What's wrong with this audio amplifier and stereo to mono circuit?

Question

The image below presents part of a schematic of an MP3 decoder (VS1003) with stereo -> mono conversion and audio amplifier to connect a speaker.

The problem is that after assembling the PCB barely anything could have been heard. I (kinda randomly) shorted R17 and C28 node to AGND and after this I can hear the music (this basically removes C28 and LEFT audio channel), but the sound quality is still terrible with a lot of noise.

I guess I did something wrong on the schematic. Can someone find a mistake here? What could be corrected to get this right? I would be happy to have even 1 audio channel.

Answer 1

The circuit is designed to operate from a positive only rail so lack of a negative rail shouldn’t be an issue.

I think your problem is the DC biasing between the VS1003 and TPA2005.

The VS1003 outputs are biased around GBUF which is about 1.2V.

The TPA2005 wants its inputs biased around VCC/2 = 2.5V.

Since these are not matched you must put AC coupling capacitors in series to block the DC component.

The TDA2005 app note shows the preferred circuit for a single ended input:

The RI in this schematic is your R25. The gain of the amp is 150K/RI so you may want to lower your value way down (from 150K) to get more gain. Make sure you change RI in both legs.

Choose RI low enough for desired gain, but CI high enough to maintain low frequency response fc at least 20Hz, as per formula in same datasheet:

If you end up using a polarized cap for CI make sure the "+" side faces the TDA2005, because DC level is higher on that side.

Answer 2

There are two ways that the sound may become muted. Firstly, the power supplies to the VS1003 don't include a negative voltage source. That usually means that the 'ground' is not in the midrange of the AC output signals, and only a coupling capacitor (to block DC) will make a suitable connection to an external device (amplifier input or speaker). The DC bias can cause an amplifier to saturate (hit its voltage rail, outputting only a constant voltage with negligible signal content).

That DC-bias problem is not consistent with the observation that the sound can be heard after removing one channel (and if the amplifier/speaker on the output has GBUF biased correctly, there is no need to modify the circuit as shown). GBUF should correct this problem, but that depends on the common-mode range of the output amplifier, so a DC measure of both the absolute voltage, and GBUF voltage, is advised. The other way that combining stereo signals can result in zeroing the output, is if the 'stereo' is in fact monaural, and the two channels are in opposite phases (basically, different polarity copies of the SAME signal).

Check the outputs for DC content, and if you have an oscilloscope, put left channel on X axis, and right channel on Y axis, and see if the display is a diagonal line, rather than the more nuanced squiggles of a known stereo sound source.

Answer 3

I've found the problem on my own - in the end it was very simple, rookie mistake. Along with this C31 capacitor of 1uF, I added in parallel 100nF and 1000uF and all the problems went away. It was just not filtered enough...