Dynacomp tone mod

My Dynacomp clone lacked a little high end, so I decided to add a simple eq circuit to it. While I was at it I also wanted to boost the output of the pedal a bit.

One of the simplest ways to change the frequency response of a circuit is to add an RC or CR filter.

In the RC version the higher frequencies are cut, as they are grounded through the capacitor. The other version allows higher frequencies to pass through, but the lower ones are cut.

A common circuit used in many pedals combines these two filters and a potentiometer to blend between the outputs, usually referred to as the “Big Muff Tone Control”, after the famous fuzz pedal.

The original Big Muff had something of a mid scoop, as both filters cut the midrange. In my version its possible to get a flat frequency response, a mid boost or a mid cut.

Please note that the volume pot (R7) was included in the original circuit. The eq/boost-circuit connets between the compressor pcb output and the volume pot.

The tone control still controls which filters output the signal is taken from, but it’s combined with a 3-way switch. The switch is a 2xON-OFF-ON switch, which is basically the same as a DPDT-swich, but in the centre position, both switches are off. The first half of the switch adds a second capacitor in parallell with the capacitor in the high-pass (CR) filter. The other half of the switch bypasses a capacitor in series with the capacitor in the low-pass filter. This changes the mid response from a slight boost in top position, flat in middle position and quite a large mid-cut in the down position. Values of capacitors where tweaked using the duncan tone stack calculator, a very useful free software for simulating eq circuits.

The second half of the circuit (from C5 onwards) is a buffer/output booster to compensate for the gain loss that is inherent to every passive eq circuit. It’s a common design using a J201 JFET. The gain of the output stage is determined by the ratio of resistors 5 and 6.

Here’s everything except the switches and the potentiometer placed on a piece of stripboard:

And here’s the board crammed into the already crowded pedal box, with no less than 11 (!) cables attached to it:

How does it sound? It sounds good. This modification made an already good pedal even better.

The circuit described above could be added to pretty much any pedal, as long as it’s connected to a source with a reasonably low output impedance. (that is, not directly to a guitar pickup, but to anything buffered). It should work on its own with a proper input buffer also, as a simple buffer/booster/tone control.

Guitar tone circuit mod

Today’s post is about a very simple yet effective modification of the tone control on a strat. I personally am a strat man all the way, but i have to admit that I find the tone control(s) a little unnecessary at times. The neck and middle pickups are very balanced when it comes to treble, and the bridge pickup, where a little high-cut might come in handy, isn’t even connected to the tone control. What I did to my strat was to wire all three pickups to one tone pot, and as I really don’t use the tone controls that much, I added a switch that bypasses the tone circuit completely when desired.

The original wiring looked like this.

And in reality, it looked like this:

What I did was to remove the middle pot, add a switch in its stead, and change the wiring to this:

 

N.B. Output wire removed. Connects to centre lug of left pot.

The result?

A tone control that loads all three pickups equally, which in itself balances the treble content between the three. And it can be completely taken out of the circuit, which is the way it is most of the time when I play.

On another guitar I did the same thing but went even one step further and replaced the tone pot with a trimpot inside the guitar. More on that guitar another time, as it’s work in progress still.

P.S. As You might notice, I also changed the volume pot (from 500k to 250k). Partly to get a softer sound, but also because of the old pot becoming noisy when turned. The tone pot turned out to be a quality ALPHA pot, which is suprising as the whole electronics package (pre-installed on pick-guard) was very cheap. (The pickups were replaced earlier from mediocre generics to Fender Texas Specials).

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)

Pedal bypass surgery

A few years ago I bought a Rocktron Big Crush compressor pedal. It’s a nice transparent compressor and it sounded great right away. As long as it was active. When in bypass mode, it was the worst pedal ever. It had buffered bypass, which isn’t necessarily a bad thing, but in this case it was terrible. There was a big volume drop and a major loss of bass frequencies. So I’ve thought about turning it into true bypass for a while, and now i finally had the time (and courage) to do it.

Why true bypass? True bypass means that the circuit is mechanically removed from the input and output when the pedal isn’t active. This is usually done with a DPDT-swich (two on-on swiches in one package), but because most pedals have a led that lights up when pedal is activated, there’s need for either a third switch (which calls for a 3PDT-swich) or one of the existing switches has to be used for this. Another solution is the millennium bypass, of which I already wrote in the fuzz-post. When the signal path is completely removed from the circuit in bypass mode, there’s no extra load on the signal, and therefore no loss of any frequencies, which might be the case when using (bad) buffered bypass.

So the pedal already had a DPDT-swich, which isn’t always the case in buffered bypass pedals (most of them use FET-switching). What I had to do was to disconnect the inputs and outputs from the rest of the circuit, and because I intended to use millennium bypass, I also disconnected the led. This was done by simply cutting off the copper trace on the circuit board in the right places (red lines).

I then soldered wires to the right places to the underside of the board, green wires for bypass and red for current to the led.

Because of the way the cicuit board is held in place inside the pedal, I had to cut a small “bay” in the side of the board so the wires could pass through.

The led switching circuit was placed right next to the switch using double sided mounting tape. The circuit is roughly the same as in the fuzz, it uses a different diode and a smaller resistor, but the principle of operation is exactly the same.

I found the schematics online (bad resolution, but it’s readable), here’s what it looks like after the modification:

(Note that R24 needs to be connected to C25. The easiest way to do this is to solder pins 5 and 6 on the ribbon cable base togehter). The parts used in the bypass circuit are: Q = 2n5457 FET, R = 560 ohm resistor, D = 1N1418 silicon diode.

One thing that I noticed is that when there is no plug in the input of the pedal, the led stays on all the time (if the power cord is connected). First I thought I had done something wrong, but this i simply because the ‘ground’ of the whole cicuit isn’t really grounded until there’s a plug in the input, a design meant to save batteries. This is not a big deal, because I never run my pedals on batteries. Apart from this, it works totally fine. Now as the pedals major flaw has been corrected, it’s a really good compressor, and will find its way back to my pedalboard!

EDIT: By request, I added three more pics with more detailed information on which wire goes where (see numbers) and a stripboard layout for the fet switching circuit. I hope this will be helpful!

N.B. the pin order on the fet might differ depending on the fet used. Check the datasheet.