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We compare products from their published specs and features; the verdicts below are our independent editorial opinions.
If you want a do-everything hi-res hub for your car or home stereo, choose SLASH 4; if you need a pocket travel adapter for in-flight and gym listening, choose Avantree Relay & Voyager. That is the whole call in one line, and the rest of this page is the reasoning behind it. Both devices lean on Qualcomm chipsets and both advertise aptX Adaptive, but they solve completely different problems — one is a fixed installation, the other lives in your bag.
• SLASH 4 — biggest strength: ESS ES9219 quad DAC plus a TI OPA1692 op-amp, Sony LDAC at up to 990kbps, aptX Adaptive, aptX HD, and 24-bit/96kHz output with a claimed 130dB SNR.
• SLASH 4 — biggest trade-off: It pairs with one device at a time by design, needs USB power, and needs the included 3.5mm cable to actually pass audio — it is not a grab-and-go gadget.
• Avantree Relay & Voyager — biggest strength: Bluetooth 5.3, aptX Adaptive and aptX Low Latency in something pocket-sized, with on-board volume, pairing and battery-level controls and LED status indicators.
• Avantree Relay & Voyager — biggest trade-off: It is built for 3.5mm AUX sources like airplane seats, and Avantree states plainly it is not compatible with phones, car stereos or hearing aids.
• Who each is for: SLASH 4 for someone permanently upgrading a car stereo or home setup; Avantree for a traveller or gym-goer who wants their own headphones working on any 3.5mm jack.
SLASH 4 is a fixed-installation Bluetooth DAC, not a dongle. The core of it is the ESS ES9219 quad DAC paired with a Texas Instruments OPA1692 op-amp, feeding a 24-bit/96kHz signal out with a 130dB SNR figure. On the wireless side it runs the Qualcomm QCC5125 chipset and supports Sony LDAC at up to 990kbps alongside aptX Adaptive and aptX HD. It is powered over USB and housed in a navy metal body measuring 72mm x 22mm x 13mm — small enough to tuck behind a dash or sit on a shelf without becoming the centrepiece of your install.
What it does best is clean up a noisy signal path. There is a built-in ground loop isolator plus an LDO regulator, which is the part that matters if you have ever chased alternator whine or a hum through a car audio build. The single 3.5mm port pulls triple duty: analog AUX for vehicles, digital optical for home and DSP amplifiers, and a headphone amp output. That one port is why I would call this a tool for installers and commercial background-music setups as much as for hobbyists — the same box can serve a car, a receiver, or a rack.
Who should skip it: if you want to switch between your phone and your laptop on the fly, this is the wrong device. SLASH states the one-to-one pairing is deliberate — one connected device at a time, to protect bandwidth and stability. And note the practical setup points: the USB port is power only, you must use the included 3.5mm AUX cable for audio, and you should unpair from your car's hands-free profile to avoid conflicts. SLASH also suggests leaving your phone volume near 80% to avoid overloading the input.
Avantree Relay & Voyager: Who It's For
Avantree Relay & Voyager is the opposite philosophy: small, battery-minded and built to travel. It runs Bluetooth 5.3 on a Qualcomm chipset with aptX Adaptive and aptX HD, which is the same codec family SLASH 4 uses — but here it is aimed at headphones rather than a stereo. You plug the adapter into a 3.5mm AUX jack, and it streams that audio to your Bluetooth headphones. Avantree's own example is the airplane seat jack, and that is exactly the use case: your AirPods or Bose stay on your head instead of the airline's disposable earbuds.
The feature list is about convenience, not conversion. There are built-in controls for volume, pairing, reconnecting, clearing pairing history and checking battery level, plus LED indicators so you can see connection and charge status at a glance. aptX Low Latency is supported too, which matters if you are watching a seatback screen and do not want the audio trailing the picture. Avantree backs the wireless side with 20 years of expertise in this category, and the adapter is described as lightweight and pocket-sized — the kind of thing that disappears into a jacket pocket.
Who should buy it: frequent flyers, train commuters, and gym users who want to keep their own headphones on and their own volume controls in hand. It is compatible with AirPods, Bose and Bluetooth headphones generally.
Who should skip it: anyone hoping to use it as a car Bluetooth receiver or a phone accessory. Avantree explicitly states it is not compatible with phones, car stereos or hearing aids. If your goal is upgrading a car stereo or a home amp, that is SLASH 4's job, not this one.
Side-by-Side: Which Should You Choose?
• Best for home stereo and DSP amplifiers → SLASH 4. The 3.5mm port doubles as a digital optical output, which the Avantree adapter does not offer.
• Best for headphone listening at home → SLASH 4. The same port works as a headphone amp, and the ES9219 quad DAC is the reason to care.
• Best for flights and travel → Avantree Relay & Voyager. Pocket-sized, battery-level indicators, and made to plug into a seatback 3.5mm jack.
• Best for gym sessions → Avantree Relay & Voyager. Lightweight, portable, and controlled from the adapter itself.
• Best for multi-device switching → neither, honestly. SLASH 4 is deliberately one-to-one, and Avantree's adapter is not intended for phone or car use at all.
• Best for installers and commercial BGM → SLASH 4. Metal housing, heat-resistant design, 24/7 rated, and optical output for rack equipment.
• Best for low-latency video watching → Avantree Relay & Voyager. aptX Low Latency is the specific feature you want when audio has to stay in sync with a screen.
On the codec question that brings most people to this comparison: both support aptX Adaptive, and both use Qualcomm silicon. SLASH 4 adds Sony LDAC at up to 990kbps and the quad DAC stage; Avantree adds Bluetooth 5.3 portability and low-latency support. Neither is "better" at aptX Adaptive — they apply it to different jobs.
In my opinion, the decision comes down to whether the device stays put. If it does — a car, a home receiver, a DSP amp, a commercial rack — I would buy the SLASH 4. The quad DAC, the ground loop isolator, and the triple-role 3.5mm port are features you cannot replicate with a travel dongle, and the metal, heat-resistant build suits permanent installation. If the device travels with you, I would buy the Avantree Relay & Voyager; it does one job well and fits in a pocket. Check the current price on Amazon for whichever one matches your setup before you commit.
what is aptx adaptive headphones
aptX Adaptive is a Qualcomm Bluetooth audio codec that adjusts its bitrate and latency based on what you are doing — higher quality for music, lower latency for video. Both products here support it: SLASH 4 through its QCC5125 chipset, and Avantree Relay & Voyager through its Qualcomm chipset, where it sits alongside aptX HD and aptX Low Latency. You need headphones that also support the codec to benefit.
can Avantree Relay be used with car stereo
No. Avantree states the Relay is not compatible with phones, car stereos or hearing aids — it is designed for 3.5mm AUX sources like airplane seat jacks, feeding Bluetooth headphones. If your goal is adding Bluetooth to a car stereo, the SLASH 4 is the product built for that, with its ground loop isolator and AUX output.
does SLASH 4 work with home stereo
Yes. The 3.5mm port can output analog AUX or digital optical, so it can feed a home stereo receiver or a DSP amplifier, and it also works as a headphone amp. Keep in mind it is USB-powered, needs the included 3.5mm cable for audio, and pairs with only one device at a time by design.
which sounds better, SLASH 4 or Avantree Relay
They target different chains, so the comparison is not apples to apples. SLASH 4 runs a 24-bit/96kHz path through an ESS ES9219 quad DAC and TI OPA1692 op-amp with a 130dB SNR figure, plus Sony LDAC at up to 990kbps on supported headphones. Avantree's strength is aptX Adaptive and aptX Low Latency in a portable adapter. For a fixed stereo or car install, I would expect the SLASH 4's DAC stage to matter more; for travel, the Avantree wins on convenience.
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