In reply to:
Interesting....what would a person look for on a spec sheet indicating this?
Shielding and isolation are built into the build quality of the amp. There's really no technical spec for either... but they will show up good or bad in the THN+d graph of course.
As for whether or not there's gate circuitry - that would be kind of a "cheat" to make an amp sound cleaner - you'd have to look through the schematic for that.
Maybe I should talk a bit about C/L/E/G - compressor/limiter/expander/gate. I bring these up a lot, not sure if everyone "gets" them.
Limiter - we keep talking about amps going into "clipping" where the waveform peaks are cut off. A limiter works by sensing when a signal will enter clipping and adjusts the gain (volume) to prevent this. It's like having Superman ride your levels for you - when he senses the amp will go into clipping, he turns it down to, say 99% of full range, just below clipping levels.
More technically, it works by reducing levels above it's threshold by a certain ratio "crushing down" the highest levels.
Of course, this changes the divisions of output (which is a distortion of the sound). Say if program audio that's 80% as loud as the loudest sample in the program plays out at 95% of the range of the amplifier before clipping, the top end will be compressed (that top 20% of program loudness has to be made to fit into the remaining 5% of available amp headroom) hope that makes sense.
Gate - the opposite end of the spectrum. Think of a gate as, well... a gate... audio waiting on one side to get out into the speakers, but the gate doesn't open until it's triggered by a signal at least as loud as whatever it's threshold is. A hard gate cuts in and out immediately at that threshold, a soft gate fades into and out of the program material to make a smoother transistion. These are especially useful for analog sources, where tape hiss and groove swish can be nullified to pure silence in "silent" parts.
Compressor/Expander/Compander - A compressor reduces dynamic range of program audio above it's threshold. Again, using our percentages of total program loudness (I'll coin an abbreviation here for my theorical scale - TPL), say the sound of a flutes keys being worked is at 5% of TPL with the trill between a D and D-sharp note the flotist is playing is at 90% of TPL. Say the compressor is set to be 5:1 with a threshold of 50% of TPL. The sound of the keys is untouched, but the note being played is reduced to 58% of TPL (40% over threshold of 50, compressed 5:1) meaning the master gain can be cranked up to make up that 32% quieter that the note is back at 90% of TPL, but that will also bring the sound of the keys up by a relative amount. Voila - the wall of sound. Sick, isn't it?
An expander works just in reverse - expanding the dynamic range of program audio.
A compander does both in the same circuitry - for instance, most CB Radio mics are companding mics - which trade off dynamic range for high RMS (root mean square) signal strength, which gives you a more powerful signal to send out over antenna.
Bren R.
