Sunday, 28 October 2012

Dolly the Tube Screamer

All this talk of guitars set me thinking about the old days when, as a kid, I used to make replicas of my MXR Distortion Plus ...


Unlike most other areas of audio engineering, guitar amplification is NOT about sound reproduction - rather it is about sound production. The distinction is important; sound reproduction (as in monitoring and what was once called "Hi-Fi") is about generating a faithful acoustic representation of an electronic signal. Guitar amplification, in marked contrast, is part of the system creating the signal. The sound of the guitar amplifier is not (necessarily) at all neutral, introducing components of the overall sound that are important. Most important of these is the family of sounds when (particularly valve) amplifiers are pushed into overdrive, when the resulting distortion can have a pleasant, musical sound in the right context.

Recognition of this fact led to the production of effects pedals (like my old MXR, above) to create (or, at least, emulate) the sound without having to go to the inconvenience (and noise exposure) of over-driving an amplifier. It also led to the appearance of a whole lot of amplifier emulations, exemplified by those built into my new amplifier. Between these extremes of the early, simple distortion (or "fuzz") box and modern complex DSP emulations of classic amplifiers in overdrive there was a whole generation of more complicated distortion effects which I missed over the past few decades. I decided to "catch up" by making a clone of an Ibanez Tube Screamer - thus "Dolly" was born.

The Tube Screamer has three controls; "Drive", "Tone" and a final "Level" control, as seen on my cloned unit...


The schematic can be found at lots of sites over the 'net - start your search here. The most important part of the Tube Screamer (and many other distortion pedals) is the exploitation of the clipping effects of a pair of back-to-back diodes, as seen here in the "clipping" stage of my "clone"...


I used 1N4148 diodes and am (at present) using a TL072 op-amp (I'll change it for one of the original JRC4558s when I get round to making a final version).

Where the Tube Screamer is distinguished from other distortion pedals (including, for example, my old MXR) is in partnering the diode clipping section with filtering circuits - including an adjustable shelving HF section, giving lift or cut to high frequency components both of the original signal and of the harmonics generated by the clipping. The tone section and other aspects of the circuit are seen "round the back"...


Having made the unit, I thought I'd take the opportunity to show some waveforms illustrating its effect on a sinusoidal input (at 2kHz).

Here's the response of the device in "Bypass" mode (the original Tube Screamer has electronic "bypass" switching implemented by some n-channel FETS in a configuration I used before for the Tx audio mute of my Funster Plus rig) controlled by a push button (Dolly uses a 555 in a toggling bistable circuit, whereas the original uses a discrete flip-flop)...


Turning on the unit, with the "Drive" control on max (giving greatest clipping effect from the diodes) and the "Tone" on max (giving HF shelving boost) gives this response (in all the following images I adjusted the "Level" control to keep the output amplitude roughly constant)...


Rolling off the "Tone" to minimum, whilst keeping the "Drive" at maximum makes the waveform "smoother" - and has exactly the same effect on the sound...


Turning the "Drive" to minimum reduces the distortion effect, influencing the sound only when the guitar is played loudly - this sounds more like an amplifier close to its limit, being pushed over the edge into saturation only by the loudest signal components. It sounds nice!

With tone setting on "Max", the effect on the 2kHz sinewave is seen as a tendency toward a triangular wave...


Backing off the tone to minimum, all but removes the visible distortion of the 2kHz waveform...


In truth, all the sounds available from this clone of the Tube Screamer are pretty close to sounds already well-covered by the amplifier emulations built into my new Roland Cube amplifier - but it has been fun to clone Dolly. I might make a PCB and knock up a few copies for friends, just as I did in the old days!

 ...-.- de m0xpd

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