Wednesday, November 16, 2022

Using an inexpensive PT2399 music reverb/effects board as an audio delay (for repeater use)

Figure 1:
Inexpensive PT2399-based audio delay board
as found on the usual Internet sites.
Click on the image for a larger version.

In an earlier blog post (Fixing the CAT Systems DL-1000 and PT-1000 repeater audio delay boards - LINK) I discussed the modification of a PT2399-based audio delay line for use with the CAT-1000 repeater controller - and I also hinted that it would be possible to take an inexpensive, off-the-shelf PT2399-based audio effects (echo/reverb) board and convert it into just a delay board. 

While the uses of an echo-less delay for more mundane purposes may be apparent, it would be fair to ask why might one use an audio delay in an amateur radio repeater?  There are several possibilities:

  • The muting of DTMF ("Touch Tone") control signals.  Typically, it takes a few 10s of milliseconds to detect such signals and being able to delay the audio means that they can be muted "after" they are detected.
  • Reducing the probability of cutting off the beginning of incoming transmissions due to the slow response of a subaudible tone.  By passing COS-squelched audio through the delay - but gating it after the delay, one may still get the benefits of a tone squelch, but prevent the loss of the beginning of a transmission.  This is particularly important on cascaded, linked systems where it may take some time for the system to key up from end-to-end.
  • The suppression of squelch noise burst at the end of the transmission.  By knowing "before-hand" when an input signal goes away, one can mute the delayed audio such that the noise burst is eliminated.

Making good on the threat in the previous article, I reverse-engineered one of the PT2399-based boards available from Amazon and EvilBay and here, I present this modification, using one of these boards as a general-purpose audio delay.

The board:

Figure 2:
Schematic diagram of the audio delay board, with modification instructions.
This diagram is reverse-engineered from the board depicted in Figure 1.
Click on the image for a larger version.

The PT2399 boards found at the usual Internet sellers like EvilBay or Amazon are typically built exactly from the manufacturer's data sheet, and one of those found on the Internet for less than US$10 is depicted in Figure 1.  (Note that the chip may have another prefix in front of the number, such as "AD2399" or "CD2399")

The pictured board is surprisingly well-built, with plenty of bypassing of the voltage supply rails and a reasonable layout.  Despite the use of small, surface-mount resistors, it is fairly easy to modify, given a bit of care, and most of the components have visible silkscreen markings, making it easy to correlate the reverse-engineered circuit diagram (above) with the on-board components.

A few of the components do not have visible silkscreen markings (perhaps located under the components themselves?) and these are labeled in the circuit diagram and the board layout diagram (below in Figure 3) with letters such as "CA", "CB", "RA", etc.

Figure 3: 
Board layout showing component designations of the board in Figure 1.
Note that some of the components have no silkscreen markings and are labeled with letters
that have been arbitrarily identified as "CA", "CB", "RA", etc.
Click on the image for a larger version.

Removing the echo, making it delay-only

This circuit is the "bog standard" echo/reverb circuit from the app note - but it requires modification to be used as a simple audio delay as follows:

  • The output audio needs to be pulled from a different location (pin 14 rather than pin 15):
    • Remove R22, the 5.6k resistor in series with the output capacitor marked "CC".
    • A jumper needs to be placed between the junction of the (former) R22 and capacitor "CC" and pin 14 of the IC as depicted in Figure 4, below.
  • The feedback for the reverb need to be disabled and this involves the removal of capacitors C15 and C17.

Figure 4:
The modified PT2399 board, showing the jumper on pin 14
and the two flying resistors on the potentiometer, now used
for delay adjustment.  Note the deleted C15 and C17.
Click on the image for a larger version.

Figure 5, below, shows the schematic of the modified board with the changes described above.

At this point the board is converted to being a delay-only board, but with the amount of delay fixed at approximately 200 milliseconds with the value of R27  being 15k as seen in table 1, below.  This amount of delay is quite reasonable for use on a repeater to provide the aforementioned functions with no further modifications.

Optional delay adjustment:

By removing the need to be able to adjust the amount of echo/reverb, we have freed the 50k potentiometer, "RA", to be used as a delay adjustment as follows:

  • Remove R27, the 15k resistor, and replace this with a 47k resistor.  This is most easily done by using a 1/4 or 1/8 watt through-hole resistor and soldering one end directly to pin 6 and the other to ground, using the middle "G" pin along the edge of the board.
  • Remove R21 and using a 1/4 or 1/8 watt leaded 4.7k resistor, solder one end across where R21 went (to connect the wiper of potentiometer "RA") to pin 6 of the IC.
  • The 4.7k resistor (and parallel 47k resistor) sets the minimum resistance at about 4.3k while the maximum resistance is set by the parallel 47k resistor and the 50k potentiometer in series with the 4.7k resistor at about 25.3k.  These set the minimum and maximum delay attainable by adjustment of the potentiometer.

Of course, one may also use surface-mount resistors rather than through-hole components, using jumper wires rather than the flying leads of the components. 

Figure 5: 
Diagram of of the '2399 board after the modifications to be a "delay-only" circuit.
Click on the image for a larger version

This modification provides a delay that is adjustable from a bit more than 300 milliseconds to around 80 milliseconds, adjustable via the variable potentiometer. 

It's worth noting that if you do NOT  require a variable delay, using fixed resistors may offer better reliability than an inexpensive potentiometer of unknown quality - something to consider if the board is to be located on a remote repeater site.

If variable delay is not required, one would not use the 4.7k resistor at the junction of R21/"RA" - or use the potentiometer at all, and R27 would be replaced with a fixed resistor, the value chosen for the desired amount of delay as indicated in the following table:

Table 1: 
The amount of audio delay versus the resistance of R27.  Also shown is the internal clock frequency (in MHz) within the chip itself and the THD (distortion) on the audio caused by the delay chip.  As expected, longer delays imply lower bit rate and lower precision in the analog-digital-analog conversion which increases the distortion somewhat. 
This data is from the PT2399 data sheet.
Delay (ms) 
Resistance (R27)
Clock frequency (MHz)
Distortion (%)

The chart above shows examples of resistance to attain certain delays, but standard resistor values may be used and the amount of delay interpolated between it and the values shown in the table.  

While not specified in the data sheet, the delay will vary with temperature to a slight degree as the onboard oscillator drifts, so it is recommended that the needed delay be chosen such that it will allow a slight variance while still providing the amount of delay for the needed task.


If this is to be powered from a 12 volt supply, it's suggested that one place a resistor in series with the "+" input to provide additional decoupling of the power supply.  The (possible) issue is that the 470uF input capacitor ("CA" on the diagram) will couple power supply noise/ripple into the ground of the audio delay board itself - and associated audio leads - potentially resulting in circulating currents (ground loop) which can induce noiseAdditionally, an added series resistance provides a modicum of additional protection against power supply related spikes.

The board itself draws less than 50 milliamps, and as long as at least 8 volts is present on the input of U4, the 5 volt regulator, everything will be fine.  A 1/4-watt 47 ohm resistor (any value from 33 to 62 ohms will work) will do nicely.

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