WHat is headphone impendance and what does it affect on the sound with an amp ?

As far as I remember there was another thread on that topic - even with almost the same title - only a while ago... The search-function will help...

Greetings from Munich!

Manfred / lini

Kryogen,

There are two main effects of impedance on an amp:

1. Current drive. The amp has to put a certain voltage across a driver to make it move. The amount of current necessary to develop a certain voltage depends on the impedance of the driver. Ohm's law says: I = V/Z, where I is the current, V is the voltage, and Z is the impedance. As you can see, the lower Z is, the more current is required.

2. Because drivers are driven by voltage, you have to worry about the voltage divider formed by the output Z of the amp, and the input Z of the driver, ie. the impedance of the driver. The voltage presented to the driver is multiplied by Zd/(Za+Zd), where Zd is the impedance of the driver, and Za is the impedance of the amp. This assumes that cable impedance is negligible, which is not a safe assumption with headphones because many cables for 'phones are so small that they have significant resistance. If cable Z is significant, it's in series with the amp Z, so the multiplier becomes Zd/(Za+Zd+Zc). For example, Etymotics cables seem to be customized for each different model. And all you Sennheiser HD-600/-580 cables upgraders, take note!

The tricky thing about the voltage divider effect is that Zd often varies over frequency (because it's not just a plain resistor), and Za can vary over frequency. This means different voltage divider values at different frequencies, leading to an altered frequency response that's specific to a particular headphone/amp combination.

What does this mean to you? If you want to drive headphones with the amp being as little in the way as possible, get a headphone amp that has as low an output impedance as possible, and make sure its output impedance is as resistive as possible, meaning, it doesn't change with frequency. This is doubly important for low-Z phones like the Grados, because the voltage divider effect kicks in at lower Za for those phones than high-Z phones like Senns.

(Voltage divider effects kick in at lower Za, because Zd is lower, and lower Za have greater effect, numerically --- plug in a few numbers and try it out, eg. compare Za = 10 Ohms with Zd = 60 Ohms and Zd = 300 Ohms, and see what the multiplier is).

--Andre

Kryogen, I wouldn't worry too much about impedance of the headphones. What you should do is buy the amp that sounds best to you which means auditioning headphone amps. If you can't audition any commercially sold amps, you might want to do a DIY amp or have someone build you an amp tailored to your headphones. There are many knowledgeable people here who build headphone amps for specific headphone brands and models.

Impedance is a measure of the resistance in the coil inside a pair of headphones. The resistance is a result of inherent resistance because of the nature of the materials used in construction as well as the back emf induced in the coil because of the changing voltages during operation.

Btw, AndreYew, it's the other way round - the higher the resistance (impedance) the higher the current needed to drive them.

I=V/R means that the higher the resistance in a circuit, the smaller the current, so you must increase the current.

Also, drivers aren't "driven" by voltage, they're driven by current. The interaction between the magnet and the magnetic field created by the changing voltage/current in the coils makes the driver vibrate which transfers this energy to the air in terms of sound waves.

Kuja,

"I=V/R means that the higher the resistance in a circuit, the smaller the current, so you must increase the current."

That's not right. In order to keep V constant, for a smaller R, I must increase. This is very easy to test in the real world with both speakers and headphones. For example, why do 4 Ohm speakers need more current than 8 Ohm speakers?

We assume V is constant because amps are constant voltage devices.

"Also, drivers aren't "driven" by voltage, they're driven by current. The interaction between the magnet and the magnetic field created by the changing voltage/current in the coils makes the driver vibrate which transfers this energy to the air in terms of sound waves."

Yes I know that. But the information is encoded in the voltage. The current is there to maintain the voltage.

--Andre