Circuit analysis: Jan Ray (Timmy)

The Jan Ray overdrive is one of the many scandals that shook the boutique pedal community recently. Indeed, the Jan Ray has been proven to be a Paul C. Timmy slightly modified as we will see by studying the circuit. However, Vemuram pretended that it was an original design he made up listening to old Fender amps during 3 years... And sold the pedal at an indecent price for an analog pedal: 370 euros (400 dollars) ! This is a typical marketing strategy: "the price represents the quality", which is totally absurd for a basic electronic guitar pedal... He sold a lot of them whereas Paul C. produced the same pedal for 120 dollars. This a pedal that deserves to be cloned, and if you want to buy this sort of overdrive, go for the Timmy!


Here is the schematic of the Jan Ray circuit as it is on the veroboard (a bit different from what you can find online, but functional !)
Jan ray schematic
Vemuram Jan Ray schematic (Made with Eagle)

It is quite a classical layout, using the 2 OP-amps included in the LM4558 IC. The first OP-Amp is used to clip the signal to generate saturation, whereas the second one is used as a basic volume boost. Let's divide the circuit in different sections.
Vemuram Jan Ray schematic

The first "input" section has different roles. First, there is a 47 uF coupling capacitor to eliminate any DC current that could go in the circuit. There is also a 1M pulldown resistor used to avoid popping noises when the effect is turned on. These noises are due to a small charge that accumulate at the entry of the circuit and cannot go through the first coupling capacitor. When the effect is turned on, this charge goes through the circuit and causes the awfully loud "pops"... To avoid this, a 1M resistor is linked to the mass to absorb the excess of current.


The signal then enters the gain/dirt section, which role is (obviously) to generate saturation. It enters in the first OP-amp of the LM4558 (it is a double OP amp), which has an inverted feedback loop to amplify the signal. The signal goes in the loop. High frequencies can go through quite easily with de 47 pF capacitor. The remaining signal will change depending on many parameters :
  • the gain potentiometer acts as a variable resistor connected that will reduce more or less the current going through the loop. The amplitude of the voltage going through the diodes will varies, and thus will be more or less clipped.
  • A trim potentiometer combined with the 9.1k and 600 ohms resistors allows you to tweak the maximum possible gain. This is useful to adapt the maximal gain setting depending on the output levels of your pickups.
Diodes are the main actors in creating saturation. They will more or less crop the signal in the feedback loop. This is a classical "soft clipping" system that you will find in many many guitar pedals, like the famous Tubescreamer from Ibanez. Depending on the nature of the diodes, the signal will be more or less clipped. Silicon diodes usually clip at 0.7V whereas germanium diodes clip at 0.35V approximately. This is why saturation with germanium diodes can be more compressed, and why there is volume loss with germanium diodes : the signal amplitude is smaller.


Fig. Diode clipping system in an amplification feedback loop

Using two diodes following each other like in the Jan ray makes the clipping less important, and thus "uncompresses" the sound. Your playing and pick sensitivity is still audible because the signal is not as clipped as in other saturations like a big muff. A very simple mod to ear that is to use a switch to choose between 2 or 4 diodes in the loop. The Timmy by Paul C has this option for instance.


Then, the signal goes through the output/volume section. A potentiometer is wired as a variable resistor to act as a low pass filter to adjust trebles. The signal then goes through the second OP amp that is just used to amplify the signal. The 3.3k resistor determines the gain of the amplifier, which is set quite high.

Finally, a coupling capacitor filtrates DC current that could have gone through the loop and thus only allows the signal to go through. Then, a potentiometer wired as a variable resistor to the mass allows you to diminish the amplitude of the final signal, constituting a classical volume knob. This almost the same volume control than in the fuzz face !


Last section of the pedal: stabilization of the power supply and "creation" of the VREF ("power supply section"). The diode at the entry protects the circuit from polarity inversion. Two grounded capacitors are there to get rid of the ripples that might be present in the power supply. Low frequencies are eliminated with the large 47uF capacitor, whereas the 100 nF capacitor is used for high frequencies. In the end, we have a proper +9V voltage that can be used with the OP amp. Another part of the current is used to create the reference voltage (VREF) by going through a 9,1k resistor.

To go further:
Pulldown resistors: http://www.muzique.com/news/pulldown-resistors/
Tube Screamer circuit analysis (circuit is closed to the timmy / Jan Ray) : http://www.electrosmash.com/tube-screamer-analysis#power
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