]

To wire the switch like Mark suggested, you need to connect the circles next to where it says "Sw1a" and "Sw1b" on the schematic to the vertical lines between the 6n8 and 10n caps.
Basically you just need to flip the switches upside down.

[fuzzo]

thanks I think I got it !



I think it's good , no ?

[fuzzo]

bump

So my last schematic seems to be good , I made a stripboard :



no Verified so If you see mistakes say me .

[fuzzo]

No one to confirm my schematic and layout ?

[fuzzo]

I need some help. I did the veroboard posted in my last post but the Wah pot doesn't work . The sharp pot increase gain.

Do you see mistakes or errors on my layout ?

[slacker]

Your layout looks good. I think the problem is that R7 is too big, I simulated the circuit and the difference between 47k and 500k hardly moves the frequency of the filter at all, that's why it sounds like the wah pot is not doing anything.
Try replacing R7 with a jumper and see if that makes it work.

[fuzzo]

Hi,
I've just tried removing R7 , nothing change and there's a weird sound when the pot is at max (or min) a big "strich", at this postion  the guitar sound disappears.

I also tried to put a lower value for R7 (1K) always nothing.

I put a 470k and 1MA for the sharp control .

[Eb7+9]

Votres doutes sont bien placés.

nice try Mark, that should read ... "Vos doutes sont bien placés."

carry on ...

[fuzzo]

It's no really typically french to say "Vos doutes sont bien placés" , we say more "vos doutes sont fondés" . Actually the word "placer" doesn't work very well in this sentence.

Anyway, I worked on it today, and I didn't find the problem.

the couple 6n8/10n and the other one must have a connexion ? C5/C4 and C7/C6 must be together to go to the wah pot or  just one couple ?