Another Electronics Blog

I was able to quickly put together a unity gain audio amp using an extra filter kit board. For a fair comparison, I will step this up to a gain similar to the LM386 audio outputs we are familiar with and post those results. However, the results for this test were very promising.

As shown above, the noise floor came in at about -138 dBFS. In practical terms, that’s zero hiss in a pair of sensitive ear buds.

When applying the 1 kHz signal at a level 0f -10 dBFS, the THD+N came out to 0.0058%, which is excellent. This corresponds to a fairly high level input with a low amplification factor of 1.0. In other words, this is a pleasant volume level in earbuds and the noise contribution was minimal.

A quick follow-up to my previous post.

Since I had an assembled 650 Hz CW filter kit handy, I decided to use the QA400 with a 650 Hz tone to make a simple THD/THD+N measurement.

Below is an image capture of the baseline filter board powered-on with no signal input. As you can see, the Bandpass characteristic of the filter will allow noise within its passband to exit the output stage. Beyond the passband, the baseline noise floor sits about -142 dBFS, i.e. where the QA400 nominal noise floor level sits. The center frequency of the filter passband shows up in this image at 662 Hz, which is what I previously had measured by other methods, so this cross-checks that result very nicely.

Now I add a 650.39 Hz tone (as close to 650 Hz as the QA400 software allowed me) @ a level of -18 dBFS to achieve the nominal measurement output level of -10 dBFS.

As you can see, the THD+N figure of 0.063% is very close to the THD reading of 0.062%, which is expected since the TL972 is a low-noise Operational Amplifier and significantly newer than the LM386 I previously tested.

Next I will construct a TL972 audio amplifier circuit as a direct replacement for a typical LM386 audio circuit, then measure and post those results.

I recently purchased a new piece of test equipment; a QuantAsylum QA400 Audio Analyzer. The specs of this unit get close to high-priced audio analyzers from Keysight and Audio Precision. For $200 USD, you get 0.00055% THD inherent, which means the generator wrapped to the input produces a baseline reading like the one below.

As you can see above, the THD is exceeding spec and the THD+N at 0.002% is exceeding its spec of 0.0038%, both at an input level of -10 dBFS. You will also note that the noise floor of a QA400 sits at about -140 dBFS.

I wanted to measure the THD and THD+N of the ubiquitous LM386N audio amplifier module that is almost guaranteed to be a part of a QRP receiver kit you may own or would buy. The LM386N is also commonly used in low-cost audio amp boards from China via ebay.

The image below shows the result of driving the input of one such LM386 audio amplifier board I bought on ebay several years ago.

The main issue is the noise floor of these amplifiers which sits at about -100 dBFS or 40 dB above the input stage of the QA400. I’ve always felt these amplifiers hissed far too much, even though a -100 dBFS noise floor might seem good on paper.

In a future post, I will offer a better alternative to the LM386 as a low-cost, easy to use audio output stage capable of driving earbuds or an amplified speaker system.