FETs have one distinct advantage over relays and switches; If they're in an automation system and you may be cutting or re-making the signal during (for example) a high-spot in a LF waveform, relays or switches WILL put a click in the signal. It's just a fact of life. the sudden interruption or resumption of signal introduces a large, vertical stair-step.
With FETs, you can slug the gate slightly and get a fery fast fade-in or fade-out. There might be some 3rd harmonic distortion for a millisecond, but it's too brief to identify, and the shift in the level of the signal also helps mask it. -It's essentially indetectable to the ear.
If you wanted to go all audiophile, you could use time-delayed relays to bypass the FETS in "full-short-only" mode, (
Q. -What's shorter than a short circuit???
A. -Two short circuits in parallel!) but why bother, really, if it's the only FET in the system?
The E and G-series problem was that you had FETs at the output of every input preamplifier (for logic and status-driven input selection and input muting during solo), then FETs at the input to the VCA, and FETs at the output of the VCA (for master status and logic-controlled fader swap) then FETs at the monitor path feed (for logic-controlled group/tape/group-&-tape-at-minus-3dB "supercue" monitoring) and FETs into the monitor fader and out of the monitor fader (for the rest of the fader reverse) then FETs in the master path, then FETs in the monitor source selection...
That's just off the top of my head... I'm sure that's much less than half of them!
In the E series they ran into (positive? -I forget...) waveform excursion part-turn-on distortion issues, even the disable for the buss compressor was a FET, which used to -at high signal levels- kick in slightly even when switched out, until we did a modification to hard-kill the detector output when the buss comp. was switched off.
With such a long cascaded chain of Fet switches, even subtle FET-related issues begin to become rather more significant. In addition, the E and G both had literally hundreds of mother-to-daughter card edge connector transisitions, which made servicing the module an absolute dream for techs (like me... when I had my "fixing hat" on) but -specially with age- made the sonic performance just a little less then stellar for engineers (like me... when I had my "mixing hat" on).
The card design of the 9k very sensibly reduced audio signal edge transistions to a tiny fraction of what they were on the E/G, (thanks Andy! :wink: :thumb
and used a seperate, elevated rail (as retrofitted to the G series on the later logic card design) which moved the FET gate-switching rail further away from the maximum audio signal excursion level. This made the remaining FET switching vastly better and basically eliminated the punch-through.
Incidentally, Neve V-series boards used the same FET switching (though not quite so much) so it can't be all that bad!
The 9000 series used SSM2142s for the fader reverse switching, if I recall correctly. There was also a VERY cunning use of a 5534 that managed to pass signal despite both of its + and - inputs being tied to the negative power rail... -Anyone here smart enough to figure out how?
Andy M, -I think that
might have been a Dave Mate-ism... though I don't wish to wrongly attribute the design. -Do you remember it? -It's the large fader (non-VCA path) wiper follower/buffer circuit...
Keith