Bottlehead SEX 4 Kit for Headphones & Speakers

Name: Bottlehead SEX 4 Kit for Headphones & Speakers
Brand: Bottlehead
SKU: COMBOK-SEX-4-0
Price: 775.0 USD
Availability: InStock

$775.00 $875.00

Say hello to the new Single Ended eXperimentation (SEX) kit!

The SEX 4 has a ton of changes under the hood. For starters, it uses two 6FM7 tubes and a refined circuit to drive more than double the power of the SEX 3. We’ve heard from builders that they’re increasingly using preamps and digital sources, so we’ve removed the volume control. This plus the roomier chassis provides ample opportunity for experimentation.

Speaking of experimentation, we’ve added a useful experiment to the stock kit. Sitting front and center is a new switch. Flipping it controls the amount of local, plate-to-plate feedback applied in the circuit. What does that mean? The higher feedback allows the SEX to act as a low-gain power amplifier with tighter mid- and low-end performance and a blacker background. Switching to lower feedback mode gets you performance closer to a traditional desktop tube headphone amp. You should see more warmth, emotion, and character.

In either feedback mode, the SEX 4 feels a lot more controlled than the SEX 3 in the bass department, and the lower gain option mates solidly to a Moreplay and avoids hair trigger volume control issues. We’ve built a bundle so you can buy both the SEX 4 and the Moreplay at a discount. We’re also offering the SEX 4 at a $100 introductory discount until September 30.

SEX 4 has a universal power transformer and can be used worldwide by following the wiring instructions in the manual. Please note that international customers will need to provide their own power cord.

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Bottlehead SEX 4 Kit for Headphones & Speakers

$775.00 $875.00

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Thirty years ago, Bottlehead created a legendary DIY amplifier: The Single-Ended eXperimentation (SEX) Kit. Over the years, that kit has evolved with the times to culminate in the release of the SEX 4. The all-new SEX 4 captures the spirit of the original Bottlehead kits, though this time it’s got some new tricks up its sleeve.

For starters, at about 4W per channel, the SEX 4 cranks out double the power of the SEX 3 (or series strap them to double your output again)! We’ve also added a brand-new toggle switch, front and center. Removing the first-stage cathode bypass cap by flipping the switch decreases the transconductance and increases the plate impedance of that stage. Those changes will reduce the voltage gain of the amp, add some local feedback in the first stage, and increase the feedback of the output stage. Overall, low feedback mode adds about 2dB of feedback around the output stage, while the high feedback mode adds 4dB of feedback to the output stage plus 6dB of feedback to the driver stage.

Sometimes you need to experiment a bit, so it comes with a roomier chassis plate and without a volume pot to take up space, leaving you plenty of room to run your own SEX-y experiments. Our world-class, detailed manuals offer a couple suggestions on places you can start experimenting.

And of course, the SEX 4 uses the same high-quality components you’ve come to expect from Bottlehead, because nobody wants SEX with inferior parts. A little sensitive or looking for some more “alone time”? The SEX 4 comes complete with headphone jacks for even your most power-hungry headphones in addition to binding posts for speakers. We think they’re great for desktop and nearfield setups You can configure the output transformers for use with 4 ohm speakers and lower-power headphones, or for 8 ohm speakers and hungrier cans.

Oh, and who doesn’t love a good SEX story? The SEX 4 was originally designed to launch with a C4S upgrade available for purchase, but after installing it and shooting the manual, we realized it sounded and performed better without what we'd designed. Thirty years in and still time for a first: a manual completed for a kit we won’t sell, because sometimes, you just need to go with the SEX that works. After all, Bottlehead is here to set you up with the best SEX around, we’re not trying to screw you. We left the mounting holes on the chassis plates. We're continuing to evaluate and will launch it when and if we have something we know you'll love.

Product specifications

Tube Complement: 2x 6FM7
Frequency Response at 1 Watt output: -3dB at 17Hz & 36.5kHz (low feedback mode), -3dB at 17Hz & 35.5kHz (high feedback mode)
Input Sensitivity for 4 Watt output: 800mV (low feedback mode), 1.53V (high feedback mode)
Output Impedance at 8 Ohms: 2 ohms (low feedback mode), 1.5 ohms (high feedback mode)
Damping factor at 8 Ohms: 3.96 (low feedback mode), 5.44 (high feedback mode)
Gain: 17 dB (low feedback mode), 11 dB (high feedback mode)
Input Impedance: 100K ohms
Power Output: ~4.5 WPC
Assembled footprint (LxWxH): 9 in x 13 in x 6 in

Graph showing total harmonic distortion versus watts per channel for SEX3, SEX4, and SEX4 (Feedback Mode).

We've doubled SEX 3's output

We've made refinements to our already-excellent SEX 3 circuit. These improvements bring about double the watts per channel at a much lower total harmonic distortion. This means you can have loud SEX any time you want.

And remember: overloaded single-ended triodes clip soft gradually. That means you won't get the same sharp or hard clipping you see in solid state circuits. You can see the softer clipping in this chart's data above 10%.

Enjoy SEX with partners or by yourself

SEX 4 can be used with all sorts of headphones and speakers. Our instruction manual details exactly how to wire output transformers for 4 or 8 ohm configurations. It's the ideal amp for desktop, nearfield, or headphone listening.

Get more with Moreplay

The SEX 4 was built with the Moreplay Preamplifier kit in mind. Use the Moreplay to control volume, switch between sources, and provide a bit of gain. We love the pairing so much that we created a bundle.

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Person using a screwdriver to attach hardware to a Bottlehead amplifier kit chassis plate.

Yes, you can do it!

Bottleheads have been assembling our kits for over thirty years. Every kit includes the parts and components you need to build the kit plus an incredibly detailed manual with step-by-step instructions and close-up photos. Check out this sneak peek into what every manual includes. We've also put together a handy list of the tools you'll need to bring along.

Need some assistance or inspiration? Check our user forums.

FAQs

Please read our FAQs page to find out more.

Does the SEX 4 have a volume control?

No, volume should be handled by a preamp or through your source. With the rise of digital sources and preamplifiers, volume controls provide little utility on power amps. We've taken the hassle out and reclaimed the space in the chassis.

Are tubes from the SEX 3 compatible with SEX 4?

No, while the 6FM7 is similar to the 6DN7/6FJ7, they have a different gain structure. This makes those tubes not exchangeable without revising the operating points.

What's the deal with the switch?

SEX 4 features a switch to toggle between high and low local feedback. More feedback provides lower output impedance and better bass extension, while the low feedback setting provides more power.

Specifically, the low feedback setting adds about 2dB of feedback around the output stage. The high feedback setting adds about 4dB to the output stage plus 6dB to the driver stage.