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13 March 2015

A discourse on Power

Today's post is one giant step in my ongoing mission to build a Piezo Buffer. I've got a Mark 2 circuit on a perfboard and sort of half fitted in an enclosure. I'm getting close to being able to explain the circuit to you, but first, I talk about disaster...

This was how I left things in my last post where I was talking about my problems with circuit noise. Can you spot a schoolboy error in the picture above? No? I never spotted it either.

I plugged everything in and switched it on. The background hum appeared and then it disappeared... and then it was back... and then it was gone for good. What? Had I magically fixed the noise problem? Wahay!

Seconds later my elation turned to despair as I realised that I'd only gone and blown the power supply. Damn, damn, damn! That's why there was no noise... there was no power! Oh dear! What an idiot!!!

While I waited for another supply to arrive in the post, I still couldn't figure out what I'd done wrong. It was only when I plugged in the new one many days later that the penny dropped and I scrambled to unplug it as quick as I could.

The de facto (non-battery) standard for powering effects pedals these days is the 2.1x5.5mm jack, supplying 9V of DC (the equivalent of a single "square" 9V battery).

See the CE marking on adaptor pictured above. The image below the letters "CE" indicates the polarity of the jack. This one is negative centre with a positive sleeve, and is the typical arrangement for most pedals you'll come across. I think this standard was set by BOSS some time in the 70s which is a real shame because I think that it would have made a whole lot more sense for the polarity to have been reversed. Let me explain for why...

See the socket I'm using. The black wire is to the negative centre-pin. The red wire is to the sleeve and here's the rub... the sleeve is connected to the metal jack front. Not a problem until I fit it into the metal enclosure. As soon as I do this, power is supplied to the metal box itself! Bad!

Because the audio jacks are wired the opposite way around with ground connected to the metal box, the end result is a short circuit via the metal enclosure... and untold tears. Damn, damn, damn!

A metal power jack in this instance was a poor implementation choice on my part. Ah well... lesson learnt.

Having finally got myself a working adaptor, I jumped right back into trouble-shooting my circuit noise issues. My first port of call was to return to power supply "ripple" noise. You might remember that I ruled this out last week, but I had something I wanted to try...

"The most common meaning of ripple in electrical science is the small unwanted residual periodic variation of the direct current (dc) output of a power supply which has been derived from an alternating current (ac) source. This ripple is due to incomplete suppression of the alternating waveform within the power supply." ~ Wikipedia

Hidden at the back of this picture is an LM317T which is a voltage regulator transistor. These devices are meant to be low-noise. Would one of these perform quieter than my collection of resistors? There was only one way to find out.

Long story short... the breadboard above sounded like a bloody machine-gun when I plugged it in to the buffer circuit. I don't know if I did something wrong, but I didn't spend any time stopping to find out. I shelved any thoughts I might have had of using an LM317T on this project and moved on. Pah!

Hold on a second, I almost forgot to share this wonderful video from The Lightning Stalker with you, where he shows us how to break into "Wall Warts". I'm new to the term "Wall Wart" and breaking into them.

I bet you can guess what I was thinking while I was watching this video...

TLS couldn't find his hammer when he needed it... but I found mine! Ha ha!

I wanted to salvage the cable, but like TLS, I was curious to take a look inside. What I found was a circuit with a large metal coil at the centre.

I've heard that you can make a crude instrument pickup using one of these metal coils. This is a possible project for the future (assuming that this one isn't fried). Waste not, want not.

Damn! This picture has turned out all blurry.

I'm going to close off now with the revelation that I think I might have "largely" solved my noise issues by being a bit cleverer with my pull down resistor arrangement. I'll explain it in another post, but I have a switchable set of basic "gain" levels with an additional fine-tuning pot; The idea being that I can tune the pedal to the impedance of the source instrument to try and get the best sound possible out of it. It's a great idea, but will it work... Yeah, I think it will. I'll knock together some audio tests for you once I've got the circuit properly boxed.

Here are my current thoughts on what this piezo buffer pedal might look like on the outside. I'm thinking of calling it an "Ultra-Z Pedal", because this is what it is. And, because I'm in charge, I think I'll put all the plugs and knobs on the top. Why not?

I reckon I might just stick with my recycled Data Switch box for it.

Ah... I'll have a pint before I make up my mind. 

I think for my own sanity that my next post will have nothing to do with electronics. Perhaps I'll tell you about my ambitions to become an expert in Washboard skiffle? Oh yessss! Ha ha.

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