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Mad_K 27th October 2005 06:17 PM

SEWA - Seven Watt Amplifier
2 Attachment(s)
Here is the prototype of SEWA, my latest power amplifier creation:

Inspired by Nelson/First Watt, this is my take on a simple-as-possible
power "amplifier". There is nothing special about it really, just a power mosfet in follower (common drain) mode. It has 0dB voltage gain, so it's really just a buffer/current booster. One of my design goals was to use only one transitor. Within its power range the performance is actually quite good.

I buildt the prototype in my power amplifier cradle (a retired XSOZ), and it contained almost all of the parts already mounted. No doubt about it, this project is all about chassis, power supply and heatsinking. My prototype runs on 24V, so it's more like a 3W amp. I used a 6,8 ohm 50W resistor for the 10R/100W drawn to get 2A bias and a little more power. My Klipsch RB75's (97dB/1W) are quite happy with this power output in my 22m3 room.

It requires a preamp that can swing +/-10-12V and has low output impedance.

So, here you have it: A couple of resistor and caps and a mosfet.

Mad_K 27th October 2005 06:23 PM

2 Attachment(s)
Here it is, playing sweet music, together with my RIAA pre and preamp.

Mad_K 27th October 2005 06:24 PM

2 Attachment(s)
I don't really want to show you this, but I know you'll ask, so here it is -the inside mess..

moe29 27th October 2005 07:28 PM

That's pretty cool Mads! :)

good to see you posting and sharing.


Mad_K 27th October 2005 07:42 PM

Thanks Moe, long time.. I've been very busy at work, but also I've been messing A LOT with chips (opamps) a few years now. A lot of time aslo spent on perfecting my PCB layout skills (just ask my girlfriend :( ). I've produced a nice little collection of single opamp stage designs. Now it's time for some single transistor designs. That means single ended class a all the way. After listening to this little amp for a week, I'm very motivated. I'm going to try a switching supply on it some time before christmas. I'm a huge fan of Nelson's work/ideas, so I try to keep things as simple as possible, but no simpler...

Nelson Pass 27th October 2005 08:02 PM

Very cool. If I may make a suggestion, you only need one of
those Zener diodes, and you might want to think of either
adding 100 ohms in series with the input, or in series with
the zener, or eliminate the zener.

I have seen a good number of instances where a preamp will
oscillate when presented with a bare zener diode, and if you
buffer it with some series resistance, this usually prevents that.


demogorgon 27th October 2005 08:06 PM

Hi Mads!

I could rant on about how very nice the amp is and such, but since i really dont have much knowledge about discrete amplifier design, i'm gonna ask you a couple of questions instead, and hope you'l answer ;)

why the two 9v zeners?
why the 2.7r res. from 0v to GND? is it there to improve the amps behavior with a speaker VS a pure resistor? (inductance, capacitans)
why the 1k resistor from out to 0V? to discharge the caps in case a loudspe3aker is not precent?

-Trying to learn :)

Mad_K 27th October 2005 08:17 PM

Thanks Nelson, I will toss out one of the zeners and add another 100R.

Actually, the amp in the pictures doesn't have any zener ;)

Mad_K 27th October 2005 08:26 PM

Hei Marius.

-No reason to use two zeners, as Nelson said.

-The 2R7 is there to help eliminate ground loops, and can be omitted.
-The 1K is to there to drain off the cap, and can also be omitted unless you plan on connecting speakers while the amp is on. At the moment I'm not using either of them ;)

Mad_K 27th October 2005 08:39 PM

2 Attachment(s)
Here is the revised schematic. Oh, the 47K at the input may also be omitted (and, of course, the prototype doesn't have one either) :D

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