rko 0 Report post Posted January 31, 2009 I have an question on amplification that i hope someone can answer. Why is it that sometimes we had to crank up the volume to "open up" the sound? My reference phones- CD900ST(rated 63 ohms,300mW, MAX 1000mW) suffers this problem when driven by the benchmark DAC1. I found out that the phones "open up" when i go over 10 O'clock. But the volume is too loud for my listening pleasure. My other pair of phones, AKG 501(120 0hms, MAX 200 mW) do not have this prob. I usually go over 11 O'clock. I find this rather weird. Technically speaking, by cranking up the volume dial, we are increasing the voltage to the phones, effectively increasing the current/power of the system. I do not have an oscilloscope on hand atm, i might try this at the lab though. My only conclusion is that, since the CD900ST is rated at 300mW, it means that the phones work best at 300mW. Whereas the AKG rated MAX at 200mW, easily reached its optimum performance at a lower wattage. I believe i can add a resistor in series to increase current flow thus effectively increasing the power of the system. Is this what is commonly known as impedence matching? I know some amplifiers have impedence selectors at the amp-out line. The benchmark have the for the RCA and XLR but i not too sure about the headamp section. Thanks in advance for any replies. Share this post Link to post Share on other sites
audison 0 Report post Posted February 1, 2009 (edited) adding a resistor in series reduces the current and attenuates the signal. it does not increase. if i not mistaken, impedance matching is such that the Zo of the output stage of the amp matches with the Zh of the headphone to gain maximum power transferred to the headphone. good headamps have this capability to maximise the output transferred to the headphones by adjusting the output impedance selector, which is typically a range of values of Z, eg, 8ohms < Z < 32ohms 33ohms < Z < 64ohms 65ohms < Z < 120ohms 121ohms < Z < 300ohms select the range that best matches the rated impedance of the headphones. normally it would be better to overpower the headphone rather than to underpower the headphone. that does not necessarily mean u will fry the cans, it just means u have more headroom (typically 6~10dB more) extension in terms of the reserve power. my headamp puts out a 2Wrms in the hdph out which far exceeds any rated max power of any cans. just that it's easier to drive even the most "difficult" cans w/o introducing any distortion or artifacts. Edited February 1, 2009 by audison Share this post Link to post Share on other sites
rko 0 Report post Posted February 1, 2009 (edited) adding a resistor in series reduces the current and attenuates the signal. it does not increase. if i not mistaken, impedance matching is such that the Zo of the output stage of the amp matches with the Zh of the headphone to gain maximum power transferred to the headphone. good headamps have this capability to maximise the output transferred to the headphones by adjusting the output impedance selector, which is typically a range of values of Z, eg, 8ohms < Z < 32ohms 33ohms < Z < 64ohms 65ohms < Z < 120ohms 121ohms < Z < 300ohms select the range that best matches the rated impedance of the headphones. normally it would be better to overpower the headphone rather than to underpower the headphone. that does not necessarily mean u will fry the cans, it just means u have more headroom (typically 6~10dB more) extension in terms of the reserve power. my headamp puts out a 2Wrms in the hdph out which far exceeds any rated max power of any cans. just that it's easier to drive even the most "difficult" cans w/o introducing any distortion or artifacts. Hmmm, i get what u mean on the impedence matching. I'm not fully aware of the idea behind it, but i roughly know the concept. About the resistor, what i mean is that i increase the input impedence, then turn up the volume dial to increase line-out voltage(am i right here?). I should be able to to get back my initial current value. And with a higher impedence/voltage now, wont my power ratings be increased? I'm not too sure what u mean by reserve power/more headroom. Possible to explain in more detail or you have some links for further reading? thanks! Edited February 1, 2009 by rko Share this post Link to post Share on other sites
audison 0 Report post Posted February 1, 2009 (edited) what i tink about your case here is, you shouldnt even be adding any resistors or doing any modding at all. why would you want to add a resistor in the input section to ensure that both your hdph play at the same loudness? power is wasted in the resistor when it can be better used to drive the load. also, if u do add in a resistor in series, and u attempt to crank up the volume more, there may be more snakes (hiss) heard thru the phones. the rated power handling of headphones is just a number. senstivities (dBspl/W/m) tells u how efficient the drivers are singing given a known applied power. some manufacturers will quote their sensitivities at certain spot-frequency. ceteris paribus, a driver with a sensitivity, say, 100dB/W/m plays softer than another with 115dB/W/m. this has nothing to do with the rated power handling. on the subject of over/under-powering ur headphones, there are equally contentious schools of thoughts... google is ur best fren, you can find the info there. Edited February 1, 2009 by audison Share this post Link to post Share on other sites
rko 0 Report post Posted February 1, 2009 hmmm, if that is the case. It means that the cd900st is supposed to be driven to a volume past my listening pleasure? Volume is the voltage magnitude. Please correct me if i'm wrong. The idea of increasing the input impedence came about when i thought of the Xin Supermacro series and the etymotic er4s/p. They did increase the impedence of the input via a small circuitry and the er4p is bascially a sr4s with less impedence(okay, they might have did more stuffs i dont now ). Okay, i just checked the DAC1 manual. In the manual, they mentioned some very very interesting stuffs. I shant quote cause of copyrights(XEDO!) 1) resistors are placed in sereies to prevent short-circuitsand maintain stability. 2) resistors in series are generally not good for headphones. 3) generally, distortion increases when headphones impedence decreases. One more thing that i understand is that although Benchmark states that the headamp is capable of driving both high and low impedence loads. It includes a gain selector so that users can use the full out-put power of the headamp. Benchmark also states that the user should be able to get a "comfortable listening volume" from 10 O'clock and above. I guess this is what i am missing here. Share this post Link to post Share on other sites
