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Active crossover network using 555 timer as class D amp

Posted 9th July 2010 at 08:17 AM by ledmania
Updated 13th July 2010 at 06:07 PM by ledmania

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.

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!

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
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  1. Old Comment
    regiregi22's Avatar
    An original approach!
    Posted 13th July 2010 at 02:23 PM by regiregi22 regiregi22 is offline
  2. Old Comment

    I think that maybe the real circuit would benefit from having some capacitances between pin 8 and pin 1 of each 555. They would help to satisfy sudden demands for pulse currents, which the inductance of the DC supply wiring would tend to prevent. You could start with 10 uF electrolytic and 0.1 uF ceramic (not NPO or C0G type), with the ceramic connected within a millimeter or so of the pins and the 10 uF in parallel with that.


    Posted 17th July 2010 at 04:32 AM by gootee gootee is offline
    Updated 18th July 2010 at 01:24 PM by ledmania
  3. Old Comment
    ledmania's Avatar
    Yes, you are correct, it is always standard practice to put by-pass decoupling caps from Vcc to ground, and I've been too lazy to put that to most of my schemo. I just copy and paste the drawing from Ltspice sim to clipboard and it made me to forget that very important stuff. I will include it next time. . Thanks for a nice reminder.
    Posted 17th July 2010 at 04:59 AM by ledmania ledmania is offline
    Updated 17th July 2010 at 07:11 PM by ledmania
  4. Old Comment
    wintermute's Avatar
    Hi Tom, just wondering why you say to NOT use NPO or GOG? I thought for anything audio NPO and GOG were preferred due to much lower distortion, are there tradeoffs for this application that make them less suitable?

    Its a very interesting design ledmainia, but a bit beyond my capacity to understand at the moment... too many unfinished things to try learning something else for now

    Posted 19th July 2010 at 11:29 AM by wintermute wintermute is offline
  5. Old Comment
    ledmania's Avatar
    Take your time, wait for somebody to test it by their own curiosity. If somebody ring the bell saying "gotcha!" then you know what they mean

    Posted 20th July 2010 at 06:21 AM by ledmania ledmania is offline
    Updated 20th July 2010 at 08:23 AM by ledmania

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