Bits & Pieces 3

This Bits & Pieces post covers a few effects modules I’ve recently made.  These are:

1.  Active 3-way tone control – the ‘Tardis’

2.  Active tone control of unknown origin

3.  Active Low Pass Filter

1.  The active 3-way tone control is standard of its type, I imagine.  I don’t know where I found it – years ago I used to scour back numbers of electronics magazines in the local library and copy out interesting circuits.  No doubt it was one of those.  The circuit diagram looks like this:

3 way tone control

and the finished article looks like this:


which is why it’s called ‘The Tardis’ – not because it’s a time-manipulation circuit, which would have been cleverer.

2.  The tone control of unknown origin is unlike anything I’ve seen before or since – or, rather, since the heart of it is quite a high-value inductor, it most resembles a variable bandpass filter – a wah circuit – but is evidently not intended to be swept up and down like a wah wah pedal.

Another of my finds in a very old electronics magazine, it was originally called a ‘Passive Tone Control’, but the reduction in the volume of the input signal was so drastic that I added an amplification stage before it to boost the level to something like the original, and it became an ‘Active Tone Control’.


Moving the single control from one extreme to the other varies the tone considerably, and it’s very useful with sounds rich in harmonics, like the various Stylophones in my collection.

This was another project after the Touch-Radio which I housed in one of the transparent jewellery cases I had recently acquired.

Active Tone Control Insidesm

Active Tone Control Outsidesm

3. I’d heard nothing but praise for Ray Wilson’s simple 741-based low-pass filter – and indeed the whole ‘Wacky Electronic Noise-maker Thingy’ which it forms part of – so I decided to make one and try it out.  In fact, I made two, and I’m glad I did, because they’re great!  One of them went inside an Optical Theremin project, which I’m in the middle of, and which I’ll be describing as soon as I’m finished [Edit: the Opto-Theremin is described here]; the second one went into another jewellery box project.

I hope I’ve interpreted correctly what it says on the Music From Outer Space website, where it comes from, and it’s OK to reproduce the circuit diagram here:

MFOS low-pass filter

You can read about Your First Wacky Electronic Noise-maker Thingy here: http://musicfromouterspace.com/ – just look for links to ‘WSG’ and you’ll find it. In fact, I just looked at it again and discovered that the circuit there is a slightly more advanced version of the one I built, incorporating fine adjustment of the filter cut-off frequency and a resonance control: looks like I’ll have to go back and make some modifications! . . . Later on I may have to build the whole thing . . .

Note in the diagram above the correct way to wire the cut-off frequency potentiometer.  I used a logarithmic pot, because that’s what I happened to have, which exaggerated the effect of my error the first time I put it together of wiring the pot the wrong way round – no effect throughout most of the travel, then a huge effect in the last quarter-turn: wire it the right way round and you get the full effect through the whole travel of the pot.  Adding a 100k pot, wired the same way round, in series with the 1M pot, at the end marked ‘1’ – which is what the slightly more advanced version includes – would help to make more precise adjustment of the tone.

I should add that the whole Music From Outer Space site is an absolute mine of information and worth reading in its entirety: you can learn about synth modules, study circuit diagrams/schematics and buy circuit boards and so forth associated with the projects described.

Once I get round to doing the modifications, I’ll add a comment or edit the post and show a picture of the finished article.

Edit: I finally got round to adding the extra parts of the circuit – a fine control for the filter cut-off point and a resonance control.  The revised circuit looks like this:

New MFOS Filter

and the finished unit looks like this:

Revised Filter IMG_1477





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June 2012

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