AKG & Crack

[fewjr]

Hello All,     
     I'm sorry for another headphone question. I just would like to know one thing. Where is the cutoff for what would be considered low impedence or high impedence headphones? I have a pair of AKG K240 mk II's, rated at 55 ohms- 91 dB SPL/V. I know from what I've read so far that the SEX is a better choice then the Crack for low impedence headphones, but it is out of my price range at this time. I just want a nice headphone amp real soon and look forward to building something. I really enjoyed putting my Seduction together. Thank you in advance folks.

fewjr

[Grainger49]

Some folks are enjoying low impedance headphones with Crack, so regardless of the official cut off you might like it.  You might not.  Sadly it is a personal taste thing.  Well, maybe not so sad.

You can also improve Crack's performance with low impedance headphones by increasing the output capacitor value.  IIRC it is now 100uF@160V.  But check it.

I'll bet there are other tweaks that help too.

[Doc B.]

The output impedance of Crack is about 120 ohms. So our suggestion is to use the Crack with headphones of at least 120ohms impedance or higher for optimal performance. You can build the kit stock, then upgrade the output tube to a 5998 when budget allows. That will lower the impedance to around 75 ohms, and be a better (if still not optimal) match for 55 ohms phones.

Crack sounds very good to some folks who have used it with lower impedance phones. I have to stick to my recommendation because otherwise someone will post somewhere that we don't understand impedance matching. If the sound pleases you, that is all that really matters.

[fewjr]

Thank you for the quick reply. Sorry I didn't research more on my own. That said, what you say makes plenty of sense to me. I believe I'll be fine building the Crack and seeing how it sounds with my present cans. I guess I have a good excuse to pick up another pair of headphones now. Thank you much! Doc...I'll place my order when we get back from vacation.

fewjr

[blake]

I'm currently building a Crack for my AKG240S  55 ohm (it's taking longer than I'd like as time for building it has been hard to come by lately). I'll let you know how it works out. Worst case scenario is that I have an excuse to buy a new pair of headphones :-)

[tdogzthmn]

You can always buy a K240 M on ebay which is the older 600 ohm version of the K240.  They can be found mostly for under $100.  I also have a K400 which is rated at 120 ohms, maybe it will be more compatible since it matches the output of the amp?

[Paul Joppa]

Just to reiterate, most headphones regardless of impedance work well with a 120 ohm source impedance - it is after all the approved (if somewhat obsolete) standard. A few low-impedance phones get all wooly and flabby unless the source impedance is lower; tose are the ones that do not do well with Crack. Those also need some adjustment in the 120-ohm resistor that is in the SEX amp; the difference is that the SEX amp can be adjusted to a much lower source impedance if needed, while Crack cannot.

[blake]

So I got it up and running earlier this week (I'm working on finishing the base at the moment). My AKG 240S work great on it - no complaints.