Once you start exploring desktop audio, it’s easy to fall down a rabbit hole of amps, digital-to-analog converters, and headphones. I’m going to be posting a list of the gear I use regularly, but it’s been nice to explore something new.

The latest bit of gear I got to test was the $199 $159 Fosi Audio ZH3, a desktop DAC designed for headphones and powered desktop monitors. The DAC is on sale now for Black Friday at 20% off.

First, a bit of explanation. A DAC is, as the name says, a digital-to-analog converter. It takes the digital signal from your computer, from iTunes or Spotify, and turns it into an analog signal that real-world gear can use. That signal then goes out through the jacks on the DAC so you can plug in headphones or speakers.

On a basic laptop, this job is handled by a tiny chip on the motherboard. It works, but it is usually noisy and weak. A separate DAC gives that job to a dedicated box with better parts and more power. You get a cleaner signal, more detail, and often less hiss when you turn the volume up. In this case, the DAC is aimed at headphone use, so it is tuned to drive headphones well, with enough power and control for larger or more demanding cans. You can still run it into powered speakers or an amp if you want, but the main goal here is to make your headphones sound like they cost more than they did.

On the back is a set of RCA in for analog sound, a USB-C, coaxial in, and optical in. It also supports a 12V trigger system that can turn all of your devices on and off at once with the right gear. It’s a nice-to-have addition.

Tired of robotic sound and sore ears, I tried the Fosi IM4 and got my music back

John Biggs · Nov 14

I’ve been out of the personal audio space for a bit simply because I thought I found the right mix between comfort and quality. My daily drivers, my AirPods Pro, however, have become problematic. When folks call me, I sound like a robot, which is cool for scam calls but not so great for doctors and probation officers. Further, my

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Inside the box, you get a few very interesting chips. The short version is that it is a modern AKM plus XMOS box with a lot of clean power on tap, and it sounds like that reads on paper.

The AKM4493SEQ is one of AKM’s current mid to upper chips. It does Pulse Code Modulation up to 768 kHz and DSD512 and is known for a smooth, slightly relaxed take on neutral rather than a hard, clinical edge. Fosi backs it with an XMOS XU316 USB receiver and four OPA1612 op amps, then feeds the whole thing from a fully balanced dual rail power stage with low noise regulators. That is why they can quote noise down around a couple of microvolts and Signal-to-Noise and Distortion in the 110-plus range on the balanced out.

The interface is quite simple. You tap the volume dial to manage inputs and outputs, and there is some basic treble and base management along with gain. There are some filter modes that change the sound slightly but not in ways you’d expect. This isn’t some kind of Bass Boost button. Instead, it changes the way the DAC handles the audio.

One thing to not: the RCA in — something I use regularly — is limited in volume, which ensures that you’ll probably stick with USB in to maximize the audio headroom.

It also includes a cute little remote control for quick input switching, allowing me to put this thing back into my standard audio chain as needed.

Power is not a problem. The 4.4 mm jack is the balanced output. At 32 ohms, it can give each channel, left and right, up to about 2.5 watts of power. That is a lot for headphones. It means it can drive hard-to-run planars and big dynamic headphones to very high levels without strain.

The 6.35 mm jack is the single-ended headphone output. At 32 ohms, it tops out around 0.64 watts per channel. On paper, that is less than the 4.4 mm balanced jack, but it is still strong enough for most headphones you are likely to own.

For normal dynamic headphones in the 32 to 80 ohm range, that much power gives you a lot of headroom. You can reach very loud levels without the amp sounding strained. With my planar headphones, it still has no trouble. I can turn the volume up to the point where it feels almost uncomfortable, and the sound stays clean. No crackle, no obvious distortion, just more level. That tells you the amp is not running out of steam.

In daily use, you do not need to go anywhere near the limit. A sane setting is around 50 on the volume knob, with your computer’s output set to roughly three-quarters of maximum. That way, the DAC has some room to work, the noise floor stays low, and you still have extra volume in reserve if a track is recorded quietly. If you use very sensitive headphones or IEMs, you will sit lower on the dial, but the idea is the same. Get to a point where the volume feels right, you have some space to move either way, and the sound stays clean and relaxed.

So what does it sound like? These are clean, neutral, and quiet. Bass is tight and controlled, not fat. The middle band is clear and honest, especially with higher impedance cans, and treble reaches up without turning sharp. Some listeners hear a hint of that classic AKM smoothness, a bit more calm than the sharper ESS house style, but nothing syrupy or messy. Stage and imaging are solid, slightly wider on the balanced side, and there is no extra warmth or glare added by the amp itself.

The device can also drive other devices, including powered speakers, but you’re probably going to want to use these for headphones only. I typically use the standard 6.35 mm jack and cable for my headphones, but I’m going to swap out to balanced cables in order to experiment with this DAC.

I’ve been using the XDUOO TA-10R headphone amp for a number of years in a primarily analog way thanks to the way I have my desk set up. That’s a $271 hybrid tube amp that was perfect for everything I did with the headphones, which was some music listening and Zoom calls. This DAC, on the other hand, is inspiring me to try to listen to much more headphone audio, something that the TA-10R never encouraged.

The ZH3 sits comfortably at the entry-level audiophile system. It’s a solid DAC with modern conversion algorithms and loads of power for headphones and speakers. It’s a lot of fun to use, has a clear and open soundstage, and doesn’t mess with the audio beyond conversion and output. At $199, you’re going to get something that will eventually make your cheaper headphones sound pretty bad, which, in turn, will force you to upgrade your cans and then your speakers and then everything else in your life. Until then, you can definitely enjoy this amp without fear of becoming obsolete or missing a single beat.

Most people never think about the battery until it dies. Charlie Welch has spent his whole career thinking about nothing else.

On The Innovators, he walked me through what he is building at Proper Voltage. Before this, he was doing applied research in battery chemistry at Northrop, trying to get “interesting and exotic” chemistries into real military systems, from underwater gear to aircraft to special operations kits. The problem he kept hitting was not physics. It was integration.

Every new battery team heard the same thing from the customer. “We’d love to use your tech, if you redesign it for our system.” Every big OEM said the opposite. “We’d love to use your battery, if you redesign your system for us.” No one wanted to move first. No one wanted to touch legacy hardware. That standoff kept better chemistries on the shelf instead of in the field.

Proper Voltage is his attempt to cut through that. The company is not betting everything on one magic chemistry. Instead, they are building a kind of battery operating layer that sits between the cells and the product. They work with sodium ion, lithium titanate, niobium and other “odd” chemistries that each have their own strengths. Then they add a hardware and software block they call a voltage command unit. That unit makes voltage a programmable interface. One pack can now present itself as whatever the device expects, without the device designer having to rip up their boards or add twice as many cells.

He gave a clear example. Standard lithium ion cells sit around 3.7 volts. Many sodium ion cells sit closer to 2.3. If you try to drop sodium into a system that expects lithium and do nothing else, you need almost twice as many cells in series. That means more size, more cost, more weight, and energy you do not actually need. With a programmable voltage layer, the system sees what it expects. The chemistry can change underneath without touching the rest of the stack. The product team gets to pick sodium for safety and life, not walk away because the nominal voltage is “wrong.”

This matters a lot in defense and aerospace. There are standard formats like the 6T battery for vehicles or the soldier’s small tactical universal battery. There are missiles and aircraft that went through years of testing and certification. No one wants to open those designs just to squeeze in a new pack. Welch told me the only way that community moves is if the new unit is truly drop in. Same form factor, same pins, same expectations. If Proper Voltage can let new chemistries look and behave like the old packs from the outside, while giving better power and life on the inside, that is a real wedge.

We also talked about the state of the art. Phone batteries have been roughly the same “spicy pocket brick” for a long time. The gains are slow. Roughly a few percent each year in energy density. The reason you do not feel a huge leap is that every time battery teams squeeze out another watt hour, the chip and software teams spend it on more compute, more video, more background tasks. The pack improves. Your day of use feels about the same.

The big jumps happen when you change chemistry outright. Welch mentioned a humanoid robot project that switched from its old pack to a lithium titanate system. Charge time dropped from three hours to about six and a half minutes. Peak power went up by a factor of four. That is not a small tweak. That is a new class of behavior for the machine. Proper Voltage sits in the middle of moves like that, making sure the robot still sees smooth, stable voltage even when it is sprinting or jumping. When robots do high dynamic moves, voltage usually swings all over the place. Their system flattens that, so the robot sees the same “fuel” from full to empty. There is a small efficiency hit in the power electronics, but because the device can use more of the pack’s range without tripping over low voltage issues, you often end up with more usable energy, not less.

The near term focus at Proper Voltage is not sci fi robots, though that work is clearly a proving ground. The team is leaning into three markets. First is infrastructure, backup power for telecom sites, data centers, LNG plants, all the places that still sit on old lead acid banks. Sodium ion and other chemistries are well suited there, and Welch likes that the United States has the raw materials to build a domestic supply chain around them. Second is defense, from 6T vehicle batteries to drones and soldier gear. Third is “industrial” in the plain sense, robotics and machines where power, charge time, and lifetime are the difference between a lab demo and a real business.

There is a quiet lesson in how he talks about all this. We like big claims about a single breakthrough that will rewrite the rules. Proper Voltage is going after something more boring and more important. The connective tissue. The thing that lets new battery chemistries plug into old systems without fifteen rounds of redesign and risk. If they pull that off, we will not see it in a flashy consumer product first. We will see it in a Humvee that has reliable backup power, a cell tower that stays up during an outage, a robot that can run all shift without a fragile pack.

The future of batteries will not arrive as one perfect cell that solves everything. It will come as a lot of different chemistries, each strong in its own narrow way, finally getting a path into real hardware. Proper Voltage is betting that if you want that future, you need to fix the link between the lab and the product. That is not a hot topic on social media. It is the kind of ugly, necessary work that moves the field forward, one pack swap at a time.