remote OPT, stupid idea ?

[Jaap]

I know a guy how has put his output transformer next to the loudspeakerboxes and uses 12 meter cable to connect the OPT to his KT66 PP amp. He tells me this is the better solution because the long cable has less influence with the high impedance than with 8 ohm.

False or true ?

[dhaen]

.
The high impedance just creates a different set of problems:

Susceptibility to stray capacitance - causing HF loss, or instabiliy if feedback is used
High voltages - up to 2 x the HT (+B)

[Sch3mat1c]

Yeah, but how much is the capacitance?

More importantly, what is the impedance of the *line*?

Mind that since both are dramatically low at the wavelength and distance concerned, neither will affect it.

So let's see, what are the pros and cons.
Pro - thinner wire.
Con - thicker insulation.

Speaker wire is cheap enough for 8 ohms under 200W...just go with 8 ohms.

However, if you were doing a large distribution system such as a stadium, you might have one large amplifier powering the whole thing. I^2R losses would be horrendous at 8 ohms, so you'd keep it at say 600 ohms (70V line, or at this power level, maybe more) and run that around to the speakers, each [set] of which has a transformer to sap its power allotment from the supply.

Same distribution theory that applies anywhere else, especially mains power distribution.

Note this has absolutely nothing to do with frequency, distortion or "sound quality", assuming you aren't letting psychoacoustics sway you. Sure, bad OPTs will sound bad, but WOW, they'll sound bad ANYWAY...

Tim

[HDTVman]

Why not just put the amp. next to the speaker and run the preamp output on some good interconnect cable to the amp. You know, kind of like powered speakers.

Myself I don't think I'd like to run the high voltage all over the room like that. Small kids you know.

Later BZ

[dhaen]

.
Well it's feasible to make a 600R interconnect, but try making a 10K one, so 6C33 is in, and 211's are out. On an unmatched line at 10K, just a nF would be disasterous to performance.

A push pull output stage would be a real balanced source.
It would be just feasible to run the output of a low power, 600R PP amp down a screened balanced audio cable (such as used in professional audio) to a remote OPT. Best run the HT down a separate cable and keep the shield at ground.

[Johan Potgieter]

For my instinct: Bad idea, Jaap. I can only echo the ideas by John and Tim. Furthermore, possibility of noise pick-up by the KT66 leads, plus possible h.f. instability with feedback with whatever L is added by these lengths. And he now needs 5 wires instead of 2 with feedback (7 with UL).

I seem to recall some 20 milli-ohm/meter/lead for moderately thick speaker cable; that gives 0.48 ohm for 12m or 6% power loss (less than 1 dB) - is he worried about that? It is surely inaudible!

The above plus high voltage risk, etc. would make me say: Nay! (otherwise back to distribution practices as Tim suggested).

[rdf]

How about a teflon twinax? Under 15 pF per foot, insignificant series L and R relative to the OPT. The capacitance to shield is double, which could be left floating or resistor coupled to B+. I still much prefer short path point to point construction but would a remote OPT work under these conditions?

[SY]

I guess I have trouble understanding why in the world a run from a high source impedance is a Good Thing.

[gingertube]

SY has it right - given the choice of a medium/high impedance (primary wiring) and a low impedance (secondary wiring) run of ANY distance it a "No Brainer". Always use the lowest impedance possible for wiring/cabling runs of any distance. Less noise, less losses, less phase shift into reactive loads.
Cheers,
Ian

[anatech]

Thanks gingertube! I was about to say the same thing. I can't imagine why anyone would risk sending HV across the room in a house.
The speaker output will suffer less running a distance than line levels. That's another trend that escapes me entirely.

-Chris