Footswitch with LED indicators

I used to have a Peavey Classic 30 that had switchable distortion and reverb. However the included footcontroller lacked LED indicators to tell if the dist/reverb was on or off. There are LED-equipped footswitches on the market, and I even believe Peavey’s newer versions have LEDs.

Well, why spend money on a new switch when you could simply get into DIY-mode?

The circuit used here isn’t guaranteed to work on all amps, but if the amps uses relays for the switching, chances are this should work on other amps as well. Be sure to check polarity on the footswitch jacks.

The Peavey switch connects to the amp via a cable with two wires + what appears to be a common ground in it. However, the naked wire is actually the positive supply, and the two other wires supply current to the relays for distorion and reverb. Roughly like this:

So it’s really just two spst (1xon/off) switches that turn on the current to the relay coils. The key word here is current. The coils need a certain current to achieve a sufficient magnetic field an thus be able to move the mechanical switch in the relay.

Now, as I actually don’t own the amp anymore, I’m not that sure about the numbers here, but I measured the voltage from +V to coil with switch open, and I think it was around +30V. The more important thing however is the current, the currents were sligthly different for the two relays, but both were somewhere around 20-25 mA. I’m not sure how much the relays need, but this is important as the leds used were rated at max 30 mA, and now there’s no need to add series current-limiting resistors.

I opened the switch by removing the bottom plate. It wasn’t glued or anything, so a sufficient amount of brute force worked fine. The only thing I needed to do was to add LEDs in series with the switches. Like this:

The naked conductors need some sort isolation of course, normal electrical tape works fine.

Done!

As I sold the amp after finishing the mod, I never got to test it on a gig. But it worked just fine at home, and as long as the LEDs don’t fail, I’m sure it will go on working fine. (if a LED fails there will be problems, as they are in series with switches. But not harder to fix than replacing the LED, they are really cheap)

Update: part layout and schematic picture:

Some more bypassing

Hello again! It’s been a while, I’ve been busy with family and stuff. Well mostly family. But the other day I decided to finally convert my CryBaby into true bypass. This is a common mod, and it has been explained in depth many times all over the internet, but I took a slightly different approach to things. The CryBaby (gcb95) basically has two circuits connected in series, a buffer and a filter. The filter part is what makes the wah-wah sound, it’s a resonant filter that amplifies mid frequencies. The rocker on the pedal changes the center frequency of this mid boost. The problem with the filter is that it

• a: isn’t bypassed properly
• b. has a very low input impedance

One of these two wouldn’t necessarily be a problem but the two of them combined is trouble. When the pedal is bypassed, the input is still connected and as the input impedance is low, it will load down the guitar signal. This is why the buffer is added to the circuit. The buffer lifts the input impedance to acceptable levels (ca 1 megaohm). However, if the circuit was bypassed properly, the buffer wouldn’t really be needed at all. Here’s how I did it: First of all, I tend to be somewhat reluctant to making too drastic changes to my pedals. By this I mean that want to be able to mod the pedals in such a way that they can easily be returned to their original state. So I wanted to keep the buffer but still have true bypass and be able to play without the buffer when needed. Sounds reasonable, right? What I did was to disconnect the two circuits (buffer and filter). This was the most drastic thing i did to the pedal, as it included cutting traces on the circuit board. I cut the trace between the input of the pedal and the input capacitor of the buffer circuit. The other cut i had to make was between the output of the buffer (at the emitter of the first transistor) and the 68k input resistor of the filter part of the circuit. I also had to add wires to the input and output of the buffer and the filter input at the mentioned points. See picture below. ( sorry for the bad pic) There was no need to add a wire for the pedal input, as it already existed (the green wire on the ribbon contact). This was really all i had to do to the circuit board, the rest was done around two switches. The original bypass switch is a single pole switch, so this had to be replaced. I happened to have a couple of double-pole switches so it was an easy operation. Here’s a schematic of the switching: As you can see there are two DPDT switches, the first one (S1 in the schematic) bypasses the buffer, and the second bypasses the filter (this is the switch that is in the place of the original SPDT switch). The first switch is a small one, placed on the left side of the pedal, right next to the output socket. The capacitor on the small switch is simply an output capacitor for the buffer circuit. It was easier to solder it directly to the switch. And that’s it! Now I have two bypassable circuits in one. The buffer can be on all the time or bypassed by the flick of a small switch. The filter part can also be used on it’s own. The reason why I wanted it this way is of course mainly to prevent loading on the guitar pickups in bypass mode, but also to be able to use the pedal as an input buffer for the whole pedalboard. The wah now sounds slightly different, not necessarily better (or worse), but the big change is in how the pedals connected after the wah sounds. My fuzz sounds way better now when there’s nothing stealing signal between the guitar and the fuzz. When I don’t feel fuzzy, I can add the buffer and have some more treble to use with other overdrive pedals. There’s one tiny problem still. When the buffer is activated, there’s an audible thump noise when the filter is activated/bypassed. Can probably be fixed by adding a large resistor in a strategical place. I’ll get back to you on that one. Stay tuned (as always!) (and yes, I will continue with the tremolo project soon…) Edit: Corrected some major spelling errors (seriously, in what state did I write the original post?). I added a 1M resistor between buffer output and ground. This drains any build-up voltage on the output capacitor to ground. This solved the click-problem mentioned above. (kind of, there is still a bit of a thump when the filter is turned on and off, but it’s no worse than you would usually expect from a mechanical switch)

Designing an effect pedal, part 2: The boring parts and a little overview

This time I’m going to look closer at the part of the effect pedal that really shouldn’t affect the sound in any way, but still is essential for proper functioning. Let’s start with the least cool but maybe most important part of the build: the power section!

Because the LFO is going to be based on a schmitt trigger (square wave generator) there is a major chance there will be a ticking noise in the pedal, unless the power section is executed properly. My first version of the tremolo (the hardly, but still, working one) had a major issue with this. Back then I had installed a potentiometer that seamlessly mixed the square and triangle waves (I now consider mechanical switching between the two waveforms) and unless I used only the triangle, the ticking noise from the LFO was almost louder than the audio signal. Enter the RC-filter!

This filter is connected between the power source and the power input the the circuit board. Basically it’s a frequency dependent voltage divider. If the signal is strictly DC (could be considerd AC at 0 Hertz), no current will flow to ground through the capacitor. As the frequency grows the capacitive reactance through C will decrease, and this way higher frequencys will be more dampened. This will keep the voltage at the output as even as possible. Suitable values for R and C are 100 ohms and 100 microfahrads.

In the case of the LFO, the schmitt trigger will momentary draw a lot of current as it changes between its two states. An ideal power source would be able to keep the voltage constant under any conditions, but real power sources have a tendency to lower the voltage when the current draw is large. These short voltage drops tend to spread through the audio circuit as loud ticks. The RC prevents this to some extent. The capacitor stores some charge and thereby keeps up the voltage when the power source isn’t able to keep up the voltage at an even level. The resistor on the other hand puts a limit on the total current that can pass through the system and thereby also on how fast the capacitor can “reload”. This might sound counter-productive but as it turns out, the combination of these two filters out most of the LFO-ticking.

Another good idea is to place a diode reversed between the voltage input and ground. This prevents voltage of wrong polarity to pass through the system as it will conduct any negative voltage to ground.

I’m going to use separate RC-filters for the LFO-section and the audio section (seriously, this is how annoying  I find the ticking!), so the whole power section will look something like this:

At last, a block diagram of the whole circuit:

Next post will be all about the in- and output buffers. Stay tuned!