Bottlehead Crack - A Custom Headphone Amp!

[Mikey]

Hi Gang,

This thread will document my assembly of the Bottlehead Crack OTL Headphone Amplifier.

Step 1 - Preparation:

The package arrived the other day, and the first order of business was to check that there were
no missing parts....everything was accounted for.  A few deviations from stock were planned right
from the get-go, so my next task was to modify the chassis plate to accomodate those changes!

They include:

- Vampire style gold plated RCA jacks
- Amphenol octal tube sockets
- Neutrik locking headphone jack
- PEC volume potentiometer

Once the chassis plate was completed, it (along with the transformer end bell) was sent to the
powdercoater for my typical black textured finish.

While waiting for the chassis plate, I painted the power transformer with a high-heat flat black
paint after cleaning the lamination stack with a rag soaked in laquer thinner.  A few strokes with
a Scotchbrite pad prepared the transformer for proper paint adhesion.

After painting the transformer, the wood base was glued up.  Once the glue dried, I ran the base
through my table saw to remove about 1/2" of overall height.  I also used the table saw to form
a 'reveal' along the bottom edge of the base, in an effort to make it look a bit slimmer.  The scraps
from this operation were used to make four blocks that were glued into the corners of the base.
These blocks are present to afford a place to mount four rubber feet.  The finished base was sanded
and given three coats of Watco Danish Oil for a killer finish!

Stay tuned, mechanical assembly is next....

Mike

 

[HF9]

Wow, can't wait to see what's next Mike

 

[Mikey]

Hi Gang,

I made a bit of headway since my last post, here are the results....

Step 2 - Mechanical Assembly:

As mentioned previously, the chassis plate and transformer end bell were powdercoated
with a black textured finish.  Unfortunately, this finish is very difficult to photograph, but
looks WAY better in person!

The various parts were attached to the chassis plate as decribed in the manual.  Stainless
steel button head socket cap screws were used in place of the supplied hardware.

Stay tuned, wiring is next....

Mike

 

[Grainger49]

Mike,

Wow!  The underside of the top plate looks as good as the top.  I'm very impressed with the drilled holes around the tube sockets.  There is just no way I could do that and have them come out perfectly placed like you did.

Your build thread will be a great for those who follow.

 

[STURMJ]

Looks great! However, I hope you have taken into account that some of the chassis hardware grounds to the plate. The paint (on the bottom side) may prevent a electrical contact from being made. If you got that taken care of; then onward!

 

[Grainger49]

I just reread the first post.  The top plate is powder coated.  As long as all ground wires are tied together it will be safe.  I think powder coating is a pretty high voltage insulator.

 

[STURMJ]

Cool. I have started using a black-spongy shelf liner, available at Lowes. on the back side of plates ( glued with contact cement ). Looks nice, and at least in my head, adds vibration damping.  And the best part is I can easily and cleanly remove discrete areas with a scalpel.

 

[HF9]

Looks great! However, I hope you have taken into account that some of the chassis hardware grounds to the plate. The paint (on the bottom side) may prevent a electrical contact from being made. If you got that taken care of; then onward!

In my experience with powdercoated plates, the electrical contact is made even without taking the time to expose the metal with a dremel. I've run resistance tests in a few different situations and found that to be the case at least.

 

[Grainger49]

I'm confused.  Are you saying that a meter from two points on the powered coated top plate will read no resistance? 

 

[howardnair]

grainger-yes i wonder at that also--i have never really paid any attention to powdercoating-but after your response some reading was in order--electrostatic particles that are either thermoplastic or thermosetting are sprayed on a object-the materials are polyester,polyurethane or epoxy based-i am not disagreeing with hf9 i am just amazed-actually the whole thing is amazing-the longlasting finish -sweeping up the overspray and reuse of it- the thickness of the coat--and one can do this in his shop with some practice and seems not much of outlay for tools -not sure how much the product is--more reading to do--

 

[HF9]

I'm disappointed.  Are you saying that a meter from two points on the powered coated top plate will read no resistance?  

No, but if you screw a solder strip into a powercoated chassis, the lug that's connected to the screw will conduct through the powercoat, probably because some of it is scratched away as you are tightening the screw. I just built the S.E.X. and everything screwed to the plate conducted to it without issue, no difference in resistance between dremelled areas and non. 

 

[STURMJ]

Sounds reasonable, but I'd still check just to make sure. It seems like one of those things that can be relied on

 

[Mikey]

Hi Gang,

I started wiring the amp this week, here are the results....

Step 3 - Wiring:

I wanted to show some of the wiring layout before it became obscured by some of the larger components.

Stay tuned, final wiring is next...

Mike

 

[intojazz]

Nice post and follow through. I've been following along and it looks great. What headphones and cable are you using?
regards,
John

 

[Mikey]

What headphones and cable are you using?

Hi John,

I've got a pair of JVC HP-DX1000's, and a pair of Sony MDR-SA5000's.
Neither is an ideal match for the Crack, so I've got my eyes peeled for some Sennheiser's...

Mike

 

[Mikey]

Hi Gang,

I finished up the wiring this weekend, here are the results....

Step 4 - Final Assembly:

Stay tuned, the listening test is next....

Mike

 

[Mikey]

Looks great! However, I hope you have taken into account that some of the chassis hardware grounds to the plate. The paint (on the bottom side) may prevent a electrical contact from being made. If you got that taken care of; then onward!

Hi Gang,

Just a quick note to follow up on the powdercoating issue...

In this case, I did not have continuity between the 'safety ground' and the
'signal ground' because of the coating on the chassis plate.  This resulted in
a low level buzz in the amp.  The issue was resolved by adding a jumper wire
to connect the two grounds....the amp is now dead quiet!

I wanted to make note of this potential problem so it's archived with this thread.

Mike

 

[HF9]

Hmmn, must be that my powdercoater is a slacker and he doesn't spray it on as thick

 

[STURMJ]

Looks GREAT! Glad you got it sounding Great too.

 

[Mikey]

Hi Gang,

I've been listening to this amp at the office for the past two weeks, here are some thoughts...

Step 5 - Preliminary Listening Impressions:


Although the headphones I've been using with the Crack (Sony MDR-SA5000, 70 ohm) are not
an ideal match, they still show off the incredible sound quality of this little amp!  First and
foremost, the amp is as quiet as a tomb....a very pleasant surprise considering the AC heaters.

The music that leaps out from that pitch black background is very dynamic, with excellent balance
from one frequency extreme to the other.  The bass is taught and deep, very well defined and
powerful.  The mids are revealing, but not harsh.  The highs are detailed, but not in a way that is
shrill or fatiguing....I can listen to this amp for hours without interruption.

Most importantly, this amp is incredibly 'musical'!  I don't have to crank the volume to be able to
hear deep into the recording, and sonic cues normally lost with other amps flow right through this
simple circuit....now I understand why Doc B. named this thing 'Crack', it is intoxicating!  ;-)

I've got a few upgrades planned for later, but I'm really not in much of a hurry to get 'em done,
as the amp sounds fantastic as is.  When they do get done, I'll keep this thread updated.

Stay tuned....

Mike