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Old 7th November 2005, 12:20 AM   #1
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Default Small class D amp with 555

This may have been discussed here before,if it was I couldn't find it.

Here's an interesting circuit..

It seems to work well on the breadboard,for such a simple setup.
The 1mH choke was actually the full secondary of a 9-0-9 power transformer that was laying on the bench.

I believe the most power from it was about 4.5W! (peak)
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Old 8th November 2005, 11:38 AM   #2
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You can increase marginally simply by increaseing the supply on the 555 towards i's max rating.

You can then move onto a little more fun by adding an output stage.
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Old 10th November 2005, 03:24 AM   #3
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Ok - this atrocity was my creation.
I left it on patrick's workbench, with a schematic drawing.
I wasn't sure he would post it, since it's not very high quality...

Yes - higher voltage does help.
But I think an output stage might ruin the whole effect, you
know: "Ohh my god, you're using WHAT for an amplifier?!?!"
Very "DIY" kinda project.

I did say the calculation of 4.5W peak was wrong though.
I neglected to notice that the speaker had a 6uf cap soldered
in series with the wires on it. (DUHH?)
An 8 ohm speaker with a 6uf in series is NOT 8 ohms at 150hz.
I think it had 18vpp input from a function generator, which
was most probably way too much.
Output was something like 6vpp into the speaker
(through the cap.) That was at 150hz from a function
generator. Higher frequencies were less voltage
and got rather ugly on the scope above 700hz.

I was hoping for something better, but it does work.

I think the problem is the duty cycle is changed more in
one direction when a signal is put in pin 5.
A voltage on pin 5 just throws off the internal voltage
divider that the comparators use for reference voltages.

I tinkered around for at least 4 hours with the bloody
thing trying to modulate it differnetly, with little success...

Anyone have an idea on how to get more modulation
(and symmetrically) onto this little sucker?
Might have to get tricky with some transistors on the
charge/discharge circuit... dunno.
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Old 10th November 2005, 02:03 PM   #4
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I've seen another way of doing it, by using the audio to charge the timing cap, and I think pin 5 takes the feedback.

I lost the link though and have never been able to find it again.

At any rate I don't think a 555 will ever get you much quality, it is fun to play with though, and now that you have a cheap modulator you can use it as a springboard and start playing with output stages.

When you get sick of that you can start messing with discretes, window comparator type amps are similar to the 555 but without the limitations.
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Old 11th November 2005, 03:43 AM   #5
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Hmm... charging the timing cap with audio; good idea.
I don't quite get the pin 5 feedback thing though.
I'd definitely be interested in seeing a schematic on that, if
you can figure it out...

Meanwhile, i'll take my own stab at it.
If I get anywhere on it, i'll post the results.

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Old 13th November 2005, 06:56 PM   #6
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YeeeeHaaaww!! It works!!

I'd be posting a schematic if i had a scanner...
(phatty, fire up yer nifty circuit software!)

So I went with a simple 555 oscillator, 124k discharge resistor -
I think the other one was 3k - and I applied audio to the timing cap
through another 124k (yes i have lots of them) and a random
capacitor. It runs just over 100khz at 15v supply. Maybe 110khz.

It needed more gain so I added a ba4558 for a preamp.
Then it was doing a wierd "motorboat" kinda spazz thing, so
I added a totem pole to the output - EXR191/192 i think.
Not really the best for output devices, but it handled the
spazzing, and gave it some more punch.

I am absolutely amazed at the clarity of sound attainable from
such a haphazard creation!!
Enduring several gripes from the next room, i hooked it up to
an old Sansui mid-range "can" speaker, and piped sound into
it from my portable CD player.
It reproduced background nuances in the music, which id first
noticed when i was using headphones one day, and which I
could barely hear in my "less-than-desireable" crap equipment
in the car. So I'd say it's doing pretty good, for some parts
thrown on a breadboard...
I did notice some harmonic distortion when i cranked up the
input - mainly high frequency artifacts accompanying the music
that's playing - but up to a certain point it's very clear.

This is actually my first success in the realm of Class D, and
I think this would make an excellent first project for anyone
interested in Class D.

One technical detail (which is the one thing i was trying to get
away from, since i began this) was the frequency drops
when modulation is applied to the timing cap.
This can be seen very clearly on a scope if you balance a
potentiometer between + and -, and hook the wiper of the
pot to the timing cap, through a resistor. As you turn it either
way, the pulse width changes, and the frequency drops.
This also happened on the earlier variant, applying signal
to pin 5, but like i said, it only goes in one direction, away
from a 50% duty cycle.

Hey - I'd be curious to see distortion ratings on this
little sucker... anybody?
Or maybe this is TOO haphazard to grace the workbenches
of real audiophiles? hehehe

Whatever - it was fun.
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Old 14th November 2005, 12:02 AM   #7
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Give it a shot guys,it's simple,and actually sounds fairly good! (considering it's simplicity.)
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