// feedforward
Impulse i => Delay delay => dac;
// feedback
delay => OneZero lowpass => delay;

// set delay
500::samp => delay.max => delay.delay;
// set attenuation factor
.95 => lowpass.gain;
// set the zero (to make the filter lowpass)
-1 => lowpass.zero;

// go!
while( true )
{
    // fire impulse
    1 => i.next;
    // advance time
    1::second => now;
}