OK so maybe I haven’t spelunk’ d the forums enough, but I have never found an answer that even remotely satisfied me concerning this topic.
So I have a degree in electrical engineering. One of the classes I took was about transmission lines and the importance of matching your output and your load impedances. The simple version being the following:
When you match impedance from your source (amp; ZS) and your load (headphone/speaker; ZL) you are just fine. (According to audio literature, the reactive part of impedance is ignored i.e. the inductive/capacitive part of impedance. So we only care about the resistance i.e. Ohms)
When you have an impedance mismatch you have signal reflections in your transmission lines that cause destructive interference.
I was wondering why no one talks about this when talking about equipment synergy. Is it a non-factor? Does dampening factor matter more? (DF = ZL/ZS) Or is it a large part of it and people don’t realize it?
It seems like when people talk about amps that work well with certain headphones, upon further research these amps have an output impedance closer to the impedance of the those headphones (less reflections/destructive interference). Or maybe it is the result of some magic DF ratio, but I didn’t look at that closely.
As a slight tangent, signal reflections affect certain frequencies differently; supporting the claim that certain amp/headphone pairings improve certain frequency responses, like improved bass response or extension.
My understanding is everything else being equal (and it isn’t) that in amplifier design, output impedance as close to 0 as possible without being negative anywhere in the frequency range (yes this is a thing) is ideal.
That maximizes damping factor.
People give general rules of thumb of at least 4x or 8x driver impedance to output impedance, but they are just that “rules of thumb”, most Tube speaker amps have damping factors under 2 for example.
BUT it’s not that simple, none of this is in isolation the tools used to reduce output impedance (excessive feedback among others) can have a detrimental effect on the overall sound.
IME using a lot of tube amps, often with multiple choices for output impedance, output impedance has an impact, but it’s not worth worrying about in isolation. I like the Hyla Saarda an IEM with a 7 Ohm impedance on the SW51+ with an output impedance of 25, which is a damping factor of 0.25, but dislike the D8000Pro a headphone with 60 Ohms impedance on the same amp.
And not all amps with say 100 Ohm output impedance sound similarly fast or sluggish into a 300+Ohm load.
It might be a part of the picture with equipment matching, but it’s certainly not all of it.
Because that is “hard” physics. Not just simple numbers you get from clicking a nice button on a GUI.
It is very possible this is a big factor lurking in the shadows.
From my (limited) understanding, depending on the amplifier design, dampening can be “faked” via the feedback to the main amplifier stage. So your output impedance can be (= has to be) low for the electronics to compensate for the load impedance.
I am pretty sure if you were to take FFTs from various amplifier/headphone combinations people like vs those combos people dislike, you would find something. I sadly have neither the budget to do this type of experiment. Best I can do is DIY an amplifier and see what I learn along the way.
Man, so many great responses here, get my tech side really happy.
I think what they says pretty much sums it up, but I will add with what I learned/use in amplifier design on communication: generally speaking, if you’re in “lower” frequencies exact impedance matching is less important than general “good design choices”. Things like minimizing the amplifier output impedance, and guarantee load driving capabilities are more of a parameter. I don’t know 100% why, but I believe this comes from you having lease reflections since capacitance and inductance are less relevant at lower frequencies.
That said, as @Polygonhell said, with tube amps this is a lot more relevant. So much so that some speaker tube amp designs have two different sets of outputs for different ohm loads. I believe this designs use transformer outputs, so they may tap windings in different points so as to match the inductance to the desired Ohm load.
I also believe people are generally more worried with speakers than headphones because it’s always harder on the amp to drive low ohms loads. But an interesting approach that kind try to “optimize” the impedance matching is the Cayin HA-1A amplifier, that has a switch that changes the output impedance on it. It’s also a tube design, alth I’m not quite sure which (I think it’s either an OTL or a hybrid). But this can also lead to different results, as some people discovered having a higher than desired impedance in the amplifier leads to more prominent bass. This is again, coherent to the ideas of higher reflection loss with higher frequencies.
Here’s what Benchmark has to say about it (they talk about damping factor but that’s the by-product of impedance matching so basically the same thing).