- 87 Posts. Joined 5/2012
- Location: Philadelphia
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Low current to drive high impedance cans?
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Take a look here, if you still need explanation, we can try again:
Yes, less current, but higher voltage.
Power = Voltage X Current, so if your phones need a certain power for a certain volume, if their impedance is high they allow less current to flow, so higher voltage is needed to achieve a certain power level. For the same power, if impedance is low, then more current flows, and less voltage is needed for the same power.
It's actually not quite that simple, because the exact volume is also dependent on their sensitivity, or how much power it takes to produce a specific sound pressure. So you can't always exactly compare low and high impedance phones based on required voltage and current, with the resulting power.
If you're picking an amp, the key is low output impedance, because just about any amp can drive high impedance phones, they pretty much all have the voltage. But to drive 30 ohm headphones the amp's output impedance needs to be around 3 ohms or less, and not every amp has that.
Ok I think this is making a bit more sense now, even though the whole concept is not as simple as I am phrasing it. Because high impedance headphones have a high level of electrical resistance, less current from the source is able to reach the drivers, thus reducing the amount of power received by the headphone. Therefore, an amp is used to boost the voltage to help push the current to the headphone, providing more power to drive the headphone.
For low impedance headphones however, the level of resistance is relatively low, allowing more current to reach the drivers; less voltage is needed to push the current.
Is this somewhat correct?
Yes, somewhat. The missing factor is sensitivity or efficiency, which is typically a measure of the sound pressure level produced by 1 milliwatt of power. Some manufacturers specify sound pressure at a voltage, but the reference power of 1mw is fairly standard.
There are fairly large differences in efficiency, like 112dB at 1mw vs 97dB at 1mw. But again, that's not the whole story.
For example, one headphone may produce 100dB at 1mw, and have an impedance of 300 ohms. Another may produce 100dB at 1mw, with an impedance of 100 ohms. Sounds pretty much the same, right? But it takes more voltage to drive 1mw into 300 ohms.
But the reality is, no need to obsess about current, or impedance, or even voltage. Pick the headphones that give you the sound quality you like, then see if the device you want to use them with works with them (hint: most do just fine). Most DMPs and amps have low output impedance, and sufficient voltage to drive all but the least sensitive headphones.
The less desirable combination of low headphone impedance with high amp impedance is actually fairly rare.
Ok I understand now, thanks a lot guys.
One more thing; is it a general rule of thumb that the headphone impedance should by about 8-10x that of the amp's output impedance? I read this somewhere but can't remember where...
- 1,453 Posts. Joined 10/2012
- Location: Van/BBY
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its just a rule but a better way to go by is getting an amp with as low output impedance as possible
Even better to actually know what you're getting. For example, a 5 ohm amp is fine for 50 ohm phones, but not as good for 16 ohm phones. An amp at less than 1 ohm would be best, and work well in both cases. On the other hand, if the 5 ohm amp has some features you like, such as EQ, small size, or great battery life, AND you have 50 ohm phones, you're golden. More info is always batter.
We do get that this stuff is a bit difficult to understand for a nubie, but trust me, it's worth the effort. The link does handle the subject pretty well. Just take your time and read it, then if there's something specific that you don't understand, we'll have a go here.
A good part of the value of knowledge is in the effort it takes to obtain it.