Crack modding - for lower impedance headphones?

[Walther]

Hello folks,

I would like to know if the Crack headphone could be modified a bit to support my beloved AKG-K271 mark II headphones at the impedance of only 55 ohms without a significant loss of audio quality. I do know that the Crack amplifier is designed for the higher impedance cans, but I beg you pardon my scepticism towards the S.E.X. amp - I don't like compromises. I have read about the upcoming Smack amplifier, but again, I would rather order a Crack kit because of its age and popularity - I know that if I'll have a problem, someone will help me to sort it out, and in case I want to modify it, again help is easily found, whereas the Smack will be hot and new but less tested and experimented on by the wide community.

I hope nobody gets me wrong, I definitely do not want to question the quality of Bottlehead products, I just want the best of the best

So, is it possible to modify the Crack amp to not only play sound on my AKG's but to get close to the quality of the higher impedance setup?

Another question is related to the other mods, particularly the capacitors. From what I've read from the forums, I might want to add some capacitors to improve the sound quality - but I have no idea where to connect them and what kind of capacitors to buy. Of course, I don't have the amp yet, but then again, it would be nice to be able to order stuff asap to be able to build them in right away with the kit. The speedball upgrade seems very tempting too - I would appreciate some comments on its effect on the sound.

Phew, quite a long post for the first post. Small disclaimer in the end - you might have already noticed that I'm not a native English speaker. I hope my English is good enough for you to understand my points though.

And hey, many thanks in advance for the great, helpful replies!

--
Veeti Haapsamo
Magician

[Grainger49]

Walther, (you didn't sell model RR kits did you?)

Lookie here:

http://www.bottlehead.com/smf/index.php/topic,526.60.html

This gives a nice look at the capacitance needed for different headphone impedances.  Since Crack is intended for higher impedances "reply #69" on this page shows that a 50 ohm headset and a 470uF capacitor yields a 3dB down point of 7 Hz.  That means it is essentially flat at 70 Hz, a decade above the 3dB down point.

A 4700uF cap will yield a flat response at 7 Hz.

[Doc B.]

Hi Veeti,

Your English is, in fact, very good. Let me address some of your concerns. First, the Crack will function with AKG-K271s. But it will not be completely optimal. There is no easy way to lower the Crack's output impedance to the level necessary to make 55 ohms headphones sound their best. I understand your interest in the Crack as a proven design vs. the Smack as a new design. But unfortunately one thing that has been proven over time is that the Crack is excellent for high impedance headphones and not always the best match for 55 ohm headphones.

Now, if I was going to modify the Crack to be optimal for 55 ohm headphones, I would first change from a cathode follower output to a lower impedance type of output topology, an output transformer. This is used in the SEX kit, but with some compromises to allow the amp to also drive speakers. If we take away that requirement of being able to drive loudspeakers we have gained the freedom to use what we feel to be our most premium circuit, the one used as the driver stage in our long established Paramount amp, with some minor modifications to optimize it for headphone use.

That is what the Smack is, a proven circuit topology that will make K271s and other medium and low impedance headphones sound much better than the Crack will. I could just tell you Crack is fine and maybe get a sale today, but I really think the best choice is to wait a few weeks while we get the Smack kit ready to sell. Those who have heard the prototype will hopefully offer their impressions.

[Walther]

Wow, that's some really fast replies!

Now that I've been thrown some technical details, I might go for the Smack amplifier after all. Originally, another point for the Crack side was the ability to run the higher impedance cans too - if/when I will upgrade my headphones. Though, I've been relatively happy with my AKG's, and I think I'll spend my next pile of money on the upcoming Bottlehead DAC rather than a pair of HD600's or similar.

So, a couple of weeks until the release of the Smack - wonderful! I'll be one of the first in the line for that.

ps. No, I'm not towards guns, my second name just happens to be Waltteri, and I use the English variation of it as my nick all over the internet - with the H stolen from my last name

[Doc B.]

Smack will have settings for 16/32/64/128 ohms so it will work with just about any headphone impedance. The Crack still has just a slight advantage sonically with headphones above 200 ohms. And it will probably be more like four to six weeks rather than two before we can begin selling Smack - a few final details still need to be worked out before we can order chassis plates, and they usually take at least three weeks to be made.

[braubeat]

Tubesandmore.com sells a "collins replacement" transformer for $26 which advertises 600 ohms to 8 ohms with good bandwidth. Probably not optimum but cheap and easy to experiment with.

Michael