Inside Boost

A gain knob for a quiet build · A TL072 on eight strips · By Onion Madder · Mess O' Pedals

Some builds come out too quiet. The Octave Avalanche is one, and there are others. This is a small board that goes inside one of them, between its circuit and its output, and makes it louder: one half of a TL072 with a gain knob from ×1 to about ×22, the other half holding a half-rail reference so it runs from the host's own supply. It has no jacks and no supply of its own. Four wires go to the host - supply, ground, in and out - and two to the knob.

The Inside Boost layout drawn as a stripboard diagram, titled Inside Boost, 17 by 8 stripboard. Eight orange copper strips lettered a to h run left to right with seventeen columns numbered above them. A pale rectangle marked IC1 TL072 sits right of center across strips c to f, and a dark blue link runs straight down through it from the top strip to strip f. Left of the chip, standing across the strips, are the resistors R6, R7, R5, R3 and R4, the small boxes C4 and C3, which are the ceramic and film capacitors, and pink circles for the electrolytics C6 and C5; R5 and C6 each lie along a strip over a cut. Right of the chip are the resistors R1 and R2, the small box C1 near the top right, the electrolytic C2 at the bottom right, and three more dark blue links. Holes carrying wires off the board are labeled in the margins: GND, GAIN_W, GAIN_A and IN down the left, and OUT and plus V down the right. To the far left a potentiometer marked POT1, A100K, is drawn with wires from its lug 1 and its wiper to GAIN_A and GAIN_W. Below it the same board is drawn a second time from the solder side, mirrored, with the column numbers running right to left, every solder point as a dark dot, the cuts as X marks and the chip as an outline with pin 1 boxed. A parts list runs underneath, then an Off the board line reading POT1 A100K gain, lug 1 to GAIN_A, wiper to GAIN_W, and a credit line reading Made with Copper Bottom. — The board as the layout file draws it, with the gain pot beside it. Every part, link, cut and off-board pad in one picture. The file itself is below. The lower drawing is the solder side, mirrored - the side you actually work on.

What you'll need

tap to expand / collapse

1× TL072 dual op-amp, 8-pin DIP (IC1) - spot it: TL072 on top, a notch or a dot marks pin 1. Socket it: an NE5532 sits in the same holes if you want to hear the difference
2× 1M resistor (R3, R4) - spot it: bands brown-black-green. R3 holds the input at ground and stands up; R4 biases the op-amp's input at the reference and lies across three holes
3× 100K resistor (R1, R2, R7) - spot it: bands brown-black-yellow. R1 and R2 are the half-rail divider. R7 holds the output at ground so it does not thump. R2 and R7 stand up
1× 4.7K resistor (R5) - spot it: bands yellow-violet-red. It sets the top of the gain: 1 plus 100K over 4.7K is about 22. It lies along its strip, over a cut
1× 1K resistor (R6) - spot it: bands brown-black-red. On the way out, so a long wire or a shorted jack cannot upset the chip. Stands up
1× 100n film capacitor (C3) - spot it: a small boxy cap marked 104 or 0.1. The input coupling cap. Ceramic works; film is quieter
1× 100n ceramic (C1) - spot it: 104. Across the chip's supply
1× 100p ceramic (C4) - spot it: 101. Across the feedback; it takes the fizz off the very top at full gain
3× 10 µF electrolytic (C2, C5, C6) - spot it: a small can with a stripe down one side; the stripe leg is the negative one. C2 holds the reference, C5 is in the gain leg, C6 is the output coupling cap. All three have their stripe toward ground or toward OUT: their plus sides sit at half the supply
Stripboard, 8 strips × 17 holes or larger, and solid wire for four links, one of them under the chip

Off the board

1× A100K pot, the gain knob - spot it: the A on its back means log. Used as a variable resistor: lug 1 and the wiper, the third lug empty. A log pot puts noon at about ×3, where most of the useful range is; a B100K works and is crowded at the bottom. A 100K trimmer in the same two pads is the set-and-forget version
Hookup wire for the four wires to the host. No supply and no power switch of its own: the host's switch is already upstream of +V

The layout file is the guide

Like the rest of the stripboard shelf, the build document for this one is the file itself: ↓ Download Inside_Boost.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.

One link runs under the chip. J1 goes straight down the column where the strips are cut, from the ground strip along the top to pin 4's strip, which is why pin 4's cut sits one hole further over than the others. Fit it before the socket. Pins 6 and 7 are tied together by J3 on purpose: that half of the chip is a follower.

The netlist

Quoted verbatim from the layout file. It was written from the board, so checking the board against it can only pass - 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.

# Inside Boost - written from the board # 8 strips of 17 holes, 17 parts, 1 chip, 7 cuts # # A TL072 gain stage that goes inside a quiet build. Op-amp B follows # the R1 R2 half-rail divider. Op-amp A is non-inverting around it: the # gain pot from OUT_A to IN- and R5 with C5 to ground make the gain # 1 plus pot over 4k7, from 1 to about 22. C3 and R4 bring IN in at # the reference, and R6, C6 and R7 take it out again with no DC on it. # Pins 6 and 7 are tied on purpose: the follower. +V: @+V C1.A IC1.8 R1.A GND: @GND C1.B C2.B C5.B IC1.4 R2.B R3.B R7.B VREF_RAW: IC1.5 C2.A R1.B R2.A VREF: IC1.7 IC1.6 R4.A IN: @IN C3.A R3.A IN+: IC1.3 C3.B R4.B IN-: IC1.2 @GAIN_A C4.B R5.A OUT_A: IC1.1 @GAIN_W C4.A R6.A GN: R5.B C5.A OUT_R: R6.B C6.A OUT: @OUT C6.B R7.A

The off-board wiring

Six pads leave the board. Four go to the host, two to the gain knob.

Pad	Goes to
+V	The host's supply, after its power switch, so one switch turns both off. 9V is fine; anything up to 18V gives more room before it clips
GND	The host's ground
IN	The host's output: lift the wire that went from its board to its level pot or its output jack, and land it here instead. If the host has an output cap, keep it; this board has its own as well and does not mind two
OUT	Where that wire used to go: lug 3 of the host's level pot, or the output jack tip. With a level pot after it, the gain knob sets how hard it goes and the level pot sets how loud it comes out
GAIN_A	Gain pot, lug 1. The third lug stays empty
GAIN_W	Gain pot, wiper. All the way down is ×1, the host as it was; all the way up is about ×22

How it works, in one breath

An op-amp with feedback works to keep its two inputs at the same voltage. The signal goes into the plus input, and the output has to swing far enough that the slice of it coming back to the minus input matches. The gain pot and R5 are that slice: with the pot at zero the output is wired straight back and the stage is a follower, ×1; turned up to 100K, the output has to swing 1 plus 100K over 4.7K times as far, about ×22, or 27 dB. C5 in series with R5 means none of that applies to DC, so the output sits at the reference however the knob is set, and the knob has no DC across it to crackle.

The other half of the chip only holds a voltage. Two 100Ks split the supply in half, C2 steadies it, and op-amp B follows it, so the input is biased to a reference with an op-amp behind it. That is what lets it run from one 9V: the signal swings around four and a half volts, C3 keeps the host's DC out, and C6 keeps the four and a half volts in. Pins 6 and 7 tied together is the follower, not a mistake, if the checker mentions it.

Headroom, unmeasured. A TL072 is not a rail-to-rail chip: on 9V it swings about 6 volts peak to peak before it clips. A source that already comes out at a couple of volts reaches that at about ×3, and a square wave clipped mostly just gets squarer, so you may not hear it. If you do, run the board on 12V or more, or turn the gain down and let the host's own level pot do the rest.

Things to try

Darker. C4 at 1n instead of 100p rolls the top off from about 1.6 kHz at full gain: a quick way to round off a fizzy square wave
A fixed gain. Leave the pot off and put a resistor across GAIN_A and GAIN_W: 10K is about ×3, 22K about ×6, 47K about ×11
A bypass. A DPDT that sends the host's signal either through the board or around it gives you before and after on one switch
A different chip. Anything with the dual op-amp pinout drops into the socket. An NE5532 is quieter and swings a little further; an LM358 runs lower but sounds rougher at the top

The circuit is the single-supply non-inverting stage from every op-amp datasheet's application notes, with the reference buffered the way the Bug Mixer does it. The layout is Onion Madder's, drawn in Copper Bottom.