Lepai LP-S60

This is another small amplifier I picked up at a Parts Express Tent sale.  I think they retailed for about $35 about 5-10 years ago.  This amp includes an FM tuner, line input, and inputs for an SD card or USB stick and also came with a remote - so the idea is it could function as stand alone receiver.  It doesn't have built-in bluetooth, but that speaks more to the era it was produced and sold in rather than anything else.  Like many car audio head units, it is spec'ed at 25Wx4, which is probably optimistic.  So, let's take a look at the dashboard.

This looks a lot like the Kinter.  A distortion factory, but notably even order distortion is very high.  If we were looking strictly at odd order distortion, that would be about -60 dB, which is 0.1% and in the realm of not terrible.  With even order distortion thrown in, THD+N sits in the -30 dB range which is greater than 1%!  Signal to noise ratio is in the high 70's, which is better than the Kinter, but not quite as good as the Lepy, which was close to 90 dB when running off of my budget regulated power supply.  There is a lot of gain available, so at least the LP-S60 will sound loud

Next, let's look at frequency response.

You'd certainly hope for better here.  This amp doesn't have discrete tone controls, but you can push a button and adjust.  It looks like the default is a little off, but at least mostly balanced.  Another issue is high frequency ripple that is seen througout the audible range.  Maybe the biggest issue is the low frequency rolloff, which starts around 100 Hz and is 6 dB down at 20 Hz.  High frequencies are nicely extended and there is no dependence on impedance.

For power testing, I noted that the amp states a range of DC power can be used (12V-18V), so the first round of tests is to see what impact the input voltage has on output power.

At 12V, 4 ohms, this looks very much like the Kinter, running out of steam starting at about 7W with the requisite 2% THD.  It makes one think we are dealing with the same amp chip and general implementation.  Bumping up the voltage get us about 11W at 15V and 15W at 18V.  At 18V, you can almost get 20W at 10% distortion - not quite the advertised 25W, but sort of close...

At 8 ohms, we should also see a lot of benefit of higher operating voltage, so let's look at that.

Sure enough, we can get up to 11W at 8 ohms with an 18V power supply.  Distortion is also lower at 8 ohms - around 1% THD+N.  Conceding that 18V is probably the right place to run this amp, we'll continue with that and put together the distortion curves at that operating voltage.  I'd say the best scenario for this amp is running 8 ohm speakers with an 18V power supply.  Lower impedance doesn't buy much additional power and costs a fair amount of distortion.

Finally, let's also look at distortion vs. frequency.  Some amplifiers have more issues near the low and/or high ends of the audio band.  WIth the Lepai, there is some increased distortion above 5 kHz, but it is very consistent (consistently high) with respect to power level.  This would generally be considered good behavior for this test if you ignore how high the baseline distortion is.