Elehobica
The EQ-100 sets out to close two gaps at once. The first is size. A turntable is already a large thing on the desk, so it’s natural that the size of the phono preamp simply stops being a concern, and most preamps reflect that. Wanting a genuinely tiny one is really a matter of taste, not necessity. But push the size far enough and a clear practical advantage appears: a preamp this small can disappear behind the turntable, or anywhere out of sight, in a setup where the music, not the gear, is meant to be the star.
The second is power. As covered above, battery operation is a long-running theme in DIY phono builds, but far less common in something you can buy off the shelf, especially in a small, affordable form that runs on a rechargeable Li-Po and charges over USB-C. That modern middle was the part we wanted to fill.
Small and battery-powered: that combination is the whole idea behind the EQ-100.
The EQ-100 didn’t start life as a product. Back in February 2021 we posted a project here on Hackaday.io called Mini Phono EQ, not a product but an idea: could you hand-build a proper RIAA phono equalizer in a genuinely miniature size? The trick was mounting SOP-8 op-amps on 1.27-to-2.54 mm pitch-conversion boards, squeezing the whole RIAA stage (built around the LME49721 reference circuit) into roughly the footprint of the RCA connector board itself. It worked. But it was very much a builder’s one-off, assembled by hand, wired for our own bench, and never really meant to leave it.
The EQ-100 is what happened when we took that idea seriously as something other people could actually own and use. A proper PCB replaced the hand-mounting. A real enclosure replaced the bare board. USB-C and Li-Po charging went in. And the power scheme was rethought from the ground up. The 2021 build used a rail splitter, and there’s a story in why we deliberately moved away from it (more on that below). Four years on, the core idea is unchanged. Almost everything around it is different.
At the heart of the EQ-100 is the LME49721, driving an RIAA equalization stage based on the reference circuit from its datasheet. It’s a clean, steady starting point, and the reason it earns its place here is specific. The LME49721 is a low-voltage op-amp, with a supply range of 2.2 V to 5.5 V, and it has a solid reputation for sound quality within that range. That low-voltage operation is exactly what makes the rest of the design possible.
A Li-Po cell sits at around 3.7 V and only sags to roughly 2.9 V as it nears empty, comfortably inside the LME49721’s range, right up to the point you’d want to recharge. The whole signal path runs on what the battery actually delivers, with margin to spare.
We know some listeners are wary of running op-amps at low voltage, and that’s a fair instinct. The obvious alternative would be to swap in a higher-voltage op-amp and feed it from a boost converter that steps the battery up: a perfectly valid approach. On the other hand, a boost converter is a switching circuit, a small noise generator sitting right next to a stage that exists to amplify millivolts, and dealing with that noise is a project in itself. For this product, we wanted the simplest path, so we kept the boost converter out of it and run the LME49721 directly from the battery.
Once the circuit topology is settled, what’s left is the parts. Within what makes sense for a preamp in this class, we used good ones wherever they reach the signal.
The capacitors that set the RIAA curve are film types. These are the parts that actually shape the equalization, so they have a direct hand in how the preamp sounds. The input and output coupling capacitors are Japanese electrolytics, again chosen for the sound. In the power supply, smoothing is handled by a Japanese electrolytic for the low frequencies paired with a multilayer ceramic for the high frequencies, a common and sensible split that keeps the ceramics out of the audio path. The signal path is physically laid out as...
Read more »
