If you wanted more power from this, you could connect each output to a couple of complementary power MOSFET's for a power cmos invertor.

QuoteI bet it's the simplest audio amplifier circuit you can find.

Let's not forget it's actually very complex, but the IC hides the complexity. Of course, we know what you meant, but still. I guess I'm in a nit-picking mood.

I have to agree with you, phatt; I don't think there is a need to over-engineer a speaker cab. I've got a MOD-8 in a sh*$tty old school intercom speaker box, and I'm surprised by how much low-end it has, UNLESS, I plug my ValveKing Royal 8 into it. But the ValveKing Royal 8 has crappy low end regardless of what I plug it into (I've got a fix for that on the way). There are some general things to do to tweak the sound a speaker cab, but don't worry about exact placement of anything. Then again, I don't exactly have a lot of experience building speaker cabs, so maybe I'm full of crap.

You might even want to consider why you like the sound of your amp and what makes it unique? If all that sets it apart from the sound of other amps is the speakers, then there may be no need to replicate the power amp at all. In fact, you might can just duplicate the preamp/tone controls, etc, use an appropriate chip amp for power gain, and use similar speakers rated for the higher power.

I think if it were me, I'd leave it alone and build a separate amp. You could bump up the power supply and add some beefier heat sinks and stuff, but if you fry the output transistors on an old 1978 amp, I'm guessing you'll have a hard time finding a replacements. If you really want a higher powered version of the amp you have, maybe you could find the schematics for it and try to modify the design for more power, and build the modified design from scratch.

Sorry, I actually meant grid. Actually, when I wrote that response, I almost called it a gate. Sometimes the words I write don't quite match the ideas in my head, like when I used the word buffer instead of booster in the original title for this post. I'd insert some sort of smiley face here if the toolbar icons were working. It's easy to see how grid-cathode current can make input impedance drop. That would certainly load down any tone controls in front of the tube.

QuoteHi Rowdy, I liked your contraption very much.
That's what I recommend everybody: make some kind of "usable" chassis with jacks, pots and switches bolted somewhere , not just hanging from loose wires, and take it to the battlefield (ahem! , practice room or even Pub stage), to be "combat tested".
Modify at will and commit it to PCB only when happy with it.
Lab is great but live sound is the real test.
I must confess being guilty of bringing terrible kludges on stage, although rather than plug-in protoboards I use the old nails-on-wood breadboards.
Your real world measurements show the only (big) problem FETs have; the terrible unpredictable spread in parameters they have, which makes them unusable in production amps.
There *are* tight controller (almost sadistic) biasing schemes that force them into a certain bias point automatically no matter what Fet you plug there, but they rob them all the "valve/tube-ness" they may have.
C

Thx, JM. Using my protobox, I can also put the top on the enclosure to minimize electromagnetic noise from power lines, etc. which even enhances lab testing.  Except in this case, I scavenged my power jack for the more permanent build, so I had to plug the battery cable wires into the bread board. The noise still wasn't bad. I haven't had any live performance tests yet, unfortunately. I haven't been in a live performance since I played bass in a band in my high school days. Between work, school(lots of calculus homework lately), and family, I haven't found the time to get myself into a band. If I ever do, I'm sure I'll be bringing strange looking contraptions on stage with me.

After taking some classes related to automated manufacturing, I've wondered how hard it would be to get a pick-n-place robot to pick up a JFET, place it on a measurement rig, and automatically select the proper resistors for biasing, and reject JFETs that are too far out of specifications. If I ever have the time for such a project, I might try it. I might even have access to some pick-n-place robots at school. The professor in the Technology dept. is pretty cool, and I might can talk him into letting me play with one of theirs.

If I remember right (and I might be full of crap), the impedance dropping you are talking about is when the plate becomes more positive than the cathode, causing a cathode-to-plate current and a major drop in input impedance. This obviously cannot be replicated with a fetzer valve stage. Probably not with a regular depletion-mode MOSFET stage either. Though they can operate in enhancement mode, I imagine their input impedance remains high. However, maybe with a diode between the gate and source, you could get the input impedance drop. But a triode does not have to be pushed to that point to be overdriven.

Whatever the case, the purpose of this circuit is to provide clean or mostly clean gain either as a booster pedal or a clean preamp. That's why there is no tone shaping. Of course one could place a separate tone control before or after the circuit. If someone is using hot pickups or a tone control with some gain before the circuit and happens to overdrive it, I want it to overdrive gracefully. It might be useful for "fattening" up the tone a bit, too. But distortion is not the main goal. Also, I want the circuit to be damn simple, and this circuit is.  When I use it for a preamp with my Crate GFX-212 (bypassing the regular preamp via the effects loop) it sounds great.

I put another one of these together in my protobox (cheap aluminum enclosure with a small solderless breadboard inside). This time, I measured the Vth and Idss of both JFETs. Both JFETS are 2n5457's. Q1 had a Vgs(off) (called Vp in the Fetzer valve article) of -1.59 V and an Idss of 3.4 mA. Based on these measurements,  the "Fetzer Valve Calculator" recommended a Rd of 3923 Ohms and an Rs of 388 Ohm. Using resistors I had on hand, I used a 3.3 kOhm resistor and a 680 Ohm resistor in series for Rd (3980 Ohm total) and a 390 Ohm resistor for Rs. Q2 had a Vgs(off) of -1.16 V and an Idss of 2.1 mA.

The recommendations for Q2 was 6720 Ohm for Rd and 458 Ohm for Rs. I used a 5.6 kOhm resistor and a 1 kOhm resistor in series for Rd (6.6 kOhm total) and a 470 ohm resistor for Rs.

The resulting preamp sounds very similar to the one mentioned above. I could have used trim pots for exact values for Rs and Rd, but I doubt it would be worth the trouble. For one thing, I do not think the measurements from my cheap multimeter are accurate enough to warrant precise values for Rd and Rs. Also, the calculations for Rd and Rs are based on a supply voltage of 18V. I'm using two 9V batteries for my supply voltage, so the voltage is rarely going to be right at 18 V.

For those who do not know how to measure Vgs(off) and Idss and would like to, checkout the Fetzer Valve article at runoffgroove.com. The test setup in that article works well. If I get around to it, I'll replace the Rd's and Rs's in my protobox with ones based on Findleton's biasing formula. If I hear a noticeable difference, I might record comparisons of the two.

If you want to use an LM386 for a preamp, perhaps you should also checkout the Grace and Big Daddy circuits on Runoffgroove. http://runoffgroove.com/grace.html . Also, lookup the smash drive circuit. http://www.aronnelson.com/gallery/main.php/v/Daves-Layouts/Smash_Drive.gif.html circuit. or maybe the Distortus Maximus http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/Krank+Distortus+Maximus.jpg.html?g2_imageViewsIndex=0 . I've not tried the Ruby or Little Gem as preamps, and they may work quite well. However, they are not desiged to be preamps. If you need a good clean preamp using an LM386, the Grace circuit might be your best bet.

Thanks, JM

The first stage has an Rd of 6.9 k (actually 4.7 k + 2.2k in series) and an Rs of 517 (47 + 470 in series). The second stage has an Rd of 10k and an Rs of 680. I do not remember the measured Idss or Vth for either JFET. I'd rather not unsolder the JFETs to measure them to find out(Yep, I didn't use sockets). I'll try to remember to throw another one of these together in my protobox and measure and write down the Idss and Vth of the fets for a better example. Then maybe for comparison, I'll replace the Rd's and Rs's with ones using Findeton's biasing guidelines from this post: http://www.ssguitar.com/index.php?topic=1518.0

Recently, I built a simple two-stage preamp circuit. Each stage is basically built using the basic criteria for the Fetzer Valve (see runoffgroove.com). If I were to use a more general biasing scheme (in other words, not using the Fetzer Value calculations for Rd and Rs), I imagine I could get all the gain I need with one stage. I chose the Fetzer scheme based on its supposed tube like qualities. There have been recent discussions on this site about possible better ways to emulate a valve/tube with a JFET, but I wanted to keep the circuit simple. I also have not yet compared this design to one using a general JFET biasing scheme. Because it is very simple and makes a good clean preamp or booster, I figured some on this site might find this design useful. The design is certainly not god's gift to guitar-related electronics and isn't all that original, but might be simple enough for a beginner to build while still providing good results.

If anyone uses this design and sticks with an 18 V supply, you may want to scale it down to 9 V if anything you put after this circuit cannot handle an output voltage approching 18 Vpp (The output of this will probably be a volt or two less).

For those who care, more info is available at http://www.riemer.us/two-stage-jfet-preamp-buffer/two-stage-jfet-preamp-buffer

Edit:  Oops, I should have used the word "booster" instead of "Buffer". I've got this fixed in the post title

Thx, dude. I can open those samples just fine.

The links to the sound clips aren't working.

If going with a Class A power amp, I'd definitely use discrete components, especially since Class A stages are very simple. Even if you found a way to make a chip amp operate as a Class A amp, it might sound like crap. One of the supposed  benefits of a class A amp is having fewer components to distort the sound (Teemuk's book has made me doubt the veracity of this claim). Whatever the case, unless a chip amp is designed for class A output, forcing it to behave that way doesn't sound like a good plan.

Check out http://www.ssguitar.com/index.php?topic=861.0. While I'm sure there are many better class a amp designs(like the zen), my source follower design demonstrates how simple a class A amp can be. If you have high wattage resistors, you can obviously get much simpler. I've seen one design that used an LM317 in a current limiting configuration as the load for a source follower stage. If you're going for class A, theres no need for chips.