[lousyreeds1]

Hi all,
 
I know this is well-worn subject, but I'm hoping someone can help me understand what may be causing my Grado HF-2 headphones to hum terribly when connected to my Coincident Dynamo 34SE amp (scroll down the page a bit for details on this model).  The Grados have an impedance of 32 ohms.  The instructions for the Coincident read as follows: "The headphone amp input impedance is 300 ohms.  Changing the value lower will diminish amplifier power and will be sonically inferior as well as causing hum."  Have I "changed the value lower" by connecting low-impedance headphones?  Is Coincident saying that I can only use high-impedance headphones with this amp?  An email to the manufacturer yielded the following, which confuses me more: "The impedance should be about 150 ohms. The amp can power any suitable headphones."  
 
I had hoped to move to a HifiMan or MrSpeakers model in the near future, which have nominal impedances of around 25-50 ohms.  Before I start experimenting with different options, I'd appreciate your collective wisdom here on what's going on, and whether I should be limiting my search to only high-impedance headphones.
 
Thanks!

 

[thatonenoob]

I'm not familiar with your amp or the HF2. However, I would look into the sensitivity of your headphones too. The amp input impedance is different from its output impedance btw. However, my gut is telling me that a low impedance high sensitivity headphone into a high z amp is causing the issue.

In general your headphones should have a higher impedance than 8 x your amps output impedance.

 

[Raketen]

I think by "change the value lower" that means changing the output impedance of the amplifier, not using low impedance headphones. AFAIK it's not unusual for speaker amplifiers to have somewhat high impedance on the headphone out. Not sure what would cause the humming, often that is the result of a ground loop somewhere in your system, but I don't have much experience with this stuff.

Some in-depth reading:
This page has a little section on impedance optimization: http://www.meier-audio.homepage.t-online.de/tipstricks.htm
This is more in depth by same author: http://www.innerfidelity.com/content/musings-headphone-amplifier-output-impedance

 

[lousyreeds1]

Thanks, all, I appreciate the help. Now I have this from the manufacturer:

"Output impedance is 300 ohms. Power output is 4 watts. Any headphones that are 150 ohms and 88 db sensitivity or higher will work well."

So does this take me out of the running for planarmagnetics from HifiMan, MrSpeakers, or Audeze? It would seem so. Are there any others I should be considering? I guess I can shift over to looking at Sennheiser and Beyerdynamic high-impedance headphones, but that's not my preferred sound.

 

[ProtegeManiac]

So does this take me out of the running for planarmagnetics from HifiMan, MrSpeakers, or Audeze? It would seem so. Are there any others I should be considering? I guess I can shift over to looking at Sennheiser and Beyerdynamic high-impedance headphones, but that's not my preferred sound.

 
Well the HD600 driven by an amp that can deliver a lot of voltage will sound closer to the HF2 driven by a decent amp than a HiFiMan on an amp that's falling short on what it needs.

 

[lousyreeds1]

Wait a second - I thought the HifiMan was more sensitive and easier to drive than the Sennheisers...  
 
And now I see this review, indicating that Grado headphones are a great matchup with the Coincident amp.  So confused...

 

[ProtegeManiac]

Quote:

Originally Posted by lousyreeds1 /img/forum/go_quote.gif

Wait a second - I thought the HifiMan was more sensitive and easier to drive than the Sennheisers...  

 

Where'd you read that? Every HiFiMan with model number 5 at 90dB down to around 86dB on the older models, and the HE300, HE350, HE400(i) are at 93dB. The HD6x0 and HD580 are at 97dB, the HD700 105dN and HD800 is at 102dB. Even if most amplifiers (barring primarily OTL tube amplifiers) produce more power at 32ohms than 300ohms, the if you're talking a sensitivity difference of over 3dB, the amp will always be able to get louder with what power it can dish out to the more sensitive headphone. Between the lower tier HiFiMans and the HD6x0, the extra could be negligible as the amp will get them loud enough still, likely without much more distortion vs driving the Sennheisers, but once you start comparing the HE-5 to the HD800, that gap is just too wide. Of course, being this easy to drive won't mean the HD600 will sound fantastic with a mainstream device with a 5mW at 32ohms output.

 

The exceptions are the HE400S and the HE1000, but again if you're talking in general terms then these are just two out of all the others they made.