TS-303 MK2 ACID SCREAMER RELEASE NOTES

This document describes what the TS-303 MK2 ACID SCREAMER does, what the buttons and knobs do and how it does its thing.
First a convention used on the drawing:
Parts with a name in CAPITALS letters = everything on the outside that is user accessible: input, output, knobs and switches
Parts with a name in normal letters = everything on the inside: non user accessible electronics

Ooooooo. What does this button do?:

BYPASS SWITCH:

Selects between the undistorted signal and the distorted signal.

DRIVE knob:

Sets the amount of distortion

DYNAMICS knob:

One main disadvantage of using distortion or overdrive on a TB-303 or a guitar for that matter is that you lose dynamic level. Almost everything sounds equally loud. On a guitar you get way longer sustain and on a TB-303 you lose the accents. Everything drowns in a distorted brown soup. When this knob is turned counter clockwise you get this typical loud distorted sound but when turned clockwise you get the same 'type of sound' with the entire dynamic level restored. This makes the TS-303 mk2 more powerful and aggressive than any other distortion or overdrive effect.

FREQUENCY knob:

This knob affects the frequency selectivity of the distortion engine. Turned clockwise it sounds more growling with a LOT of sub, Counter clockwise it sounds more screaming (hence the name acid screamer) enhancing the and adding more upper harmonics. Do not mistake this knob with a filter. It is not. This is actually influencing the distortion process itself with some magic involving a voltage controlled variable capacitance multiplier.

MODE switch:

This switch sets the operation mode of the distortion engine. It lets you choose between distortion and overdrive. Overdrive is the softer mode with emphasis on high frequencies and has the most dynamics with a typical hollow sound. Distortion has a more harsh sound with a very broad spectrum with a lot on the low end but less (although still a lot) on the high end compared to overdrive and almost no dynamics. The first edition acid screamer was overdrive only.

OUTPUT LEVEL knob:

Controls the output volume.

SIGNAL IN:

The input jack

SIGNAL OUT:

The output jack

Explanation of the electronic parts:

amplitude extractor:

The output of this is the amplitude of the input signal. This means the output is a DC signal that shows how loud the input signal is at that moment.

analog divider:

Plain mathematics: output = input 1 / input 2

constant:

Nothing special, just a plain DC voltage. Only drawn for explanatory reasons.

crossfader:

Controlled by the DYNAMICS knob. OUTPUT is constant DC level when knob is turned counter clockwise and is the output of the analog divider when turned clockwise. Everything in between is possible.

distortion engine:

As the name suggests: This is the part that does the distortion. It is a asymetrical bilateral logarithmic amplifier and clipping stage. The gain/clipping level of it and thus the amount of distortion is controlled by 4 parameters:
1: The DRIVE knob.
2: The signal itself: quiet signals get more gain than loud signals (logarithmic behaviour. The gain is also frequency dependent. Some frequencies get more gain than others.
3: The FREQUENCY knob.
4: The MODE switch

input buffer:

This is a very low noise input amplifier. The output is an exact copy of the input signal with a bias added so that it can be used and be loaded by the rest of the circuit.

output amp:

Sets the output signal and makes it low impedance so that it can drive almost anything.

precision rectifier:

The output is the rectified signal of the input. This circuit is here because the amplitude extractor works best (fastest and most accurate) with positive signals.

vca:

Voltage Controlled Amplifier. The Output Signal is the input signal multiplied by the control voltage. It is drawn here as a VCA for simplicity reasons but is in fact a current controlled current amplifier in a translinear configuration (most vca's are transconductance amplifiers). It benefits are the acceptance of much higher input signals so that it can cope with the signal of the distortion immediately without leveling it down before entering and amplifying again when coming out. This results in seriously lower noise and less distortion. Now you should be thinking: "Why less distortion? This is a distortion effect". The signal coming out of the distortion engine contains a lot of higher harmonics and the type of distortion of a transconductance amplifier (tanh to be exactly) would compress these a bit and this is unwanted. Don't be alarmed, the distortion engine produces more distortion than you and your speakers can handle.

Signal flow:

The input signal gets buffered and gets a bias level. It enters the distortion engine and also precision rectifier 1 which rectifies it for the amplitude extractor 1.
The output of the distortion engine (the distorted signal) also enters a precision rectifier (here nr2) followed by amplitude extractor 2.
Wanted was a signal which had the sound characteristics of the distorted signal but the amplitude (envelope) of the original input signal. Altering the amplitude of a signal is done by an amplifier. Wanting to control the amount of amplitude alteration requires a voltage controlled amplifier. Now, how much (gain) does the distorted signal have to be amplified to get the same amplitude of the original signal? It has to be multiplied by an amount that is the amplitude of the original divided by the amplitude of the distorted signal.

Original Amplitude = Distorted amplitude x GAIN
or GAIN= Original Amplitude/ Distorted amplitude

The amplitude of the original signal is present at the output of amplitude extractor 1 and the one of the distorted signal is present at the output of amplitude extractor 2. These signals get divided in the analog divider and thus the output of it is the necessary gain to give the distorted signal the amplitude of the original signal.
This gain level is fed to one side of a crossfader and a constant level is fed to the other side, so one can choose how much of these dynamics have to be restored. The output of this crossfader goes to the vca which does the multiplication.
The output of this vca (or the bypassed signal if selected) goes to the output amplifier which sets the level and makes suitable for loading.

deliberate error:

The above description on how the envelope is restored (the circuit controlled by the dynamics knob) is based on the first ideas and the first prototypes for this MK2 version. I left it this way because it is understandable and it gives you an idea of what I wanted to do and how I wanted it to be done. This worked very well but it required a lot of components, precise adjustment and it actually worked too good (read: sounded too clean). For these reasons an alternative, much simpler circuit was designed which introduced some error. After all, when working with sound levels an amplitude error factor x2 or /2 gives only a 6dB difference, so no need to be accurate here. The circuit now exaggerates a bit at both ends but is quite accurate in the middle. This is very useful because it amplifies loud parts more and silences quiet parts. This gives you a lot more dynamic range and is a very effective noise reducer.Even with no distortion (drive anti-clockwise and overdrive mode) it can be used to pronounce accents even more.

About Noise:

The TS-303 mk2's own noise contribution is historically low for a distortion effect. I am quite confident that it is a this moment the most silent distortion built so far. However, you will notice that when using it an amount of noise will be present. This noise is the noise that comes from the 303 and that gets amplified and distorted. Depending on the settings of the TS-303 MK2 there will be more or less audible noise. It will be the most present when the 303 is on but not playing, because this way the gain of the distortion maximizes due to not getting any signal. A good way to reduce it is using the dynamics knob. Also some 303's produce a hell of a lot more noise than others. With some of them you can actually hear the leds blinking. This is a problem that can be easily fixed by replacing the power transistor Q45. These are known to fail but often don't fail completely and just get noisy.

Construction:

No compromise anywhere:
-Translucent polycarbonate impact resistant case.
-Solid aluminum knobs.
-High quality swithes with 50000 operations lifetime.
-Vishay Spectrol Conductive Plastic Potentiometers.
-Nickel plated closed (to keep dust and durt out) jack sockets.
-2.4mm thick (double the thickness of a normal PCB) FR4 laminate PCB with immersion gold plated pads & thru-holes
-All components are industry standard and have been available for at least a decade and thus have proven reliability and are easy to obtain in case of unlikely failure