My pedal board is flush with overdrives, but sorely lacking in distortion. I’ve never loved my RAT – it has scurried on and off my pedalboard over the years, never finding a permanent home there. I’ve really been needing a solid traditional distortion lately, to use with my band covering tunes by Green Day, Blink-182, Weezer, etc.
I love my Vox Ice-9 overdrive, so I decided to try out the Vox Satchurator. It’s apparently a boutiquey version of the classic Boss DS-1. When I received it, I immediately plugged in and lost myself in its glorious tones for an hour or so. It sounds really really good. Very different from my overdrives and fuzz. It has a big, saturated, overtone-rich distortion. Excellent for thick heavy leads, but also equally good for massive chunky rhythm.
So, when I first played with the Satchurator in a loud rehearsal context with my band, I was utterly disappointed to find that the thing just isn’t loud enough. No, it doesn’t go to 11- not even close!! Searching for answers, I see a number of other Satchurator users with similar complaints. Even with the volume up full, I find there is very little volume boost when switching on the pedal. And unlike the Ice-9’s 12db volume boost when engaging the “MORE” switch, the Satchurator’s “MORE” switch instead kicks up the gain voltage, providing more gain and distortion, but not much volume boost.
Reluctant to bail on the Satchurator, I instead decided to modify the circuit. There’s no simple component change that’ll boost the signal, so I added an extra clean boost stage:
The Satchurator tone control is a passive tone matrix, rather like the single-knob passive tone control in the Big Muff – it cuts bass on one side, and cuts treble on the other, with the middle position being scooped. It’s a great tool, but as I mentioned in my Treble and Bass article, there’s quite a bit of signal loss in this type of tone circuit.
Furthermore, the 1N914 diode clippers in parallel with the tone circuit will clip anything above about .6V. So there’s no way the signal can be louder than .6V going into the final volume control. A signal coming from a guitar’s passive pickups can peak as high as 1V, so you can see that .6V is not a particularly strong signal.
Those diodes at the end of the signal chain are the limiting factor, and they make up an important part of the Satchurator’s sound – changing them for another diode type with a higher forward voltage would boost the volume but would dramatically change the pedal’s sound. And there’s no other components after the diodes that might be tweaked to boost the signal. So, we need to add an entirely new clean boost stage, between the tone pot and volume pot.
I selected a JFET op-amp- the TL071 single op-amp is a good choice for audio. The TL072 dual op-amp is equally good – you’ll just use one of its two op-amps. The TL072 is the one I had in my box o’ tricks, and I’ll be showing the TL072 in the pictures throughout this tutorial.
Note, the TL071 single op-amp and TL072 dual op-amp have different pinouts. I showed and described the method for both, but please be careful. Specifically, the op-amp output is pin 6 on TL071 and pin 1 on TL072. And VCC+ is pin 7 on TL071 and pin 8 on TL072.
Wiring up an op-amp for boost is a fairly simple undertaking. A pair of resistors form a voltage divider between the inverting input and output. The ratio of the two resistors specifies the amount of gain: G = (R1+R2)/R1. If R1=R2, you get a 2x volume gain. I chose a 10k and 22k resistor, giving about 3x gain.
You need to be careful to avoid adding TOO much gain, or you’ll start adding undesirable clipping in the op-amp. Since BIAS is at VCC/2 or 4.5V, and the incoming signal is about .6V (because of those diodes), you’d start to clip if you add over 7x gain. In any case, 3x is more than enough, but you can experiment with resistor values or use a trim pot to select your preferred amount of gain.
WARNING: This is a fairly simple mod, but does involve cutting a trace on the PCB, and adding some wires and a small additional circuit. All the usual disclaimers apply: proceed at your own risk, don’t blame me if you ruin your pedal or hurt yourself, etc.
Here’s what you’ll need:
- a small piece strip board (VeroBoard)
- a few resistors around 10k-30k, or a 50k trim pot
- a few inches of wire (22 – 24 gauge stranded)
- a TL071 or TL072 JFET op-amp. 8-pin DIP package.
- 8-pin DIP socket
- some non-conductive wrapper for the circuit, like heat shrink or electrical tape
- something to secure the circuit inside the enclosure- hot glue, electrical tape, or a rubber band.
- 60/40 solder
- Tools: soldering iron, wire stripper, cutter, razor blade, Phillips screwdriver, crescent wrench, multimeter, 3/32 drill-bit, helping hands
The new circuit will be placed between the current tone pot and volume pot.
First let’s prepare the PCB:
- First disassemble the pedal. Remove the chicken heads by pulling them directly off. Use the crescent wrench or socket to remove the nuts for the pots, pad switch, and the input and output jacks. You can leave the two footswitches in place.
- Unscrew the back, and carefully remove the PCB from the enclosure, removing the screws with a Phillips screwdriver. Note that the footswitch board is connected to the main PCB using a rather delicate wire harness. Be careful not to stress the connection points- try not to pull it or let it twist. These wires easily break and are a pain to re-solder. This happened to me when I was repairing my Ice-9, so fair warning!
Locate the trace between lug 2 of tone pot and lug 3 of the volume pot. Use a razor blade to thoroughly cut through the trace. Confirm using a multimeter that there is no longer a connection between tone lug 2 and volume lug 3.
Solder a wire to the the test-point marked TP1 (it’s the yellow wire in this picture). This is 9V, VCC+. Be careful to avoid shorts- strip only a small amount of wire insulation, only exposing about 2-3mm of wire. Tin it, and then angle the wire up a bit when soldering it so it’s not laying down on the ground plane. Orient the wire so that it is facing towards the through-hole for the battery wires. We’ll be routing this wire through to the other side of the PCB.
Flip the PCB over, remembering to be careful not to stress the footswitch-PCB wire harness. Solder wires to the other connection points, as shown – the picture on the right has annotations. The volume pot solder point is very close to the input jack, so be careful. The colors are optional, but can help you stay organized. I used yellow for VCC+ (routed through from other side), brown for BIAS, gray for GROUND, and blue for audio signal.
Next, let’s prepare our boost circuit on strip board:
Refer to this schematic and strip-board layout as you go through the guide below. Note, I’m building my circuit on strip-board with the TL072. The board layout and wiring is a little different with a TL071, as you can see in the layout picture above. I haven’t actually tested the TL071 layout, but it should be ok.
On the strip board, cut the traces for 4 of the strips, so we can mount our 8-pin DIP socket. You can use a razor, but it’s tedious. I use a small 3/32 drill-bit and just twist it by hand in the hole, until the copper trace is removed. Leave one row of holes connected at the top to connect to BIAS later. (I accidentally removed one too many, as you can see in the next picture, so I’ll need to add a jumper there later).
Solder the socket across the removed holes.
If you’re using a TL071, there’s only a single JFET, and several of the DIP8 pins are unused. But if you’re using a TL072 like me, there’s a second JFET that we don’t need, and it’s best to tie it off so it’s benign. We need to connect its inverting input (pin 6) to its output (pin 7) using a jumper, and tie the non-inverting input, pin 5, to BIAS. I made a little jumper using a leg from a resistor, bent into shape with needle nose pliers and connected pins 6 and 7. And I used a short piece of brown wire to connect pin 5 to the top row, which we’ll connect to BIAS, in preparation for the next step.
Next, we’ll wire up the active JFET. First, add the resistors to set the gain. I’m using a 10k and a 22k, but you can experiment, or even use a trimpot. I solder the 10k resistor from the top row (BIAS) to the inverting input of the op-amp (pin 2). And I solder the 22k resistor between the op-amp inverting input (pin 2) and its output (pin 1 on TL072 or pin 6 on TL071). Note, I’m standing the resistors up on end so they can fit between two adjacent pins.
Finally, it’s time to connect the new circuit board to the main PCB:
I’ll be positioning the new circuit on the lower part of the back of the PCB. So first, I clip the wires to be a good length to reach that position. Then I solder the blue wire from the tone pot lug 2 to the op-amp input (pin 3). Then the other blue wire from the volume pot lug 3 to the op-amp output (pin 1 on TL072 or pin 6 on TL071). Then the yellow VCC+ to the op-amp V+ (pin 8 on TL072 or pin 7 on TL071). The gray ground wire is connected to op-amp pin 4. And lastly, the brown BIAS wire is connected to the top row of the strip board.
Next, I wrap the new circuit in something non-conductive so there’s no chance of a short-circuiting against the PCB. Here, I’m using a piece of large 1” heat shrink. I won’t actually shrink it- it just makes a nice envelope and is easier to work compared to electrical tape.
Finally, I use a spot of hot glue on the PCB to fix the new circuit down to prevent it rattling around inside the enclosure. Alternatively, you could use electrical tape, or a rubber band.
Power it up and try out your much louder Satchurator! This one definitely goes to 11. I usually have the volume knob at about noon or 1 o’clock for a nice boost when engaging the distortion.
If you decide to build this mod, let me know how it goes!