Crack;Bottlehead OTL

[Beefy]

Quote:

Hmm... at this point I'm thinking I have a misunderstanding of the term damping factor... what does it mean in technical terms?

I don't fully understand the nuances myself, but I personally think of it this way...... if the damping factor is high, the amp has a very strong hold over the headphone driver. Helps stop resonance, overshoot, etc. If the damping factor is low, the amp has less control over the driver. It overshoots and otherwise flops about.
 
In headphones with heavy and stiff drivers, low damping factor is not that big of an issue because the mechanical effects hold it in check. Orthos, however, have a very light driver that can flop around a lot if the amp doesn't have tight control.
 
And yes, Wikipedia article is quite helpful. Basically, damping factor is driver impedance divided by amp output impedance. Your 50R ortho from a 120R OTL tube amp has a damping factor of 0.42. The same headphones from a stout solid state amp like the B22 with an output impedance below 1R would have a damping factor of greater than 50.

 

[Doc B.]

 
Quote:

I'm curious about this...... I've been looking at your kits, trying to decide on my next project. I'm interested in S.E.X. for my low impedance phones, but am a little confused by the fact that it uses 120ohm resistors in series with the output.
 
Also, I have seen some complaints of the gain of S.E.X. being too high, even with high impedance phones. Can you possibly tell us the overall voltage gain of the Crack and S.E.X. into 32 and 300 ohm loads?

The gain of both amps is 15 dB. The Crack is measured into 300 ohms and the S.E.X. into a load that matches the output tap, i.e., 8 ohm load on the 8 ohm tap. There is no problem at all with removing the 120 ohm series resistors from the S.E.X. and wiring the output directly to the headphone jack if that is your favorite flavor. The resistors are there to meet the standard for headphone jack output impedance established by the IHF many years ago, and they serve simply to keep the output level fairly even when you swap from low impedance headphones to high impedance ones.
 
I do wonder if those who complained that the gain is too high noticed that there is a volume control on both amps. As we have no control over the source output level we have to allow for lower source levels in some systems. In other words if some guys say they have too much gain, others will say they don't have enough. The only standard in audio that is adhered to with any conviction is that manufacturers pretty much ignore standards.
 
Also re soft bass: I believe that there may have been a semantics crosswire, largely my fault. By soft bass I meant uncontrolled, not rolled off. Small caps do indeed cause bass rolloff. Way small caps can even cause a bass hump above the rolloff in some systems. And yes, low damping factor causes flabby, slow bass. In some cases speakers can be designed specifically for amps with lower damping factors. Most are not designed this way.

 

[Beefy]

Quote:

Originally Posted by Doc B. /img/forum/go_quote.gif
 
I do wonder if those who complained that the gain is too high noticed that there is a volume control on both amps.

HA!

 
Thanks for the reply Doc, it is very helpful to myself and other prospective builders.

 

[milosz]

OK  I finished building the Crack with Speedball.  Even though I goofed and at first it didn't work the folks at the Bottlehead Crack forum pointed me in the right direction and I found what I did wrong. (I didn't read the model numbers on the sides of the TO-18 small signal transistors, I just said to myself  "these 4 transistors are all the same"  well guess what there are two 2N2097  and two 2N2222's!  And they are not interchangeable. DOH! Oh well I put them right and now it works.)
 
I also replaced the 100 uF electrolytic output caps with 100 uF / 250 v  Dayton polypropylene metalized film caps, which do sound a bit nicer in the highs.
 
This is a GREAT sounding amp for the Sennheiser HD800's, IMHO.
 

 

[HiGHFLYiN9]

I'm digging the gloss white finish. Did you take any pics of your handiwork on the inside?

 

[ironbut]

Yeah, I like the white on white and the different knob!
The "through the blinds at sunset" photo is pretty cool too.
Have fun.

 

[milosz]

The inside is not anything special. Maybe I'll post a picture of it later.
 
Paint is Rust-oleum gloss white with a coat of Rust-oleum clear gloss laquer over. (Home Depot)
 
Wood is Minwax "chestnut" stain (3 coats) with 5 coats of Minwax rub-on poly.
 
The knobs are Kilo from Digi-Key, solid machined aluminum, under $4. http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=226-4131-ND
 

 

[HiGHFLYiN9]

Finally finished with mine (mostly, I want to add a vintage pilot light at some point).
 

A pic of the inside. A few variations to the kit, including:
 
Cardas Rhodium RCAs
DACT 100k stepped attenuator
Kimber TCSS wiring
Kiwame Resistors
Neutrik Locking Plug
Obbligato 1uF Copper coupling caps (hidden under Speedball board)
Solen 100uF Capacitors
Teflon Tube Sockets
Woo Audio Tube Socket PCBs
 
 
 
 

Attached to the bottom are EAR feet for ample ventilation
 
 

Here's a photo of the bypass caps nestled under the vent holes. Do note that if you use these caps, the shell is conductive.
 
 
 

Here's the whole shebang
 
 
 

Made especially for the kit is a 10 gauge power cable with Marinco plug, Right angle IEC and DHLabs power cable.
 
 
EDIT: I've had countless people ask about how to get a badge made like the one above. Unfortunately my contact is no longer in the promotional products business so I do not have any referral information. 

 

[Skylab]

WOW! Nicely done - very, very nice looking.

 

[TheWuss]

i think i just messed myself.
that amp looks amazing.
highflying, it took you a while, but you should be very proud of the results.
best looking crack i've seen. 

heck - one of the best looking amps i've seen...

 

[HiGHFLYiN9]

Haha, thanks fellas. It did take forever, most the of the time waiting was for random parts to arrive, but the actual build was pretty quick. I really like the hammertone paint, but I've decided that future builds will be powder-coated for increased durability. I actually had to second coat the plate after the amp was completely built to cover a couple of small scratches, carefully masking everything off with painters tape.

 

[Armaegis]

If I ever build another one, I'm either going to layer the plate with painter's tape to protect it or cut out a sheet of cardboard and tape that on top. I spent way too much time polishing mine by hand (I don't have a hand sander), only to scratch it up carelessly. There's one particularly bad gouge where I slipped up with a dremel because one of the holes was slightly out of line so I tried widening it (I know, I know, I should have been going from the back side, gimme a break this was my first time doing this sort of thing

).
 
Sigh, I also botched the finish. Note to self: spray on poly is terrible stuff.

 

[Doc B.]

Quote:

I actually had to second coat the plate after the amp was completely built to cover a couple of small scratches, carefully masking everything off with painters tape.

Yup, hammertone finishes really need some time to harden up. To be safe it's best to let the plate sit for at least a week, maybe more. Hardly anyone has that much patience, including me!
 

 

[Canuck57]

Beautiful job Highflyin9! Love the new Bottlehead nameplate, couldn't find it on their website though?

 

[Calypso]

Quote:

Beautiful job Highflyin9! Love the new Bottlehead nameplate, couldn't find it on their website though?

Try checking out page 16. That might give you a hint

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