Bottlehead S.E.X. DIY Amplifier

The Bottlehead Single-Ended Experimenter’s (or S.E.X.) kit amplifier is the mid-level headphone amplifier in Bottlehead’s range. The S.E.X. also happens to drive speakers which is a nice piece of flexibility, but I’ll get to that later.

This first part of this review refers to the non-C4S modded version of the S.E.X. with a section below dedicated to the C4S modification (a Bottlehead add-on kit similar to the Speedball for the Crack amp).

The S.E.X. is a step up in price from the Crack and sells for around $600 (incl. shipping) depending on your local currency and shipping costs. For the extra money it offers increased versatility over the Crack and maybe better sound, but I’ll save those details for later.


Before I discuss the build stages it’s worth noting that the S.E.X. is an incrementally harder kit to build than the Crack. The same techniques are used, but there are many more connections, many more wires, some slightly tricky components, and various wiring options. This means you need to keep your wits about you and allow plenty of time. I built the S.E.X. with 2 friends and we completed the assembly stage in around 3 hours. It then took us from 11am until 2am to complete the hook-up and soldering stage (including a long lunch and moderate length dinner). That’s 15 hours in total, but realistically we were on the tools for about 12-13 hours.

All up, I’d recommend allowing 2 relaxed days worth of work to complete the hook-up and soldering of a S.E.X kit, plus a few hours for assembly and an hour or so for testing (assuming no issues arise).

Parts & Assembly

The carton containing the S.E.X. kit is a heavy beast, and when you open it you quickly discover why. In addition to the aluminium chassis plate, the S.E.X. kit includes a power transformer, two output transformers, and two plate chokes that look a lot like the output transformers so be sure to note which is which (it’s easy to tell by the fact that the plate chokes have wires already attached).

As per the Crack (and all Bottlehead kits as I understand it), instructions are provided on a CD so you can print it out or view the manual electronically. Personally, I recommend printing the manual so you can cross off each step as you go. It’s far too easy to miss a step if you’re looking at an electronic version. Once again the instructions are exceptionally clear and well thought out.

Also just like the Crack, the first step for the S.E.X. is to assemble the components such as the chassis plate, terminal strips, RCA terminals, and tube sockets (plus a few other parts). This is just a simple case of following the instructions and wielding a screw driver and pair of long nose pliers from time-to-time.

Once all of the assembly is complete, you’ll have something that looks a little like the image to the right except yours will be a silver / aluminium colour, not browny bronze like mine, but I’ll get to that.

Now, when you see the image to the left, you might be thinking “Hang on, he said he’d only done assembly so far, but I can see wires!” and you’d be right. If you want to stay solder free in stage 1 you could just mount the power transformer only, but if you’re happy to include about 5 minutes of simple soldering then you can pre-wire and install the output transformers and plate chokes.

At whichever stage you choose to install the chokes, etc. you need to make a decision. Unless you’ve ordered the optional impedance switching kit (not reviewed here, but a great addition), the wiring of the output transformers will directly influence what the amplifier is optimised to drive. You can wire the S.E.X. to drive a 4, 8, 16, or 32 ohm load and that basically means you choose to wire it to suit speakers and headphones (4 / 8 ohm) or for more dedicated headphone use (16 / 32 ohm). I chose 8ohm wiring because I’d like to be able to use it with some bookshelf speakers I have, but my main priority is headphones. If using the S.E.X. with particularly power hungry headphones you would probably want to wire for 32ohms or buy the impedance switching kit that allows you to change impedance by simply turning the amp off, lifting the chassis plate and throwing a switch or two (as opposed to rewiring the output transformers which is what I would have to do).

Edit: after some listening and reading and asking around on forums I’ve discovered that the 4 or 8 ohm wiring is ideal for almost all requirements except the hungriest of headphones (HiFiMan HE-6 or AKG K1000) and will provide ample power with minimal noise.

Once again similar to the Crack, the final step of assembly is attacking your chassis plate with a permanent marker to number each terminal for the upcoming hook-up stage. This numbering system is truly a godsend and helps both with wiring and trouble-shooting if needed. For example, if something’s wrong with the tube heater power supply it allows one of the gurus from the Bottlehead forum to tell you to check the C-strip (the terminals numbered C1-C5) rather than trying to explain it component-by-component.

Custom Appearance

When I built the Crack I was too excited to pause for any kind of custom painting or finish (except for staining the wooden base) so the S.E.X. gave me a chance to play.

If it’s your first ever kit you may find summoning the patience difficult, but it can be very rewarding. I was fortunate that I had to wait while we found a date that worked for all 3 of us in the build group so I had idle time to play with the looks of my S.E.X.

I really wanted a vintage, art deco look for my S.E.X. so I ended up choosing an bronze colour anodised finish on the aluminium chassis plate (costing around $60) and I painted the visible metal parts on the plate chokes and transformer bell using simple, cheap spray paints from the local hardware store. All up, the customisation cost around $80-90 and a whole lot of patience and prep-work.

If you’re painting the metal parts you really need to do the prep well as many of the parts will have resin on them as part of the manufacturing process. The resin doesn’t play nicely with primer or paint so take the time to remove it thoroughly first time or you’ll have to do what I did and strip back your first attempts (yes, that’s multiple attempts – it took me more than one go to get this right) to start again from scratch.

In addition to anodising the chassis plate, other options are to polish the plate or paint it. It all depends on the type of look you’re after so don’t be afraid to get creative and you can check out the Gallery thread over on the Bottlehead forums for plenty of inspiration.

One final tip: if you choose to paint or anodise the base you’ll need to make sure you connect the earth terminal to bare metal by sanding or filing the metal around the screw hole for the earth terminal.

Hook-Up & Soldering

There’s not a lot to say here that I haven’t already covered in the Crack review. As already mentioned, the S.E.X. kit has more components and more wires so it is a trickier process than the Crack, but the techniques and steps are essentially the same.

One thing I will mention is that working with the Schottky rectifiers on the power supply is quite tricky because space is limited and the rectifiers have very thick leads which don’t easily bend. The only real tip I have for this section is plenty of patience. There are 4 rectifiers to install and not a lot of space so it can get mighty frustrating and a break and a few deep breaths may well come in handy.

That said, now is probably a good time to include some general tips which apply to both the Crack and the S.E.X., but which became increasingly important in the more complex S.E.X. kit where space is at a premium and your concentration may fade as the hours go by.

Top Building Tips

  1. Check and double-check the orientation of all components that have specified directions / alignments. This includes the tube sockets, power switch, diodes, and capacitors. One error here can completely ruin your day and may destroy the component in the case of capacitors. Not only that, but de-soldering is a real pain in the proverbial, especially in cramped spaces with lots of components.
  2. Measure wire lengths based on the path they will travel. We found a few times that the lengths specified in the manual were a little too generous or a little tight. This might be because we were working with the metric measurements (mm not inches) and a few of these had minor errors or, more likely, we made errors with the routing of the cables and so the lengths seemed wrong because of the path we took. My approach next time will be to check the origin and destination of each wire and trace its path (around obstacles and components) to get a tidier and easier solution.
  3. Read the instructions carefully. All of the information and guidance is there, but it can be easy to miss a key detail so always read each instruction again before finalising your work. Once again I recommend working with a printed copy of the manual that you can write on and tick / cross off each step.

I’m not going to discuss the testing stage here so please refer to the Crack review for details about testing.


The S.E.X. is a single ended triode design which essentially uses one triode tube per channel in combination with an output transformer. SET designs are simple (i.e. not lots of components in the signal path) and clean, but not particularly efficient in the scheme of things. For our purposes though (driving headphones and sensitive speakers) they appear to be a good choice and knowing the amount of testing and planning conducted by the crew at Bottelehead, the SET design has no doubt been chosen for its combination of performance and affordability (with neither being compromised, just maximised).


By including an output transformer in the S.E.X., Bottlehead have given us an amp that can drive all range of headphones (including lower impedance models) as well as speakers. That’s a pretty versatile offering and although I won’t be using my S.E.X. for speakers, I love that it can drive them and a brief test proved that it drove my bookshelf speakers admirably (but not set up properly or for long enough that I’m able to effectively review the sound).

The S.E.X. comfortably drives headphones of all different impedances although I’d recommend drawing the line before you get to the sensitivity and impedance of IEMs. I tried my GR07 MkIIs and found the hum to be pretty bad (amp wired for 8ohms) so to me this is a headphone and speaker amp, not an IEM, headphone and speaker amp.

As for using it with various headphones, I’ve tried 600 ohm Beyerdynamic T1s, 120 ohm Fischer Audio FA-011 LEs, 75 ohm Ultrasone HFI-680s and some old, cheap 32 ohm AKG K-66s. In all cases, the S.E.X. drove the headphones comfortably and well, but I’ll get to the sound in a few moments.


The S.E.X. utilises the very utilitarian 6DN7 tube which is apparently an old TV tube and no longer made, but still fairly readily available as new old stock (NOS) it seems.

From everything I’ve read, there is no benefit to be had with tube rolling on the S.E.X. because all 6DN7s are essentially the same. Having tested this with some very affordably priced tubes I can confirm this to be true which makes my wallet feel a lot more secure than when I’m dealing with the Crack with its never-ending range of new, used and ultra-rare tube options.

Back to the 6DN7 tubes… There are some variants floating around where the base is much thinner. These are referred to as coin-base tubes and may look preferable depending on your tastes, but are no different in sound as far as I can tell.


So here we are at the part that really matters. I’ll start by talking about the S.E.X.’s sound in general terms before comparing it directly with the Crack. Remember that the following comments are based on the S.E.X. in 100% stock form and without the C4S add-on.

One aside before I break down the sound: when I first went to test the S.E.X. I plugged in my RWAK100 as a source and my old, cheap AKG K-66s as a test headphone. The K-66s were never a particularly inspiring listen (hence why they’re my sacrificial test headphone), but I was absolutely gob-smacked when I heard them from the S.E.X. I have literally never heard such an amazing transformation caused by an amplifier.

Now, I’m not suggesting that the S.E.X. will magically transform your crappiest headphones into world-beaters. What the K-66 experience showed me was that the S.E.X. has a great presentation which will lift many headphones to a higher level than other similar-priced amps, and that it synergises really well with the K-66s for some unknown reason. My other headphones still sound clearly better, but where the gap from the NFB-5.2 is exponential between the K-66 and other headphones, the gap becomes much narrower with the S.E.X.


The treble from the S.E.X. is crisp and clean with nice bite. Through the T1s (which can be aggressive with the wrong treble signal), the S.E.X. presents outstanding detail and clarity in the upper registers, but without any fatiguing edge. Where the Crack seemed to offer a slight bump to the lower treble, the S.E.X. seems to be more balanced across the whole treble range. At first I felt it might have had a slight bump to the upper treble, but I think that might be more that I’m used to a little roll-off from the Crack.

The S.E.X. presents some really nice sparkle with the treble. Across a variety of tracks and with all different headphones, the treble presentation is consistently excellent. Somehow the S.E.X. manages to present the treble clarity of a solid state amp combined with the smoothness of a tube amp – very impressive. Cymbals and percussion have really nice energy with the S.E.X., but it’s not overdone or in-your-face.


The mids from the S.E.X. are very similar to the treble – crisp and clean. There’s a transparency from the S.E.X. that pairs beautifully with the T1s and I’d love to hear what it can do for headphones like the HD650 which are great, but can sound a little opaque at times.

Turning to Sierra by Boz Scaggs, the S.E.X. produces lovely, warm, liquid mids for the vocals and guitars. The presentation is clean and clear with no sense of slowness or blur. Boz’s voice has texture and air while still being smooth and lush. Once again I’d describe the presentation of the S.E.X. as akin to the clarity of a solid state amp, but with the smoothness of a tube amp.

One thing I really like about the mid-range from the S.E.X. is the texture it grants to the instruments. I’ve been flat-out stopped in my tracks a few times while listening to music as I work. I listen to music via headphones for around 4-5 hours per day while I work so I’m used to hearing the detail and clarity that headphones can provide. Having used the T1s with the Crack for some time now, I thought I was hearing everything in my music, but the S.E.X. proves me wrong almost daily, not by showing me any extra details as such, but by revealing the subtle character of those details.

For example, the other day my music was playing on shuffle when a was stopped dead by a piano chord in a track I know really well, but I can’t remember the name of as I write this. What stopped me wasn’t the detail in the rendition, but the sense of realism conveyed by the texture and resonance I could hear in the piano. As a piano player myself, I find it rare to hear a system truly recreate the proper acoustic sound of a piano with all of its richness and subtlety. What I find amazing about the S.E.X. (especially as a $600 amp) is that it does this really well. I guess it comes back to the transparency I mentioned earlier – if it’s in the recording and you have headphones to do it justice, the S.E.X. will let you hear it by simply amplifying the signal and getting out of the way.


If there’s one weak spot for the S.E.X. it might be the bass, but not because it does anything wrong per se, just because it is very transparent and therefore may not give some people the “oomph” they’re looking for.

I personally like the S.E.X.’s bass presentation because it’s clean, tight, quick and punchy, but there’s no denying that it doesn’t have the body of the Crack (full comparison shortly). If I’m honest, I think that the S.E.X. is probably the more accurate representation of the recording, but some people might want a little more if they’re looking for a warm, smooth, tubey sound.

On the plus side, the S.E.X.’s well-controlled and accurate bass is a great foundation for it’s brilliant mid-range and treble presentation because there’s no bleed between the bass and mids. The bass hits and rumbles when it needs to, but it never blurs the edges of other frequencies. For that reason, it’s great with headphones that have a good bass presentation to begin with, but may not pair so well with headphones that people traditionally might use tube-warmth to “fix”.

Staging & Imaging

Good golly does this thing image well!

I mentioned the K-66 experience at the beginning of the “Sound” section. It was staging and imaging that I heard with the K-66s that told me I was in for a treat. The S.E.X. presents a soundstage that is both wide and deep. With the T1s the soundstage from the S.E.X. extends beyond each ear by about an inch or two. It also has good forward depth, but I think the key difference I’m noticing is that it extends out diagonally better than anything else I’ve heard so far with the T1s.

What I mean by that is that the soundstage from the S.E.X. is more semi-circular than triangular. To the left and right of the centre there is equivalent depth to the centre and sides. Other amps I’ve tried, specifically the NFB-5.2 and Crack as my references from this review, seem to compress the stage at the 45o mark to each side before expanding again at the sides and centre so the result is a kind of triangular stage. The S.E.X. doesn’t do this and it makes the stage feel massive and spacious even though it probably doesn’t extend much further (if any) in the centre and to each side when compared to the Crack.

Sometimes a large stage can feel too big and the results seems somehow incoherent or unrealistic, but I don’t find that at all with the S.E.X. Everything is perfectly placed with space around each sound, but without separating one instrument from the next. As you’ll read in the comparison to the Crack. It does everything the Crack does well and a little bit more when it comes to staging and imaging.

I used Good Excuse by John Butler Trio to check the S.E.X.’s ability to layer the stage vertically and was pleased to find that it manages the vertical equally as well as the horizontal. John’s voice was clearly slightly higher than the cymbals behind him and higher still than the sound of his guitar. Bravo Bottlehead!

While completing this review I got a bit tired of the same songs over and over so I switched to a different playlist. One of the tracks that followed was Clint Eastwood by Gorillaz. In the background of the track is a old piano that sounds like it’s recorded from an adjacent room (not to be confused with the main synth which is piano-like, but not a piano). Listening on the S.E.X. actually made me take off my headphones to double-check that the distant sounding piano wasn’t a mobile phone ringing in the next room – it was eerily lifelike.

Sound Summary

To summarise the sound from the S.E.X. I’d have to say things like clean, transparent, accurate and detailed. It’s definitely a smooth listen, but not at all warm like you might expect from a tube amp. It’s not sterile, analytical or dry, but it also doesn’t gloss over everything with a rose-coloured hue. It just finds a really nice balance that has instantly placed it as my favourite amplifier of all time (so far).

Comparison to the Crack

I’m basing this comparison on the stock Crack + Speedball with stock tubes. I will also briefly compare the Crack with the GEC 6AS7G brown base, but want to start with the 2 kits in essentially their baseline states.

Moving between amps, both using the same DAC, RCA cables and headphones (Beyerdynamic T1), the difference is stark, but not in the way you might expect. The S.E.X. doesn’t wipe the floor with the Crack, they just kind of shake hands and walk comfortably in different directions.

Where the S.E.X. is clean, transparent and open, the Crack is warm, smooth and intimate. Both are enviably good so my aim here is more to help you choose which you might prefer.


The Crack’s signature is undoubtedly warm. The bass is more present, the mids are lush and creamy, and the treble is ever-so-slightly rolled off to a smooth, but detailed finish.

Returning to the S.E.X. I find a cooler presentation, but not cold. Both amps are probably on the warm side of neutral, but the S.E.X. is almost right on the line whereas the Crack is 2 steps into warm territory. Bass is reduced on the S.E.X., but still present. The nearest analogy I can think of for the bass is the HiFiMan “RE” range of earphones. I’ve owned the RE-0 and RE-272 earphones, both of which are beautifully transparent, but often left me wanting a little more in the bottom registers. Anyone who’s heard these earphones probably knows what I mean and might be getting worried about where I’m going with this, but don’t worry!! My point in raising the RE earphones is to clarify that the S.E.X. does not go as far away from the bass as something like the REs. Is it leaner than the Crack? Yes, without a doubt. Would I describe it as lean or analytical? No, not at all. Accurate, clean and quick, but not lean or analytical.

To use a really inaccurate, but illustrative method of description, I would describe the Crack’s signature as sounding like a gently diagonal frequency response starting a little above neutral in the bass, staying much the same through the mids and trending downwards in the treble. The S.E.X. on the other hand starts basically neutral and continues flat across the board – end-to-end flatness. For some that will be desirable. For others, it may be a detractor.

Staging & Imaging

Once again, the two amps are different here, not better or worse.

The S.E.X. presents that beautiful, open and spacious stage I’ve described above while the Crack is much more intimate. Despite the Crack’s intimacy, it doesn’t feel crowded or congested, just intimate. I know many people who like one or the other so this is once again a question of taste.

Jumping back and forth, there is no way to choose a winner based on staging because it’s a preferential choice. Imaging though is a little different. There is absolutely no doubt that the imaging from the S.E.X. is a step ahead of the Crack. It’s not miles ahead, but definitely a good step ahead. Where the Crack has accurate imaging, the S.E.X. has crystal clear and almost holographic imaging. This might be in part due to the extra space available in the S.E.X.’s soundstage.

Resolution and Detail

I’m jumping back and forth between the two amplifiers as I write this and although I hadn’t originally planned to include this heading it became abundantly obvious that it is necessary.

The reason this is so important is the difference in resolution and detail retrieved by each amp. The Crack is very good with detail and clarity, but the S.E.X. is on a whole other level. There are sounds I hear from the S.E.X. that I can’t hear from the Crack even once I know they’re there. I think this is in part due to the extra treble extension from the S.E.X., but it may also be a difference in the coupling of the headphones with the output tube versus an output transformer – I’ll leave it to more qualified minds to answer that question though.

Reality Check

I’m calling this a reality check for 2 reasons. Firstly because I wanted to confirm if I can match the S.E.X.’s level of performance just by rolling tubes in the Crack and, secondly, because the tubes involved bring the total cost of the Crack to the same level as the S.E.X.

I began by installing just the Cleartop 12AU7 in the Crack to see what the affordable little marvel could do…

The sound was clearly better defined once the Cleartop was in the circuit, but was it as good? Nearly… very nearly. The Crack still didn’t quite have the top-end extension of the S.E.X., but it was near enough that you might not miss it if you weren’t directly A/B-ing the two amps. The Cleartop didn’t bridge the gap in terms of imaging and the detail level was still a fraction behind the S.E.X., but it was definitely closer.

Next step was to pull out the big guns with the GEC 6AS7G and the results were actually very interesting. The GEC refined the Crack’s sound a bit, but I don’t feel like it pulled it any closer to the S.E.X. in the areas that it falls behind. What it did do though was take the Crack’s mid-range to a new level and perhaps a level not achieved by the S.E.X. in terms of creamy, liquid goodness. I guess what I’m describing here is that it made the Crack better at everything it’s already good at without really changing the overall presentation.

Simplified Comparison

We all have different tastes and preferences so here’s a summary of where each amp excels (remembering that this comparison was completed with the Speedball installed on the Crack). I’m only including factors where there is a clear difference. If something’s not there it probably means they’re about the same, but feel free to post any specific comparison questions in the comments.

  • Bass quantity / body – Crack
  • Bass speed / refinement – S.E.X.
  • Lush mids – Crack (esp. with GEC tube)
  • Treble extension – S.E.X.
  • Detail retrieval – S.E.X.
  • Intimacy – Crack
  • Stage size – S.E.X.
  • Imaging clarity – S.E.X.

Closing Statements

It feels like there’s a lot to summarise here so hopefully this crystallizes it effectively. In my mind the S.E.X. clearly deserves to cost more than the Crack. Not taking into account the fact that the cost of materials is undoubtedly higher, the sound from the S.E.X. is more refined, more detailed and just plain impressive.

I still love the sound from the Crack and if I was told tomorrow that it was the only amp I could use for the rest of my life, I’d be completely OK with that. Since building the S.E.X., though, the Crack has had close to zero airtime because the S.E.X. is just too good.

I love the subtleties in music. I love to hear the little textures that the musicians and producers chose to include, but that we often don’t hear during day-to-day listening. These subtleties help me to continue rediscovering my music collection and the S.E.X. shares new subtleties with me daily.

The Crack is an incredibly enjoyable amplifier and is flat-out unbeatable for the price (when driving high impedance cans), but the S.E.X. is on another level overall.

If I had to recommend just one of these amps, I’d ask if you want warm, musical sound (Crack) or smooth, detailed sound (S.E.X.). Ideally though I’d suggest that you try both – start with the Crack as a practice run for the S.E.X.

Even if you don’t spend a further $200+ on tubes, the Crack is an exceptional buy and one that still holds it’s weight against its twice-the-price big brother.

Of course, if you want to drive speakers and headphones or if your headphone collection consists of <300ohm cans like HE-500s, AKG Q / K series headphones, or Audeze LCDs then go straight for the S.E.X. and love every second of it!

C4S Upgrade

What’s a C4S?

My understanding of the electronics theory is a bit limited so, in very much layman’s terms, the C4S is a circuit which applies extra load (impedance) to the tubes. The extra load helps the tubes to operate in a more stable range and also helps to prevent the influence of external noise sources.

The predicted result of the C4S upgrade is better resolution, quieter background and tighter bass.

Is it Hard to Build?

No. This is the easiest Bottlehead kit I’ve built so far. You do have to work on a circuit board, but it is with simple components and with nothing that poses major risk of damage or failure. You can see in the image that it has very few parts and the one part that can cause trouble (the 4 tiny LEDs – middle right of the image) are safely hidden on the other side of the circuit board when you’re doing the soldering of their leads.

This kit is so simple that I was able to successfully build it in front of the TV in about 30-40 minutes. I still checked and double-checked everything to ensure no errors (and recommend others to do the same), but it’s just too easy and cheap to not do this upgrade!

The Sound

It’s all well and good for me to sit here and say you should do this upgrade, but there’s no point if it doesn’t significantly improve the sound so let’s chat about that…

Having read my comparison of the Crack and the S.E.X. you’ll know that I prefer the S.E.X. over the Crack by just a small margin. The area that the Crack (with Speedball) held my attention was in the bass department. The Crack consistently delivers more impact and body in the bass compared to the standard S.E.X.

On one hand I was hoping that the Crack would still win the bass battle after the C4S upgrade to the S.E.X. because I don’t want to make it a redundant item in my collection. On the other hand, the S.E.X. garners the majority of my listening time and therefore would be great if it could be the clear winner in every category.

Let’s start with the bass

As the one area that I felt the S.E.X. was a little lacking (only by a hair) I am pleased to say that the C4S completely rectifies my concerns. The bass on the S.E.X. is now sweet, solid and DEEP. To my ears, the S.E.X.’s ability to produce sub bass frequencies has significantly improved. The bass is tight and clean, but deep and powerful in a way I feel was missing before.

Everything Else

I know I normally break down the individual details (mids, highs, staging, etc.), but I’ll just be saying the same thing over-and-over. Simply put, the C4S does everything that Bottlehead promise. The sound is cleaner and noticeably more resolved. To my ears it also seems smoother – more relaxing, but without losing any of the detail and transparency that I loved about the stock S.E.X.

As a result of these improvements there is now a better presentation of the micro details and texture in recordings, I am rediscovering my music yet again! In addition to the greater detail retrieval, the stage is now better defined, but probably no larger (it was already great so this is completely fine). Instruments jump out of recordings because they are so beautifully defined, but the music stays wonderfully coherent and together as a whole even when you can pick out each individual piece.

In short, my Beyerdynamic T1s have never sounded so good. I was up very late last night because I always wanted “just one more track”.

S.E.X. + C4S vs Crack + Speedball

I’m sorry to say this, Crack addicts, but S.E.X. is better than Crack. It’s a no contest.

The Crack is still an outstanding amplifier that punches massively above it’s weight (or price), especially with the Speedball installed, but the S.E.X. is just on another level once the C4S is attached.

What the Crack does differently to the S.E.X. is a warmer, fuller mid-bass which can make the music a bit more mellow and smooth overall (similar to how the HD650s are very enjoyable for their warm, mid-bass and midrange presentation). The bass from the S.E.X. is leaner in the mid-bass area, but natural and solid. Where it excels, as I’ve already mentioned, is the sub-bass region. While the Crack may have this same sub-bass, it’s harder to notice behind the mid-bass. The S.E.X. keeps everything so beautifully balanced that every single sound and frequency is right there for you to feel, hear and experience to the fullest.

Could I be happy with just a Crack + Speedball having heard the S.E.X. + C4S? Yes, Crack+ is still SO good that it can be an end-game for many people (so long as you have >300ohm cans). The S.E.X. is for those who want the versatility to drive lower impedance cans and / or speakers, and for those who want to take another step towards audio nirvana – I’m getting close now – I think I can see it from here!

DAC Upgrade Update – 27th October 2013

A recent upgrade to my DAC has brought some very interesting changes.

The new DAC is a Matrix X-Sabre based on the very detailed ES9018 Sabre DAC chip. The extra separation and clarity offered by this DAC (compared to the previous Audio-gd NFB-5.2) has brought the Crack (with Speedball) very close to the S.E.X. in terms of overall sound performance. It’s so close now that I could actually be completely happy with either amp when driving the Beyerdynamic T1s. For lower impedance cans, the S.E.X. is still the only choice due to the Crack’s high output impedance, but with high impedance Beyerdynamic or Sennheiser cans I would probably choose the Crack simply because it is cheaper and still excellent.

I think the key to this change is the signature of the DACs. The NFB-5.2 was slightly warmer than the X-Sabre. The X-Sabre is still musical, but a little closer to neutral than the NFB-5.2. The result is a cleaner sound from the Crack – still warm and rich, but not smoothed over at all. The S.E.X. can sound a little dry at times in comparison which makes the Crack a bit more seductive – it soothes the ears with detailed, mellow tunes while the S.E.X. presents oodles of details and clarity, but isn’t always soothing.

Attenuator & Capacitor Upgrade – May 2014

During May 2014 I began to modify my S.E.X. and Mainline amps beginning with the volume pot in the S.E.X. which I replaced with a stepped attenuator, assembled using high quality Dale Vishay resistors. I chose this particular attenuator because the Dale resistors are known to be very transparent and are actually the same as those used in the Mainline kit. By all accounts, you can achieve similar results with other options – the key is transparency.

Attenuator Installation

The first thing to note if you are changing your attenuator (or potentiometer) is that you might need to carve out a bit of extra space from the locking channel where the pin / tab of the attenuator sits into the chassis plate in order to prevent it spinning. Thankfully, the aluminium chassis plate is easy to work with using either a hand-file or a dremel. You can see in the image to the right that I lengthened and slightly enlarged the stock groove in the chassis plate. This isn’t visible from the top once the volume knob is in place so don’t stress if it’s not the tidiest job ever (so long as you don’t overshoot the width of the volume knob, but that would take some extreme carelessness).

Once the hole is ready, mounting the new attenuator is as easy as the stock one. Remove the supplied nut and washer(s), insert the spline through the hole, align the locking tab / pin and tighten the nut from the top side of the chassis. That’s it for mounting – simple! The next step is wiring and that can change depending on the model of attenuator you choose so please refer to the provided instructions for your attenuator of choice and the instructions from the S.E.X. kit. If you get stuck, the wonderful people on the Bottlehead Forum can also usually help.

Do be aware at this point though that you may find the stock volume knob sits a little higher than it used to. It’s not a major drama, but for the aesthetically inclined, you might need to modify the stock knob or track down a different one if you want the knob sitting flush against the chassis.

Attenuator Sound Improvements

You’re not going to make a change like this unless there’s a payoff so let’s discuss what happens when you change the attenuator. The exact changes may vary depending the brand / model used, but if you go for something known for transparency (like I did) then you’ll get a nice increase in the detail and clarity of your S.E.X. The signature of the amp doesn’t change significantly other than perhaps a slight bit more brightness, but it’s subtle enough that I’d say to expect no real change in signature and just an increase in detail retrieval and texturing of sounds.

As an upgrade, I would say it is absolutely worth it. Including the time taken to modify the hole in the chassis I would say this is a 1-2 hour project and delivers a sound upgrade second only to the C4S mod. You can complete this mod on a budget of about $50 USD, but can also spend a bit more for more exotic attenuators if you want to.

Capacitor Upgrades

Next on my list was a double-barrelled capacitor upgrade. I approached the upgrade in 2 stages at the excellent advice of some fellow Bottlehead enthusiasts over on Head-Fi, JamieMcC and mcandmar. Both deserve kudos for their input and support over the course of multiple mod discussions around capacitors and the like. The reason for the 2-stage approach was to isolate the impact of each pair of capacitors, but the problem was that the sound was still limited by other caps in the system so I am going to summarise the whole upgrade in one hit because I can’t truly describe the full potential of the FT-3 teflons I installed first without having upgraded the other (output) caps which then brought their own influence to the sound. Hopefully that makes sense…

I started off installing some FT-3 0.1uF teflon capacitors (the same as the ones used in the Mainline upgrade) and immediately noticed improvements even though they require significant burn-in. These caps replaced the pillow-shaped brown / orange capacitors that sit over the tube sockets. Early improvements I noticed were cleaner, clearer textures in the sound and better layering of instruments in the soundstage (i.e. different instruments were now more clearly defined ‘in space’ within the soundstage). As I said above though, the full impact of the teflon caps couldn’t reveal itself because they were hidden behind the stock output caps (which are good, but able to be improved).

Stage 2 was the installation of output capacitors – the ones that connect the sound to your ears (kind of). I chose the Mundorf Supreme Silver / Gold / Oil caps for this purpose after doing a lot of reading about this particular model’s excellent warmth and midrange. I decided that the Mainline should stay as neutral as possible while the S.E.X. would become my fun, organic, musical amp. Don’t get me wrong though – this doesn’t mean I was trying to turn the S.E.X. into some kind of thick, syrupy mid-monster. I wanted musical, enjoyable sound that was still nimble, agile, liquid and accurate. The good news is that the mods were a resounding success.

With the Mundorfs installed, the S.E.X. instantly took another small step in a very positive direction. All of the sound was warmer and fuller, but not at all heavy or thick. Music is still well-paced and lively, but with a little more note weight (not quite as good as the Mainline in this regard, but closer) and more present mids.

The staging from the S.E.X. is still excellent, but the Mundorfs added to what I was already hearing from the FT-3 teflons by helping to deliver greater fine detail and textural cues.

No doubt there are even more (subtle) changes that took place as a result of my upgrades, but the inability to truly A/B makes a direct and detailed comparison impossible. What I can tell you by way of a summary is that the S.E.X. post-upgrade does everything good that I wrote about above only now slightly better and with a dash more soul and warmth. I imagine you could get similar improvements with different cap combinations too and encourage you to give it a go. Changing these 2 caps on the S.E.X. is a relatively easy process and it’s fun to hear what results.

Lachlan Fennen Written by:

Facilitator, training design consultant, blogger / writer and amateur photographer

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