What a DIY Low-Noise Amplifier Gives You
A DIY low-noise amplifier is a high-gain audio tool that boosts tiny signals, trims unwanted frequencies with selectable filters, and provides a true RMS DC output for accurate level monitoring, all in a compact, bench-friendly box suitable for hobbyists with basic electronics skills.
Think of this project as a Swiss‑army knife for your DIY audio amplifier experiments. You get a low noise amplifier (LNA) with three different gain settings, six filters for bandwidth restriction, and a True RMS voltmeter that gives a DC voltage at its output. Finished, it becomes “a handy companion in the test bench of any audio designer.” If you already like tinkering with an audio filter circuit or waveform generator, this box pulls those worlds together into one practical RMS voltmeter project. The only real caveat: you should be comfortable reading schematics and soldering through‑hole parts on a single PCB, because almost all components sit on one board for easy construction.
Inside the Design: Gain Stages, Filters, and RMS Output
Before you build, it helps to know what you are assembling. The device combines a low noise, high gain amplifier with three different gain settings, six filters for bandwidth restriction, and a True RMS voltmeter that outputs DC. According to the project’s designer, the low noise amplifier stages can switch between gains of x100, x1000, or x10000 before the signal hits the selectable filters.
The core low noise amplifier uses top-quality op-amps, with the first stage based on an ultra-low-noise device and the following stages using audio-grade OPA2134 op-amps. Almost all components are contained on a single PCB and powered from an external low noise supply. Filters play an important role for audio noise measurement and are applied to reduce signal components at selected frequencies. The built-in options include an A-weighting filter, a 20 kHz low-pass, a 20 Hz to 20 kHz band-pass, a 10 Hz low-pass, and a 1 kHz low-pass audio filter circuit. The True RMS voltmeter section measures the signal and gives a DC output voltage directly analogous to its RMS value, so you can monitor levels with confidence.
Step-by-Step: Building and Using Your Low-Noise Amplifier
You will approach this project like assembling a trusted bench instrument: start with the foundations, then add the features that make it a precise low noise amplifier instead of a simple preamp. Take your time with each stage so noise performance stays as designed, especially around the sensitive input section.
- Populate the power-related parts and op-amp sockets on the single PCB, following the provided schematic and keeping the external low noise power supply connections clearly marked for later.
- Assemble the first ultra-low-noise gain stage, using the specified op-amp and low-value feedback resistors, which define the x100 gain and help achieve the excellent noise performance.
- Build the second and third amplifier stages with the OPA2134 op-amps, wiring each for selectable gain of x1 or x10 via the front-panel switch that controls the three overall gain settings of x100, x1000, or x10000.
- Add the filter networks and rotary selector so the amplified signal can be routed through the A-weighting filter, 20 kHz low-pass, 20 Hz–20 kHz band-pass, 10 Hz low-pass, or 1 kHz low-pass filter as needed.
- Integrate and wire the True RMS voltmeter section to the main signal path, so that it measures the filtered signal and outputs a DC voltage directly proportional to the signal’s RMS value.
- Mount the PCB, rotary switches, and connectors into an enclosure sized to sit comfortably on your bench, similar to a waveform generator or other test gear, and label gain, filter, monitor, and RMS output controls.
- Power the unit from the external low noise supply, then verify each gain setting, filter selection, monitor output, and RMS DC output using a known audio source and an oscilloscope or external voltmeter.
The big gotcha here is that noise performance depends heavily on layout and source impedance. The design’s first stage is optimised around very low source resistance; keeping leads short and shielding the input pays off in a quieter DIY audio amplifier. Treat it like building a small lab instrument, not a casual utility box, and you will get the performance you are expecting.
Why This Project Suits Hobbyists and Audio Tinkerers
Although the circuit looks serious, the project was conceived as a DIY build that many readers will want to explore, and almost all the components are contained on a single PCB for easy construction. That makes it an accessible RMS voltmeter project if you already enjoy building a DIY audio amplifier, filter, or waveform generator for your workbench.
Once completed, “the LNA-100 device will be a handy companion in the test bench of any audio designer,” in the same way a compact waveform generator is more than adequate for everyday audio work. You gain well-behaved gain staging with three adjustable settings, six built-in filters to clean up the band you care about, and a True RMS voltmeter output that turns abstract audio levels into a stable DC reading. In use, you will probably find yourself reaching for it whenever you need to compare noise floors, verify a new preamp design, or check how an audio filter circuit behaves at different bandwidths.
Takeaway: A Bench Tool Worth Building
Finishing this low noise amplifier turns a pile of parts into a reliable bench companion: high gain with three selectable settings, six useful filters to restrict bandwidth, and a True RMS voltmeter output you can trust. It sits naturally alongside a waveform generator or other small test boxes and feels at home on a workbench shelf. If you approach the build carefully, pay attention to the noisy front end, and keep the wiring tidy, you get a DIY audio amplifier that doubles as a precise measurement tool. The main thing to watch for over time is how you use the gain and filters; treat the highest gains and narrow bands as special tools for low-level signals, and this project will stay helpful for a long time.
