My Killer Little Amp

[AAK]

Here's my latest project nearing an end I'm happy to
say after several weeks of working with many different
designs and board layouts. I soon will be building
seven of them for my home theater systems. The design
itself has been posted on this forum before, though
the values may differ some. My objective for this
project was to find a simple Class A/B design with
a low parts count, that would allow me to fit over sized caps plus power supply caps on a small board for my home theater amp. Of course it had to have great stability and sound. I think exceeded my expectations by far. The board measures 4.8"x2.95" or 122 x 75mm. Depending on the voltage of the power supply caps the amplifier can run on +/-50v rails for 100 watts into 8 ohms or +/-42v for 60 watts.

I call it a killer little amp because of the way it
performs, and measures. I compared it to three 100
watt amps that use 4 output transistor versus the KLAs two.In a blinded A/B test with three separate subjects using a broad range of music the KLA was selected 80% percent of the time. And it was preferred 100% for it's bass, which I found interesting considering my speakers are rated at 4 ohms, and dips to 3.6 ohms at DC. The amps where running using 42 volt rails, with 50 volt rails the THD is lower, but not by much.

After how well it performed I decided to check it out
on the scope, check it's THD, and anything else that
might explain why it stood out.

The first thing I found is that the amp doesn't need
an L/R on the output. It handles reactive loads with
ease. I'll post some pics at 10Khz @ 10 watts with 8
ohms||2uf cap. I even tried 100 watts at 20 Khz with
8||4.7uF for a second or two and it held up.

The amp has an output resistance of 0.005ohms for a
damping factor of 1600 into 8 ohms at 1Khz.

It's frequency response is down 3db at 220Khz at 1
watt.

The S/N < 100db

It's THD measures using 50 volts rails as follows:

8 ohms/50 watts @ 1Khz, THD = 0.008%
4 ohms/100 watts @ 1Khz, THD = 0.01%
8 ohms/100 watts @ 1Khz, THD = 0.02%

These measurements look really good, and tells part of
the story as to why the amp sounds so good. The major
factor I believe is the board layout. Also I used high
quality Nichicon ES/FG caps, and a Auricap input cap. But the board layout is key, having the power supply caps on the board allows for a short speaker ground connection which I believe makes a big difference. I always wondered why I don't see more amplifier kits with power supply caps on board. If done correctly it makes a lot sense. Anyway,

I'd love to here from the experts out here as to what
they think.

I'll be placing in order shortly for my home theater
amplifier, and I should have some extra boards available very anybody interested.

Here's the schematic without the signal ac filter caps or power supply caps. I also snubberized the power supply. I'm not sure if it made differnce but figured I had the room so why not. I'll post some waveform pics shortly.

Best regards,

Al

[AAK]

Wrong schematic. Here's the actual.

[AAK]

Here's a pic of the board silk. The schematic and board part numbers don't match.

[AAK]

Here's a square wave output at 1khz into 8 ohms at 1 watt.

[AAK]

11Khz into 8 ohms at 1 watt.

[AAK]

5Khz at 10 Vrms with a 8||2uf cap

[AAK]

10Khz at 10 Vrms into 8ohms||2uf cap.

[pinkmouse]

Excellent work. Looks like a nice little project, though I hope you're going to heatsink those poor output trannies well.

I'd be tempted to give it a go, but for two reaons I can't:

A) I have too many projects on the go already

B) I have far too many projects on the go already!

Still, maybe one day...

[AAK]

Thanks Al. Glad to here that your as busy as I am, isn't this hobby great!

[spind]

Nice work! Though I don't have the skills to critique your circuit, I'm impressed by it's simplicity. I'd be interested in boards if you do some type of group buy.

Congrats,
Steve.