Pete's Guitar Reverb Circuit -- Hallelujah!

This PT2399 guitar reverb circuit is very much based on all the information found in the documentation; however it has some modifications to make it more suitable for the guitar. The basic circuit configuration is that of the echo circuit; however the RC values of the filters are that used in the surround/delay circuit. I decided to use those values as they are more favourable to better sound quality. In this circuit the output is taken from pin 14 of the PT2399, like the way shown in the surround/delay circuit, hence the signal there contains only the delayed output which goes to a 500 kΩ potentiometer that is part of the 2N7000 MOSFET mixer circuit. The other potentiometer in this mixer circuit receives the initial input signal that is in phase and unadulterated. I decided to design a MOSFET mixer because It has good frequency response and it will not alter the initial input signal too much thereby keeping it high quality. Usually designers tend to use a high-cut op-amp mixer however I do not like to reduce the treble, especially since the reverb signal has already passed through a plethora of low-pass filters. I tend to prefer mixing the highest possible treble from the initial signal, with the bass from the reverb. Consequently, this MOSFET mixer is more to my taste.

The initial input signal first goes through a phase splitter circuit consisting of the 2N7000 MOSFET. I have designed this myself and have not seen anyone using anything like this in this type of a circuit. There are many advantages of using a MOSFET input with phase splitter, and the main ones are, 1. The gate impedance of the MOSFET is extremely high, and it will accept an input signal voltage as large as 1 V p-p without having to use a resistor at the gate junction. If you are using a ceramic pick-up then the last thing you will want is a resistor there as the capacitance of the pick-up ends up making an RC filter that will reduce the treble!

2. The phase splitter produces two signals of equal amplitudes but opposite phases. The in-phase signal is tapped out to the mixer; hence the mixer will always receive this signal. However, the delay circuit can receive either an in-phase signal or out of phase signal, which the user can select with the phase selector switch. 3. By avoiding op-amps we keep the transistor count low thereby keeping power consumption low. This might be beneficial if the gadget is to be portable and battery powered.

Finally, the foot switch controls the input signal to the reverb circuit. Closing the switch produces a reverb output that is mixed with the initial input signal by the mixer. However opening the switch prevents new reverb to occur, but allows the existing reverb in the loop and decay naturally (known as tails/trailer).

I call this guitar reverb circuit halleluiah because when you hear the guitar sound, you will fall off your chair and praise, "Halleluiah, I have just heard a technological musical marvel!"

Component List

In filter circuits it is a good idea to use the most stable and accurate values for the components. In that regard, I am using ±1 % tolerance for the resistors. Whenever possible, and if affordable, ±10 % or better tolerance for the capacitors is customary for these types of circuits.

This Article Continues...