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Self-built digital echo device

With the integrated circuit PT2399, a digital echo device can be set up without much effort, as shown here. The IC contains according to data sheet a 44kbit RAM with AD and DA converter as well as the necessary clock generator. So it is a real digital echo, not a delay circuit in BBD technology. The circuit can be used well as a guitar effect device, for example. Much more pleasant compared to analog delays in BBD technology is the calm sound thanks to the much larger signal to noise ratio (up to about 90 dB).

The high-impedance input through the FET input stage ensures little load on the pickup in the circuit shown so that it can fully develop its sound. If the 1kΩ source resistor at the FET is bridged by an electrolytic capacitor of for example 47μF in parallel, the input sensitivity increases so that also a microphone can be connected. If a trimming potentiometer (for example, 2.2kΩ) is connected in series with this electrolytic capacitor, the input sensitivity is adjustable within certain limits. The output level can be adjusted to the required value with the gain pot. Echo volume and repeat can be adjusted separately (Depth / Feedback). In addition, the echo time is adjustable over a wide range. Using small times, reverb-like effects arise.

According to data sheet a delay time of 342 milliseconds is achievable (K = 1%). By accepting a still acceptable harmonic distortion, this can be exceeded, up to the value of about 0.5 sec. Such delay times could previously only be achieved with devices that used an endless tape loop. Here, distortion levels of 10% were not uncommon and you had to be content with a much smaller frequency range. The circuit shown here was particularly dimensioned as a guitar effect device. By considering, that for guitar speakers above a frequency of about 4 to 5 kHz, the transfer range drops steeply, the low pass for the echo with the 2.7nF capacitors are designed accordingly. A balanced sound of the echoes is achieved by lowering the depths with a 0.1μF capacitor at the output of the PT2399. This also means that multiple echoes are becoming "thinner", similar to a tape loop echo. With this dimensioning, echoes can be clearly differentiated soundwise from the original signal, an undifferentiated "sound mash" is avoided. Even in nature, echoes practically never have the sound of the causative sound source. In this respect, this circuit can also be used well for other sound sources. With this limited frequency band and a 100kΩ potentiometer with parallel connected 220kΩ fixed resistor to set the echo time ("Time") can be set delay times up to about one second with usable quality.

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