The Agrajag is yet another distortion pedal, this time based on the JHS Colour Box, which is, in turn, derived from the channel amplifier of the Neve 1073 mixing desk channel amplifier. The Neve 1073 has a line-level input and a microphone input, both of which have transformer inputs. For microphone use, an additional amplifier section is used to provide the additional gain, and this is the essence of the circuit as a distortion pedal – the Colour Box design simply treats the microphone input of the 1073 as an instrument input, thus allowing the prodigious gain to overdrive the two amplifiers. This is the reason I was drawn to the circuit, as I have long wanted to find a distortion pedal that could emulate the sound of Robbie Krieger’s tone in “When the Music’s Over”. This was allegedly created by plugging the guitar straight into the desk with the engineer soldering a resistor to link two channels in series for lots of gain.
The amplifier circuit is a simple discrete amplifier that uses three BC184C NPN transistors. The “C” designation on the transistor simply means that it is a high-gain part, and other high-gain NPN transistors can be substituted without affecting the sound, which is useful for reasons that will become apparent.
One of the challenges with the 1073 in the context of a pedal design is that it uses a positive rail of 24V. My prototypes first used a switched capacitor voltage trebler based on the 7660S/MAX1044, but it quickly became apparent that the high output impedance of this power supply arrangement simply could not deliver anything like enough current. To get around this, I instead used a boost converter circuit that uses a switching transistor filtered by a small inductor and a capacitor.
The other challenge with the circuit is the use of an audio transformer on the input, as these tend to be quite bulky and, hence, difficult to get into a small enclosure. The Colour Box uses a Lundahl audio transformer, which is a bit pricey and hard to source. I chose instead to go with the Vigortronix VTX-101-002 as it is readily available and provides plenty of flexibility due to its 1+1:2+2 configuration, and it is designed to be used in either a step-up or step-down mode. This arrangement gives access to 1:1, 1:2 and 1:4 turns ratios, both forward and reverse. The PCB includes a number of pin headers that can be used to configure the transformer for all of these options, as well as allowing the transformer to be bypassed altogether.
While the origin of the Colour Box is clear, actual schematics are hard to find. This circuit has an input stage based on the NE5532 dual op amp, with the op amps in parallel so that they can easily drive the transformer. Once again, the PCB can be configured to bypass this input stage, and this allows the guitar pickups to interact directly with the transformer (or just the channel amplifier).
To return to the transistors, this circuit definitely needs all of the PCB space afforded by the 1590BB arrangement. Not only does the transformer take up a fair amount of space, the circuit also calls for a number of bulky electrolytic capacitors. This means that surface mount is a necessity, but the BC184C is a discontinued device, and the typical replacement, the BC547C, is not available in SMT. The BC847C is another good choice, though and is available in a SOT-23 package. It’s possible that the PCB could be laid out for TO92 devices, thereby allowing a choice of transistor, but I don’t believe it would be worth the effort, as the devices are sufficiently similar for there to be no impact on the tone.