What?! Another headphone amp kit?! Smack me!

[Doc B.]

We just did the test firing of a new headphone amp prototype that I plan to take to the Bay Area Head Fi meet this weekend. The amp is designed as a complement to the Crack OTL headphone amp for high impedance headphones. This new design is a WOT (With Output Transformer) design that has been developed for low impedance headphones. I decided to call it Smack. It uses two 5687 tubes. One half of each 5687 is an active loaded single gain stage that is parafeed coupled to an output transformer. The prototype has a single tap of 32 ohms, however we might alter the production transformer design so that it could be configured for 32, 64 and 128 ohms. The second half of each 5687 is used in an active loaded hybrid shunt regulator. The prototype has been built on a S.E.X. amp chassis and uses the same PT-3 power transformer used in Crack, S.E.X. and Foreplay III. The gain stage and shunt reg use the new Soft Start PC board that we use in the new Paramount.

Unfortunately I only have some inexpensive Sennheiser 201s here today to try the amp with. But even with those it's showing promise. It's dead quiet, seems pretty quick and there seems to be pretty good bass for an amp that's about 2 minutes old. Gonna burn it in a bit and we'll see what folks think at the meet. Before anyone asks, I don't have a firm price. But it would probably hit somewhere in the $350-$400 range.

[Spinifex]

That's great! But what would be the differences between this one and the S.E.X.?

[Doc B.]

Well, we just finished this amp about 20 hours ago so I can only make judgments that may change as we further refine the circuit. Right now we are exploring the best parafeed cap value. We have increased it from 3.3 uF yesterday to about 13 uF this morning and the bass is now about as deep as a SEX with MQ iron, and it seems a bit better controlled. The lower mids seem a little better balanced now too, yesterday the Smack seemed a little too bright to me. The S.E.X. may be winning in the midrange magic category and it might be a little bit more sweet on top. Might also give a little better sense of depth.

From a technical standpoint, of course the S.E.X. is much more versatile in that it can run efficient speakers as well as headphones. The Smack has theoretical advantages for headphone listening in that it is a single gain stage amp and it is shunt regulated. And if we decide to go into production on the output transformer with the higher impedance taps that might make it a little more universally good for both lower and higher impedance cans. All 6DN7s seem to sound equally good so tube selection is pretty easy for S.E.X. The 5687s and 7119s that work in the Smack seem to come in different flavors and it may take some rolling to find a pair you like. Some like that opportunity to play around, others don't want to bother. S/N may be a bit better in the Smack. I will need to cut the HVAC in the building to make a judgment about that, as the noise floor is pretty low in both amps.

[hasafraker]

omg you're the devil... I'm working on a little headphone amp for work at the moment but my guess is I might just have to try some smack... if only my speedball would ever show up   ...but I must say that my crack is hands down the best sounding headphone amp I've listened to. I might have to offer build services to some of the guys at work after I take my Crack to work and give them a little taste.

[Doc B.]

A little followup here - I took the Smack prototype with me to the Bay Area Head-Fi meet the weekend before last. It was very well received and the general consensus was that the amp did indeed work very well with Grados and other low impedance cans. So I think this one is a "go".  We will start to refine the layout a bit and see just how fast we can bring this puppy to fruition.

One question that was asked a lot was how Smack fit in the line relative to S.E.X. IMO this is a little bit better headphone only amp because the single gain stage and the shunt voltage regulation give it a touch better resolution. Of course the advantage of the S.E.X. kit is that it can run headphones and speakers, so it is quite a bit more versatile.

[xcortes]

You said before you were using a pair of Stereomour opts. Those are 4k to 8 ohm, right?

I ask because I have a pair of Nickel Exo 45s waiting for a project (would be drop in!) and two sets of low impedance cans begging for an amp.

No plate chokes, right?

:^)

[Doc B.]

The Stereomour output transformers can be configured as 4Kohms : 2/4/8/16 ohms, and 8K ohms : 4/8/16/32 ohms.

[xcortes]

And in this case you're using?

[Doc B.]

We wired the Smack in the 8K:32 ohm configuration. But I think we will go ahead and shoot for a custom version of this transformer with additional higher impedance taps for the production version.

[xcortes]

Thanks for the answer.

Makes sense.

[Jim R.]

Dan,

At the Bay Area meet, did anybody try a pair of the audeze lcd-2s with this amp?  Do you think this will be an appropriate amp for the audezes?

Thanks,

Jim

[Doc B.]

I think the LCD-2s need a lot of power. I have posted elsewhere that I have yet to hear an amp that does them justice. I haven't had a chance to try Paramounts with them, but that may be the best bet.

[Jim R.]

Dan,

Thanks.  I have seen posts where people say these will work directly from their ipods, so figured that maybe the s.e.x or the smack would work too.  And while they may work, I'm getting that the full-on experience will only be realized with a higher power amp.

Maybe I'll just keep the hd-800s as my cans to shoot for and then the crack will be the best answer ther.

-- Jim

[Armaegis]

I may be misreading something here, but would it be possible to incorporate a switch to easily switch between the 32/64/whatever taps?

[Doc B.]

That is the plan.