18 Feb
2012
18 Feb
'12
1:23 a.m.
On Friday, February 17, 2012 07:07:39 PM Fons Adriaensen did opine:
On Fri, Feb 17, 2012 at 12:32:01PM -0500, gene heskett
wrote:
But at a 60 ohm impedance, that capacitance is considerable. I could put
20 volts P-P into a cable headed for our news dept, at say 15 kilohertz,
and 200' away, I could only see 6 or 7 volts.
[bolometers]
Either of those methods costs 500-5000 USD to accomplish.
The average house & garden multimeter is indeed completely useless
for measuring anything audio. OTOH
* There are quite good handheld audio RMS meters which don't
cost a fortune (but they are in the higher price range).
They use analog integrated circuits which can be quite
accurate - at least for normal audio use. They are not
laboratory standards of course.
* Any pro-quality audio card, once calibrated against a known
signal and combined with some simple software will make a
near-perfect RMS meter *for the audio band* and as long as
you don't drive it into clipping.
[cable impedance]
Why? The common two wire & foil shielded audio cable, used in
broadcast and studio facilities in miles per studio quantities,
actually has an impedance in the 60 ohm area! Feed it with a 600 ohm
source and 300 feet of cable later its rolled off like a Ma Bell
telephone circuit. Your audio DA's, to drive that, need to source
terminate at 30 ohms per wire, from a very low impedance amplifier.
The concept of cable impedance makes sense only if the lenght
becomes a non-trivial fraction of wavelength. For audio that
means that for anything shorter than a few hundred meters it's
only capacitance that matters. And yes you need hefty line
drivers and low output impedance to push 20 kHz, +20 dBu on
a long line. Which is one of the reasons why real pro quality
analog audio remains expensive.
And often sensitive to both crossover distortions and slew rate limits, one
of the reasons I built a DA card years ago, 2 channel in 4 out each
channel, and used TLO84's for all of it. I was replacing a rig that had
heat sinked TO-5 bugs in it that had both crossover and slew rate limits
much worse than the near gain of 1 op amps had at half the output P-P.
Those were also a home brewed copy of something one of my employees made
before I came in the door in '84, and in my attempts to build a working
television station, that problem finally filtered itself to the top of the
must do list about a year later.
The biggest problem with the TLO84's was that those long cable runs made
excellent antennas for EMP's, which caused then to self destruct in bags
full every time the STL tower, a 255' pyrod self supported tower about 20
feet outside the back door, got tapped by a wandering lightning strike, so
I had to keep TLO84's around in 3 stick quantities, but TLO84's are dirt
cheap. The lightning strike was a many times annually event when you have
255' of steel sticking up.
Ciao,
Likewise Fons, have a good evening.
Cheers, Gene
--
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