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why low impedance phones are hard to drive? - Page 2

post #16 of 47
its easy to wrong when manufacturers are changing sensitivity reference from dB/mW to dB/V - and not indicating which they're using

I believe K701 are 93 dB/mW - making them less sensitive than HD600/650 @ 97/8 dB/mW

the differing impedance however puts the dB/V numbers as you have them

it takes ~3x the power to drive the K701 to the same loudness as the HD600/650 - even though they require similar V

an amp for the K701 has to have that much more current output to drive them without distortion - as well as more V swing than most 32 Ohm (Grado) optimized amps

so people should quit recommending the Gilmore Lite for K701 - since it was designed for Grado load/sensitivity it doesn't have the Vswing
post #17 of 47
Quote:
Originally Posted by jcx View Post
its easy to wrong when manufacturers are changing sensitivity reference from dB/mW to dB/V - and not indicating which they're using

I believe K701 are 93 dB/mW - making them less sensitive than HD600/650 @ 97/8 dB/mW

....
Could you give a reference for those numbers?
post #18 of 47
Quote:
Originally Posted by op2003 View Post
Could you give a reference for those numbers?
http://www.akg.com/akg_personal/medi...9b2523d088.pdf

If you look at the simple web page on AKG's site, you'll see that the specification lists both dB/mW and dB/V for the same value. However, the link above (on the same page) makes it clear that the 105dB value is for dB/V, just as jcx says.

Likewise, Sennheiser actually claims 115dB/V, much higher than the K701's 105dB/V:
http://www.sennheiser.com/sennheiser.../HD_650_GB.pdf
post #19 of 47
Thanks for your effort. But where do the efficiency values come from (93 / 97dB/mW)? Is there a simple calculation to transfer the db/V values to db/mW ?
post #20 of 47
The K 701 has slightly higher sensitivity than HD 600 and 650. I can tell this from own experience and reproduce it anytime. This applies to an output impedance near 0 ohm. With higher output impedances, it will change, since the K 701 is 62 ohm, the HD 600/650 is 300 ohm, hence the latter will «suffer» less from additional serial impedances.

The Sennheisers are clearly more efficient, though (e.g. need less current). It's the AKG's lower impedance which makes up for that and leads to a higher sensitivity over-all (again: with low-impedance amps).
.
post #21 of 47
Quote:
Originally Posted by op2003 View Post
Thanks for your effort. But where do the efficiency values come from (93 / 97dB/mW)? Is there a simple calculation to transfer the db/V values to db/mW ?
1Vrms generates 16mW on K701: 1V*1V/62ohm
10*log(1/16) = -12db
105db - 12db = 93db/mW

Compare HD650 and K701, assuming we need 110db of maximum loudness (I can't imagine who would ever need something louder than this ...)

HD650 300 ohm, 103db/mW, 5mW, 1.22V/4mA(rms), 3.43V/11mA(p-p)
K701 62 ohm, 93db/mW, 50mW, 1.76V/28mA(rms), 4.93V/79mA(p-p)

HP with a low sensitivity is always more difficult to drive, just like those less sensitive loudspeakers. In the case of K701, I think 4.9V/79mA is hitting the limit of most OPAMPs, in term of both V swing and output current. Also keep in mind that, impedance is not the same as resistance. It varies with frequency of signal. The value on spec is the impedance at 1KHz. It tends to be smaller with lows, and bigger with highs, which means more current is needed to drive bass properly. My two cents.
post #22 of 47
This is a very interesting discussion. Also confusing. My headphones are 38ohm and 99db sensitivity. What kind of demands do they place on a amp?
post #23 of 47
Quote:
Originally Posted by Bones2010 View Post
This is a very interesting discussion. Also confusing. My headphones are 38ohm and 99db sensitivity. What kind of demands do they place on a amp?
It sounds confusing, because we have two variable factors here: sensitivity and impedance. If we only talk about low impedance phones, we'd better freeze the sensitivity for the sake of easier understanding. If sensitivity does not change, the anticipate output power remains the same, then low impedance phones drain more current, while high impedance phones require higher V swing, which should be easy to understand. I have a spreadsheet to calculate these numbers I posted in my previous post. It's attached in case anyone is interested.
By the way, a solid-state amp works under low voltage and high current, and has a lower output impedance, so it drives low impedance phones better, if enough gain is there. However, a tube amp works with high voltage and low current, and has high output impedance, thus it's better matched with high impedance phones.
LL
post #24 of 47
Thanks for the explanation.
post #25 of 47
x2 Thanks for your effort! I was looking for a calculation just like that!
post #26 of 47
so, given that impedance/sensitivity measurements *are* vital to determining how well an amp will drive K701s, is it possible to determine which amp(s) most closely match the requirements of these feisty cans?
post #27 of 47
x lk: Thanks for the spreadsheet! This certainly makes it easier to understand things
post #28 of 47
Quote:
Originally Posted by x_lk View Post
It sounds confusing, because we have two variable factors here: sensitivity and impedance. If we only talk about low impedance phones, we'd better freeze the sensitivity for the sake of easier understanding.
I guess the dB/V is the conventional way of specifying sensitivity right? So when a given phone has a specified sensitivity of say 108, it normally means 108dB/V, have I got this right?
post #29 of 47
Quote:
Originally Posted by x_lk View Post
HD650 300 ohm, 103db/mW, 5mW, 1.22V/4mA(rms), 3.43V/11mA(p-p)
K701 62 ohm, 93db/mW, 50mW, 1.76V/28mA(rms), 4.93V/79mA(p-p)

HP with a low sensitivity is always more difficult to drive, just like those less sensitive loudspeakers. In the case of K701, I think 4.9V/79mA is hitting the limit of most OPAMPs, in term of both V swing and output current. Also keep in mind that, impedance is not the same as resistance. It varies with frequency of signal. The value on spec is the impedance at 1KHz. It tends to be smaller with lows, and bigger with highs, which means more current is needed to drive bass properly. My two cents.
I don think opamps as such have a problem with the voltage swing, though the current is too high for most. The larger problem is that if feed from a 0.447Vpp source, most amplifiers will not have enough gain from K701 - you would need a gain of 13 ... Worse, as it is a "low impedance" headphone, people would set the gain switch (if there is one) to "low", further disadvantaging the amplifier.
post #30 of 47
That's why you need more power than you will ever use. A good all around amp will have the power reserve to handle mismatched impedance. Cheap amps won't do the same thing.
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