Wah bias issue

[Brymus]

My wah was making horrid noise at last practice so I decided to mod it.
Mine is the Boss FW3 similiar to the rest of the wahs with the typical boss electronic switching.
Anyway after reading alot I did all the popular mods ,made it true bypass ,all on the stock PCB's.

With the emiter resistor of Q1 a pot I found anything belowe 470R causes a thump noise in the heal postion.
This can be negated to some degree by increasing the resistnace of the inductor to ground or increasing the 1k5 resitor of the base of Q1.

I noticed Doug Hammond recomends a 100R for Q1's emitter,others 270R,or 330,390R 
Can someone clue me in to why mine thumps when going belowe 470R ?
Is it the stock trannies maybe? They were running at 8V,but I removed the electronic switching and raised them to 9V supply.
everything else works fine.
Has anyone else had this problem?    It does it when used as a volume pedal too.
I raised it back to 470R no thump,but am wondering why it thumped with lower values.

[Gus]

Without a schematic and transistor voltages it will be guessing.

You need to read the wha patent and R.G.s writeup and try to understand what each section does.

As you change the emitter resistor value the operating points of the transistor(s if a crybaby like circuit)and gain of the stage before the wha pot changes.  You can measure this with a DMM and measuring the transistor pins.

Why did you remove the Boss Switching?

[Brymus]

Thanks for the reply Gus.

I removed the Boss switching because it really sucked the high end and sounded weak to me.
I got around this for quite awhile with a bypass looper,but when the pedal got noisy I decided to redo it.
I wll post a schematic with my changes and some voltages later tonight.

It's basically a Clyde McCoy clone with input and ouptut buffers (both 2N5457 - 1M from Vref(instead of ground) to gate, 5k7 at source to ground ,drain to 9V)

[Gus]

Ok if you reduce the emitter resistor value you increase the Q2 gain stage that "feeds" the wha pot.
AND you shift the operating points of Q2 and Q5
When the wha pot is at max bass you are at the max output from the wiper of the wha pot to the emitter follower stage.


I found a schematic of the wha on the web and the following references the schematic I found

Where is the slider pot when you have the thump?

What I would do is measure the transistors pin voltages as you move the slider pot(I would measure two times with the slider at the opposite ends)

If you find that the collector of Q2 changes too much with adjusting R6 adjust R7(100K) for Q2 and Q5 stock bias

Also the stock circuit looks nice I don't see how it would hurt the tone.  It has a Jfet input buffer and a emitter follower output buffer.

An interesting thing I see is Q8 and R24 that looks to be a sim of the input load of a non-buffered crybaby I looks like Boss went as far as to switch in 82K across the 1meg when the wha was switched in.  

[joegagan]

http://www.nicosonic.com/schems/BossFW3.pdf

[Brymus]

Again thanks for the reply Gus,my apologies for not documenting everything before posting.
I really need to get in the habit of getting it all documented before starting a thread.

Here is the stock version


Here is mine after modding it,with R6 set lower the thump is lower in pitch,as R6 gets larger in value the pitch of thet hump gets higher until it dis-appears.
Raising the value of R4 or increasing the resitance between the inductor and the C12,R4 junction,will decrease the affect a little.
The reason for biasing the Fets with Vref is because I read at AMZ that gives more headroom on the AC swing.
EDIT: There is a .33uf input cap where R26 is supposed to go and R32 is 100K not 1M as I wrote on the schematic,oops.

The Wah pot is a 30mm slider with 22.5 mm of travel if anyone wants to make a clone of a FW3.

[Brymus]

Ok according to data sheet I found the 2SC3378GR is a low impedence, low noise NPN the GR denotes an Hfe range of 200-400
I tried some 2N4401s lots of noise no wah.
tried some NPN Ge (hfe 300) no sound.
I tried a 2n3904 and a 2n2222A it wahs AND no thump !!!
All control combos and no thump,so it must be something with the stock trannies and them running at 8V?
I am going to pull the 330R stop resistor I installed on the bias control and see if it still doesnt thump at lower settings.

[Brymus]

Ok according to data sheet I found the 2SC3378GR is a low impedence, low noise NPN the GR denotes an Hfe range of 200-400
I tried some 2N4401s lots of noise no wah.
tried some NPN Ge (hfe 300) no sound.
I tried a 2n3904 and a 2n2222A it wahs AND no thump !!!
All control combos and no thump,so it must be something with the stock trannies and them running at 8V?
I am going to pull the 330R stop resistor I installed on the bias control and see if it still doesnt thump at lower settings.

Well I have tried a few different sets of trannies and 270R is the lowest I can go without the "thump" coming back.
No big, I am still able to bias it alot warmer with different trannies (glad I put in those sockets)
If I can still get R7 replaced with a 100K pot and a 47K stop resistor I will.
And see if that will negate the "thump" and allow even warmer bias.
Otherwise I am pretty happy with the tonal variety the new mods have given me.
The last one will be a Bass/Guitar mod by adding a 56n in parallel with the 2P6T rotary switch.
That will give 12 different sweeps 6 bass and 6 guitar.

[Gus]

Why did you reduce R2 from 68k to 39k? this increases the gain

Why did you replace the emitter follower with a source follower at the output?

Why did you remove C16 and R26? I did read you still have an input cap in your post

Is the wha now louder than when bypassed?

Have you tried reducing C14 value?

[Brymus]

Why did you reduce R2 from 68k to 39k? this increases the gain

Why did you replace the emitter follower with a source follower at the output?

Originally I wasn't going to have any buffers at all,just the McCoy circuit,but then after reading decided to try the Fet buffers.

Why did you remove C16 and R26? I did read you still have an input cap in your post

I removed R26 to use its pads for the input cap (.33uf) ,I removed C16 to wire the 3PDT to the PCB.

Is the wha now louder than when bypassed?

That depends on where (V)R6 and (V)R4 is set at.
R6 is wired so it gets lower turned cw,R4 is wired so it increases (adds more mids) thus getting louder when turned cw.
I like it so that its slightly louder than bypassed so yes,it is slightly louder, but it can get noticably boosted.

Have you tried reducing C14 value?

No I did try with both C14 and C13 = .22uf didnt make a difference,I really don't notice any difference in taper either.
The reason I made C13.33uf is because its supposed to mimic the ICAR taper when using a linear pot.

I raised R9 because I like the sharper peak sound,I wish I would have made it 22k or 33k even.

I ended up with Q2 and Q5 as 2n3904, earlier I must of had the pin out wrong on the 4401's I tried.
Basically 2n2222,2n3904,2n4401,2n5088 and pn3565 all worked.
Just the higher in gain you go the more noise that is added,so 2n3904's sounded good without adding any hiss.

I can't get to R7 anymore,it is in the middle of where all the wires enter the pcb from the variable resistors I added.
I hot glued the wires to the board after testing it to keep from breaking any or lifting any pads from stress on the wires,as the PCB mounts components down.
If you think adjusting R7 will negate the "thump " I will try the circuit on my breadboard.

[Brymus]

I forgot answer your first question,I lowered R2 to 39K because originally the pedal was less than unity and if I wasnt able to compensate with R6 I wanted to make sure it was at least unity gain when in use.
What I read on the net, said that making R2 lower to 39K usually solves the less than unity gain issue.


Interesting thing I discovered - that bypassing R6 with a cap kills the wah effect.
Leaving you a volume pedal.
This might be a better way to do a volume pedal mod than lifting C4 from ground.
Since it had better frequency response,than the C4 method.

[ayayay!]

Interesting thing I discovered - that bypassing R6 with a cap kills the wah effect.
Leaving you a volume pedal.
This might be a better way to do a volume pedal mod than lifting C4 from ground.
Since it had better frequency response,than the C4 method.

Huh.  Never would have guessed that.  *scratches head*

[Brymus]

Interesting thing I discovered - that bypassing R6 with a cap kills the wah effect.
Leaving you a volume pedal.
This might be a better way to do a volume pedal mod than lifting C4 from ground.
Since it had better frequency response,than the C4 method.

Huh.  Never would have guessed that.  *scratches head*

I know I was expecting a boost,but it just killed the filter effect instead,no boost at all.