Originally Posted by scuttle
Ok. But in this case it looks (ie from xonr's diagram - thanks for that) like adding a resistance in series is has an effect that is never achieved by varying iem resistance (unless maybe you have a very low iem resistance - in this case loweer than 8 ohms, which is not going to happen.) So who cares?
Going back to xnor's diagram:
Let me put names on some of this. Vs is the amplifier. Zs is it's internal impedance of the amplifier. Zl is the load, in this case the Shure SE530. Vl is the voltage measured across the load, which is what is being graphed in your original graphs. This circuit is essentially a voltage divider made up of Zs and Zl. I found this link about voltage dividers, it seems to be one of the more simple explanations out there:
Now for a few details that are very important to understand. In any amplifier Zs is not really just a resistor. It actually is a complex impedance made up of a resistor, inductive reactance, and capacitive reactance. The latter two may be very small and insignificant, or may have an easily measured effect. We don't really know what's in the Hifiman, but most solid state amps, and many tube amps too, can be considered to have mostly resistive source impedances. This is a small but important detail. The Hifiman amp output impedance is about 32 ohms. That isn't actually specified, but it can be inferred. But we run into a small problem here. The 801 isn't current product, and there are no published specs for it on the Hifiman web site. Their current 602, however, shows this information:
Output level: 1.1v at 32 Ohm; 2.2v at 150 Ohm
We can estimate the output impedance of that unit to be around 32 ohms. Why? Because the combination of the output impedance of an amplifier and the load create a voltage divider. When the two are equal, exactly half the unloaded amp voltage appears across the load. The specs show 1.1V at 32 ohms, 2.2V at 150 ohms. 150 ohms isn't technically "unloaded", but it's high enough to get close to the right answer. The 602 has an output Z of approximately 32 ohms, and is likely almost purely resistive. There is a high probability that the 801 used a similar output stage. Sorry, I thought I'd seen the 801 specs somewhere, can't find them again.
Now for Zl. In the case of the Shure SE530, it's not a just a simple resistor, but in reality it's a complex load made up of a resistance, inductive reactance, and capacitive reactance. But, more complex than even that, the result is an impedance that varies with frequency. The graph from headphone.com is not very detailed, but at least shows what's going on:
You can see that it's somewhere around 30 ohms at the bottom, swings up at 1.2K, then dips quite low at between 5K and 6K. Recall that the amp Z and the load Z create a voltage divider. The load Z is varying with frequency. So the frequency response measured across the load will vary too as a result, with dips at the minimum impedance points and peaks at the maximums.
Now lets look the same circuit, but this time with the Clip as the source. It has a source impedance of about 1 ohm or less. Looking at that 1 ohm Z and the load again as a voltage divider we can see that the 1 ohm resistor has very little loss, so nearly all of the amps voltage will appear across the load. The fact that the load varies with frequency doesn't alter the measured response across the load much because the source Z is so low. Even at the lows point of the SE530's impedance curve, the source Z is still about 1/8 of the load Z.
Now, if you wanted, for some odd reason, to make the Clip measure just like the HM-801, you just need to look at their biggest difference: source impedance. Clip, 1 ohm, 801 32 ohms. My suggestion was to add, at the output of the Clip, a series resistor of 27 ohms (again, suggested because it's the closest 5% standard value to 32 ohms, and easily purchased). The circuit would be like this:
The total source Z is now 1+27=28. Yes, I know, it's not 32, but you could make it 32 if you wanted to, it's just a little more trouble. Again, I chose the value in case somebody actually wanted to try this and didn't want to buy a lot of little resistors to make up 31 ohms.
Now see how it compares to the equivalent circuit using the Hifiman:
From the standpoint of the load looking back at the amp, the two are nearly identical. The results of a frequency response test across the load would also be nearly identical.
I home that helps to clear up some of your confusion. If not, let me know, I'll try again.
Thanks to xnor for the original drawing and discussion, and mikeaj for his post as well.