Something new is coming

[Paul Birkeland]

I know I will regret asking, I am in danger of becoming an addict:

Has it been tried with low sensitivity AKG 240Monitor phones? The autoformer Quickie works with my 240M cans, but just barely.     

 

I managed to score some K240M's locally not too long ago.  I just plugged them into the first prototype, and you will certainly have more power than you can use.

[vuntruong]

Would it be wise to order 2 Crack and use them as a balance heaphones amp? 

[xcortes]

I just ordered mine! I'm getting fond of headphone listening and the coolness factor of the Crack amp is something you just can't pass on.

[Doc B.]

If I can get it by 3/17 that would be awesome.

It's probably going to be a week or two longer than that before we can ship. This new kit has a new chassis plate design and those typically take about three weeks to receive from the laser cutter.

[Paul Birkeland]

Would it be wise to order 2 Crack and use them as a balance heaphones amp? 

I have tried this before, and I wouldn't discourage such an attempt, but I would warn that the 6080/6AS7 is notorious for having pretty poorly matched sections. 

In a single ended circuit like the Crack, this is not particularly bothersome.  In a differential circuit, it will be far more pronounced. 

[arveedub]

What would you recommend as the minimum impedance for a set of headphones to use with the kit?

[dmannnnn]

Just ordered mine.  Happy birthday to me.  If I can get it by 3/17 that would be awesome.  Has anyone come up with a crossfeed circuit for it yet?

Here are a couple of projects I have been eyeing.

http://www.tangentsoft.net/audio/mlxfeed.html

http://www.meier-audio.homepage.t-online.de/
click 'crossfeed filter' next to 'projects' at the top of the page.

[Doc B.]

What would you recommend as the minimum impedance for a set of headphones to use with the kit?

The output impedance of the 6080 cathode follower is around 100 ohms. So in theory the most ideal load will be 300 and 600 ohm headphones. You could push this down to 100 ohms and just expect somewhat increased distortion - in fact we tried it with 32 ohm phones and it sounded pretty darn good, but that is not the intended usage. The current wisdom seems to be that a S.E.X. amp with the 120 ohm resistors removed from the jack works best with low ( like 32 ohm)  impedance headphones. The curves of some 6080 relatives might give a little lower output impedance in the given circuit, but I have not pursued that yet. Remember, this kit is only about 10 days old.

[Jim R.]

You can also try increasing the output coupling capacitor value to get a little more mileage out of some borderline cans.

So Doc, where you expecting so many orders for a kit that is only 10 days old and has not had a lot of pre-release anticipation built up around it?  Pretty cool if you ask me.

-- Jim

[Doc B.]

Definitely cool. It's interesting to see more and more folks go for higher impedance cans these days. Since PB brought up this idea I been looking for precedents and in the process had noticed that there had been some attempts to revive a similar but different kind of 6080 based OTL headphone amp by some folks over on Head-fi that ultimately weren't able to happen for reasons of cost. The fact that we could use mostly parts that are already in stock (including an existing, great power trans design by PJ that we have made in quantity) allowed us to put together something for a really low price. Perhaps the word will get out over there that we are offering something for a very low price that is easily tweaked and could fill the bill.

[ironbut]

Hey, funny you should mention it.
I started a thread over there yesterday. You should get a few early adopters from there. I've noticed an undercurrent of interest in DIY on Headfi lately and I think this'll be a dandy kit for those folks to get their feet wet.

https://www.head-fi.org/forums/f5/crack-bottlehead-otl-476650/

[mrarroyo]

Hi, my name is Miguel and I saw the post in Head Fi on the Crack and came over to read about it. I have soldered in the past, most of the time successfully but I have had a couple of mishaps w/ circuit board type of amps. Anyways I am looking forward to this new amp but I have a few questions if I may.

1. Will the aluminum plate be pre-drilled to accept the components, plus will it be sized to fit in the wood base.
2. Will the wooden boards be pre-cut and ready for assembly?
3. How much work to finish the wood? I like to use beeswax w/ a lite stain on many of my projects.
4. Does the wood bass come in pine or just alder?

Thanks.

[Paul Birkeland]

1. Will the aluminum plate be pre-drilled to accept the components, plus will it be sized to fit in the wood base.
2. Will the wooden boards be pre-cut and ready for assembly?
3. How much work to finish the wood? I like to use beeswax w/ a lite stain on many of my projects.
4. Does the wood bass come in pine or just alder

The aluminum plates are laser cut and ready to go.  No machining is required. 

The wood pieces for the base are cut and have a groove for the top plate to rest in. 

Finishing the wood is completely optional.  Plan on maybe a little sanding, then stain away. 

The wood bases are alder only.  A Google search may find wood workers who have made Bottlehead bases in other finishes, and sometimes they offer custom bases for a fee. 

[roger_s]

Having only four headphone amps (including my S.E.X.) I went ahead and ordered a Crack.  It only makes sense with three pairs of Grados, a pair of HD650 Sennheisers, and a pair of AKG 702s lying about...

Good work, Doc.

[Grainger49]

 .  .  .      4. Does the wood bass come in pine or just alder?

Thanks.

Just to add a little.  Think of alder as Pacific Northwest pine without the pitch.  It has prettier grain than the pine I am used to in the South.