[zhenya]

I have a question about the pairings of some of my equipment that regards volume levels, impedance, gain, etc.  The two amps in question are my Woo Audio Custom, an OTL amp based on the old Woo Audio 1 design, and my newly acquired Headamp GS-1.  The phones are my Sennheiser HD-580's and Denon D7000's.  The GS-1 was purchased as an upgrade to, and as a better match to the Denon's, which at 25 ohms are not considered a good match for an OTL amp.  What is odd, and that I'd like to better understand is why the phones both have approximately the same volume level (for a given position of the dial) when driven by the Woo, yet when plugged in to the GS-1, the Denon's must be run on low gain (as expected), yet the Senns are almost silent on that setting.  In order to drive them to appropriate levels, I must switch to high gain and turn the volume up. 
 
Why such different behavior from the two amps?
 
Thanks!

 

[hodgjy]

You are comparing apples to oranges.  Sensitivity levels play a more important role in "driving the headphones" than does impedance.

 

[nikongod]

This is almost entirely about the load impedance & output impedance. Sorry hodgjy.
 
The Woo-1 (and other circuits that share numerous design similarities) has a decent chunk of output impedance. If memory serves, the Woo 1 has an output impedance of 60 or 120 ohms depending on the output tube.
 
This output impedance means that the output voltage will sag in relation to the load when compared to the output voltage into infinity ohms. The HD580 have an impedance several times higher than the output impedance and dont cause very much sag at all. The D7000 have a very low impedance, several times lower than the output impedance, and cause a LOT of voltage sag. 
 
The saggy voltages cause weird things to happen with the power, and effective gain of the circuit. With the HD580 the gain could be like 8 or 12, but with the D7000 the gain could be 1 or even less! 
 
The GS-1 on the other hand as a VERY low output impedance, less than 1ohm (from memory) and the voltage output does not really change depending on the load. I wouldn't necessarily call this better, as there are some subjective advantages to amps like the Woo1, but the rules first brigade will be here soon to tell me what I should like. 
 
In the end, I would not pair the d7000 with an amp like the Woo1 despite the fact that I think the Woo1 sounded GREAT with HD580. On the other hand, the GS-1 would not be my first choice for the HD580 despite the fact that the GS-1 (actually an older version I have heard a few times) makes the D7000 sound about as good as I have heard them.