ricardo said:
> claim that common collector and common emitter output stages could both be stabilized just as easily (not in my experience).
Which do you prefer John, and what has been your experience?
I am not a power amp guy (think jack of many trades, master of none), but I have dabbled a little.
In my experience common collector power stages seem easier. With common emitter stages any voltage on the power supply rails looks like an input to that stages transfer function, so inductance in rails can lead to HF instability and even ripple crosstalk seems more problematic. That said common emitter stages are very attractive to amp designers because they are easier to drive rail to rail without boot strapping or higher voltage driver power supplies. I've done some crude one-off common emitter designs. One a hifi I built for my sister back in the 60's where I used an opamp's power supply pins to drive a pair of common emitter power devices up at higher voltage rails. This was modest power (<50W) and probably pretty low fidelity (since I had to compensate it heavily to not sing), but it made enough sound to keep my little sister happy for years when she was a poor college student.
Prior to working at Peavey the only production amp I did was a simple 2x35W job right out of a National app note using one of their IC front ends. Most of the conventional amp stages I was involved with at Peavey were cut and paste from their extensive library of standard amp designs... There was a whole series from 5W (IC) up to 300W used in powered mixers, install amps, guitar amps, whatever, with little reward for reinventing the wheel. But being me I was inclined to wonder what if? The oddest amp I designed that made it into production (AMR PMA70+) was really different. It started out as a simple 2x35W amp using a single pair of to-3 outputs each. Then I added cap doublers to each PS rail, so my cute little 35W amp, could put out 2x the voltage and 4x the continuous power transiently. I rated it at 100W sine wave burst power for 20 mSec, 60W sine wave power for something like 15 seconds, and 35W continuous all night long. The instantaneous transients were close to 4x the 35W but voltage decayed quickly. It turns out the semi-continuous sine wave power was mainly limited by how fast I recharged the cap boost circuit. If I didn't current limit the recharge circuit it made full power, and full heat, and drew full current, so it didn't really save anything. I used some PTC fuses to current/thermal limit the boost charging circuitry, so it timed out after 15 seconds of hard sinewave, but it was all but impossible to thermal with music unless very hard clipped. Playing dynamic music it was as loud as a 100+ watt amp, but with the size and weight of a modest 35W amp.
I looked at scaling this up to higher power points but the complexity of multiple high current boost circuits vs. economics of only saving marginally on PS iron and heat sink didn't pay off. You can look at this as kind of a Rube Goldberg class G/H multi rail amp, but the conventional G/H was simpler/cheaper/easier to make. If i were to revisit this today, perhaps modern mosfet switching devices might simplify the boost circuits (I used bipolar devices back then). It was hard to get truly hifi spec's because the PS rails being modulated by the audio on peaks talked into the miller capacitance of the output devices, so there was trade off, between slowing down the edge rates of the PS modulations for low distortion, vs making them faster for higher efficiency. It sounded far better than a 35W amp clipping, but not better than a standard 150W amp, but it was different. Note: The distortion performance for the early class G/H amps was pretty rough too... Modern stuff is so much better.
JR
