Systech Envelope Follower

Schematic of the Systech Envelope Follow - Note there is more than one version!

The Systech Envelope Follower is an unusual pedal, with several interesting features. This is an "auto-wah" type of effect. There is a voltage controlled bandpass filter that is swept by the amplitude of the incoming signal. The sweep goes up with amplitude, and it is fixed that way. The range, or starting point or the sweep can be calibrated, as can the "Q" of the filter.

I noticed that the photos of the guts of a unit here have a different design than I traced here. The op amps are 1x 741 and 1x 1458, and there are 3 trim pots. I do not know what else might be different. So please note that this article (so far) only refers to the 2 trim pot version.

This pedal uses two dual op amps which are relatively uncommon to see: SN72L022P. SN was the first prefix Texas Instruments used for ICs, and it stands for "semiconductor network." 72 refers to a family of chips. (74 being the very popular 7400 logic series.) Finally, "L022P" reveals what we call this today: TL022. Unlike the most "TL0XX" chips, the TL022 has bipolar inputs. It is called a low power op amp, and Systech is making great use of that here by powering the audio path with a well regulated +6V power supply derived from the raw +9V input. This isolates the power connections between the two ICs, so audio and control signal are (hopefully) well separated.

How it Works

Voltage Controlled Filter

The filter chosen is a Voltage-Controlled Voltage-Source Filter, or VCVS for short. From the output of U1A to the input to U1B there are two 10k resistors in series. These work with the capacitors to form the time constants that ultimately decide the frequency response of the filter. In parallel with each 10k resistor is a JFET. The JFETs will act as variable resistors that shift the time constants as their drain-to-source resistance lowers due to changing the voltage applied to their gates. U1B is configured as a non-inverting amplifier, and the gain of the amp can be set with a trim pot. This has the effect of adjusting the "Q" of the filter, and the filter will self oscillate if this is set too high.

Peak Follower (or Envelope Follower)

U2A is a peak follower with adjustable gain (DRIVE), dc offset to set the starting point, and an adjustable time constant (DECAY) to allow for quick releases or slow releases of the peak voltage. The output of U2A is like any amplifier. When set at maximum, it should clip with even moderate playing. The diode and cap turn the ac into dc. The higher the amplitude of the the signal, the higher the dc voltage. This is buffered by Q3 and applied to the cathode of the next diode.

How the JFETs Turn On and Off

The drain and source connections are biased via the 10k coming in from the +6V, then through the 470k that connects to the gates, through the 1N4148 diode, finally completing the circuit at the 91k resistor to ground. When Q3 is off, there should be enough voltage drop across the 470k resistor to bias the FETs off (the gates need to be significantly lower voltage than the source to turn it off... there is no exact amount, varies widely, but a few volts is usually good in effect pedals). As Q3 raises the voltage below the diode, this decreases the voltage drop across the 470k, and the FETs turn on. The trim pot connected to U2A will adjust the idle dc output of the op amp, and you can manually sweep the filter with this trim. It sets the filter's starting point.

Input and Output Buffers

Filters like this have changing output and input impedances as they sweep, so it is a very good idea to buffer them from the variable impedances that might be connected to the input and output. This ensures reliable operation between different instruments and different amplifiers.

U1A is arranged as a voltage follower. It is unusual to place 100k in series with an op amp input when it is arranged as a voltage follower. This increases the input impedance by 100k, and provides protection to the input, but this is probably unnecessary, except that the 200k biasing resistors do present a relatively low 100k input impedance by themselves, so the series 100k makes the total input impedance about 200k. A 200k input impedance was considered OK in the 1970s, but modern designers would push that value up to 1M or more when designing for passive electric guitars. The output drives both the filter input and the envelope follower input. The input to the envelope follower is relatively low at 1k.

Q2 provides a little gain and buffers the output of the filter. Output impedance is relatively high, roughly equivalent to the 10k collector resistor. This is expected to connect to a high impedance (1 Meg) input. A modern circuit would probably use another op amp section to provide gain along with a low impedance output, allowing more flexibility as to where this goes next.