246 Chapter 8
8.1.2.3 Audio is Not RF
Precise “ impedance matching, ” where specifi c impedances (often 50 or 75 ohms) must
be adhered to, is correct for radio frequencies, where cables above a meter or so act as
a transmission line.^4 But at the highest audible frequencies (20 kHz) even a 200-m-long
input cable in a stadium PA system doesn’t behave as a transmission line.
Where the wavelength (the dual of frequency) is a fair fraction, say 20 or 10 times
greater than the cable, cables look mainly like the respective sums of their resistance,
capacitance, and inductance. As the ratio falls, the cable begins to behave increasingly
like a transmission line.
8.1.2.4 Voltage Matching
Since the widespread use of NFB (50 years ago), the majority of power amplifi ers ’ inputs
have been voltage matched. This means that the source impedance is low—much lower
(at least 10 times less) than the total destination, or load impedance. 5,6 The intention is
to transfer the signal, which is encoded as a voltage “ wiggle, ” without signifi cant loss of
headroom, dynamic range, or detailing.
The source’s impedance—whatever’s feeding the amplifi er(s)—also has to be low enough
and remain so at hf to support a fi at hf response into the capacitative loading of likely
cable lengths. Voltage matching is defi ned by de facto industry practice, in the IEC.268
standard. Here, recommendedinput impedances are 10 k Ω or over and equipment source
impedances are 50Ω or less. This is easily memorized as
Lookingback from amp: Looking up from amp:
50 Ω⇐⇒10 kΩ
With voltage matching there is no sharply defi ned “ right ” impedance. Except that in high
common mode rejection (CMR) balanced systems and high resolution stereo systems
Table 8.3 : Power Amplifi er Input ImpedancesType of power amplifi er Z (^) in range
Domestic, seperated. and integrated 10 k–200 k Ω
High-end domestic, esoteric 600–2 M Ω
Professional 5 k–20 k Ω
Vintage professional 600 Ω