SMSL DA-1 Gain Fix

The SMSL DA-1 amplifier is probably the smallest Class D amplifier I've ever seen. It's not much bigger than a tin of Altoids mints. It fits easily in the palm of your hand and it packs a pretty surprising audio punch.

Based on the TPA3118 chip and sold with a 19 volt power supply it produces plenty of clean power for a desktop setup. In my testing it ran a pair of old school Minimus 7 speakers very nicely and should work well with most smaller desktop speakers.

Although generally well reviewed, a couple of online reviewers noted a bit of hiss and lack of travel in the volume control before distortion set in. This was also evident in my testing, so now it's time to fix the problem.

The internals

Getting inside this little amp is a bit tricky. You need to remove the volume control knob and the nut underneath it, then undo 4 long bolts in the back. The board will then slide out of the back of the case, with the back cover still attached.

Given how small this amplifier is, I was surprised to find a very spacious and well laid out board. The workmanship was good and the parts were of decent quality. In testing nothing got unduly hot or showed signs of being driven outside it's design parameters. So, no big changes to be made.

The TPA3118 chip is a "pad down" device that actually uses the circuit board as it's heat sink. While it does get warm when pushed, it did not get unduly hot or show any signs of breaking down. There are drill holes in the circuit board on either side of the TPA3118 chip, so an auxiliary heat sink could be fitted if necessary.

My one grumble is that there were no part identifiers printed on the board, which could make it difficult to communicate service or warranty issues. But then, for the price, it would almost certainly be cheaper to replace this little guy than to pay to have it fixed.

Hissssssssss

As a couple of online reviewers noted there is a faint but audible hiss when this amplifier is sitting idle. In my testing I noted that while not loud by any means, it can be quite annoying in a quiet room when listening in the near field on a desktop setup.

The TPA3118 chip is a variable gain amplifier. There are no op-amps used in this design. So I did a little circuit tracing and worked out the gain setting. As I suspected it was set to the chip's maximum of 36db (63x by voltage). Then while the hiss is visible on the outputs, it is not on the inputs to the chip, so this is chip noise.

Testing the input level for clipping with the maximum 24 volt supply, revealed that the DA-1 would clip it's outputs on less than 200mv of input. This is way too hot for modern devices, most of which are capable of producing 2 volts of output or more. This accounts for the commentary about limited range on the volume control.

So, the answer to both the limited range of the volume control and the background hiss is to reduce the amplifier's gain.

The gain mod

The gain of the TPA3118 chip is set by a voltage divider running between GVDD (pin 7), GAIN/SLV (pin 8) and ground, setting the voltage at pin 8 to various levels as defined in the data sheet's Table 1 (page 14).

With a 2 volt input, the chip needs 20db of gain (10x by voltage) to clip at about 90% of the volume control's range.

For 20db gain we want the GAIN/SLV to be at 0 volts (R2 open). Studying the data sheet's schematic at the right we can see that simply removing R2 will collapse the voltage divider to ground through R1, giving 0 volts on GAIN/SLV. Which is what we want.

Doing a bit of circuit tracing locates the two resistors just to the right of the group of 4 input capacitors behind the volume control. The middle resistor in that group of 3 is R2 from the schematic.

So, we can simply unsolder and remove R2 to set 20db of gain.

Note: The wire jumper was installed at the factory.

Summing up

A simple modification transformed the DA-1 into a much better amplifier, which now has better sound quality, no hiss and no clipping distortion below about 90% rotation of the volume control.

The hiss is gone. The sound quality is very good. So this is a win-win situation.

To complete the tiny desktop setup, I coupled the DA-1 with the FXAudio FX03 DAC, which is the smallest DAC I've ever worked with and the combination works wonderfully with Windows.