Now something completely different: Coron Guitar Effect pedals
Yea right! A couple of years ago I was doing music and like any other guitarist I have amassed a pile of guitar effects.
If you search the net for guitar effect pedals you’ll usually find tons of information including schematics and review. I have a wah which I wasn’t able to find much information for, so I decided to take it apart and do a technical review. The pedal was in need for a good clean-up anyway.
The Coron Wah-Pedal W-220.
If you open it up you’ll see that it’s mechanic is quite sturdy and similar to higher praised brands like Vox or Dunlop.
Also at the first look the circuit board looks like just another Vox style two transistor Wah. I wanted to find out exactly which schematic they’ve copied, so I dug a little bit deeper and I found:
It’s not a 100% clone! There is more going on inside. Quickly I drew out the schematic:
What you see here is mostly the two transistor Vox style Wah.
They use 2SC732 transistors. Nice high gain transistors with excellent hFE linearity. The inductor only has half the inductance usually found in Wah pedals, but that has been compensated by doubling C2 from 4.7µF to 10µF. Quality film capacitors for all remaining caps in the audio path. The big 220nF caps have for sure been a lot more expensive than ceramic disk style caps.
The Feedback Path
Remarkable however is the negative feedback path right at the bottom (R12, C6, R13).
This is something I haven’t seen in Wah circuits before and it’s quite clever: It is frequency dependent, so it linearizes the frequency response and lowers distortion a bit. The frequency dependency makes it much more pronounced in the low frequencies, so it kind of works like a presence control giving a tighter low-end. It also moves the wah bumps a bit up in the frequencies.
Here is a simulation of three pot positions with (green) and without (red) the feedback path:
I have them as well: I did a small test by generating white-noise using my mbed LPC1768 board, dividing it down to around 0.3Vpp and running it into the Wah. I changed the pot values: Both extremes and some in-between values. The output was sampled using my PC sound-card and converted to a spectrum using FFT:
The output is noisy, true, but you can see that the bump in the frequency response end up roughly at the same spots as the simulation. Nice.
But wtf is this?
Somehow it makes me wonder if the Coron engineers actually knew what they’ve been doing: Notice the resistor R12 (4.7k) and R13 (220k): They’re in series and have 10% tolerance. Whatever R12 is supposed to do, it has no effect because R12 is smaller than the 10% tolerance of R13. Also it’s not simply used as a jumper because there is no trace to jumps over either. This resistor is completely useless.
And while we’re at it: The R9, the 47 Ohm resistor right after the power source does nothing as well. It’s tiny compared to the 22k and 1k collector resistors, and these have a local bypass in C5 anyways. I could see it if the pedal would be powered from a wall-wart. In this case R9 and C5 would build a low-pass filter to get rid of power-source hiss, but this pedal doesn’t even have a plug for external power. Only battery is allowed.
There are a few wtf’s on the trace side of the PCB as well like needless ground loops and so on. But hey, not critical, it works, doing PCB in the 80’th was harder than today.
Measuring the Inductor
No technical review of a Wah would be complete without taking a look at the inductor: I measured mine as about 320mH with 12.8 Ohm series resistance. The bobbin is type EE-1916 and the core looks like to be laminated steel. The copper wire is very thin. I haven’t measured it, but it looks like 0.1mm style wire.
For measuring the inductor I did nearly the same setup as for taking the frequency response curves. It’s super simple and doesn’t need any special parts:
Here is how:
You take your inductor and a high quality capacitor of know value. You’re likely have an idea in what range the inductance is, so pick a capacitor that would result in a notch in the low audio range around 1Khz or so.
Then connect these as a notch filter as shown below:
Now you use an audio editor like Audacity, generate a few seconds of white noise and play them into the filter while sampling from the output. Do a spectrum/FFT analysis of the sampled data and a very deep notch will appear. This is the resonant frequency of your LC filter.
To get from that frequency to the inductance just calculate as:
L = 1/(4 pi^2 C F^2)
And you’re done..
The Coron W-220 is not a straight clone of the Vox Wah but a variation of it, it’s tighter in the low registers. This may be just what you’re looking for. In a band context you don’t want to compete with the bass player and if your guitar or rig is already on dark sounding to begin with this Wah may be a good fit as well.
Build quality is on par with the flagships like Vox and Dunlop. They didn’t tried to squeeze the last penny out of the parts cost.
These cheap 80th Guitar pedals from Asia tend to have bad reputation overall. Sometimes this is justified, especially for the boss style knock-offs with the plastic housing (does your still has a working battery compartment?). Sometimes they’re pretty good though and always worth a try.
Oh, for completes sake, here are the images I’ve used to trace the schematic from. You can directly overlay them to see the components and traces at the same time: