November 28, 2008
about 3 years ago
The audio output of mp3trigger works well with isolated loads (like a headset). It can be troublesome when trying to use it with an amplifier, or any other device with a single-ended input referenced to power supply ground. The reason is that both the left and right outputs as well as the return line at the mp3trigger headphone connector all have a DC bias. You will need to build an isolation filter (a 1 micro-farad non-polarized capacitor is OK) in series with the right and left outputs as well as the return line. Note that the return connection of the mp3trigger is not the same as the power supply ground. More details on this issue can be found at the makerjam.com forum.
about 4 years ago
The calculation of the coupling cap value is described in Para 3.1 (just above Fig 8).
I had a similar problem with garbled/distorted audio. The problem was the bit rate was too high. The MP3Trigger should work OK with 192kbps (and lower) sample rates using the latest firmware. First, check the sample rate by right-clicking your file in explorer and choosing Properties. If it is 192 then you may not have the latest firmware. Go to makerjam.com for files and instructions. If it is greater than 192 then convert the file to a lower sample rate. There are several PC utilities available for this purpose.
You need to be careful when connecting the MP3 Trigger to any non-isolated electronic equipment. The 1/8" stereo jack on the board is wired specifically for use with isolated devices, like a headset. The shell is connected to the GBUF line on the VS1053 Audio Codec Chip. GBUF is NOT ground and is biased at 1.25 volts (as well as the Left and Right outputs). You must provide dc isolation on all three lines if you are going to connect the MP3 Trigger to any grounded equipment with single-ended inputs in order to prevent damaging the VS1053 chip. However, you might be able to get away with a direct connection providing the MP3 Trigger has a dedicated, transformer-isolated power supply.
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