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Toolbag 29th September 2009 12:08 AM

Headphone Impedance
 
So I have some Sony MDR7509 headphones which sound great but people keep blowing. I have AKG K240's which sound thin and Audio Technica ATH-D40fs that are muddy (because of the fs) but good.
The Sony HPs keep blowing and they ain't chep to replace the drivers ($120). It looks like they're blowing because their impedance is so low and people either are completely deaf or they leave the HP cranked all day long ... or something I haven't figured out that's different than when they use either of the other HPs.
So, is there an efficient way to increase the impedance of the Sony headphones? I can't do anything to the PrivateQ (or optional que amp) because that would affect the AKG and/or AT headphones.

I originally bought the Sony HP's because people were complaining that the other HPs didn't get loud enough. Now they're blowing these things and I've had them for only a month and now have 3 drivers that are dead.

thanx

tINY 29th September 2009 02:15 AM


You could add a series resistor in-line with each driver (on the "+" side).




-tINY


nosebleedaudio 29th September 2009 12:52 PM

Quote:

Originally Posted by tINY (Post 4624968)

You could add a series resistor in-line with each driver (on the "+" side).




-tINY


Good engineering.. when NOT in use turn OFF...
Adding the resistor will not solve anything IMHO...People will simply turn up the volume even MORE...
This is an old problem..

ripple_fx1 29th September 2009 02:45 PM

Headphone amp clipping and excessive low frequencies are a popular method of driver destsruction - use a limiter and hi-pass filter in your chain if possible. Fuses are cheap - find out what the power handling and nominal impedance of your driver and you've got the two numbers you need to find "I" and the starting point to build some protection into a small box with a couple of screw cap fuse holders and trs jacks. Possibly some fuses rated less than "I" would be appropriate as well.... Cheers! JR

Toolbag 29th September 2009 07:29 PM

Guess I hadn't cleared up that I did my research to recognize the differences between the HPs that are blowing (Sony) and the one's that aren't (AKG and AudioTechnica's).
Anyway, here's the result of data I collected before posting:
..........................Freq.......Imped..Max Power...Sensitivity...Driver
AKG K240.............15-25kHz...55......200 nW.......91dB/mW....40 mm
AudioTech D40fs...20-28kHz...66....1600 mW.....102dB/mW...40 mm
Sony MDR7509HD....5-80kHz...24....3000 mW.....107dB/mW...50 mm



tINY: hat's what i was going to do, but thought it best to see what the consensus from my other techies would be. I'll test both in series and parallel and see which sounds better
I'll probably stick em inside the TRS connector if I don't get a "Don't do that because.." response

dale116dot7 29th September 2009 07:46 PM

I wonder what Pete Townshend would have done with his headphone mixes (and stage volume) had he known he'd not expire before his hearing. I use a fairly cheap EQ and an Alesis 3630 as a limiter in line with my headphone amp, set just before clipping.

nosebleedaudio 29th September 2009 09:26 PM

Quote:

Originally Posted by dale116dot7 (Post 4627386)
I wonder what Pete Townshend would have done with his headphone mixes (and stage volume) had he known he'd not expire before his hearing. I use a fairly cheap EQ and an Alesis 3630 as a limiter in line with my headphone amp, set just before clipping.

A Compressor is not a bad idea IMHO, also prevents those POPS when someone pulls/plugs in a cable and forgets to tell everyone...

tINY 30th September 2009 01:19 AM

Quote:

Originally Posted by Toolbag (Post 4627334)
Guess I hadn't cleared up that I did my research to recognize the differences between the HPs that are blowing (Sony) and the one's that aren't (AKG and AudioTechnica's).
Anyway, here's the result of data I collected before posting:
..........................Freq.......Imped..Max Power...Sensitivity...Driver
AKG K240.............15-25kHz...55......200 nW.......91dB/mW....40 mm
AudioTech D40fs...20-28kHz...66....1600 mW.....102dB/mW...40 mm
Sony MDR7509HD....5-80kHz...24....3000 mW.....107dB/mW...50 mm



tINY: hat's what i was going to do, but thought it best to see what the consensus from my other techies would be. I'll test both in series and parallel and see which sounds better
I'll probably stick em inside the TRS connector if I don't get a "Don't do that because.." response



That's a big difference. I bet the sensitivity is voltage sensitivity too...

You may need to get a jumbo connector if you add resistors in series - with a 20 ohm series resistor, you'll probably want to use the 1 watt size. You could add 50-100uF caps in series for a HPF, but that would be best done at the cans where you have more room.




-tINY


SANDS 30th September 2009 01:30 AM

Wow, how are you blowing these things? I used to run a rap studio and that is all I ever used and still use at home. A few engineers that worked at the studio used to leave them on in the booths and forget about them cranking them at unreal volumes for long periods of time everyday. Never once blew one up.(Still made me mad though) At one time I think we had 15-20 pairs of theose things at the studio never a blow up.

I'm curious what headphone amp you are using?

I still use the 7506's and just my roomate just got a pair of the Equation rp21's I think they are called, really cool too. 7506's still beat them on volume, but they will kill your head after long use, not very comfortable, but make the cut everytime.

Toolbag 30th September 2009 02:39 AM

Thanks for the added discussion.
Like I said, I have no clue how they're blowing them, but I wish I knew. Different assistants on different sessions. The assistants use the PrivateQ (MYTEK DIGITAL USA) 99% of the time, so limiters and filters on the inputs won't work. I can't run 12 limiters/filters just for headphone mixes in every room. The assistants have been using the PrivateQ for the past few years with the AKG and AudioTechnicas headphones and not blowing them at all. So the issue is obviously not the amp or patches or feeds or anything other than the headphones.

I used to run Sony 7506's all the time in a multi-room facility running que's off of Hafler 6000 amps and only blew one driver every month or two. I figured the 7509's had higher power ratings so I'd have no issues. WRONG. Must be the 63Ω impedance on those 7506's and only 24Ω on these damn 7509 headphones. Everything keeps pointing back to the low inpedance.

At the next staff meeting I'll explain to the assistants that it's easier to press "st mix" in the center section of the console and use a regular HP box than to patch the mix and a talkback feed into the PrivateQ when 80% of the session work with just the st mix to cues. (I assume the 100Ω power resistor in the que box will help the Sony 7509's in the same way that tINY and I have been discussing)

Thanks again guys!

rodabod 3rd October 2009 04:02 PM

Amp distortion (which will occur when sinking a lot of current into lower impedances...) will upset the drivers. That's often why speaker manufacturers recommend amp power ratings for speakers; so that they don't clip and pass square waves into your drivers.

tINY 4th October 2009 07:10 AM


That's a good point. If the amp doesn't have enough current output capacity, the headset will clip earlier.

But, excessive HF energy doesn't usually blow up full range drivers (just the HF device in multi-way speakers).

Anything is possible, though...




-tINY


ripple_fx1 5th October 2009 02:30 PM

Have a listen to the cue system while in operation - there may be some horrifying transients created by the talkback or or other switching that you're not aware of - dropping the lifters on the tape machine while cueing or scrubbing a waveform may be creating massive HF or LF energy too. Any chance somebody is patching/unpatching the cue system while it's hot? Cheers! JR

JohnRoberts 5th October 2009 04:30 PM

While adding resistance in series with any transducer is not a recipe for highest fidelity, it was common practice in studio headphone systems. To get a decent signal into hi or lo Z cans, just use a modest sized power amp and dropping resistor build outs at every jack. That way parallel cans would load down the output and high z cans get more voltage than lo z cans.
-----
Current limiting, if not perfectly symmetrical could introduce a DC component into the headphone driver that is undesirable. Lower power headphone systems often cap couple to avoid this.

It seems the simple solution is to add resistors... I think I used 200 ohms or so in some build out boxes I did a couple decades ago.. 100 ohms should surely help, and do less sonic damage.
----------

Note I recently read about proposed EU legislation to limit SPL in personal playback systems. Can studios be far behind?

JR

Jimbo 13th September 2012 09:49 PM

Quote:

Originally Posted by JohnRoberts (Post 4647190)
While adding resistance in series with any transducer is not a recipe for highest fidelity, it was common practice in studio headphone systems.

Sorry for hitting ye olde thread, but I'd like to know why adding resistance in series is not a recipe for high fidelity?

Thanks for 'splaining!

Lotus 7 14th September 2012 04:55 AM

Quote:

Originally Posted by Jimbo (Post 8260538)
Sorry for hitting ye olde thread, but I'd like to know why adding resistance in series is not a recipe for high fidelity?

Thanks for 'splaining!

The impedance of a transducer like a headphone or a speaker driver typically varies with frequency*. Adding any series resistance means that the drive power will change with frequency because the series resistance will provide a different amount of attenuation as the frequency varies. That will significantly change the apparent tonal balance of the sound.

The second factor is that tranducers (like headphone drivers) really need to be driven from a source impedance that is much lower then their own intrinsic impedance in order to be properly damped. Adding a series resistor dramatically raises the source impedance and virtually eliminates the proper damping of the transducer usually resulting in "muddy" and "boomy" bass frequencies because of the internal resonance of the driver.

* A published impedance curve for the Beyerdynamic DT-800 headphones shows an impedance curve that varies from 55 ohms at 2kHz to 220 ohms at the LF resonance point of 95 Hz. Having an impedance curve that changes by a factor of 4 to 10 over the audio band is not uncommon.

Richard Crowley 14th September 2012 01:54 PM

Quote:

Originally Posted by Lotus 7 (Post 8261527)
That will significantly change the apparent tonal balance of the sound.

Would that even be perceptible to people who have blown their ears from years of those kinds of excessive SPL?

Jimbo 14th September 2012 02:26 PM

Thanks for the well written explanation L7.

JohnRoberts 14th September 2012 02:32 PM

Using build out resistance in studio headphone networks is pretty common, so apparently acceptable for monitoring.

Answer above is accurate regarding my fidelity comment.

JR

Lotus 7 14th September 2012 04:08 PM

Adding 100-150 ohms to pad down a set of low-Z set of phones like a MD7509 can result in an approximate frequency dependent change at the phone's resonance peak of about 4 to 5 dB. This, of course, is in addition to the already far-from-flat response of most phones, so although the tonal balance change is easily audible, it probably doesn't matter much to most players. The bass "muddiness" might be a concern for a drummer who wants to hear his/her sound accurately mixed.

Lotus 7 14th September 2012 04:12 PM

Quote:

Originally Posted by rcrowley (Post 8262323)
Would that even be perceptible to people who have blown their ears from years of those kinds of excessive SPL?

Sadly, those players probably don't want any pads added to their phone. Those poor souls are the people who keep asking for "more phones" until the drivers or their heads go up in smoke.