Crack with low impedance cans?

[MagicCat]

Hi all,

Just wondering who is using the Crack with low impedance cans, and if so how have you done so?

Best,

David.

[Laudanum]

Crack is really designed for higher impedance cans.  Originally 100 ohms and up but Doc modified the recommendation to 200 ohms plus.   I know some have used it with lower impedance phones but it's best with hi impedance cans.   It sounds pretty good with a pair of 64 ohm M-Audio phones that I have (and like) and do use with Crack sometimes,  but it's clearly better with my high impedance Beyers.  Not overly impressed with a pair of lower end,  lower Z Senns that we own and the Crack doesnt endear me to those phones any more that the soild state amps do but I cant blame that all on the amp.  Crack does do fine with my AKG's but they are 120 ohm.     

Using a tube like the 5998 on the output lowers output impedance of the amp a bit (down to around 70 ohms as opposed to around 120 I think)  but still not an ideal match with low z cans.   I know the guys were working on a modification to the circuit but I dont know if that is/was intended to make it an amp for low z phones.   You may want to consider the SEX amp.  It fits the bill for both low and high impedance phones.   I think you mentioned Grados in another thread.  Maybe someone who has tried them with Crack will chime in. 

[MagicCat]

Thanks for your input, much appreciated. My only reservation about buying the s.e.x amp is the cost. I'm sure this is warranted, it just may price me out of a purchase, or asking for it as a Christmas gift.

[Grainger49]

The SEX is not just a headphone amp, it drives speakers through output transformers.  It produces about 2WPC which is more than enough for my speakers in my room.

So, for the additional cash you get a headphone and speaker amp.

[Laudanum]

MagicCat ... Go to advanced search and type in Grado.  Search only the Crack forum.   It turns up several threads that may be very useful.

[MagicCat]

Thanks to you both for the info. I hadn't realised it was also an amp  I'll give the search function a whirl also.

[Paul Joppa]

Because the Crack has no output transformer, it is not well matched to low impedance phones. There is still work going on to increase the current through the 6080 which will enable it to handle lower impedance phones, but only slightly lower. I estimate it will revise the recommendation to 150 ohms minimum instead of 200 ohms, but the actual number will depend on listening tests after we've built the final version.

SEX was originally conceived as a speaker amplifier. At the time it came out we had no headphone amp at all, so we added that feature (and spent some extra effort to make it quiet enough for headphones!).

[MagicCat]

Because the Crack has no output transformer, it is not well matched to low impedance phones. There is still work going on to increase the current through the 6080 which will enable it to handle lower impedance phones, but only slightly lower. I estimate it will revise the recommendation to 150 ohms minimum instead of 200 ohms, but the actual number will depend on listening tests after we've built the final version.

SEX was originally conceived as a speaker amplifier. At the time it came out we had no headphone amp at all, so we added that feature (and spent some extra effort to make it quiet enough for headphones!).

Thanks for taking the time to reply Paul. Is there an easy way I can modify the Crack to run some Grado 225i's without altering the sound too much? What I was thinking was asking for it to be supplied with the necessary components to do so; providing the cost isn't radically different (?)

[Doc B.]

If there was an easy way we would offer it. But the OTL circuit simply has a high output impedance. One could experiment with adding an output transformer after the cathode follower. But we have not done so, and in the end you would end up with something that probably costs close to the S.E.X. amp and doesn't sound as good.

[Grainger49]

Here is Dan's recommendations on amplifiers given headphone impedances:

http://www.bottlehead.com/et/whichheadphoneamp.htm

The above link mentions the Smack but it is in semi-retirement.  I would ignore it right now. 

I guess it is coming down to whether you want to change to a high impedance headphone and to to the Crack or keep the low impedance Grados and have to pony up for the cost of the SEX.

Trust me, there are a lot of guys out there happy with the above combinations.  Crack was most appropriately named.  I think it is the best selling kit.  It gets the most posts.  That is just my guess.

[Paul Joppa]

I looked but did not find any off-the-shelf transformer that would be suitable - and as Doc B says, it is unlikely to sound that great anyhow.

About the only non-transformer method would be a resistive divider or L-pad. Using 180 ohms in series and 22 ohms in shunt (for example) would give an output impedance of around 20 ohms, which would suit the Grados and other low impedance phones reasonably well. And it would present a 200 ohm load to the Crack, about as low as you'd want to go. However, it would cut the available power drastically, to maybe one mW instead of 100mW. That makes it hard to recommend!

[MagicCat]

Thanks so much for your help on this guys. I guess my option is the s.e.x. Kit then 

[Wormwood]

I am in the same camp as MagicCat at this point in time - I have a desire to build one of these units but I 'only' own Low Impedance - AKG Q701 & AD700. Of course I will be aiming to trundle down the High Impedance path as time moves on.

So is the S.E.X the only option open for me? Will this unit keep me if I do get High Impedance phones?


Cheers and thanks for a wonderful resource!


Wormwood

[Noskipallwd]

I have the Crack and the S.E.X. both, I highly recommend either amp. The low imp. phones I own are; AKG702, HiFiMan HE-400s, and modified Fostex T50-RPs. They all sound great on the sex, surprisingly the Crack does a decent job driving the HE-400s and T50-RPs, most likely due to their high sensitivity, but by no means is it optimal. The S.E.X. does a great job with my high impedance cans as well. I am loving the HE-400 and S.E.X. combo, the amp and the phones go well together. Personally I would have no problem with the S.E.X being my only amp.

Cheers,
Shawn

[Laudanum]

Wormwood ... I dont own the S.E.X amp so cant comment on the sonics but everyone seems to love them.   And the versatility is great with the amp being very well regarded with both efficient speakers and headphones, both high and low impedance.    So it's not wasted if you purchase high impedance phones in the future.   The problem with high impedance phones is that there arent many of them available.  Used to be lots of choices, now, not so many.   AKG has gone to lower impedance phones leaving Beyerdynamic and Sennheiser as the two big names in high impedance phones.    Koss still makes a couple pairs but arent super popular and I think Fischer audio still has a model.  Maybe some companies I dont know about.  There is always used/vintage though.  For example, there are tons of used AKG high impedance phones available.   The fairly recently discontinued AKG K601  still seems to be available new in abundance.  Mixed reviews on these and maybe just a little low z for Crack at 120 ohms.  I have one, a little light on the low end but I love it with live recordings.

Just some thoughts.