Single Ended eXperimenter's Kit 3.0 for Headphones & Sensitive Speakers

The Bottlehead S.E.X. 3.0 (Single Ended eXperimenter’s) kit is a versatile, low-power stereo integrated amplifier that delivers 2 watts per channel of rich, three-dimensional single-ended triode (SET) sound. Designed with both headphone and high-efficiency loudspeaker users in mind, it’s the perfect choice for desktop systems, headphone listening, or even as the heart of a minimalist hi-fi setup.

Built around a two-stage, zero-feedback RC-coupled design, the S.E.X. 3.0 features parafeed output topology—a hallmark of Bottlehead and MagneQuest designs—offering articulate bass, extended highs, and reduced influence from the power supply. The result is exceptionally clear and musical sound with a low noise floor (around 83dB S/N), achieved without feedback and thanks in part to a Schottky rectified DC heater supply and precision Bottlehead-designed transformers and chokes.

This kit includes a pseudo dual mono power supply, providing isolated filtering for each channel from a single power transformer, further enhancing channel separation and sonic purity. The new PT-10 transformer incorporates internal shielding for even quieter operation.

Thanks to its transformer-coupled output, the S.E.X. amp is ideal for a wide range of headphone impedances and works seamlessly with 97dB+ sensitivity speakers, offering a flexible solution for both traditional listening rooms and compact desktop setups. An integral volume control means you can plug in a source and start listening—no preamp required.

But S.E.X. isn’t just about listening—it’s about experimenting. The included 6FJ7 dual triodes form a rock-solid base for creative modification. Builders can reconfigure the amp as a low-output-impedance line stage, a 4-watt parallel single-ended monoblock, a 2-watt monoblock with shunt regulation, or a 10-watt push-pull class AB amplifier (for the advanced tinkerer).

Skill Level 1.5, the S.E.X. kit is suitable for beginners with patience and attention to detail, or intermediate builders. Assembly typically takes one or two evenings and is guided by a thorough, photo-rich PDF manual. Construction is entirely point-to-point, with high-quality components throughout—including a brushed aluminum chassis, Pacific Northwest alder wood base, gold-plated RCA jacks, color-coded speaker posts, and a rugged Neutrik ¼” headphone jack. An IEC power entry module is included, along with a standard North American power cord.

Whether you’re chasing refined SET audio, a versatile amp for both headphones and speakers, or a launchpad for deeper DIY adventures, the Bottlehead S.E.X. 3.0 delivers purity, flexibility, and unmatched modding potential.

“Just wanted to share some thoughts on the SEX amp after completing the build in March (3rd build after Crack and Eros II).

I built the SEX primarily to use in my home office, near-field through Overnight Sensations connected to my PC (Asus Essence STX II soundcard). The decision was also driven by musings elsewhere on this forum about whether to go all tube and replace my solid state gear in my main system. PB recommended the SEX as a good way to test the SET sound before making the leap untested on more expensive amplifiers.

In the 4 months I have been listening to the SEX, I have absolutely loved it for all sorts of music and it has had more use tham my main system. What a great amplifier! A really clean sound with great seperation and what is really noticeable, is the depth of the sound stage. Not sure I was expecting this with near-field listening. I also listen through HD800s headphones so the SEX has been the perfect choice for the office. For anyone thinking of buying, I cannot recommend this fantastic little amp enough.

More recently, I had reason to plug it into my main system (trying to find source of hum in Eros – another thread).

First off, I made the mistake of turning it all the way up so I could hear the same hum, after all, how loud can 2.5 watts be? My listening position is only 9 feet from the speakers. Unfortunately, so was my wife, engrossed in a book. Oops. She almost jumped out of her chair. It’s plenty loud for most listening situations (also confirmed that hum was greatly reduced). In terms of sound quality, I am hesitant to use words such as better or worse when discussing subjective topics but there was definitely much that I preferred. There was no discernible loss of detail or seperation and the depth actually seemed greater. The thing that really stood out, though, was the naturalness of voices and instruments, especially when paired with the Eros and vinyl. Whilst the SEX did perhaps suffer from being underpowered in comparison to my solid state gear (less weight to the bass at times but 4ohm tap into 8ohm speakers might explain this?) I have heard enough to convince me that the switch to tube with Kaiju and BeePre is next. This is not where I expected to be a little over 18 months after building the Crack so some work required on the rack to make space (I have a question I will post on the Kaiju board before pulling the trigger). I have loved building the kits, love listening to them more, and am looking forward to next steps.

Not sure if my experience is interesting or helpful to anyone sitting on the fence but, despite my reservations about using the word ‘better’, I can definitely say without hesitation that the SEX will be better than you expect. Much better. I hope it is not taken the wrong way when I say it is surprising how good it is given the cost when compared to other quality hifi gear.

With thanks and appreciation to all the BH team, I leave you with a pic of my workspace which has now been converted from ‘enforced working from home hell’ to a place I can happily just sit and listen to music when I need a break.
Or even when I just want to relax with a beer. Like now.”

Chris H – Bottlehead Forum

“The S.E.X. is a SUPERB amplifier, extremely quiet, has great tonal definition and imaging, and a listen all day quality that’s very beguiling.”

–Jeff Poth, Enjoy the Music DIY

“The HD800. To my ears, they are the most speaker like and the soundstage was absolutely huge. I had the pleasure of listening to them on the Doc’s tape rig coming out the heavily modded S.E.X and damn! I can now hear why analog has such a strong hold within the audiophile community.”

— HeadFi member alphaphoenix, on the HeadFi forum