The old question asked is ofcourse how easy are headphones to drive, and the old high impedance harder to drive etc. One of the things that I’m still not familiar with is low impedance headphones and amps. Some planars go as low as 12-14 ohm. And I’m not aware what effect this has on amps. Most amps mentioned 16 ohms and upwards.
Can it be that for some amps a headphone with too low impedance gives issues? Reason I’m asking is that I plan on buying a Hifiman Ananda Nano next week, which is listed as 14 ohms. The amp I’m planning to purchase is the Schiit Midgard, which in ASR review was shutting down on the 20 ohm test. So I want to learn to know when amps don’t match well with certain headphones.
Can anybody give some insight on this and teach me?
Which headphones did they use for the load test? Or was it an artificial load using other equipment?
The ohms of a headphone is only part of the story. You also have to take into account the sensitivity of the driver. The higher the sensitivity the less power needed to get them to your desired listening level.
Planars are normally fair better with higher current amps, but I don’t think that headphone would trigger a shutoff.
I am not the best person to explain it unfortunately, but I will try.
The design of the amp comes into play. Planars put a higher current demand on the amp. If the planar in question is particularly inefficient it can draw more current than the amp can handle and causes it to exceed operational temperatures. Some devices have a current protection safety switch (chip) which will shut down the unit to prevent damage.
I didn’t have any problems driving Ananda Nano with the Midgard, FWIW. That circuit protection discussed on ASR didn’t kick in until the amp was pushing nearly 5 watts. You are not going to be using that much power to drive the Nano at anything resembling a normal listening level.
As to the underlying concept - and these are the SIMPLEST of terms - an amplifier takes electrical power from a wall outlet and combines it with the analog electrical waveform that is a music signal and makes that music waveform taller. ie it increases the amplitude of the waveform…“amplitude” and “amplifier” get it? Who says physicists have no sense of humor? Lol. But, because of that pesky law of conservation of energy, and the fact that power is the rate at which energy is delivered, an amplifier cannot put out more power than it sucks out of the wall. And in fact, no amp design can deliver 100% of the power it pulls from the wall. Some energy inevitably gets lost somewhere, often to thermal energy which we feel as heat. Point here is, amplifiers have a limit as to how much power they can deliver. Now, electrical power is the product of current and voltage. Voltage is an electrical energy differential between two points in space. Current is the actual movement of electrical energy from a high potential energy spot to a low potential energy spot. Most consumer audio amplifiers are voltage amplifiers, unless they are specified to be current amplifiers. Thus, they specialize in developing an energy differential across the speaker or headphone drivers and then deliver just enough current to make the drivers move. So at any given time during music playback the amplifier is causing a set amount of voltage to exist across the driver(s). Then, the amount of current that flows through the audio circuit is basically determined by Ohm’s Law: Voltage = Current x Resistance, or V = IR. In an alternating current circuit, which audio circuits are, the impedance plays the role of resistance. From Ohm’s Law we see that when voltage is set, more resistance/impedance means less current and vice versa. So this is the most basic of explanations as to why lower impedance headphones draw more current from an amp.
So why did the Midgard shut down? IIRC the power brick that comes with it is a 16Volt, 1 amp power supply, meaning that the Midgard circuit is designed to draw and work with a maximum of 16W of power from the wall. But, it’s likely to be a voltage amplifier which means that most of its power output goes to making electrical energy differentials and then allowing just enough current ot flow to get the job done. Given the Midgard’s price, it’s circuit design likely would burn up pretty quickly if it had to handle a lot of current. So, they put a protective shut down in there to prevent that. Amir had a test rig that found that limit and tripped it. I doubt it would ever show up in any real world use. And Amir says the same, basically.
To the electrical engineers reading this, I know there is more to it than what I’m saying. I emphasize the BASIC part of this. Breathe.
@JeroenNietDoen I hope that’s just enough to help you understand what’s going on.
@JeroenNietDoen: Adding to what wave theory said, remember that you’re not going to push a headphone or amp to its absolute limits in real world usage unless you’re cranking up the volume extremely high, which isn’t safe for the gear or your ears.
Some tests are designed to push the limits, but they’re typically done without a headphone attached to the amp.
Very well explained! Once you came along with Ohms law and the formula, remembering physics classes, I now kinda understand what’s happening. And yes, as you mention, that can only happen when looking for the extremes. Now I listen music pretty loud but I don’t see that 5W happening unless I’m already deaf
Planars are generally constant impedance whereas dynamic drivers will have a more U or V shaped impedance curve. Planars that require a ton of power are usually low in sensitivity, which means they inefficiently convert the amplifier’s electrical energy to the mechanical energy of driver motion.
