Introducing the Passive Pickup Emulator (PPE) June 5, 2013 17:45 6 Comments
What does it do?The PPE makes connecting your non-guitar gear to guitar amps and pedals sound "right." Like everything we do here at DIYRE, the PPE draws its magic from the dynamic, interdependent nature of analog electronics. When you connect two pieces of analog gear, they interact. Unlike a digital system in which data is transferred from one stage to the next unaffected, the signal passed between two analog devices is a unique result of the characteristics of both stages. So, one can't substitute one type of device for another and expect the same results.
What's impedance got to do with it?One of the most important characteristics of analog inputs and outputs is their impedance, often shortened as "Z." Modern gear is typically designed to minimize the sonic effects of impedance. Most guitar gear, on the other hand, takes a vintage approach (if it's not vintage to begin with) and embraces the effects of impedance. Guitar gear expects to see the wildly variable and complex output Z of a passive pickup and tone control. So it's no surprise that your guitar gear may sound a little flat or unnatural when fed a "modern" signal with a low, linear output Z.
How does it work?
The PPE recreates the dynamic, interactive system of a guitar output using only four components. Looking at each of these in turn will provide a fairly complete explanation of how the circuit works.
L1 - 1 Henry Inductor
In a guitar pickup, the inductor converts the vibration of the strings into an electrical current. There are no vibrating strings in our emulator, but we use an inductor to recreate the pickup's phase and frequency non-linearities. In conjunction with the capacitor and resistors, the inductor creates a resonant low-pass filter.
Just like a Fender guitar, we use one leg of a potentiometer as the resistive element of our tone control. The rotation of the pot controls how much of the high frequency signal is thrown away by being shunted to ground. 100nF/.1uF Capacitor A capacitor makes up the second half of the tone control circuit. The relatively-large .1uF capacitor sets the initial rolloff frequency around 6kHz; smaller capacitors will yield a brighter tone with a higher rolloff frequency.