Follower Rack

Audio in, CV out · A TL074 on seventeen strips · By Onion Madder · Mess O' Pedals

Patch a drum module, a VCO or a guitar into it and the CV jack follows how loud it is: up on every hit, down as it fades. The second jack is the same thing upside down, high in silence and dropping as the sound gets louder, which is what you want for ducking. Three knobs - sensitivity, attack, release - and an LED. It is the Envelope Follower on the stripboard shelf, the same four op-amps on a TL074 and the rack's real ±12 V.

The Follower Rack layout drawn as a stripboard diagram, titled Follower Rack, 18 by 17 stripboard. Seventeen orange copper strips lettered a to q run left to right with eighteen columns numbered above them. Along the top the two small green capacitors C8 and C7, the pink electrolytics C6 and C5 and the diodes D4 and D3 stand between the power strips, and the power header H1 to H10 sits in two columns at the right end of strips b to f. A pale rectangle marked IC1 TL074 stands in the middle across strips i to o, with a column of gray cut marks beside its left pins. Every resistor stands between strips: R11, R12, R9, R10 and R8 left of the chip, R1, R3 and R5 right of it, R6 and R7 at the bottom left and R4 at the bottom right; C3 is a pink circle at the upper left, C2 and C1 small green boxes right of the chip, and the diode D2 lies along strip o below it. Dark blue links run straight down in several columns, one of them from the top strip to beside pin 11. Pads leaving the board are labeled in the margins: INV, CV and LED_A on the left, and the header, GND_P, SENS_A, SENS_W, IN, ATT_W, ATT_A and REL_A on the right. Beside the board the panel parts are drawn with a wire to each pad: the INV OUT and CV OUT jacks and the LED on the left, and the IN jack and the RELEASE, SENSITIVITY and ATTACK pots on the right. Below it the same board is drawn again from the solder side, mirrored, with the column numbers running right to left, every solder point as a dot, the cuts as X marks and the chip as an outline with pin 1 boxed. A parts list follows, then a block headed OFF THE BOARD listing each panel part, what it does and where each of its legs goes. — The board as the layout file draws it: the power along the top and the header down the right, the chip in the middle, every part, link, cut and off-board pad in one picture, and the panel parts beside it with a wire to every hole they land in. The lower drawing is the solder side, mirrored - the side you actually work on.

What you'll need

tap to expand / collapse

On the board, counted from the layout file:

1× TL074 quad op-amp, 14-pin DIP (IC1) - spot it: TL074 printed on top, a notch at pin 1. A TL084 sits in the same holes
2× 100k (brown-black-yellow; R8 to R9)
2× 10k (brown-black-orange; R5, R11)
1× 15k (brown-green-orange; R10)
1× 1M (brown-black-green; R1)
4× 1k (brown-black-red; R4, R6 to R7, R12)
1× 22k (red-red-orange; R3)
2× 100n ceramic disc (104; C7 to C8)
1× 100n box film (104; C1)
1× 1u box film (105, or a small film cap; C2)
3× 10u electrolytic (C3, C5 to C6) - spot it: the stripe marks the negative leg
1× 1N4148 signal diode - spot it: a tiny glass body, black band toward the cathode (D2)
2× 1N5817 Schottky, the reverse-polarity diode - spot it: a fat black body, band toward the cathode (D3 to D4)
13 wire links (J1 to J13) - solid core, tinned
1× 2×5 pin header, plain, 0.1″ - the Eurorack power connector. spot it: ten square pins in two rows. Not the shrouded kind; a shroud is 20 mm long and would cover the neighboring strips
Stripboard, 17 strips × 18 holes or larger

Off the board

1× A1M 9 mm Alpha pot, SENSITIVITY, as a variable resistor: lug 1 to SENS_A, wiper to SENS_W, lug 3 empty
1× B100K 9 mm Alpha pot, ATTACK: lug 1 to ATT_A, wiper to ATT_W, lug 3 empty
1× B1M 9 mm Alpha pot, RELEASE: lug 1 to REL_A, wiper to GND_P, lug 3 empty
3× Thonkiconn PJ398SM 3.5 mm jacks: IN, CV OUT and INV OUT. Every sleeve to GND_P
1× 3 mm LED, long leg to LED_A, short leg to GND_P. Its 1K is on the board
A Eurorack blank panel - three knobs, three jacks and an LED fit 8 HP - and a Eurorack power cable

What it is

Four op-amps, one job each. D (pins 12 to 14) is the sensitivity stage: the input sits at ground on R1 1M, C1 takes it in, and the SENSITIVITY pot over R3 22K gives a gain of 1 to about 46. C (pins 8 to 10) catches the peaks: it charges C2 through D2, R4 and the ATTACK pot, and reads C2 back on pin 9, so the diode sits inside the loop and its 0.6 V never comes off the CV. C2 drains through R5 and the RELEASE pot. B (pins 5 to 7) is a buffer, so the jack and the LED never pull on C2. A (pins 1 to 3) flips the CV around a reference of about 4.7 V from R10 and R11, so the inverted jack sits at about 9.4 V in silence and falls as the other one rises.

What changed from the 9 V box. On one supply that board's gain stage rectifies by flattening the negative halves at 0 V, and it needs a Schottky on its input to keep the chip from being dragged below ground. Here the input swings both ways inside the rails, so the clamp and its series resistor are gone; the peak catcher only ever charges C2 upward, so it is still a half-wave follower. Between peaks pin 8 swings down to about -10 V, and D2, a 1N4148, is good for far more than that the wrong way.

C2 is a film cap on purpose. An electrolytic leaks, and the leak would be a second release knob you cannot turn. Every time on this page - attack about 1 to 100 ms, release about 10 ms to a second - is the arithmetic of C2 and the pots, not a measurement.

The layout file is the guide

As on every stripboard guide here, the build document is the file itself: ↓ Download Follower_Rack.json - import it into Copper Bottom and the editor gives you the board picture, the placement walkthrough, the electrical checks and a printable build sheet, all from the same file. No coordinates are written out on this page on purpose: the editor is the viewer, and a number typed twice is a number that can drift.

The netlist

Quoted verbatim from the layout file. It was written from the design, so the editor's check that the board matches it compares two separate records - what it is for is carrying the circuit somewhere else, and telling you what each strip is supposed to be doing while you stare at it. HOLD is the capacitor the CV lives on; PKO and PK are the two ends of the peak catcher's diode.

# Follower Rack - written from the design # The Envelope Follower on a TL074 and +/-12 V. # D (12-14): sensitivity, x1 to about x46 with the SENS pot over R3 22k. # C (8-10): the peak catcher. D2 inside the loop: positive peaks only. # ATTACK: R4 1k plus the pot into C2. RELEASE: R5 10k plus the pot. # B (5-7): the buffer, so the jack and the LED never drain C2. # A (1-3): the inverter. INV = 2 x VREF - CV, VREF about 4.7 V. # Power header H1-H10, red stripe at H1/H2 on strip b; a 1N5817 on each rail. # Attack about 1 to 100 ms, release about 10 ms to 1 s, CV 0 to about 10 V. -12RAW: @H1 @H2 D4.B -12: D4.A IC1.11 C6.A C8.A GND: @H3 @H4 @H5 @H6 @H7 @H8 @GND_P R1.B R3.B R11.B C2.B C3.B C5.B C6.B C7.B C8.B +12RAW: @H9 @H10 D3.A +12: D3.B IC1.4 R10.A C5.A C7.A IN: @IN C1.A INP: IC1.12 C1.B R1.A FB: IC1.13 R3.A @SENS_A GAIN: IC1.14 IC1.10 @SENS_W PKO: IC1.8 D2.A PK: D2.B R4.A ATT: R4.B @ATT_A HOLD: IC1.9 IC1.5 C2.A R5.A @ATT_W REL: R5.B @REL_A CV: IC1.6 IC1.7 R6.A R7.A R8.A CV_J: R6.B @CV LED: R7.B @LED_A VREF: IC1.3 R10.B R11.A C3.A INVN: IC1.2 R8.B R9.B INV: IC1.1 R9.A R12.A INV_J: R12.B @INV

The off-board wiring

20 pads leave the board. Ten of them are the power header.

Pad	Goes to
IN	Audio in, jack tip. Anything from a drum module to a VCO
SENS_A, SENS_W	The SENSITIVITY pot. Lug 1 and the wiper. Turned down is x1, up about x46
ATT_A, ATT_W	The ATTACK pot. Lug 1 and the wiper. With the 1K on the board, about 1 to 100 ms
REL_A	The RELEASE pot. Lug 1 here, the wiper to GND_P. With the 10K on the board, about 10 ms to a second
CV	CV out, jack tip, through 1K. 0 to about 10 V, rising with the sound
INV	Inverted CV out, jack tip, through 1K. About 9.4 V in silence, falling as the sound rises
LED_A	The LED, long leg. It shows the CV
GND_P	Ground for the panel: the three jack sleeves, the LED's short leg and the RELEASE wiper
H1 … H10	The power header, 2×5, pins down the strips. H1 and H2 are -12 V and take the cable's red stripe; H3 to H8 are ground; H9 and H10 are +12 V

The rack block

The power sits along the top and the right of the board, the way it does on every module on this shelf. The 2×5 header takes the rack's power cable, mounted with its pins running down the strips so each pair of pins lands on one strip: -12 V, then three grounds, then +12 V. The red stripe on the cable goes to H1 and H2, on strip b at the top right. A 1N5817 stands in series with each rail so a cable put on backwards does nothing, and a 10 µF and a 100n sit on each rail. The three ground strips run the whole width of the board, which is where every ground on it lands. No regulator: a TL074 is happy on the rails as they come.

The IDC socket on the power cable overhangs the header by about two strips at each end. Nothing tall stands there, so keep it that way if you move a part.

Check the cuts under the chip before the first power-up, with a meter. On a TL074 pin 4 and pin 11 share a strip, +12 a whisker of copper from -12, held apart by nothing but the chip's own cut. A cut that did not quite go through is a dead short across the supply. Cut them, then measure each one.

How it works, in one breath

An envelope follower listens and reports. The gain stage decides how hard it listens, the peak catcher grabs the top of each wave and holds it on a capacitor, the release knob lets that hold leak away, and a buffer hands the result to the jack without disturbing it. Into a filter cutoff it is an auto-wah; into a VCA fed by something else it is a sidechain; inverted into that VCA, it is ducking.

Things to try

Ducking. Kick drum into IN, INV OUT into a VCA carrying a pad, and the pad dips on every kick
Auto-wah. A guitar or a VCO into IN, CV OUT into a filter's cutoff, and the filter opens as you play harder
A longer release. C2 at 2.2 µF doubles every time on both knobs. Keep it film
Into the box version's world. The Envelope Follower on stripboard does the same from 9 V, for the boxes on the shelf

The circuit is the textbook envelope follower - a gain stage, a peak detector with its diode inside the loop, a buffer - with an inverter added for the second out. The layout is Onion Madder's, drawn in Copper Bottom the same evening as the Envelope Follower it comes from.