The TDA7052A is a readily available amplifier chip (Farnell # 526198) that has a DC controlled volume input. Here, the IC is used as the variable gain amplifier in a guitar compressor, so the effect can be accomplished without the hard to obtain CA3080 operational transconductance amplifier (OTA). Note that the suffix-less TDA7052 does not have DC volume control.
The TDA7052A has a relatively low input sensitivity and also a relatively low input impedance, so common source JFET amplifier T1 provides some pre-gain while emitter follower T2 provides low impedance drive for IC1’s input. The advantage is taken of IC1’s dual outputs to keep the diode pump and output loads apart, although distortion from this probably wouldn’t happen as IC1 has very low output impedance (about 0.2 ohms). The output on pin 8 is fed via DC blocking cap C7 to the level pot P3.The output on pin 5 drives phase splitter T3 whose outputs drive T4 and T5 on alternate half cycles. Both of these transistors in parallel discharge C9 which effectively holds the control voltage for pin 4 of IC1.
The input stage has a number of design aspects worthy of mentioning. The JFET stage will clip if the Idss is too high. For T1, a 2N3819 can be got away with if selected for Idss less than 5 mA otherwise the input amplifier won’t work. This may not be possible with all brands of 2N3819. The J113 shown here is spec’ed at 2 mA min. and no upper limit given on the data sheet. Source resistor R5 may be determined empirically by temporary insertion of a trimpot to set the drain to 0.5 VBATT, obviously centring the drain operating point optimizes the headroom for output swing. With lower values of drain resistor it may be possible to use unsorted examples of the 3819. It is made abundantly clear in the TDA7052A app note and data sheet that good supply decoupling is important, hence it is recommended that C6 be a good quality electrolytic. C5 is specified as 0.1 µF in accordance with data sheet advice, although if a miniature 0.22 µF that will fit in the available space is ready to hand — every little help. C5 should be fitted as close as possible to the IC1 supply pins. It is assumed that anyone who gets as far as building a board for a guitar pedal will know how to wire a bypass stomp switch, however, there are some notes that concern the placement of the pots. Ideally, only one pole of the two pole changeover stomp switch is needed, to switch the output jack between the compressed output and an additional pot on the pre-gain buffer.
It is likely that P1 (pre-gain) would be useful as a front panel control, and this is a good place to connect the bypass stomp switch. The best option is to use 2x 10 kΩ pot in parallel instead of P1 on its own, one wiper feeding IC1 pin 2 (pre-gain), the other feeding the stomp switch bypass (bypass gain). P2 (sustain) varies how much effect the voltage on C9 affects the voltage on IC1 pin 4 and therefore controls the range of gain control. The circuit is supplied by the usual PP3 9 V battery, a slide switch can be wired in series with the positive lead from the battery clip if preferred, but it is common practice in the FX industry to use stereo jack sockets with the tip of the plug carrying the signal as normal, the ring contact of the jack socket is shorted to ground when a mono plug is inserted. Provided skeleton style jack sockets are mounted in a metal case, inserting a mono plug connects the ring contact to the case. In this way if the battery negative lead is connected to the ring contact of one socket and the PCB negative lead is connected to the ring contact of the other jack socket, removing either jack plug will break the circuit between the battery negative and PCB negative.
The circuit from C1 to P1 is used in its own right as a ‘clean boost’ pedal, within reason the input impedance can be pretty much as high as you want to make it, and emitter follower T2 gives it a very low output impedance capable of driving long cables without losing the high notes and also overdriving the input stage of valve amplifiers (not so worthwhile with transistor amps!). However, depending on the JFET choice and biasing it is just possible that a good quality guitar with ditto pickups might overload the input stage to a degree.