Ultra Low Cost and simple discreet FET power amp...

[darwindeathcat]

I found a schematic for a ridiculously simple FET-based power amp on this site: http://www.oliveaudio.com/index.php?page=3
The schematic I'm looking at is the UCLA3 (I can't figure out how to attach a picture to this post). The site says it's capable of 17w with a single +60v DC supply! Does this seem right? If so, this could be a ridiculously cheap and EASY power amp project for ANYONE! Combine this sucker with any of the ROG preamps (the ROG "distortion" pedals are actually preamps) and you'd have a killer small amp that would sound really tubey!

Thoughts anyone?

[teemuk]

Sigh,

17 watts with 60V supply is not much, you should typically get about 60 watts using that supply voltage and a 4-ohm load. About half of that to 8 ohms. Class A, what Oliver Audio claims this circuit is, would explain the low power - except that the circuit is in fact class AB. I wouldn't trust even half of the stuff written on that page - and I definitely wouldn't built that amp.

SS amps were like this in the late 60's - early 70's. The only difference is the MOSFETs and they do not make that big difference. The well acknowledged fact is that most SS amps from that time period do not really stand out as being great sounding or very reliable. I haven't heard this amp but I still know that it won't have the "properties of a Triode amplifier" - not even near. In fact, the whole "transistors are horrible" reputation is due to circuits like this - and high expectations of how they would perform. If you want something easy, consistent, well-performing and reliable build a chip amplifier.

The only reason I could recommend building this circuit is for learning: Go ahead, build it, see how "easy" and "cheap" it really is to build that amp when compared to some alternatives. See how much time it takes and ponder whether it was worth it considering the outcome. Hear how the circuit sounds like when overdriven and find out if it really has any of those advertised triode amp characteristics, find out how stabile it is when you build it as is. See how it performs when you put a volume potentiometer in front of it, find out how well it handles operating in a hot environment, lower than rated impedance loads, momentary shorts etc. It can be fun, as long as you don't expect too much from it.

[fisherbim]

wow..
sir teemu
i'm gonna build this to learn
and for comparison to my
other small builds..

tnx..

[darwindeathcat]

aha! I just KNEW it was too good to be true... I'm no math whiz, but something about the amount of power being thrown at it for the return did not add up... I was hopeful for a low parts count discreet power amp project that would sound halfway decent and still deliver some decent amplification (you know, bigger than ten watts!)... Anyone know of a proven circuit like this? Something fairly simple and not ridiculously expensive that a relative newb could fool around with? I still might build this thing to see how it really does sound...

[J M Fahey]

Well, oliveaudio seems to have dissapeared from the net. Can anybody post those schematics, just for a curious look? Thanks.

[paelgin]

Well, oliveaudio seems to have dissapeared from the net. Can anybody post those schematics, just for a curious look? Thanks.

The website is indeed gone, but the Google cached entry is still available, but without the graphics. 

I agree about wanting a curious look-see.  I assume that the schematics were downloaded by someone, so please can they be shared with us here?  Thanks.

[teemuk]

I conducted a SPICE simulation of the circuit before I posted that reply here so here's a screen cap of the circuit (see attachment). It has some mods that compensate the lack of models (e.g. I think there was just one 5.2V Zener in the bias chain instead of 4.7V in series with a schottky diode having Vf=~0.3V ) and I can't rememeber what the MOSFETs were either. It should work on IRF530, IRF9530 though.

[darwindeathcat]

Hi Guys, This is the original schematic from the Olive Audio site... Cheers!

[J M Fahey]

Thanks, Teemuk and Darwin. Well, it MAY work, and could be built as a curio, but if simplicity is the name of the game, the bias can be simplified (and bettered) by pulling the zener (which does NOT track the output stage temperature) and recalculating the bias resistors , replacing the unnecessary FET "current source" by a 2k7 resistor, and some other minor changes. The "power stage" has only a gain of 6 (100K/15K),and isn´t properly stabilized. An LM3886 is as simple and much better.