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Aniket 27th April 2012 01:30 PM

Simple 100W power amp
1 Attachment(s)
You may find similar circuits in this power range, but it couldn't be more simple than this. This amp uses bare minimum components which are readily available and are cheap. There are better amplifiers out there but the main thing behind this circuit is its simplicity. No need to adjust Vas, Vbe etc.....
You can even make this circuit on a general PCB. Just solder these components and plug to an audio source and you Speakers and you are ready to roll.
However I made a stereo amplifier and sourced the PCB from a local shop which included a preamp with Volume and tone controls. I am using a 30-0-30 8 ampere transformer with 3, 4700 uF caps for both +ve and -ve rails. I would upload my amplifier pics later and the schematic is attached.

For latest design Goto:

Tekko 27th April 2012 02:11 PM

Add a second pair of output transistors before attemting >50 watts output into 4 ohms.

Stormrider 27th April 2012 04:00 PM

It could actually be simplified further, while improving the performance. What I would do:

Replace the 2N3773's with the MJ15003/04 complementary pair, or the better ON Semi pairs like MJL4302/4281. Output stage will now be complementary EF2. I would connect the drivers for class A, removing the lower 100R.

Drop the rails to +/-30 or +/-35Vdc.

Replace the 3k3/Zener with a single 12K or 15K, or jump up to a actual current source. Use better devices for the input pair like KSA992. The NFB cap doesn't need to be a 63V part, 16V or 25V is more than enough.

Replace the VAS device with something better, like a KSA3503, and recalculate/simulate the compensation.

I would also fit a bias trimpot and add the third resistor for safety. You should also have two reverse biased diodes from the output to the rails.

smartx21 27th April 2012 04:09 PM

BD139/140 and TIP122/127 are NOT interchangeable!

Aniket 27th April 2012 04:36 PM

thanx for the replies friends. Its really hard to find genuine MJ15003/4 trans. and the MJL4281/4302 are almost impossible to get. I live in India and these devices are most likely to be fake if bought from a local dealer. The parts used are easily available and are cheap.
sure we could use better trans. but it works and works good.
i used both BD139/140 and TIP122/127 as drivers and both work fine, so any of the pairs could be used, however i would recommend the TIP pair.

Tekko 27th April 2012 05:09 PM

MJE15030 and MJE15031 are good drivers too.

Aniket 28th April 2012 12:45 PM

6 Attachment(s)
Here are the pics of my build. Please help me to balance the LTP and emitter loads on the two drivers and mention the modifications, changes to be done.

AndrewT 28th April 2012 02:27 PM

Measure the current through the LTP degeneration (emitter) resistors.
They should be identical.
Oh, you don't have any?
Measure the current through the 1k collector resistor and then add the base current going to the VAS Q.
This should give the collector current.
Measure the current through the 3k3 tail resistor.
I3k3 = 2*[I1k0+Ib]

Aniket 28th April 2012 03:17 PM

thanks for the reply,
please help me out more,
what's LTP and what's Vas Q could you describe briefly,
and please advice for any more mods/changes could be done, keeping the circuit simple, to make it better
which other driver trans. could be used using same output trans.

Tekko 28th April 2012 03:28 PM

What is that board from ? Kinda looks like a board to some old kit.

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