I spent the past year building (afaik) the world’s first portable 2.1 system (2-channel + a subwoofer), in search of an alternative solution to subbass in IEM listening. The system centers around two piece of equipments:
Backbeat G2 is the closest to perfection in its category in my experience, but it is not without flaws:
Upon realizing that there is an inherent latency issue (due to AD-DA in the Backbeat) I have to deal with on top of the “splitter” problem, I began looking into “lip sync correctors” to delay the main signal (the signal to IEM) by 200ms...
Defeated, I went back to the “DAPs with SPDIF output” route, but this time I looked for “DDCs with analog output” or “DACs with digital output”. With a device like this, I thought I'd at least have one less box to deal with (the DA for the sub) than going with a DDC. There are few products in this category, and upon trying some of them, I had the biggest discovery of this journey: most of their digital outputs lag behind their analog outputs by ~200ms! I am 99% sure this is but a coincidence and would otherwise be considered a design flaw, but for me it miraculously solved 3 problems with 1 box. It'd function as
However, I present the complete system I had built around the SMSL and the Backbeat, since I didn't have a Mojo to start this project with. Overall, the goal was to
It pretty much only works when the IEM volume is sufficiently loud, such that the listening experience sonically necessitates a physical, thumping, hard-hitting bass. It does the job: when I turn off the Backbeat after listening with it for a while, it sounds like there is less bass. In what follows, I will try my best to describe the sonic experience, what works and what doesn't, of using a Backbeat.
This is easily my longest ever post on head-fi, so thanks for sticking it to the end. If you'd like me check if this setup "works" for the type of music / IEM tuning you have, feel free to comment below.
- Backbeat G2 ($399)
- SMSL PO100 AK ($89, discontinued)
Backbeat G2
Backbeat G2 is a wearable subwoofer advertised towards bass guitar players, as a stage monitoring device for when it is difficult to hear bass guitar amp on-stage. Bass players have been using the Backbeat in IEM gigs since 2018. For years I've wanted the portable 2.1 idea to work, and it's only when the Backbeat came by that I saw hope. Compared to the likes of gaming-oriented battery-powered wearable subwoofers like Woojer and Subpac, the Backbeat has several advantages- It is way more accurate/neutral in the audiophile sense. I cannot explain it better than Backbeat’s own product description: “If you play the A string on your bass, the moving mass will oscillate 55 times a second. You'll feel this through your body, and your mind will instinctively know you're playing an A.” I find that it is possible, for example, to tell what song is being played by only "listening" to the Backbeat with my body, with nothing playing in the ears. Woojer and Subpac, in comparison, sound like a rumbling mess. They promise to vibrate when you feed them bass, that’s it. From my first-hand experience with these products: the “sound” is monotone both in the input frequency they vibrate to and the frequency they vibrate at. There are online reviews about how terrible they are for music listening, most notably the one by @GoldenSound , and they are absolutely correct.
- It is designed for on-the-go usage. The only other realistically portable option is the Woojer Strap. But it is, as previously stated, not suitable for serious audio applications. The Backbeat G2 comes with a clip that is designed to clip on to 3/3.5-inch guitar straps, making it possible to be attached to the shoulder straps of a crossbody/sling bag or placed inside the compartment closest to the body in a backpack, for a true portable experience.
- It is designed to monitor an un-amplified bass guitar. It has a low-pass filter built in, but since the key range of a bass guitar is typically open E at 41Hz to D# at 311Hz, Backbeat G2's frequency response by design is even wider, and will playback *some* lower midrange content. It definitely can use the help of a lower low-pass filter. This is technically fixable with Backbeat’s upcoming Bluetooth companion app, on which the user can EQ the frequency response of the G2.
- It is not designed to mesh with the music. When the "volume" or rumble intensity is too high, the Backbeat becomes a distraction to the music playing in the ears. It is not an issue to the Backbeat's intended audience, but it is when the device is used to compliment a "main" signal. Therefore, the sub volume needs to be carefully tuned.
- It has a great dynamic range, and would reveal dynamic range problems in bass in otherwise well-mastered music. For example, when a bass instrument solo in a song is followed by the addition of other bass instruments, the low frequencies suddenly get busy and the compressor is forced to sacrifice the volume of the soloing instrument in the mixing process. The intensity of the rumble suddenly reduces. It is annoying when it happens, despite it being the "right" thing to do when the mix lacks dynamic range
SMSL PO100 AK
Now that we have a subwoofer, we need to figure out how to get a separate analog signal from a portable source, e.g. a DAP, to feed into the subwoofer, much like the LFE output on a preamp. Here are some of my failed attempts:- DAP with two analog outputs: All DAPs with balanced and SE outputs I’ve tried would disable one when both are plugged in. Chord Mojo legitimately has two analog outputs, but their volume cannot be controlled independently.
- Pro multi-channel headphone amplifier, like the PreSonus HP4, on which I can adjust the main and the sub volume independently. The problem is that none of them are battery-powered or designed to be space-efficient for portable use–no live musician needs to carry two stereo signals in their back pocket.
- 3.5mm splitter. When the Backbeat G2 and an IEM are connected to a 3.5mm splitter plugged into a DAP, the IEM would sound extremely distorted. Is G2 using all the current?
- DAP with SPDIF output. The idea here is that I can use an external DAC/amp connected to the DAP’s SPDIF out as the main signal, and the analog output from the DAP as the sub signal. If the DAP’s SPDIF out is fixed volume, then I have two volume controls: DAP controls the sub, external DAC/amp controls the main. Unfortunately, all such DAPs I've tried (iBasso, FiiO and A&K) would disable the analog output when both are plugged in, even if the SPDIF does not share the same 3.5mm jack as the SE output.
- Dual-streaming bluetooth adapter / Samsung smartphone with Dual Audio. These Bluetooth adapters are usually intended for two people playing a Nintendo Switch. All Samsung smartphones post Galaxy S8 can play to two Bluetooth headset simultaneously. The idea here is that I can DA both of those signals with two DAPs with Bluetooth receiver functionality. I was excited to get this fully wireless setup to work, only to face a bigger problem: Backbeat G2 has a delay of about 200ms. I later checked the Woojer Strap/Vest and they are worse, with about 250ms delay. This is because these devices internally do AD-DSP-DA, to support app-controlled functionalities in the digital domain. When the bass lags behind the music by so much, it is basically unlistenable.
- Lip sync corrector with analog I/O, like this small box or this battery-powered one. I quickly found out that analog-only audio delay doesn’t exist. To delay audio, it needs to be stored in a memory of some sort for a set time; this means that the main signal has to go through DA-AD-DA before the signal goes to my AMP/IEM. Unacceptable by head-fi standard.
- Lip sync corrector with digital I/O, like this big box or this bigger box. Now these are audiophile-proper products, most of them employing the Texas Instruments TPA5050 chip. But they are bulky, as they are intended for home theater use.
- DSP. I learned that milliseconds of time alignment is a common task in crossover speaker design, and DIY folks would use miniDSP for this purpose. However, they are large and not battery powered, and often are too feature-rich for my simple use case. In the end, I found this DIY Toslink delay device. Small footprint, but still requires an external power bank.
- A digital audio splitter
- A 200ms digital delay device (for the main signal)
- A DAC (for the subwoofer signal)
- It does not require additional power or need to be charged. It is powered by the USB C host.
- It is very small and slim, perfect for portable use.
- It provides a sonic improvement over my DAPs with a Toslink output, as well as other similarly sized competitor products.
The complete system
- A USB C DAP
- SMSL PO100 AK
- Chord Mojo (the smallest battery powered DAC/amp with SPDIF input)
- Your IEM
- Backbeat G2
- Maximize sound quality.
- Have all controls, charging ports and analog outputs face towards the same direction, so that everything is easily reachable when the rig is placed inside a crossbody bag. No pressure is put on any of the sockets when put into the bag.
- Do cable management!
- Tempotec V3-D
- SMSL PO100 AK
- Chord Hugo 2
- Enleum HPA-23RM
- Final E500
- Backbeat G2
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- Flat, angled 20cm USB C to USB C cable (V3-D to PO100 AK)
- Generic RCA to 3.5mm cable, coiled 3.5mm to 3.5mm cable (PO100 AK to Backbeat G2)
- Supra ZAC 15cm Toslink cable with angled Toslink adapter (PO100 AK to Hugo 2)
- Custom Gotham GAC-1 Ultra Pro angled RCA cables (Hugo 2 to HPA-23RM)
- Flat, angled 20cm USB C extension cable (to charge the HPA-23RM from the front)
All:
Top:
Front:
Back:
Left:
Right:
Top:
Front:
Back:
Left:
Right:
Wearing the Backbeat in the back:
Wearing the Backbeat in the front:
Wearing the Backbeat in the front:
What does it sound like?
Overall, it is an experience that resembles a subwoofer in a room but not quite. It sounds more “accurate” (it is a bass guitar monitor, afterall) but less “impactful”. If I have to describe it in one sentence: a real subwoofer sounds like pressure that wraps around the skin, a bass-shelved IEM sounds like pressure in the head, and Backbeat sounds like a hifi version of bone-conduction from the rumbling floor of a live concert.It pretty much only works when the IEM volume is sufficiently loud, such that the listening experience sonically necessitates a physical, thumping, hard-hitting bass. It does the job: when I turn off the Backbeat after listening with it for a while, it sounds like there is less bass. In what follows, I will try my best to describe the sonic experience, what works and what doesn't, of using a Backbeat.
- You may have noticed that I wear the Backbeat facing towards the body, whereas the bass guitar players wear it facing outwards. This is for "sound" quality reasons. Having the haptic device making direct, maximal and firm contact with the body is key to getting a good, detailed sound. The sound improvement is huge compared to the minor downgrade in comfort.
- In the middle of the collarbones: this is my favorite placement, SQ-wise. It basically does what bone-conduction headphones (which, by the way, are terrible at bass reproduction) do, but 1) only to the low frequencies, 2) from physically below my head and 3) with impeccable intensity. The fact that the "bone-conduction" happens below the head is a pretty insane premise. Imagine bone-conduction headphones with crossovers, and that low, mids, high are conducted at the low, mid and high position of the head, physically creating a vertical "soundstage". This is what the collarbone placement does, except only to the low frequency. Compared to the other placement, this one is more "heard" than "felt", in the same way bone-conduction headphones are "heard". The low frequencies are heard through the vibration of the neck, the skull, to the inner ears, blending seamlessly with the music played in the ears. The downside is that I don't know how to wear the Backbeat on the collarbones without looking like an idiot.
- On the chest: this is where the bass is somewhere between "felt" and "heard", as the skull is no longer being vibrated directly. The intensity of rumble needs to be turned up higher than the collarbone placement, but not too high to cause distraction. The experience "sounds" like a mix of the collarbone placement and the rumbling floor of a live concert, except not all of my organs are shaking, only the rib cage. There is still a good amount of bone-conduction-to-the-inner-ears going on, a plus for SQ. This is my go-to placement if I am wearing the rig out and about, and would like to maximize sound quality.
- On the back: this is where the bass is more "felt" than "heard". This is the Backbeat's intended wearing position for the bass guitar players. I almost need a bit of bad posture for this to work (most guitar players have pretty bad postures on stage, so it works for them), as the Backbeat needs to be pressed against a bone, in this case the spine. This is the placement that most resembles the vibration of a car stereo or a movie theater. It takes some getting used to, as the sensation is more localized than what it would be in a seat. But once I have gotten used to the sensation, it sounds quite similar to the chest placement, albeit requires a little more rumbling intensity.
- Bonus - On the back, while lying in a bed: Stereo imaging is all screwed up when lying down, but damn this is sensational. Imagine the experience of a 4D movie theater, but way more hifi. The vibration, to some degree, wraps around the entire body, like a real subwoofer would. The vibration is both heard and felt, as the bed now sends vibrations to the skull. Try Travis Scott - 90210 or any industrial bass music; mind-blowing experience.
- It depends on the volume of the music, but since PO100 AK outputs line-level signal, it is much hotter than the output of a bass guitar. Even when Backbeat's volume is turned to the lowest, no one would survive the rumble of a line-level input. Thus, the ability to adjust the volume on the transport is a must. Luckily, by design, PO100 AK's analog output volume is controlled by the transport, while the SPDIF outputs are fixed at max volume (great for the sound quality going into Hugo 2). On most songs, I need to turn the volume of the TempoTec to 50%-70%. On bass-heavy music, the volume needs to be lower; on bass-light music, the volume needs to be higher. All while keeping the Backbeat at the lowest volume setting.
- The exact volume needs to be tuned on a per-song or, at a minimum, per-album basis. Too low, there is no rumble felt. Too high, the rumble becomes a distraction. Generally, the more bass heavy the music is, the wider the comfortable range. Depending on the genre and the recording, the comfortable range can get quite small, and requires meticulously finding it through trial-and-error and experience. In extreme cases (<5% of my library), the comfortable range is so small that there is only 1 exact volume (out of 100 on my TempoTec) that feels right. They are usually music that has a lot of "dynamic range" in the lows, perhaps not in a good way. For example, when the kick is mixed to be much louder than the bass guitar, I'd either have to live with not being able to feel much of the rumble from the bass guitar, or bear with a distractingly intense rumble from the kick.
- My rule of thumb is that the loudest rumble in the song shouldn't "tickle" me. As long as the Backbeat is below the "tickling" threshold, it will only add positive to the experience.
- Lacking a crossover mechanism (for now; see more in Q&A) like a real 2.1 system, the natural question is, does it work better with IEMs that already have a lot of subbass or with IEM that have a bass roll-off? My experience is mainly based on Truthear Zero & Red vs Final E500, and I much prefer the latter.
- As previously stated, the top range of Backbeat G2's frequency response is too high, so it doesn't really compliment a "full range" main signal in the same way as a real subwoofer. It is weird when mid and upper bass content is heard and felt at the same time. The two sensations don't gel well, if at all. It's like "hearing" the bass twice, but simultaneously...
- The "simultaneous" subbass is just as bad as the mid and upper bass. For lack of a better term, IEM subbass (like Truthear Zero with a 10Ohm adapter) sounds like "air pressure in the head", with which the wearable subwoofer experience has nothing in common. Again, it's like simultaneously "hearing" the bass twice, but one is in the head and the other is on the body. The bass on the body weirdly makes the bass in the head sound distorted, and the overall music congested. I don't exactly understand the mechanism that causes this impression. Perhaps there is just simply too much bass when more bass is added to a subbass-sufficient IEM.
- The bass-rolled-off Final E500 does great with the Backbeat. I like it not only because it is my personal favorite even without the haptics, but also because it is a very "sensually pleasing" IEM. Its semi-open and feather-weight design checks all the psychoacoustic boxes to convince me that I should be listening to the music with my body together with my ears, if that makes any sense. However, IMO the low-pass filter of Backbeat G2 is too high even for Final E500.
- Hip-hop/Trap/DnB: Absolutely life-changing, due to how loud and monotone the kick and the bass are in the mix, all the way down to the lowest octave. From Kool G Rap & DJ Polo to Future & Metro Boomin, all are absolutely insane. IMO it is a better experience than bumping loud music in a car or live concerts. It’s like a concert if the venue has the most accurate subwoofer ever, the music is not drowned in noises and “imaging” is now a thing. It’s like being alone in a live venue, except the venue *is* the mix.
- Techno/House/Italo/Four-one-the-floor electronic music: It works, but the sub volume needs to be turned down quite a bit compared to Hip-Hop. I guess that the volume of bass (proportional to the entire mix) is less than in Hip-Hop. It works, but most of the time it is kind of boring. The Backbeat rumbles to the kick and pretty much only the kick. However, with the Backbeat, it is an entirely new experience to "feel" the texture of the kick in different tracks, especially in modern techno (e.g. Falling the Same Way) where a lot of thoughts are put into making the kicks sound interesting.
- Trance: is the only problematic electronic genre. When trance finds its identity in mid 90's, it converged to a sound where the bassline "drives" the music. The kick mostly occupies the midrange and the bassline occupies the bass while being slighly behind the beat. Backbeat rumbles to the bass and not the kick, and thus it feels as if the impact lags behind the rhythm. I suspect that the fundamental problem here is that the “crossover frequency” or the top range of Backbeat G2's frequency response is too high.
- Recorded Rock music: Once the subwoofer loudness relative to the headphone is nailed down, this is at least an improvement over no-sub. In 90's and 00's music where the kick is louder, Backbeat would mostly react to the kick. It’s only on things like Linkin Park where the mastering is closer to hip-hop than rock can the Backbeat give enough attention to both the bass and the kick. In the more grainy and bass-light 80’s music, Backbeat does little to nothing. 70's is great; you can bet that those that are often used for audiophile testing (Floyd, Sabbath, Zepplin) are great with the Backbeat. However, I do find the enjoyable range of rumble intensity is more narrow with rock music than with electronic music. The sub volume needs to be carefully tuned, almost on a song by song basis, to ensure the impact is felt without causing a distraction.
- Live Rock music: this is where things don’t work out as much as I had hoped. I struggled to find one song from Iron Maiden’s laundry list of live recordings that has any interesting subbass content. I wanted to recreate the overwhelming rumbling and loudness of a rock concert played in an arena, but the Backbeat G2 made me realize that most of the live rock recordings are not mastered to preserve that information. The more audiophile-friendly recordings like Eric Clapton Unplugged or Nils Lofgren Acoustic Live are amazing with the Backbeat, with it rumbling to drum kicks, bass guitar, guitar’s open E, piano solo. The usefulness is really up to the quality of the recording, mixing and mastering. And I never realized how many poor live rock recordings are out there until the Backbeat.
- Jazz: I listen mostly to post-70’s Jazz, e.g. ECM. Backbeat rumbles to the double bass and, if there isn’t a double bass, to the piano, all with great dynamics. Percussions are greatly enhanced too. As most Jazz mixes are less complex and more “audiophile” than the typical audiophile rock recordings, Backbeat shines with Jazz. I am unsure whether Backbeat correctly reproduces the relative loudness between the double bass and the percussion, but I guess it’s really up to the recording itself.
- Classical: no. IMO subwoofers are useful in a 2-channel system for classical music playback precisely for the thing that the G2 cannot do: creating the sense of pressure on the skin, the feeling of “being there”. Backbeat cannot do that even when pressed hard against the spine. Backbeat feels much closer to the shaking floor of an amplified concert than the natural acoustics of a concert hall.
Why TempoTec V3-D?
- This is a decision mainly based on battery-life concerns. The transport needs to supply power to the PO100 AK while serving as the USB-C host. Some DAPs would die quickly (Hiby R3 II), others could even insufficiently power the PO100 AK. TempoTec V3-D has one of the biggest batteries in its footprint (3500 mAh). It is the longest-lasting component in the entire rig, as a digital transport should be.
- Because of how big and heavy this rig is, I'd really like to be able to switch songs without taking it out of the sling bag. TempoTec / Hiby OS makes it easy to control music with a smartphone. Hiby Link, two-way Bluetooth, Airplay, DLNA all supported.
- By my ear-measurement using the DIY Toslink delay device, the accidental delay by the SMSL PO100 AK isn't perfect. The main is still about 5-10ms ahead of the sub. Please don't quote me on this number, as any digital delay device could add to the delay, and thus lead to underestimation. I could also be imagining things and I doubt 5-10ms of latency is scientifically humanly noticeable, except with very high BPM songs.
- If you take a look at Backbeat G2's product page, it has more features and complexity than what I had mentioned. It has two analog inputs and two analog outputs. One 1/4'' input from the bass guitar, one 1/8'' input from the IEM mixer, one 1/8'' output to the IEM for stage monitoring (we are obviously going to ignore it), one 1/4'' output to pass-through the bass guitar to the guitar amp. It is designed for the complete workflow of stage-monitoring, with IEM playback in mind. The Backbeat, somewhat unintuitively, rumbles to the input from the IEM mixer too. Why a bass player would want to feel and monitor the rest of the mix, I have no idea.
- The aux input does, however, provide an alternative "sound". Its upper range of frequency response is much lower than the instrument input, technically making it more similar to a real subwoofer's crossover. However, I have not found a song on which I prefer the "sound" using the 1/8'' input to that using the 1/4'' input. The rumble from the 1/8'' input "sounds" less precise and dynamic than the 1/4'' input. Perhaps the 1/8'' input's low-pass filter is too low, or my Final E500's bass roll-off is too high?
- The bluetooth EQ app promises the ability to EQ both inputs, so hopefully the two inputs are in fact identical up to the user selected FR / low-pass filter.
- As previously state, the Backbeat G2's default frequency response range is too high to be a true subwoofer. For example, when a synthesized bassline in modern electronic music plays a climbing melody, and the G2’s rumble would fade as the bassline rises over 400Hz. Here's a Temporary fix with the Dayton Audio DSP-LF Low Frequency DSP. Quite portable. Someone should give it a try. It will, however, almost certainly introduce a delay to the sub signal that the main signal now needs to account for...
- The ideal crossover frequency, between a bass-rolled-off IEM and a wearable subwoofer, is IMO higher than the common 90-120Hz in two-channel. It is a plus if a wearable subwoofer can double as a bone-conduction driver to deliver some of the mid and upper bass content, with carefully selected relative intensity. The Backbeat's FR range, given its default low-pass filter, is reproducing too much upper bass content to my liking.
- And it is for the above reason that, IMO, even when we are able to EQ the Backbeat, finding an IEM that compliments the Backbeat is just as important as EQing the Backbeat to compliment the IEM. For example, with a subbass-heavy Fatfreq IEM, one likely needs to EQ the sub signal down to only the lowest of the subbass. At that point, Backbeat isn't really doing much.
- I'd admit, however, that finding the perfect bass-shy IEM to use with a wearable subwoofer is a peculiar task, especially in 2024 when everything has a bass shelf. In addition, we deserve bass even when we decide not to wear the bulky subwoofer from time to time. Thus, EQing the main signal should be in consideration as well. There is probably an "IEM target curve" when a haptic device is in use, showcasing the ideal IEM roll-off, complimenting an ideal subwoofer low-pass filter. Going down this path doesn't require reworking the entire plan: we just need the DAC connected to the PO100 AK's SPDIF output to have an EQ functionality. The question is whether this will introduce additional delay to the main signal.
- Backbeat G2 (using the 1/4 input) is a bass guitar monitoring device. Bass guitars are mono instruments, so is the 1/4 input to the Backbest G2. You may have noticed from the photos that, in my rig, the right channel of SMSL PO100 AK's RCA is not connected to the Backbeat, as doing so will technically short the SMSL's right output channel (not sure if the SMSL's circuit has protection against that). The proper way to feed a stereo output to a mono input is through mono summing. I custom-made a cable that does it. The advantage of mono-summing over taking the left channel is obviously that in the case that there's "stereo bass", I won't miss the rumble from the right channel. The disadvantage is the potential phase cancellation. In reality, I can hear neither in my music, so for now I've settled with only the bass from the left channel. This may be problematic when the music is more complex.
- While on the topic of stereo bass, what about adding another Backbeat, making it a 2.2 system, possibly by wearing them on the shoulder straps of a backpack in some way? I don't know. Someone should give it a try, given that PO100 AK has a stereo output anyways. My initial reaction to this is that, even if the two vibrating devices on the chest somehow blends well with each other (as in listener isn't distracted by the fact that there are two sources of vibration), they aren't very separated from each other within the soundstage/headroom portrayed by the IEM anyway, since both are "below the head". So even if the bass content is stereo / directional, they'd be pretty close to each other.
- Full-sized headphones, especially the ones with bass roll-off such as the HD600, sound wonderful with the Backbeat. However, I'm in the headphones-are-not-portable camp, and if I'm going to be sitting down at home listening to headphones, I'd invest in a Buttkicker Concert. Don't know how accurate it is compared to the Backbeat, but it can't be too bad given its intended audience and the sheer size and power it has over the Backbeat. It rumbles your listening chair, and that extra contact surface over a device on the shoulder strap *should* translate to advantage as well.
This is easily my longest ever post on head-fi, so thanks for sticking it to the end. If you'd like me check if this setup "works" for the type of music / IEM tuning you have, feel free to comment below.
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Tempted to do something similar as I have a hugo 2 and love electronic bass music. Do you know of any alternative to the smsl as its discontinued?
