PT2399 Basic Echo Circuit

The PT2399 basic echo circuit is the one where the delayed output is fed back to the input to produce echoes. Furthermore, the level of attenuation in the feedback loop is set by the repeats control potentiometer. Here we look at some of the features of this circuit and pin functions.

After redrawing the basic echo circuit, we can now clearly see that the power is fed through pin 1 and the analogue ground. The designer has also placed 100-µF and 0.1µF capacitors across the power planes to ensure a clean power supply. Pin 2 exposes the internal reference voltage of 2.5V for the purposes of connecting a 47-µF electrolytic capacitor that helps to keep it stable.

At pin 6 there is a fixed resistor in series with a 20-kΩ variable resistor connecting to the digital ground exposed through pin 4. The purpose of keeping the digital ground separate from the analogue ground is to prevent any digital noise entering the analogue circuits. As you can see, apart from this one connection, all the rest of the components connect to the analogue ground. The documentation suggests R to be 10-kΩ, which limits the range of delay times achievable. Hence with the variable resistor to full resistance, the resistance at pin 6 will be 30-Ω, and with the variable resistor to zero ohms the minimum resistance will be 10-kΩ. Hence according to the table, in the documentation, this limits the delay time range between 342-ms and 151-ms. This range is consistent for achieving an echo effect; however for a subtle guitar reverb significantly smaller delays will be required.

The circuit uses a plethora of resistors and capacitors for the low-pass filters, which are usually used to filter out aliasing and quantization noise inherent in analogue and digital conversion processes. You can get a clearer idea of the filters and signal loops in another diagram showing the overall big picture.

The delayed output of the IC emerges at pin 14 and goes to the 50-kΩ repeats potentiometer. The attenuation provided by this potentiometer determines the number of echoes you get, or put in another way, how quickly the echoes fade out. The length of time from the initial signal to the last echo is usually known as reverb time, hence many designers call this reverb control. The wiper of this potentiometer feeds the signal back to the input stage at pin 16. If the wiper is set to the fully minimum position (ground side), then there will be no signal being fed back to the input and the circuit behaves as the basic delay circuit shown in the documentation.

It is important to visualise that between pin 15 and 16 there is an internal op amp used to form a low-pass multi-feedback filter. The initial input signal and the delayed signal mix at this point and enter the op-amp at pin 16. The output of this op amp is at pin 15, and this is where the final signal is tapped out from. Hence the final audio output signal is a mix of the input signal and the delayed signal. For guitar players, this poses a problem because the initial signal is mixed through the low-pass filter as well, and the low-pass filter strips out some of the high-frequency such as treble in the music. They usually express this by saying that the tone is mangled man... To solve this problem, what is needed is an external mixer that mixes the initial analogue audio with the delayed output signal of the PT2399.

Another problem with the basic echo circuit is that there is no way to make delay tails. Guitarists use the term delay tails (also known as delay trails) to refer to that part of the sound that continues to echo even after the input signal stops. To achieve this, there is usually a switch, which cuts the initial input signal to the PT2399, however the delayed echo within the loop continues and is allowed to decay naturally. An alternative to tails or trails is known as by-pass when the echoes stop instantly and this too is controlled by another switch. As you can imagine, the placement of these switches to control the signal path is not possible with the way the input signal and delayed signal enters at pin 16.

In the following pages of this multi-page article, we look at addressing the deficiencies of this circuit to make it more useful for guitarists. The following features will be added.

1. Smaller delay times
2. External mixer
3. Switches to produce Tails/trails

Which ground pin for the power supply?

The PT2399 has two grounds known as analogue ground (AGND), and digital ground (DGND), hence the first question many people might have is which ground pin should be used for the power supply. Reading the pin description table in the PT2399 documentation, pin 1 (Vcc) is known as the analogue supply voltage input, and pin 3 is known as analogue ground. Hence this would suggest that the power to the IC is fed through these two pins. In addition, looking at the basic circuit we see that the two capacitors (0.1-µF and 100-µF) on the +5 power supply line (pin 1) connect to the analogue ground. Hence this too would further suggest that the input voltage is fed between pin 1 and the analogue ground (AGND) pin 4.

Pin 6 connects to the internal voltage controlled oscillator, which dumps huge amount of digital noise to ground through the resistor. Hence the manufacturer has provided a separate ground for this known as the digital ground (pin 4). Consequently, this ground has to be kept separate from all analogue grounds and circuitry. Therefore it is important to ensure that the audio signal inputs and outputs are referenced to the analogue ground (AGND) (pin 3).

Interestingly, documentation of other similar ICs such as the PT2395 and PT2396 suggest joining the analogue and digital grounds together. Of course, if you were to do this then it would not matter which ground is used for the power supply. However there is the risk of introducing digital noise into the analogue circuitry such as the operational amplifier filters, which are very sensitive to such noise.

Many experts suggest that there are different versions of the IC and some versions require the analogue and digital grounds to be connected together. Consequently, in my schematic and board layout, the two grounds are kept separate; however there is a tiny red link that allows joining of the two grounds should it be necessary to do so.

This Article Continues...