Introducing the TDA7052A Amplifier Chip
The TDA7052A, easily accessible through Farnell # 526198, is a versatile amplifier chip featuring DC controlled volume input. In this application, the IC serves as the variable gain amplifier within a guitar compressor. This design allows achieving the desired effect without the necessity of the elusive CA3080 operational transconductance amplifier (OTA). It’s essential to note that the variant without a suffix, TDA7052, lacks DC volume control capabilities.
The Input Stage and Its Significance
The TDA7052A, with its relatively low input sensitivity and input impedance, necessitates the inclusion of a common source JFET amplifier T1 to provide some preliminary gain. This is coupled with the use of emitter follower T2 to offer a low-impedance drive for IC1’s input. Taking advantage of IC1’s dual outputs, the diode pump and output loads are kept separate. While the likelihood of distortion arising from this configuration is minimal due to IC1’s extremely low output impedance (around 0.2 ohms), the output on pin 8 is channeled through a DC-blocking capacitor C7, ultimately leading to the level pot P3. The output on pin 5, on the other hand, is directed to phase splitter T3, which subsequently drives T4 and T5 on alternate half cycles. These two transistors, operating in parallel, discharge C9, which effectively maintains the control voltage for pin 4 of IC1.
Important Considerations in the Input Stage
The input stage incorporates several noteworthy design aspects. It’s crucial to be mindful of the clipping potential in the JFET stage if the Idss is excessively high. For T1, opting for a 2N3819 is generally acceptable, provided it’s chosen with an Idss less than 5 mA, as otherwise, the input amplifier may malfunction. This may, however, not be applicable to every brand of 2N3819. The J113, depicted here, specifies a minimum of 2 mA without specifying an upper limit in the data sheet. Source resistor R5’s value may need to be determined empirically by temporarily inserting a trimpot to set the drain to 0.5 VBATT, ensuring the drain operating point is centered to optimize headroom for output swing.
With lower values of the drain resistor, it might be possible to work with unsorted 3819 examples. The importance of adequate supply decoupling is emphasized in the TDA7052A app note and data sheet. Hence, it’s advisable to use a high-quality electrolytic capacitor for C6. While C5 is recommended as 0.1 µF in accordance with data sheet guidance, fitting a miniature 0.22 µF capacitor that fits the available space can be advantageous. C5 should be positioned as close as possible to the IC1 supply pins.
Placement of Pots and Bypass Stomp Switch
It is presumed that anyone embarking on building a board for a guitar pedal is familiar with the wiring of a bypass stomp switch. However, some important notes pertain to the placement of the pots. P1 (pre-gain) can be a valuable front panel control, making it a suitable point to connect the bypass stomp switch. An effective approach is to use two 10 kΩ pots in parallel instead of P1 alone. One wiper connects to IC1 pin 2 (pre-gain), while the other is linked to the stomp switch bypass (bypass gain). P2 (sustain) allows variation in the impact of the voltage on C9, thereby controlling the gain control range.
Power Supply and Jack Sockets
The circuit is powered by the standard PP3 9 V battery, and a slide switch can be wired in series with the positive lead from the battery clip if desired. However, in the FX industry, it’s customary to use stereo jack sockets with the tip of the plug carrying the signal, and the ring contact of the jack socket is shorted to ground when a mono plug is inserted. If mounted in a metal case, inserting a mono plug connects the ring contact to the case. This way, by connecting the battery negative lead to the ring contact of one socket and the PCB negative lead to the ring contact of the other jack socket, the circuit between the battery negative and PCB negative is broken when either jack plug is removed.
Utilizing the Circuit as a ‘Clean Boost’ Pedal
The circuit between C1 and P1 can be independently used as a ‘clean boost’ pedal. Within reasonable limits, the input impedance can be adjusted as desired, and emitter follower T2 ensures a low output impedance that can effectively drive lengthy cables without sacrificing high notes. Additionally, it can be used to overdrive the input stage of valve amplifiers, though this may not be as advantageous with transistor amps. However, depending on the JFET choice and biasing, it’s conceivable that a high-quality guitar with corresponding pickups could potentially overload the input stage to some extent.