Owner`s manual

Another reason is to maximize the amount of useable range that the volume control has, which is

especially useful with units that have detented (click-stepped) volume controls. The volume control

taper (rate-of-change vs. amount of rotation) at the low end of the control is necessarily coarse.

After all, most people like to be able to shut the sound off once in a while. A system that is too

loud when the control is three clicks from off isn’t very useful unless the owner is a hardened

volume freak. Remember to explain to the owner than even though the control must be turned “way

up”, that they are still getting full performance out of the amplifier.

The impedance problem

Another old bugaboo is the “matching” of impedances. Let’s put this one to rest right now.

In a contemporary audio system, there is no need to match impedances, except where

maximum POWER transfer is desired. Currently, the only place where this is true is at the

antenna terminals of a radio receiver.

If we don’t match impedances, then what do we do? Simple...make sure that the output impedance

of the source is less than the input impedance of it’s load. Thus, a head unit with a 1000 (lK0 ohm

output impedance is fine driving an amplifier with an input impedance of 10,000 (10K) ohms. On

the other hand, a head unit with an output impedance of 10,000 ohms is not okay looking at a

power amplifier with an input impedance of 1000 ohms.

Let’s take a moment and look at amplifier and speaker “matching.” As mentioned previously, the

only place where impedance matching is desirable is where you are interested in maximum power

transfer. Impedance matching occurs where the actual source impedance equals the actual load

impedance (i.e. 8 ohm source, 8 ohm load). For a typical solid-state amplifier, the actual output

impedance is perhaps 0.01 ohm. Why do the manufacturers tell us that the amplifier is intended for

a 2 ohm load? What they are really telling us is:

This amplifier is suitable for load impedances down to a minimum of 2 ohms.

What would happen if we were to actually try to match impedances, in the strict sense of the word.

If the amplifier were perfect (technologically impossible), we’d have a very large power output

figure.

For example, let’s create a fictitious amplifier that not only has a 0.01 ohm output impedance but

can actually drive a 0.01 ohm load (speaker). This amplifier has a 4 ohm power rating of 50 watts.

Doing a bit of quick math, this means that the amplifier will develop 14.14 volts across a 4 ohm

load. Now if we drop the load (speaker) impedance to 0.01 ohms (remember, we said matched

impedances) then this amplifier will deliver 20,000 watts of output power, albeit into a 0.01 ohm

load. If you’re not afraid of the math, the power in the load is the voltage squared, divided by the

load impedance. For our hypothetical amplifier:

Power = (14.14 x 14.14) / 4, or 50 watts.

If you tried this in the real world and the amplifier was well designed, it would do the best it could

(basically it would stop at around 50 watts output) and probably get quite hot under the collar. If

the amplifier weren’t so well designed, then it might destroy itself trying to drive the abnormal

load. That’s why you can’t just keep connecting speakers to an amplifier unless you’re trying to

help the amplifier repairman buy a BMW.

Gain, Level, Sensitivity, and Power Relationships