Sennheiser HD660S on Crack

[L0rdGwyn]

First time poster over here on the Bottlehead forums.  Just wanted to bring up a topic that has been on my mind the past few days which could have quite an impact on Crack users wanted to hear your thoughts.

Sennheiser just announced their HD660S, the "successor" to the acclaimed HD650.  I, for one, am very interested in picking these up.  Before the announcement, I had planned to eventually pair the Crack/SB with my HD650's, a popular choice I know.  I am a big DIYer, but haven't jumped into building amplifiers yet, was just getting ready to pull the trigger when this bomb hit!  I know the topic of the Crack and headphone impedance has been discussed ad nauseam, but still, I am wondering how these would pair with the Crack given their lower impedance of 150 Ohms.  I had read Doc had suggested headphones greater than 120 Ohms would pair well with the Crack.  For those with experience, is 150 Ohms cutting it too close to the output impedance?  Is there a way to alter the output impedance of the Crack without significantly affecting performance?

Thanks in advance for your replies!  Given that Sennheiser has called the 660S a "successor", it almost implies that the 650s will be discontinued.  I also wonder what that could mean for the fate of the 600s.  It would be a shame for such a great HD6XX/Crack pairing to go extinct, here's to hoping the 660S can become a worthy replacement.

Cheers!

[Doc B.]

There are thousands of HD600s and HD650s around. The smart money would be on buying a pair for peanuts right now as people sell them off to get the 660 before the other kids on the block.

As to whether the HD660S will work well with Crack, how could we possibly know without trying them?

[L0rdGwyn]

Oh, I wasn't asking anyone to tell me how the 660S perform, just wondering how headphones near 150 Ohms perform with the Crack, generally, hoping to infer the likelihood that they might be well-matched.  I don't own an OTL amp (yet), just wasn't sure how far down you can run the damping factor before you run into audible issues.

[Paul Birkeland]

While we do recommend headphones greater than 120 ohms, there are plenty of folks who have used 60-100 ohm headphones and are pleased with the results.

[Doc B.]

If we say that headphones above 120 ohms will probably pair well and these new headphones are 150 ohms, then we have given you our best guess.

My advice is to listen to a pair before you make a decision. The nominal spec is just not enough to determine if you will like them. I remember the first time I heard the HD700 a few years ago. That memory is kinda like remembering the first time I got a filling. They sound so different than the HD600/650 that one couldn't begin to use one as a basis for buying the other. Though Sennheiser markets and prices them as a headphone that fits in the product line between the HD650 and the HD800.

[L0rdGwyn]

Well thank you both for your thoughts, I appreciate it.  I plan to give them a listen beforehand, early impressions from those that have heard them are calling them a "more resolving" HD650.  Looking forward to my Crack/SB build, going to do a socket rewire to roll my collection of 5670 spec tubes.  Assuming I do purchase the HD660S, and when I build my Crack, I'll post some impressions.

Cheers!

[Paul Birkeland]

going to do a socket rewire to roll my collection of 5670 spec tubes.

The 5670 and the 12AU7 are very different tubes.  The 5670 has about twice the amplification factor of a 12AU7. As it is, the Crack already has such ample gain that we provide instructions for padding down the attenuator in the Crack FAQ.

The 5670 will also bias differently, and you'll find that the plate voltage that you get with the Speedball is way too high.  While the plate voltage in the stock circuit would be a bit lower, the operating point that you'll end up with will have poor performance (increased distortion).

[L0rdGwyn]

Understood, thank you for letting me know.  If that is the case, I will stick to the 12AU7.

[L0rdGwyn]

Just wanted to add one more thing here: it is widely believed that the HD660S driver is a variation of the HD700 driver, based on its appearance and the 150 Ohm impedance.  So, the performance of the HD700 on the Crack may be a good approximation of the HD660S's future performance.