“In the old days”—oh,
here he goes again, someone pull out the liver pills—audio equipment
manufacturers paid a great deal of attention to the input and output impedances
of their products. Modern operational
amplifier circuit design has made a mockery of some of the lengths to which
these manufacturers went. Try looking at
a schematic for an old CBS Audimax, with input and output level controls made
up of three-section potentiometers configured as “T”-pads. Or for real humour, try any of the old Ampex
tape machine audio circuit designs….
The reason for all
this attention to detail was obvious at the time: unless balanced lines were sourced and
terminated at the proper impedance, “bad” things would happen. “Bad” being unpredicted and
undesired results. We were all
taught to just terminate everything properly, and life would be good. The real smart guys, we knew, could sometimes
make miracles happen by selectively breaking the rules, but beware to the mere
mortal that tried it. So we carried on
the tradition of pads, pads, pads everywhere.
All broadcast circuits started and ended at 600 ohms.
The real culprit,
mainly, was all that iron in the circuit—the input and output
transformers. And then opamps came
along… suddenly, all input impedances are bridging, and output impedances are
so close to zero that few worry anymore about the evils of double- and
treble-loading.
Today I want to talk
about a situation that you might find yourself in where you’ll need to start
worrying about impedance again, or you’ll rue the consequences: the good old telephone program line. Hey, those phone guys invented the balanced
line…
Even if the program
line to your transmitter is a digital circuit, as is likely, it most likely has
an analog loop between your studio and the nearest telephone central
office. At the C.O., the phone company
will equalize the circuit, then it’ll go into some kind of A/D, and from there
it could go on a microwave carrier, or fibreoptic link, or copper T1 or HDSL,
or a combination of all of these to get to your transmitter site. Here in
For the first 150m or
so, a twisted pair at 600, or 150, or even 50 ohms just looks like a pair of
wires. Beyond that, in addition to
copper resistance, we have series inductance and parallel capacitance, which
give us your typical low-pass filter.
See Figure 1. The audio response
of the analog loop rolls off at the high end.
The telco equalizer is adjusted to extend and smooth the passband
response. But how is the response at the
input of the equalizer affected by the value of R, the source impedance of the
signal generator?
Here’s the key: telco engineers will adjust the equalizer at
the C.O. for flat response using a 600-ohm source. When you connect your modern
processor with its 30-ohm buildout resistors to the line, it acts like a 60-ohm
source. I have personally seen circuits
that measured +12 dB at 10 kHz (ref 0 dB at 400 Hz) because of this!! The amount of the effect is determined by the
length of the analog loop before the equalizer, with longer loops causing
larger HF peaks. Many engineers will run
the processor output through a repeat coil, or 600:600 ohm transformer before
leaving the studio. Very nice, but it
won’t do you a bit of good here—the repeat coil, true to its name, presents the
60-ohm source with an image of what it sees.
The really nefarious element of this problem is that if you suspected
the line was poorly equalized, you’d likely patch in a 600-ohm generator, which
would measure this circuit as flat. Only
if you fed the tone into the processor, with the processor in the proof
position, would you truly see the problem.
The solution is quite
simple: either add buildout resistors in
series with the processor output tip and ring connections, to make up a total
of 300 ohms/leg (when added to the internal buildout resistors inside the
processor, see Figure 2), or run the output through a 600-ohm pad of 10 dB or
so. (Ahh, once again we witness the
universal curative properties of pads!)
And keep the repeat coil in the circuit—it does provide some protection
from that 48V battery that the telco types are so fond of, and keeps the
processor side of the coil ground-referenced.