Mod the Bottlehead Crack to make it nice for low impedance headphones

[drewdavis]

Hello,

Since I'm new to the DIY audio world I wanted to start with a kit for my headphone amp. I love the Bottlehead Crack, but it's not made for my Audeze LCD-X.

I'd like to mod it to make it powerful for low impedance cans.

Also add a second pair of RCA out as preamp would be great.

Can anybody help?

[Larpy]

Nope.  The Crack uses a circuit whose output impedance is higher than headphones like Audezes like to see.  It's just the way the amp is designed.  To drive lower impedance headphones like yours, an amp needs output transformers.  The Crack doesn't have any, which is why it's so relatively inexpensive.

The only thing you could do is to try to find an output tube like a 5998 or 7236 to replace the stock 6080 tube.  That would lower the amp's output impedance a bit, but not enough to make a huge difference.  You'd be better off building a S.E.X. amp.  More expensive, but if you love your Audezes (and I can relate), it will be worth it.

However, the S.E.X. is quite a bit more complex to build than the Crack.  If this is your first DIY build, keep that in mind.

[Paul Joppa]

Two limitations are peak signal current and output impedance.

Recommended power for the LCD-X is 250mW, that would be about 2.25 volts at 112mA rms, or 3.2v at 150mA peak. Crack runs at ca. 30mA, which is the maximum peak available current. One could increase that current by reducing the cathode resistance, but the power supply is limited to about 70mA peak and the temperature rise would more than double, likely exceeding the maximum safe temperature.

The output impedance should normally be significantly less than the load impedance (20 ohms for the LCD-X); Crack is a bit more than 6 times that. (This is less critical for the LCD-X, which has a flat impedance spectrum).

A more capable tube will not solve either problem. A pair of transformers could improve both issues, but I'd be surprised if Crack could make more than 100mW without further design revisions. Not to mention that the absence of transformers is one of the reasons Crack sounds as good as it does.