From: monta@image.mit.edu (Peter Monta)
Newsgroups: rec.audio
Subject: Re: Digital Amps -> next???
Date: 24 Nov 92 01:58:21 GMT
Organization: MIT Advanced Television Research Program

kirk@hpcc01.corp.hp.com (Kirk Lindstrom) writes:

> If not, it will at least be amusing! 8-)
> Digiatal preamps exist and sound pretty good (Sony TAES2000 @$1200).  They
> take either a digital signal, process it (DSP) and convert it to analog
> with D/As or take an analog signal, digitize it, process it and D/A it.

This is a "preamp"?  "Preamp" to me means the first gain stage after
a transducer, as in "microphone preamp" or "stylus preamp"---provides
gain/shaping, sets noise performance.  Audio types must have
seriously distorted this term.

For the kind of simple processing you mention (depending on what
"DSP" is supposed to mean), $1200 sounds outrageous.

> ...
> Quality power amps typically have SNRs (A weighted and converted to 1V ref)
> of 100 db or more while line preamps do well to get 90-95db.  Acurus, for
> example is 110 and 95dB (not sure if this is referenced to 1V).  DSP units
> are in the mid 80s under $1000 and reach the 90dbA range for $3000 
> (Lexicon CP-3).  My GUESS is that DSP lowers SNR because all the digital
> switches (or gates) pump transient current into the ground as well as radiate
> off the power supply bondwires.  Both sources can find their way back into
> the analog section unless you spend alot of money to isolate and shield
> them.

(What is this 1V business?  One would expect noise and distortion
specs to be cited for the entire range of the amplifier.)

Again, I'm curious about what a "DSP unit" is supposed to be.  Digital
power amps are meaningless.  "Digital" is a signal representation.
Power amplifiers drive electrical actuators, and so they are
analog.  Certainly newer techniques, DSP ideas included, can
make better power amplifiers, but why paste the word "digital" on
everything?

This remark that "DSP lowers SNR" is tautological; everything lowers SNR.
Interference from digital subsystems is an issue, but high isolation
is possible with careful design.  Certainly "radiation off the power
supply bondwires" is ridiculous, if that's what you really mean;
radiation becomes an issue only when the bond wire is on the order
of a wavelength---for a 3mm bondwire, pad to frame, this is 100 GHz.
Such wires contribute objectionable parasitics at much lower
frequencies, of course, but there's no interference issue there.

> For a mixture of decent sound with low cost and lots of "features", it might
> make sense to have powered speakers with D/As in them.  They would be fed
> with Plastic Optical Fiber (hopefully HP) links from the console that had
> DAT, CD, DSP, etc.

Sure.

> The cost savings comes from only needing one pair of
> D/As with this method as well as not needing alot of expensive cables.

Surely cables are cheap; they're just wire.  (Yes, I've seen several
iterations of the cable debates, and I'm really very unimpressed/amused.)

> We aren't talking "audiophyle sound" with this sort of system, but I bet
> you could easily get mid 80 dbA SNRs which would satisfy 99% of the
> market.  I believe SONY has gone half way to this with some of their
> "100 WPC rack systems" at the $1000-$1500 range with speakers.  SInce
> these SONY systems are 4 channel for surround-sound, I think it is 
> cheaper to have one set of D/As in the preamp box and drive the speakers
> (four) with cheap speaker wire.

We're not talking "audiophile sound"?  Why?  What's the defect in this
architecture?  I would think that several separately powered speakers
with on-board D/A and power amp(s) would perform very well.  More
flexibility for the speaker designer, less scope for interference.

Peter Monta   monta@image.mit.edu
MIT Advanced Television Research Program