Design #001 – Class-A Buffered I/O Circuit

In the realm of guitar effects pedals, signal integrity is paramount. A well-designed input/output stage can significantly enhance the overall performance of a pedal, ensuring minimal signal loss and optimal interaction with other equipment in the signal chain.

This article delves into our quest at Voltanode of designing and constructing a Class-A buffered input/output circuit specifically tailored for guitar pedals.

The Problem

Without a buffer circuit, each time you plug another pedal or a cable, you get more noise because your guitar pickups are high-impedance devices that are particularly susceptible to noise from power transformers, LCD screens, power cables, and stage lighting.

Even worse, if you extend a high-impedance cable more than 20′, the tone of the instrument will change due to the capacitance and inductance of the cable and, of course, more noise will be injected into the full circuit. Moreover, you have to consider the strength of the guitar signal itself. If the guitar signal is splitted in two to drive two amps or into three when you add a tuner pedal, the tone will be altered.

Most well-known pedal manufacturers use standard ICs (op amps) as input/output signal buffers. These little bugs operate in class-B mode, making them perfect efficient for 9V batteries and cost efficient too. Keep in mind that class-A mode is the best audio mode for minimum zero-cross, phase and intermodulation distorsion. But class-A circuit never shut offs which could drain a 9V very quickly. This one more reason to use a power supply unit in 18V and not batteries.

The Solution

To make a perfect buffered input/output circuit, we need to design stages operating in class-A mode, providing sufficient power supply in 18V in symmetric mode (for more headroom) and using premium components, whatever op amps or solide state transistors they may be.

The one-dollar question is: Should we use high grade op amps which include matched-by-design transistors inside or discrete external bipolar or FET old buddies ?

We have decided to use high-gain, low-noise transistors in bipolar or JFET or hybrid technology in emitter followers configuration.

That means what we need to bias very precisely these transistors at the optimal DC point ensuring linear operation within the Class-A region. Op amps would have been easier to use but at Voltanode, we like challenges ! We’ll have also to block DC and to have efficient coupling of AC. The impedance input should match closely a typical guitar or bass pickup (not the same !)

The circuit should be designed to maintain a flat frequency response across the audible spectrum, typically from 20Hz to 20kHz.

In summary, key components and implementation features are :

• Symmetric stable +9V / -9V power supply based on standard 18V external PSU and strong filtering and bypass capacitors.
• Class-A operating mode
• High-gain, low-noise bipolar or JFET transistors
• Biasing Network: Precision resistors establish the optimal DC operating point for the transistor, ensuring linear operation within the Class-A region.
• Coupling Capacitors: High-quality film capacitors for AC coupling.
• Bypass Capacitor: A low-ESR electrolytic capacitor provides a stable power supply, reducing noise and enhancing overall circuit stability.
• A star-ground configuration is to minimize ground loops and reduce noise