Because of these properties of amplifier and speaker systems:
Let’s simplify this to a couple of statement:
Amplifiers are current limited. This limit defines the maximum voltage at any given impedance.
As the amplifier output impedance rises, or speaker impedance drops, the electrical frequency response at the speaker inputs goes from flat to tracking the speaker impedance.
Want to understand why? Read on.
As an introduction, see the first graph on my blog post on speaker impedance for a very easy to drive speaker:
https://speakermakersjourney.blogspot.com/2016/12/crossover-basics-impedance.html
We’ll ignore phase angle, as it’s hardest to grasp. Let’s assume instead:
At low frequency, say 300 Hz where the panel is 4 Ohms, the voltage at the panel is
4/(4 + 1) = 4/5ths = 80%
of the amplifier output.
At high frequencies, say 10kHz 0.3 Ohms the voltage stays at the amp, and the speaker gets
0.3 / (0.3+1) = 0.3 / 1.3 = 23%
of the amplifier output, or about 25% of the amp output!
But look what happens with an amplifier with very low impedance of 0.001 Ohms (aka high Damping Factor):
At 4 Ohms:
4/(4 + 0.001) = 4/4.001 = 99.97%
At 1/3 Ohms:
0.3 / ( 0.3 + 0.001) = 99.67%
So in the range of speaker impedance from 4 to 0.3 Ohms, the amplifier output remains nearly rock-solid.
- Amplifiers have a non-zero output impedance
- Voltage is proportional to impedance in a series circuit.
- Speakers usually present complex impedance, with a range of impedance magnitude and voltage / current phase angles
Let’s simplify this to a couple of statement:
Amplifiers are current limited. This limit defines the maximum voltage at any given impedance.
As the amplifier output impedance rises, or speaker impedance drops, the electrical frequency response at the speaker inputs goes from flat to tracking the speaker impedance.
Want to understand why? Read on.
As an introduction, see the first graph on my blog post on speaker impedance for a very easy to drive speaker:
https://speakermakersjourney.blogspot.com/2016/12/crossover-basics-impedance.html
We’ll ignore phase angle, as it’s hardest to grasp. Let’s assume instead:
- Tube amp output with about 1 Ohm impedance,
- An ESL such as a Martin Logan electrostatic, which has a panel impedance that varies from 4 Ohms at the bottom of it’s range to 0.3 Ohms at the top end.
At low frequency, say 300 Hz where the panel is 4 Ohms, the voltage at the panel is
4/(4 + 1) = 4/5ths = 80%
of the amplifier output.
At high frequencies, say 10kHz 0.3 Ohms the voltage stays at the amp, and the speaker gets
0.3 / (0.3+1) = 0.3 / 1.3 = 23%
of the amplifier output, or about 25% of the amp output!
But look what happens with an amplifier with very low impedance of 0.001 Ohms (aka high Damping Factor):
At 4 Ohms:
4/(4 + 0.001) = 4/4.001 = 99.97%
At 1/3 Ohms:
0.3 / ( 0.3 + 0.001) = 99.67%
So in the range of speaker impedance from 4 to 0.3 Ohms, the amplifier output remains nearly rock-solid.