OUTPUT IMPEDANCE: What is the effect of an amplifier’s output impedance on the sound of speakers?

FM writes:

this is a brilliant question. The majority of audiophiles know nothing about output impedance. Since the question is specifically about the effect on Sound, I’m going to try not to get too technical.

A solid state amplifier will have an almost unmeasurably low output impedance, close to zero, i.e. .0001 ohms or thereabouts. A tube amp will typically have an output impedance between one and two ohms. There are exceptions but not germane to this discussion. incidentally this output impedance thing is tied to damping factor. A solid state amp will often have a very high damping factor and a tube amp will have a lower damping factor. I’m not convinced that this matters. My business associate believes it matters greatly. So it’s controversial.

your loudspeaker is an electric motor. As such, it’s subject to the law of reciprocity. We typically refer to this as the theorem of reciprocity. But the law states that any electric motor in operation will send voltage back to the voltage source. In our case, the loudspeaker generates the voltage because it’s an electric motor which is driven by different stronger voltage, the amp is the main voltage source, but a driver still sends some of that tiny voltage back to the amplifier. When you have such a low output impedance, the amplifier will not accept this back voltage (AKA-Back EMF). that’s an advantage.

The back-EMF can get into the amplifier’s circuitry. It can affect the amplifier stability and its frequency response. The solid state architecture of the output stage provides this type of advantageous isolation from the drivers’ back voltage.

but what exactly is in that back voltage we are able to discard? Wow, it’s the room sound.

in addition to your loudspeaker being an electric motor, it’s also a pretty effective microphone. It’s just bigger than a microphone but basically the same concept. So while the system is playing, the loudspeakers pick up the vibrations (music) bouncing around the room and of course it’s now in the back voltage. Do we really want to discard the room sound? through experimentation, some of us have determined that if this room sound is 35 to 40 dB below the main musical experience, it will not manifest itself as annoying reverberation. It will give you the speciousness that we equate with openness in loudspeakers. A good thing.

With its higher output impedance, the tube amp allows this voltage to modify its behavior, for better or worse.

many times it results in a subjective improvement, especially in the size of the soundstage, the envelopment, and the depth of the playback.

But this could also depend on how the loudspeaker and amplifier interact with each other. You would not use a four ohm loudspeaker that dropped to two ohms at certain frequencies with an amplifier that has a two-ohm output impedance! The frequency response will be very unpredictable.

but fortunately, most of the time, a modern good quality tube amp will work with almost any loudspeaker. Here’s how you tell: you listen to it.

there are fabulous tube amps. There are fabulous solid-state amps. We do not have complete understanding of the ear- brain interface and the entire endeavor is complicated by the use of transducers (loud speakers). These electro mechanical devices convert one type of energy into another with extremely low efficiency and extremely high levels of distortion compared to how we handle things in electronic circuits. So make sure you ignore all the specifications and base your purchasing decision on the way the unit sounds with your loudspeakers and familiar material.

I hope my explanation didn’t bore you. Even though the two styles of amp do sound different, sometimes the differences are not audible. If you could not tell the difference after 15 or 20 minutes, buy the cheaper unit no matter what design.


RM writes:

The maximum power transfer theorem states: “Maximum Power Transfer occurs when the resistive value of the load is equal in value to that of the voltage sources internal resistance allowing maximum power to be supplied.”

Idk the rest but I always thought this applies to dynamic resistance too (impedance). So if a speaker has 4 ohms impedance, and the amplifier also has 4 ohms, you will get the maximum power that amp can provide (half the resistance means half the power will dissipate in the amplifier, which is why amplifiers get hot). Max power is not the same thing as efficiency though (which is another topic).

ANYWAY: the impedance of speakers is DIFFERENT at difference audio frequencies.. LOL, chew on that!

I believe this would means low impedance frequencies will be louder, and high impedance frequencies will be quieter. This is one of the reasons you need to have a reference CD (or audio source that you are familiar with) when auditioning stereo equipment to buy. Then you can tell if some of the bass notes disappear, or other parts are attenuated; — overall making listening less enjoyable and leading to ear fatigue. Idk if the electronics inside speakers are engineered to minimize this effect (by weighting frequencies differently to compensate.. I also don’t know if the impedance of am-lifters vary over the audio range as well)..

Bottom line, use your reference audio source, and you’ll be able to choose your equipment much better than looking at specs (with a little learning curve; for example don’t use low quality music where a bass note sounds about the same as a drum beat). You want music to sound enjoyable over all, not harsh, and low frequencies not to sound like coming from a garbage can.

If anyone has a deeper understanding, I’m curious.

I don’t understand what someone said about super low impedance of electronic amplifiers (if somebody wants to explain it; my knowledge from college is 35–40y old.. back when portable volt meters were big as a brick)..

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