I know about the importance of impedance matching and how high output impedance can mess with the sound of low impedance headphones.
But I’ve also read about how negative feedback can reduce output impedance. Let’s say I use a high impedance headphone. Wouldn’t that be better to buy a headphone amp with relatively high output impedance to have less potential negative feedback?
I have an HD58x and the Aune amps are usually 10 ohms. I don’t think the 10 ohms would change the HD58x’s sound. So… less unnecessary negative feedback?
In isolation lower output impedance is a benefit to a point.
But what matters is how it was accomplished.
There is no direct value in greater output impedance, but amps with higher output impedance can sound better because they don’t compromise other aspects.
The “secret” here if there is one is ensure the measurables are good enough, then worry about how it sounds.
Yes, higher output impedance can have benefits, like:
- it limits the current that will flow through low-impedance headphones if you connect them to that amp while it’s at full blast, so you don’t burn out the drivers
- it ensures a correct frequency response for headphones (usually high impedance, usually old models) that were specifically designed to be powered from high output impedance amps (e.g. if made according to IEC 61938, the amp out should be 120 ohms).
I’m not sure headphone impedance really affects the amp’s negative feedback, maybe you were thinking of electrical damping. The difference in damping made by the amp-out is that you get worse damping with higher amp-out, but it’s not bad enough to worry about unless the headphones are low-impedance.
The main concern when matching impedances is frequency response. You can’t really tell what the best match will be unless you have access to the headphone’s impedance vs. frequency curve.
All of the above explained in detail with math and graphs by Solderdude from DIYAH: http://www.mediafire.com/file/82kf0r5kdckdcer/resistance%2C+impedance+and+other+issues.pdf
That is true in some sense.
I would say this to be inaccurate.
In theory: high feedback results in low distortion and high dampening factor.
→ Stray Oscillations from the voice coil cause a voltage wich the amplifier will register via the feedback generating the inverse of the signal.
→ A Negative-feedback system in general is more stable than a positive-feedback system as its output is used to adjust itself.
However: The gain of an amplifier is equal to the feedback resistance Rf divided by the input resistance Rin
Dampening factor is described as “Q factor”, I think.
Is to complex for my basic electronics understanding.
You’re right. Generally speaking using negative feedback is a good way to have a controllable amplifier, as well as a way of defining output impedance in a precise way. Positive feedback is generally unstable, especially when connecting the output stage. It can, however, be used as a “gain boost” technique, but that’s another topic.
I actually believe the reason some amps may “sound different in high gain” is because this is their “direct driving mode”, when we see no feedback control. But that’s just a theory of mine.
As for why not have just high output impedance amps, here is a nice read of it: https://en.m.wikipedia.org/wiki/Maximum_power_transfer_theorem. Basically, it’s ideal to calculate how your amp will react to different loads and choose a balance between power transfer and efficiency. However, that’s not easy to achieve with headphones because each have a different load, and you can’t use the power transfer to directly design amps. So it’s a balancing act.