Active crossover network using 555 timer as class D amp

This blog may provide interest to some newbie looking for low cost, easy to build class d amp source tuned to their listening taste. I have already tried this circuit on the breadboard and it sounds surprisingly good but I’m sure it will work much better on PCB of course. The timer chip I used is the CMOS LMC555 because it’s a lot faster than bipolar counterpart such as 15 ns rise and fall times and can oscillate up to 3 MHz @ 5V. Of course you can use bipolar 555 timer as well.
About the circuit:
The circuit consist of 2 classical yet popular configurations; The master wired as “astable multibivrator” and the slave as “one shot” monostable.

https://geekcircuits.com/wp-content/u...ive-filter.png
Figure 1.
Slave A is an active band pass filter tuned to 20 Hz with +/-7 Hz bandwidth serving as “mega bass enhancer” while slave B is a simple high pass filter with -3db roll-off at 7 KHz for the treble. Slave C will fill-in the missing center frequency having a full bandwidth. You can tweak any band of frequency that will suit your taste by selecting the Q, Bw and the center frequency of the tuned amp.The 3 pots are use for 50% duty cycle adjusts and also offset null adjust. Java script for calculating your desired tuned frequency can be found here.
The circuit above will work in multiple channel application because the master’s output pin 3 effectively syncs all slaves connected to it. Also, it has negligible loading effect of the trigger input pin 2 of any slaves thereby giving lots of opportunity for multiple band of frequencies you wish to add. This circuit is quite unique in its own right because the operating principle does not conform to a conventional setting where saw tooth is generated to sample the audio signal thru comparator’s two inputs to produce PWM. Here, the master generates continuous but fixed pulse width of approximately 98% for the purpose of two important things; To reset the internal flip flop of all slaves, and at the same time, to discharge the timing capacitor via pin7. The most attractive feature of this circuit is that they are darn cheap. This circuit can produce wider modulation from 0% to 95%. But 100% is not allowed because the master is required to pull down pin 2 to ground of all slaves in short duration fast enough to accomplish its two task as already mentioned above.
My broken oscilloscope is in the brink of extinction due to the leaky 1.5KV filter capacitor so I can show you only the simulated waveforms via LTspice. BTW, LTspice have done a good job in providing fairly accurate behavioral model of 555 timer that is identical into its real world brother. So, the waveforms shown below are just the same waveforms I’ve seen from my scope when it was still functioning LOL!
https://geekcircuits.com/wp-content/u...5-waveform.png

Figure 2.
The master produce an output of very short negative going pulse which is sufficient to change the output state of all slaves into logic 1 but the “width” of that “on” state is governed by modulation voltage on pin 5 of all slaves. The master is oscillating at 275KHz in my frequency counter.
Please take note that even though the circuit looks good and simple, proper matching of the master and slave’s frequency is very important. Improper matching will result into unsuitable for class d application. This mismatched frequency scenario I’m referring to is; instead of producing class d amp, you will end up to get a frequency divider instead. Try to simulate it so that you know what I mean.
Thanks for looking
ledmania