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:

Logos and badges, part 2

As promised, a follow-up on the badges/stickers I mentioned earlier.

They arrived quite fast and well packed, and the printing quality was as good as I had hoped for. Nothing to complain about really. I tried to save money by putting four logos on one sticker and then cutting it up. This was maybe a bit unnecessary, as they weren’t really that expensive. Here’s what they look like on my pedals:

The turqiose one is my still to be finished tremolo pedal. The others have been presented here before.

The plastic badges for my amp project were perfect!

 

New amplifier!

The big project for this autumn has been to build a guitar amplifier. After many hours of late-night-soldering, today was the day when I finally got to switch on the power.

The amp (called Uraltone jr) is built from a kit, designed and sold by Uraltone in Helsinki, Finland. It is a very reasonably priced kit, which contains all the electronics (even including high-quality tubes!) and the metallic chassis for the amp. What You need to buy apart from the kit is materials for the box, and of course the speaker. I had already built a speaker cabinet, so it was just the amp missing. I went for a head/speaker design, but the amp can be built as a combo as well. (I will not go into detail on the wooden box here, the materials and techniques were essentially the same as on the aforementioned speaker cabinet).

The first step was to install all the external parts, transformers, potentiometers, jacks, switches etc. This was in a way the easy part, as the chassis had a hole pre-drilled for every part, so it was really just a matter of getting everything in the right place, and securing everything with the right nuts and bolts.

In the picture above, some of the wires are already cut and soldered to the right place. All the wires on the left are connected to the mains transformer, while those on the right go to the output transformer. The twisted pair of black wire in the middle go to the anode choke, which is essentially just half a transformer (or a very reactive load, with high AC resistance and very low DC dito).

With all the external components in place, it was time to start working on the circuit board. The board itself is a very sturdy PCB/turret hybrid, which holds most of the capacitors and resistors (a few are mounted directly to the pots/tubes).

The board was then installed, and the last wires were soldered, with everything in place, it looked like this on the inside:

It is a mess, there is no denying that, but the instructions were clear, so I actually got everything right on the first try. The outside looked like this:

The front controls are, left to right: On/Off-switch, Master Volume 1&2 (for the different power tubes), Tone (it’s called ‘colour’ by the manufacturer), Volume (gain) and a six-position boost switch.

Back controls: Anode voltage, Feedback and Line out level. These may need some explaining. The first one lowers the voltage on the anodes, which takes down the output power without affecting the sound too much. (this requires som extra parts, which can be bought as an add-on kit). The second knob controls the negative feedback from the output transformer to the preamp. It does not have a huge impact on the sound, but it’s clearly audible, especially on higher output levels, where it does a good job cleaning up the sound a bit. The last one is simply a volume knob for the line out jack. The amp itself has a built in dummy load, which takes care of all the output power when no speaker is connected. I have not tested this feature yet, and I guess it will not be used that much, but it is a nice one to have when recording for example.

Well that’s pretty much it then:

I was a bit concered about messing around with high voltages, but it turned out all right. Common sense applies, as the voltages inside are even higher than mains voltage.

The important part then, the sound. The amp sounds quite fender-ish, with a very nice clean. It can take a lot of different power tubes, at the moment mine runs on the supplied JJ 6V6 and Svetlana 6L6, with a TAD 12AX7 preamp tube, and a JJ 5Y3 rectifier. I guess messing around with different tubes could make a huge difference in sound, but the important thing I think is that it sounds great right away. It can do distortion to some extent, but preamp distortion is not where this amp shines. Instead it does a wonderful job at the level where the power tubes just start to break up. This  combined with the ability to take down the output power makes it a really nice blues-machine. With the master volumes just above half-way the sound starts go get a little furry, with just the right amount of harmonics. The tone control is well designed in the sense that it is useful in its entire range, from ‘slightly softer’ to ‘a bit brighter’ (instead of the usual ‘useless muddy mess of low frequencies’ to ‘ear-piercing sqeal’). It also needs to be said, that this amp is REALLY quiet, virtually no hum at all at clean settings, and only the slightest hint of hum when moving into overdrive.

The mandatory ‘glowing-tubes-picture’.

Closing thoughts then. I really enjoyed building this amp, and I feel this might not be the last one I build. This is a well designed kit so it shouldn’t be too hard even for a beginner. I guess a tube amp shouldn’t be your first electronics project, but if You feel that You’re comfortable with soldering and can follow instructions, then I can really recommend this amp.

Please stay tuned for a demo!

Dynacomp clone

A while ago I bought a MXR DynaComp clone kit, but I never got started on it. Lately I’ve been trying to tidy up my electronics-stuff and finish all my half-finished projects. So I went through the parts and realized there were a few parts that were not in the kit. I ordered the missing parts together with some other stuff that I needed and sat down to finish it.

I like to try something new for every project, some new technology or way of doing something, that I haven’t tried before. This time I wanted to etch my own circuit board. I downloaded the layout from General Guitar Gadgets and printed it on Press-n-Peel film.

This turned out be a very convenient way of transfering the layout once I got the printer settings right. Be sure You print the right size! There was one line that wasn’t transferred properly, but I was able to correct that with a permanent marker. Then I etched the board in ferric chloride untill all the extra copper had been dissolved.

The cotton string is a good idea unless You like getting acid on your fingers.

Then I did the ususal drilling (1 mm drill bit) and cleaned the board from the transfer film residue using lighter fluid.

Then I drilled the enclosure (1590BB, pre-painted) and installed all the components that don’t go on the circuit board.

Once again I used the millennium bypass, as I happened to have couple of DPDT switches lying around.

I felt like being super-organized, so I taped all the components to a sheet of paper and wrote down the component values next to them. Maybe a bit excessive work, but I actually got everything right at once this time.

It was the usual routine when soldering, jumpers first, then resistors, polyester caps, electrolytic caps, semicounductors.

With everything in place I connected the board to all the external parts.

Spot the mistake in the picture above.

I found it the hard way when testing. No sound whatsoever, and my spirits sank. Until I realized I hadn’t put the OTA in it’s socket… Easy fix, and everything worked fine.

That’s it then folks. I really like the sound, much more prominent compression than my Rocktron Big Crush, more vintage if You like. I could use a little more treble, anyone know which capacitor to change for this? But it’s no big deal, I like the sound, perfect for funk and clean legato lead stuff.

Stay tuned!

Logos and badges

I was searching the web for some company that could make custom-made badges and logos for my projects, and stumbled upon this one. I’m going to give it a try, and get back to You when I have something more to report.

In the meantime, You could check it out, it seems promising! They ship to a large part of Europe.

DIY guitar speaker cab

For some time now I’ve been thinking about replacing my guitar amp. What could then be more fun than building one yourself?

As amps in general, and especially tube amps, are high voltage devices, I don’t feel like taking any risks by designing something from scratch, but rather build something from a kit. More on the amp later. This post will be about the speaker cabinet.

I don’t have a lot of experience trying lots of different cab types, so I went to the all-knowing internet for some guidance. Through reading blogs and forums on the topic, it cleared on me what I wanted. I went for a single speaker (1×12”), half-open non-resonant cabinet. My material of choice was 18mm locally produced birch plywood. This isn’t the cheapest of materials, nor is it the easiest to work with, but Finnish plywood is usually considered to be of very high quality, so you get what you pay for.

One thing I’ve learned in the process of building stuff is that careful planning always pays off. I started by making a detailed layout on paper with all the measurements scaled down to 1:10. This way I was able to get all the pieces to fit on the smallet possible piece of plywood. Once I had all the planning done, it was time to cut all the pieces. (at this point I made my only deviation from my original plans, as I used 12mm plywood or the back panels).

The pieces were joined together using wood glue and screws. As I prefer having the screwheads invisible, the whole construction is screwed together from the inside:

Once all the pieces were fitted together, it was time to round the corners using a router. Then I did a first round of sanding, painted one layer, and then more sanding and more paint:

One thing the internet community pointed out that I would have missed was to paint the front of the speaker baffle black to avoid it from shining through as the cloth is applied later:

I also added a thin wire mesh over the speaker cutout, just to be sure the speaker is protected. To avoid rattling in the future, i sealed it with an elastic filler and tiny woodscrews.

For the speaker itself I went for a Jensen Tornado. I hadn’t heard of this speaker before, but it was recommended to me by the guy who sold all the other hardware I needed, so I trusted his judgement. It is a neodymium speaker, so it’s lightweight. (Actually I had been wondering why guitarists haven’t accepted neo magnets yet, but appearently they are coming at last…)

Then I added all the usual necessities; metallic corner protection, rubber feet, handle.

The last touch was of course the front cloth. As I live in Finland I wanted to give a really Finnish finish, so I bought some marimekko unikko fabric. It was expensive, but at least my speaker looks quite unique.

So that’s it! Now I just have to build an amplifier to match the cabinet, which will of course be a completely different project, but I’ll get back to You on that one.

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!