Park - Auto Wah [schematic]

[gigelmargel]

Hello!
I finished building Park Autowah using this layout http://dirtboxlayouts.blogspot.com/2021 ... o-wah.html (thanks to Anders!).
I like it a lot, but I want to modify to my taste:
1. it sounds a little muffled (I want to have more highs) - I eliminated both 1nF caps to ground (C2 and C5) but with no significant changes
2. the minimum speed it's too high (I increased the 2.2uF caps (C9 and C10), but this decreased a lot the maximum speed).

Thank you!

[gigelmargel]

I increased the speed pot, too...but it didn't what I want...

[Ice-9]

The basis of the speed control is build around an 'Astable Multivibrator circuit, this is also the basic building block of a flip-flop circuit which can be Astable, monostable and bistable as used in Boss style switching. In the case of the Park autowah it is the astable type with a control for speed. Try altering the caps for different speed range.

You can read up about how this all works from this link, it should explain the component value changes needed to get the results you want. (smaller caps will charge/discharge faster providing a faster clock.)
https://www.electronics-tutorials.ws/wa ... table.html

[snk]

Hello,
I built it too, using the same layout posted above by Gigelmartel.
I like the sound!
I used BC547B transistors, but the output volume is much below unity gain. I also tried with 2N5088 and MPSA18 (for Q1 & Q2), but it didn't change anything...
I know that some early wah and fuzz pedals suffered low output volume, so i am wondering if low output level is to be expected (and thus i'll consider adding a booster after the circuit), or is something to be tweaked in my build to reach unity gain?

[karul]

I used BC547B transistors, but the output volume is much below unity gain. I also tried with 2N5088 and MPSA18 (for Q1 & Q2), but it didn't change anything...

2n3702 is PNP
BC547B, 2N5088 and MPSA18 are NPN

circuit is running on -9V (negative supply)

[snk]

2n3702 is PNP
BC547B, 2N5088 and MPSA18 are NPN

circuit is running on -9V (negative supply)

Hi, Karul, According to the creator of the layout i used, the circuit has been adapted accordingly :

Layout has some mods; power supply mod since original units was positive ground, CBE pinout for the transistors, power filer cap and protection diiode. As for the transistors, most low gain NPN silicons should work, I ended up with PN2222s in the 150Hfe ballpark.

[karul]

A-ha, there is a modded version. I was looking at the schematic in this thread.

[gigelmargel]

Hello,
I built it too, using the same layout posted above by Gigelmartel.
I like the sound!
I used BC547B transistors, but the output volume is much below unity gain. I also tried with 2N5088 and MPSA18 (for Q1 & Q2), but it didn't change anything...
I know that some early wah and fuzz pedals suffered low output volume, so i am wondering if low output level is to be expected (and thus i'll consider adding a booster after the circuit), or is something to be tweaked in my build to reach unity gain?

Hello!
The transistors seems OK to me. As far as I remember, my clone have enough volume, so maybe you have another problem.

[snk]

Yes, I guess i have another problem, but I can't figure out what could it be ?
I mean, which part in the schematic/layout defines the gain structure?

[snk]

Hi,
I investigated further, using audio probe : I get a loud, wet (processed) sound at the rows D and F from the left side of the veroboard layout, which correspond to the Q2 Emittor , and the 10k resistor going to Q2's Collector. It seems that something is not right in the last steps of the circuit (10k/R8 > 10µF/C7 > 100k > out).

My questions are the following :
- What do R8...10 & C7 do in the circuit?
- Would it be wiser to troubleshoot the whole circuit (I guess the wise answer is "yes"), or is it safe to just wire the output to either row D or F on the layout (skipping R8...10 & C7 ) ?

[edit]
This is where I get the "full sounding, wet sound" in the schematic, and on the layout :

[snk]

Hi, tweaks made, pedal improved!
- The 100k resistor at row 12 was responsible for the volume loss. I used a 10k instead, and now i am running the pedal at unity gain.

- I tried to tweak the 10 nF cap (row2), hoping to change the resonance : increasing it lowers the cutoff point, making the sound "darker". 22nF sounds good, 33nF starts to be quite dark, but the stock value sounds fine as it is

- I tried to use a 47µF electro cap (instead of the 2.2µF row 19) : the LFO can go much slower, but I found the modulation shape to be quite assymetric (more a saw than a triangle), thus creating a strange modulation effect (maybe because i changed only 1 cap). I think I will keep the stock value here, too.

[The G]

Yes, the oscillator capacitors (C9 and C10) need to be of the same value to keep the LFO swing symetric.
IMHO R10 could as well be zero. After the output passes through the voltage divider made with R8 and R9, it has to go through the divider made with R10 and the input impedance of the following stage. Maybe it was some kind of protection, although I cannot see any use case for it.

Being curious about the usability of the veroboard apps, I've done the dirtboxlayouts.blogspot.com layout in Eagle and DIYLC. Man, having no feedback from the schematic is hard. I don't think I'll try again to use DIYLC to do a complete layout. Anyways, here are the fruits of my labour, maybe they'll be of some use for someone. I hope I didn't make too many mistakes.

[snk]

Yes, the oscillator capacitors (C9 and C10) need to be of the same value to keep the LFO swing symetric.
IMHO R10 could as well be zero. After the output passes through the voltage divider made with R8 and R9, it has to go through the divider made with R10 and the input impedance of the following stage. Maybe it was some kind of protection, although I cannot see any use case for it.

Thank you, The G.