1/8 rule???

[Borka]

Hi-Fi newbie trying to understand things here...

I own a pair of HD650 and I´m thinking of buying a Crack + Speedball. From what I have heard this is a awesome combo so I have no worries that it will sound great. However, I recently read about the 1/8 (Headphones work best when the output impedance is less than 1/8th the headphone impedance) and can't for the life of me understand what this is all about???

Since the HD650 has an impedance of 300ohms according to the 1/8 rule the output impedance of the amp should be less than 300/8 = 37,5ohms. And the Crack has an output impedance of 120ohms (...which is a lot more than 37.5ohms).

I´m sure this is a face palm some where around here but I need some clarification =)

[Grainger49]

A "Rule of Thumb" is a guideline.  It doesn't guarantee good sound.  The design has more to do with the quality of the sound than the impedance ratio.  As you have said, the Crack/HD650 is a well known good combination.

[Paul Joppa]

Some headphones are more sensitive than others to the amp's source resistance. I doubt anyone has actually identified a specific headphone which sounds wrong when the ratio is 7:1 but right when it's 8 to 1.

There are a lot of these rules going around, with little or no basis in experience. Pretty often, they exist only to make you hate the stuff you have so you'll buy new stuff.

[Doc B.]

That was close! We almost had to switch to the 10:1 rule!