SEWA - Seven Watt Amplifier

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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.
 

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Here it is, playing sweet music, together with my RIAA pre and preamp.
 

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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...
 
The one and only
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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.

:cool:
 
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?

thanks
Marius
-Trying to learn :)
 
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 ;)
 
Marius, the zener at the input is there to prevent any static discharge from destroying the mosfet, and can be omitted if you are careful when connecting the preamp (touch RCA gound to power amp chassis before plugging it in). As always, there are no zener in my amp :bigeyes:
 
You must be a Fearless DIYer to walk in the Class A amplifier woods!
Just right, Moe:) I always prefer to have the zener input protection onboard:D You never know when someone starts fiddling with your system, do you???
The resident ("Female") is one possibility!!! Some other freak might also be tempted to do some "reorganisation":D :D
You are doomed, whatsoever. I kind of hate that ****, resoldering for the fourth time the ZVP3310 on my Zen amp:bawling:
BTW I am glad you take look at this forum, Marius;)

Steen:cool:

I nearly forgot the most important thing: Great work Mads:)
 
Thanks for the comments guys!

Moe, I've reason to be fearless: No matter how careless I have been , I've yet to blow something up. Usually, I'm quite concentrated when fiddling with my stereo :drink: And, the female never does anything but
play music on my system ;)

I've made a pcb layout, and will submit it for production over the weekend toghether with a MM-pre and preamp of the same (but common source) topology; essentially a Pacific and a BOZ).

Here's a screen dump of the pcb layout in case anyone is curious:
 

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They are called Atlantic (MM Phono, two stage JFET passive eq, like Pearl, only much simpler) and Moschatus (Pre, one IRF510 in common source mode, like BOZ). I will link up the schematics in the projects section on my webpage as soon as they're ready :cool:
 
For those of you that would like to use a zener, I would suggest adding a 1N4148 type diode in series with it.

The zener acts like a non-linear capacitor with changes in the input signal. A 100 ohm resistor in series with it (as Nelson suggested) helps, and so does an extra series diode. The extra diode allows the zener capacitance to charge, but as the signal changes the extra diode holds the charge so as not to discharge back into the input node.

I think both 47K resistors could be increased by an order of magnitude, to 470K, making the input impedance much higher. This may be of help for peoples using a tube preamp, or an input transformer (for voltage gain), or a transformer volume control (some of which can have voltage gain too).
 
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