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DT880 wild impedance difference in channels

post #1 of 5
Thread Starter 

Little background : I'll try to keep it brief, I've been getting back into audio more heavily & focusing on headphones, not a full-size rig like my previous system.  I bought a pair of DT880s, "600Ohm" off of eBay a few months back, and an Objective 2 from a member here. 

 

Wow, great bass, awesome clarity, overall huge improvement from my last loudspeaker system. 

 

I get the tube itch, and buy a Bottlehead Crack assembled from a member.  Whoa....

 

The Problem!  Big time channel imbalance with the Crack!  Left is way louder.  I measure everything, spend 5 hours debugging the Crack.  DT880s still sound balanced on the O2..hmm...

 

Then I finally ohm out the DT880s:  578 ohms on the right to 47 ohms on the left channel measured at the connector. 

 

How can this be??? Really poorly matched 600 & 32 ohm drivers swapped in by the previous owner? 

 

I haven't opened them up yet...the channels sounded the same on the O2.  Maybe I have a tin ear!

 

Should I be calling Beyer up right now and ordering a driver?  (and trying to get $$$ back from my eBay seller...)

post #2 of 5

How are you "ohming them out"?  Impedence is not the same thing as resistance... and hence can't be measured on a Fluke.  That being said, they (resistances) still shouldn't measure THAT different from one side to another.

 

Do you have another headphone(s) you can plug in and test with that amp?  Do channel imbalances follow the headphones or the amp? 


Edited by kramer5150 - 1/11/13 at 9:45am
post #3 of 5
Thread Starter 

Pardon the loose vocabulary, you're right of course: resistance at DC measured w/ a DMM.  From the tip to the sleeve/common & ring to sleeve...(now I forget which is supposed to be left/right, don't have them in front of me).  They were very different!

 

The difference follows the amp, I can plug in my ATH-M50s, or a pair of ear buds to the Crack and they sound balanced (and ohm out as matched too!!)

post #4 of 5
Thread Starter 

Closing this one out, at least for my own sanity. 

 

Opened up the Beyers to replace the driver, desoldered the driver and the diodes and found something odd -- the diodes were acting as a ~50 ohm resistor.  Their breakdown voltages were .02 and .002 volts.  Not sure how that happens.  Running with no diode on one side now....

post #5 of 5
This Crack must have huge output impedance.

Voltage across driver and diodes is proportional to 50/(Zout+50) on the damaged side and to 600/(Zout+600) on the undamaged side. If Zout is low (as in O2), both sides are close to 1. If Zout is high, they get closer to 50/Zout and 600/Zout - not exactly equal.

I'd check this amp for high DC offsets, particularly at power-up and power-down. Something must have fried these diodes.
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