stu@hpficad.HP.COM (Stu Bell) (05/06/88)
Okay audiophiles, I've been out of touch with hi-tech audio trends since I left college (and my audio fanatic roomie) 9 years ago. Whatever happened to Class D (digital) power amplifiers? I remember that Infinity (or was it Phase Linear?) came out with one in 1978, but I assume it failed miserably since it (or a reasonable facsimile) is no longer for sale. What's wrong with pulse modulation (essentially the digital approach) for power amplifiers? Do the high frequency harmonics of the square waves fry speakers? Stu Bell HP Colorado IC Division "No Clever Quotes, Just Honest Curiosity"
dya@unccvax.UUCP (York David Anthony) (05/10/88)
In article <4110001@hpficad.HP.COM>, stu@hpficad.HP.COM (Stu Bell) writes: > Whatever happened to Class D (digital) power amplifiers? I remember that > [Infinity] introduced one in 1978 . . . > What's wrong with pulse modulation (essentially the digital approach) for > power amplifiers? . . . Nothing, really. Pulse width modulated switching power amplifiers are commonly used in AM broadcast transmitters. In one such system, there are two valves connected in series, a "switch" valve and a "carrier" valve. The switch valve runs at 70 kHz or so, and is connected to the anode of the "carrier" valve at operating frequency. There is a massive transmission multipole filter between the two valves. The system is reasonably flat to DC. This is the kind of system which lends itself to precisely the kind of application above (generating large amounts of audio power efficiently). No one needs a, say, 7500 watt audio amplifier for entertainment use. Infinity introduced (supposedly) a prototype PWM audio amplifier I think, around 1975 or so. Power transistors were not wonderful then by any means (power V-channel FET's also just coming out) and I doubt there were many actually produced. The PWM approach for home use presents mostly packaging and production problems, not to mention that regular linear amplifiers can easily surpass PWM performance even in the most rudimentary of specifications. The fidelity of the PWM equipment is controlled by not just the linearity of the PWM generator (trivial to do now with ECL 100k logic) but by the tuning of the lowpass filter and various traps. It is difficult to get good supersonic suppression and a flat frequency response AND deliver this to such a low impedance load. (In the AM transmitter case, the load impedance for audio frequencies is a nice, comfortable 1000 ohms or so, and it doesn't vary much-in a well designed transmitter- over the audio range). Speakers are notorious for interesting impedance v. frequency characteristics. Yes, the switching waveform must be very well suppressed, and it will fry speakers easily even with a few watts. This is never a problem in the AM transmitter, because you suppress the switched waveform say, 50 dB, and the class "C" output stage with its various tuned circuits takes care of any residual 70 kHz component. I suppose that one could use various lookup table schemes, high speed digital signal processing, or whatever, to predistort the PWM signal so it comes out linear at the filter output, but why would anyone want to? Unless you need gobs of audio power (or, for that matter, well regulated DC), PWM just isn't the ticket; although some netter will undoubtedly prove me wrong... David Anthony DataSpan, Inc.
davea@hpscdc.HP.COM (Dave Angelini) (05/12/88)
I second that it is not the ticket !!!!
Davestu@hpficad.HP.COM (Stu Bell) (05/21/88)
Thanks for the info. I was actually thinking of the space reduction one gets in going from a linear to switching power supply as being analogous to linear versus switching power amps. Thus, audio power amps could theoretically be much smaller than their linear cousins. My interest was in a home project resulting in a SMALL, HEFTY power amplifier. Oh Well. Guess I'll just start looking at old Audio Amatuer magazines... I hadn't thought of them being used in AM transmitters, but of course that WOULD be logical. Thanks for the responses! Stu Bell