Four Transistors and the Truth (Part 2)

Power In, Power Out

At the end of the last post, we had an amplifier capable of about a half Watt of power output with about 2.5W of dissipation (at Vcc=24V.) The limiting factor causing distortion was clearly available current: the output voltage distorted around 3Vpp, far from the supply. It occurred to me to try removing R6 and R7--lo and behold more power was had! Now R5 determined the current draw of the amplifier nearly directly.

With this change, I did some simulation and found the following:

At R5 = 10k: Ivcc = 0.25A, Pout = 0.2W @ 1% distortion

At R5 = 3.3k, Ivcc = 0.75A, Pout = 2.7W @ 1% distortion (3.7W @ 10%)

At R5 = 2.2k, Ivcc = 1.0A, Pout = 5.0W @ 1% distortion (6.2W @ 10%)

At R5 = 1k, Ivcc = 2.0A, Pout = 6.8W @ 1% distortion

With a value of 2.2K, the amplifier actually beats the original specs given for the Amp Camp Amp. It also needs some serious cooling, dissipating about 15W in Q2 and 8W in Q3.

The final circuit is thus:

Finishing Touches

I was putting together a cute little box with jacks, independent 24V AC/DC converter (you can find one by Drok on Amazon for $15ish that works great!) and such, I took another look at my circuit hoping to squeeze any last bit of performance I could out of it. I also hadn't settled on the exact value of Rc, I found 3.3k worked with just passive cooling while 2.2k worked with a small fan (actually, I used a spare miniature CPU heatsink and some thermal adhesive.)

Out of curiosity, I simulated the current flow through R5 and found it was between 3 and 11mA. I replaced the resistor with a constant current source of 7mA and, what do you know, everything got better. Distortion went down, power handling went up.

Next, I replaced the imaginary current source with a pair of 2N3906 transistors and a resistor to ground acting as a current mirror. Distortion actually went down further, although total power draw went up (depending on the mirrored current.)

The amplifier now solidly outperformed the design it was based on. The new "final" circuit (for now...) is:

Six transistors, six resistors, and three capacitors. Not bad.

5W RMS, 0.34% distortion, 1.25A draw at 24V (with R5 = 3.3k.)

Gain, you say?

One thing I haven't mentioned is the gain of the amplifier. It's actually less than one. To get that 5W figure, the input signal needs to be 8.6V for an output of about 8.1V. I'm using a Fiio headphone amplifier as a pre-amp which happily supplies this. Perhaps there's an argument in there that gain really is the purpose of a pre-amplifier, but I'm not sure. Either way, the "obvious" solution of increasing R2 to boost gain in the circuit results in considerably increased distortion. A different solution is clearly in order but I'm not too invested in pursuing it right now. The current design meets most of my requirements (it requires active cooling or a heatsink I don't own) and sounds nice to my ears--I call it a win.