[Mauricio]

Quote:

Hear, measure? Relative to what? If I increase/decrease the volume is that the same?

Read this article.

 

[wuwhere]

Quote:

I'm kind of struggling with interpreting this question.  Is it about why electricity works the way it does, the general idea behind how to tell for a particular system, or specific tests to run to be able to tell for that particular system?

You should know, what is your reference? In physics its all relative, right?

 

[Isaansound]

Impedance is a result of 2 different situations: resistive, or reactive. Resistive impedance is the result of components which act like resistors. Resistive impedance is the same across all frequencies. Reactive impedance is caused by components that act like inductors (coils) and/or capacitors. Capacitors increase resistance as frequencies fall, inductors do the opposite. Electrical filters are composed of capacitors (to block lows) and/or inductors (to block highs). As most components including phones and speakers contain elements having all of the above characteristics, non-uniform (non-linear) performance across a wide band of frequencies is difficult to avoid.
Voltage is the difference in charge (potential) between one segment of a circuit and another, amperage ( current) measures the flow of electrons thru any segment. Voltage accomplishes no work. Current is a better measure of work. Take as an example a welder. !2v at high current will apply a lot of metal to the weld, but 1000v at .1ma will do nothing.
Low impedance phones are spoken of as being sensitive because a low impedance element will allow higher current, so it takes less "power" to drive them. The use of high impedance phones is that a dampening affect is provided by the resistance to the current flow. Dampening uses the idea that there is an excess supply, so some can be "wasted" to smooth response curve.
In speakers the old AR brand was high impedance and the Klipsch and Altec were low. Either can sound great IF the rest of the chain is matched properly. Impedance mismatches are a source of many problems with hum and hiss for example.
If you hope to get a good result with no amp, get low impedance phones, and choose a source (like a Fuze) that is good standing alone and likes low impedance loads. If you will be using an amp, then you can choose either type.

 

[Mauricio]

Quote:

...Electrical filters are composed of capacitors (to block lows) and/or inductors (to block highs). As most components including phones and speakers contain elements having all of the above characteristics, non-uniform (non-linear) performance across a wide band of frequencies is difficult to avoid.

 
Some primitive filters are consist of capacitors and inductors only.
 
Yet another reason to avoid passive crossovers in speakers.  An active crossover consisting of electronic components dispenses with the drawbacks of circuits that rely solely on inductors and capacitors.

 

[stv014]

Quote:

Originally Posted by Mauricio /img/forum/go_quote.gif
 
Conclusion 1:  the amplifier drives its own output stage in parallel with the headphone

 
The output impedance and the headphone impedance are actually connected serially.
 

 

[stv014]

Quote:

 
So the output impedance of an amplifier also varies with frequency?

It can, for example if its output is capacitor coupled. This is common with low end sources that have a single power supply. For example, a 100 uF capacitor would have an impedance of 32 Ohm at 49.7 Hz, and with a 32 Ohm resistive load and insignificant output resistance, it would attenuate the signal by 3 dB. The same capacitor at 1 kHz has (in theory) only about 1.6 Ohm impedance, and the attenuation is insignificant at that frequency.
 

 

[stv014]

Quote:

Originally Posted by Isaansound /img/forum/go_quote.gif
 
Voltage accomplishes no work. Current is a better measure of work.

 
Neither is a good measure of work by itself. That is determined by power, which is the product of voltage and current.
 
Quote:

Originally Posted by Isaansound /img/forum/go_quote.gif
 
Low impedance phones are spoken of as being sensitive because a low impedance element will allow higher current, so it takes less "power" to drive them.

 
Impedance and sensitivity are two separate parameters. It does not take more power to drive a high impedance headphone if it has similar sensitivity (dB/mW), but it usually needs more voltage. On the other hand, low impedance generally needs more current. However, sometimes a sensitive high impedance headphone may need less voltage than an inefficient low impedance headphone (and the other way around with current).
 
Quote:

If you hope to get a good result with no amp, get low impedance phones, and choose a source (like a Fuze) that is good standing alone and likes low impedance loads. If you will be using an amp, then you can choose either type.

 
It depends on the source what works best. The safest choice is perhaps a medium impedance headphone with decent sensitivity and not too high reactance, such as the HD25-1 II. Occasionally, high sensitivity may even be a bad thing, because it makes noise more audible.
 

 

[Mauricio]

Quote:

 
The output impedance and the headphone impedance are actually connected serially.
 

Yes, you are correct.  In series, otherwise the current would flow more easily through the output stage of the amp.

 

[choptalk]

I can sell you my month old FILO E7 for $40 bucks if you want.
I used it a grand total of 1 hour.
It still has the original charge and all packing etc..
 
Just bought an LYR desktop

 

[firev1]

Hmmm, for low impedance reactive headphones, noise levels, distortion characteristics also change with output impedance.

 

[wuwhere]

Quote:

 
The output impedance and the headphone impedance are actually connected serially.
 

So if one shorts the output of an amplifier, the only load is the output impedance of the amplifier no matter how small it is, its not 0.

 

[wuwhere]

Quote:

It can, for example if its output is capacitor coupled. This is common with low end sources that have a single power supply. For example, a 100 uF capacitor would have an impedance of 32 Ohm at 49.7 Hz, and with a 32 Ohm resistive load and insignificant output resistance, it would attenuate the signal by 3 dB. The same capacitor at 1 kHz has (in theory) only about 1.6 Ohm impedance, and the attenuation is insignificant at that frequency.
 

So what you are saying here is that if I sweep from 20Hz to 20kHz an amplifier using a 32 ohm purely resistive load, will I be able to see the dips from the amplifier's frequency response?

 

[stv014]

Quote:

So if one shorts the output of an amplifier, the only load is the output impedance of the amplifier no matter how small it is, its not 0.

Yes, although doing so is obviously not recommended with an amplifier capable of high current output.
 

 

[Mauricio]

Any amp worth its salt will have a fuse or circuit that prevents run-away current and shuts down the unit.

 

[stv014]

Quote:

So what you are saying here is that if I sweep from 20Hz to 20kHz an amplifier using a 32 ohm purely resistive load, will I be able to see the dips from the amplifier's frequency response?

 
Yes, here is an example with 10k and 33 Ohm loads (note how the bass is rolled off when driving low impedance):


 
From the voltage drop relative to the high impedance load, the output impedance of the amplifier can be calculated. On the above example, the difference is about 1.8 dB, so the approximate output impedance is (assuming that the 10k load is "infinite" for simplicity): 33 / (10 ^ (-1.8/20)) - 33 = 7.6 Ohm. Now from the total impedance of 40.6 Ohm and the -3 dB point of the bass roll-off, the coupling capacitor can also be calculated: 1 / (2 * PI * 17.5 * 40.6) = 224 uF, so the actual part used is apparently a 220 uF electrolytic capacitor. The 17.5 in the formula is the approximate frequency at which the response drops by a further 3 dB.
 
With a complex reactive load like a headphone, the frequency response interactions of course become more complex, but can still be calculated from the impedance/phase graphs available at InnerFidelity.