Further Adventures in Plywood

Here’s something not-a-pedal that I’ve been working on. It’s a stand with a built in amplifier and speakers for a Teenage Engineering Pocket Operator PO-20 (basically a musical calculator but so much more awesome than that description makes it sound).

The synthesiser can be slid in and out of the stand from the back. You connect the output of the Pocket Operator to the (stereo!) amp input on the back of the stand. The 3W+3W amp can be powered by USB or an internal battery.

I used the CAD drawings available on Teenage Engineering’s website to work out the exact dimensions to fit the synth in to a narrow slot cut into the face of each side.

I was worried about cutting a groove so close to the edge of plywood as I expected the layers to shear off, so I put together a very rudimentary jig, clamped my drill to the worktop and used a hobby drill cutting wheel to gradually make the slots deeper and wider. I used some drywall screws inserted over the bit to make micro-adjustments to the angle of the cutting wheel, and used pieces of cardboard in between the component and the wooden stop to alter the position of the cut. Somehow it worked!

Necessity is the mother of… look I know it’s ridiculous but it works, ok?

After sanding each part I used Birchwood Casey gun oil to treat the end grain and faces, then after 24 hours lightly sanded and applied a further coat of gun oil. I repeated this process every day for a week, then finished with a thin coat of gun wax. I think it turned out great!

The best part was putting together the speakers. I had some 30mm speakers I’d salvaged from a broken Bluetooth speaker. I created custom holders for them from 32mm PVC drain pipe, glued them in to the holders then made speaker grilles from 40mm PVC drain pipe and stretched speaker fabric over them. The custom speaker/drain pipe holder combo then fit exactly inside these grilles. This was a particularly good idea because I already had a 42mm hole cutter from when I did my kitchen plumbing a few years back 😄

It was great fun to use some of the building techniques I’ve learned from my recent wooden pedals for this project, and I am loving my ever expanding range of clamps. How did I ever live without quick release one handed clamps before this year?

Wood and solder

My latest adventure in the world of wooden pedal building continues with the TIMBRE, powered by the Death By Audio Reverberation Machine circuit.

The pedal uses this nifty Accutronics Belton reverb module to get deep, long echoes.

It did feel a bit like cheating using this module instead of laying out on vero board but to get the same result I would have needed to build a circuit four times the size of The Colonel, which just seemed impractical.

The enclosure was made with plywood which I cut into strips and glued so the end grain shows along the front of the pedal. This included a little bit of sanding.

New material

The pedals I’ve boxed up so far have used either Altoids tins or aluminium ‘1590B’ style boxes. While building in Altoids tins is fun they don’t allow you to fit in more than a small circuit. The 1590B enclosures present a moderate challenge to a novice like me, and I’ve enjoyed learning how to fit everything in a reasonably small space (I’m not about to try 1590As, I have no desire yet to fit everything in a tiny space just to see if I can).

Lately though I’m completing more circuits that I want to box up but finding the greatest cost is the aluminium enclosures. So, born partly of necessity and partly because I saw some amazing designs on Instagram courtesy of @glacialcreative I’ve decided to try my hand at using wood, starting with probably (as it transpires) the worst type of wood you can possibly choose for a first attempt: OSD or Oriented Strand Board.

This stuff hates not being a large flat sheet.

I’ve really challenged myself building with this, there’s loads of new problems to solve such as how to make parts of the enclosure thin enough to pass components through but still remain structurally sound, or how to cut material that’s essentially chewed up scraps of stuff and glue in a way that it doesn’t instantly disintegrate.

I’ve got some more enclosures in the works including some reclaimed kitchen worktop (!) mixed with pine (!!) and different types of plywood (!!!). Suddenly every piece of scrap wood is a potential guitar pedal material.

Watch this space! Or don’t. I mean, it’s up to you. I can’t tell you how to live your life.

Amp in a tin

When testing audio circuits I found that I was having to plug them into my big amp to test if they were working, or use my small headphone amp. This wasn’t ideal for a number of reasons. I decided to build myself a small amp with all the features I wanted to test circuits. I also wanted to be able to use it as a practice amp for use with headphones.

I found the Noisy Cricket Mk II on tagboard effects which seemed to have the features I was looking for. It has controls for volume, gain and tone, along with a ‘grit’ switch (slight overdrive) and a bass switch which flips between capacitors to allow more or less bottom end signal through. I used an LM386 op amp IC as every time I’ve used one of these in a circuit it sounds great (in my opinion, other internet opinions are available).

Noisy Cricket circuit.

As a practice amp I also wanted to include a metronome I could hear through my headphones, so I built a little astable 555 timer circuit, and attached both a white LED and a small speaker, as well as an audio output to the headphone jack.

Metronomical.

The metronome can be switched on and off, and when on I can switch between it playing out of the small internal speaker and playing out through my headphones mixed with my audio signal.

The next things I want to add are a way to play audio in from my phone to play along with, and maybe a Bluetooth receiver so I don’t need to use a cable.

Tin amp.

One of my favourite things about this amp is I can take the lid off whenever I need to mess about with the innards. As it’s primarily for testing this is ideal, as I can take readings from inside the amp if I need to.

My new circuit testing setup

One thing I’ve found repeatedly when testing audio circuits is that I don’t have a very good way of quickly and effectively plugging in and testing – both for power and signal path. I often find strange, intermittent faults appearing and sometimes everything stops working, which almost always turns out to be a loose test lead on a jack socket or a bad connection on the battery. Gah!

The olden days of intermittent signals. Whyyyy

I decided it was time to improve my setup to a sturdier, reliable, more permanent solution. I wanted something with fixed audio and power jacks, but with the flexibility to add and remove other parts as and when needed.

I found an old metal shelf bracket which was an ideal width and height although it was over 2 metres long. I cut a 20cm length then used an angle grinder to remove the rust and paint and take it back to bare metal. I then filed and sanded all edges to give a nice, smooth finish. I drilled holes to fit the audio and power jacks, and drilled extra holes for any potentiometers and switches I may want to add. Finally I drilled holes so I could attach the bracket to my existing breadboard setup.

Neat.

After installing the input and output audio jacks I added a DPDT (double pole, double throw) bypass switch and connected everything, adding in crocodile clip test leads from the in and out pins of the switch.

I installed a 9V power jack and wired it to a SPDT (single pole, double throw) switch, with the switched live pin attached to a crocodile clip test lead and the other pin attached to ground, so when the power is switched off the live test lead is automatically grounded. I also added a small LED (with a resistor) between these two pins so I can see at a glance when the board is powered.

Ready to test a circuit.

I wish I’d done this earlier. Having a solid setup for the in/out parts of my audio circuits has entirely removed one uncertainty from my builds – whether the basics of a good power supply and good signal path connectivity are in place. I can use it for both testing breadboard circuits and soldered vero board or PCB layouts, and the whole thing is portable so I have the freedom to leave my desk and go sit just over there instead.

Reverse angle, revealing the means of construction.

There are a couple of extra things which I’ve added or built to complete my workflow. I’ve stuck pedalboard Velcro to the side of the metal bracket so I can place my TC Electonics Ditto looper pedal on the board. This means I can pre-record a loop and play it into the board when I’m tracing audio paths and tweaking trimmer values.

Testing a Reverberation Machine build.

I’ve also built a little amp so I can take the board output straight to it and out to headphones, very exciting. I’ll go through the features of the amp in my next post.

Tin Amp. Something something llama.

Scrap fuzz

For quite a while now I’ve been taking components from various broken electrical items that people have given me, and have a good selection of both passive components like resistors and capacitors, along with some more specialised parts that I’m planning on using in future.

One man’s trash is another man’s useful trash.

I wanted to make a fuzz effect that I found the layout for on tagboard effects which is based on the standard Bazz Fuss circuit.

I challenged myself to build it using as many salvaged parts as possible. and managed about 90%. The only new components I added were the transistors as I couldn’t find 2n3904s in anything I took apart.

It looks a bit beaten up but it works.

It sounds great! It’s a crackly, raw, high gain fuzz that makes you think your speaker is broken but in a good way. In fact, the sound is so rough around the edges that if I put it in a fancy enclosure with cool graphics on it and a huge volume knob I could sell it as boutique for £200. 😉

Muffin

I finished a new pedal, which I’ve called Muffin.

It’s based on the EHX Big Muff Op Amp pedal. If you’re not sure what that sounds like, listen to one of the first Smashing Pumpkins albums. Here’s an internal shot:

It’s a bit messy but I’m trying to get tidier and more efficient with each build.

Not much else to say about this, it sounds great and I’m pleased to finally have this to add to my pedal board. At some point maybe I’ll record some demos of what these effects sound like.

Oh, capacitors!

I just received a big box of assorted Mylar capacitors (the funny green bulby ones). They don’t come with any sort of guide to help you decode their values, oh no.  I think they just expect you to know.

My usual method is just to Google the code when I’m building a circuit to find out if I’m using the right part. What I’ve noticed is that no-one on the internet seems to give a straightforward answer as a top result (don’t make me scroll, I’m lazy) although this calculator is pretty neat. So I’m sharing what I now know about Mylar capacitor codes. Don’t tell me I don’t know how to have a good time.

Mylar capacitors usually have a code on them that looks like this: 2A224J This can be broken down into three separate parts:

  • 2A is the maximum voltage the capacitor can withstand
  • 224 is the capacitance rating (how much it can… capacitate?)
  • J is the tolerance or margin for error – how close the actual capacitor is to its stated rating.

Sometimes capacitor manufacturers like to keep you on your toes by omitting either the voltage rating, the tolerance or both. Sometimes they’ve tried their best but utterly failed to clearly print the code on the cap. So what do the codes mean, I hear you ask. That’s a great question, don’t ask Google because I found them all for you:

Voltage rating

Code Max. voltage
1H 50V
2A 100V
2D 200V
2G 400V

Capacitance rating

Code Picofarad (pF) Nanofarad (nF) Microfarad (uF)
100 10 0.01 0.00001
150 15 0.015 0.000015
220 22 0.022 0.000022
330 33 0.033 0.000033
470 47 0.047 0.000047
101 100 0.1 0.0001
121 120 0.12 0.00012
131 130 0.13 0.00013
151 150 0.15 0.00015
181 180 0.18 0.00018
221 220 0.22 0.00022
331 330 0.33 0.00033
471 470 0.47 0.00047
561 560 0.56 0.00056
681 680 0.68 0.00068
751 750 0.75 0.00075
821 820 0.82 0.00082
102 1000 1 0.001
152 1500 1.5 0.0015
202 2000 2 0.002
222 2200 2.2 0.0022
332 3300 3.3 0.0033
472 4700 4.7 0.0047
502 5000 5 0.005
562 5600 5.6 0.0056
682 6800 6.8 0.0068
103 10000 10 0.01
153 15000 15 0.015
223 22000 22 0.022
333 33000 33 0.033
473 47000 47 0.047
683 68000 68 0.068
104 100000 100 0.1
154 150000 150 0.15
204 200000 200 0.2
224 220000 220 0.22
334 330000 330 0.33
474 470000 470 0.47
684 680000 680 0.68
105 1000000 1000 1
155 1500000 1500 1.5
205 2000000 2000 2
225 2200000 2200 2.2
335 3300000 3300 3.3

There is some logic to these codes, and it relates to the picofarad (pF) column. The first two digits are values of the capacitor in picofarads and the third digit is the number of zeroes. Fun fact: sometimes manufacturers don’t print the third digit if it’s zero!

Note: The pF, nF and uF ratings are equivalent values, so for example code 103 is 10000pF, which is the same as 100nF or 0.01uF)

Tolerance

Code Margin
F +/-2%
G +/-3%
J +/-5%
K +/-10%
M +/-20%

 

If you fancy putting your new-found knowledge to the test, here’s some of my capacitors for you to decode:

img_3658

Answers, top left to bottom right:

330pF, 6800pF, 560pF, 39000pF, 220000pF and 33000pF – all 100V rated, +/-5% tolerance.