Looking quickly at
the web page for that amplifier, my guess is channel 1 is meant for a microphone and channels 2 and 3 are meant for guitars or other instruments with magnetic pickups.
It gives a spec or -10 dBu for the aux input, which is still extremely sensitive. By my guess is that spec is with the "Level" knob turned all the way up to 10. For your first try, I'd suggest connecting the audio shield line out to the aux input and turn the knob almost all the way counterclockwise, very close to zero. Run an example program that makes continuous sound, like the WAV file player or File > Examples > Audio > Synth > Guitar which makes a repeating guitar strumming sound. Then turn the level knob up until you get a usable volume.
View attachment 38477
Thanks for going through the amp specs! Here is my sequence of things to do (when teensy is on battery power). Please correct me.
1. Set sgtl5000 volume() to 0.5. I like to keep it fixed for my application:
2. Connect teensy audio shield headphone jack to aux in on roland and set its level to 0.
3. The actual speaker output is controlled by the 'master' knob on top of the amp (image). I can't really determine 'usable volume' using just the aux level knob. For convenience, I'll set the master knob to 50% to start with.
4. With master at 50%, play audio through teensy, and turn up the aux level knob until the speaker output volume is enough.
A fun thing that I tried today, to determine if the levels of channels 2 and 3 match that of aux, was connecting the Right headphone out from audio shield to R of CH3 and the Left one to L of aux. The output from the shield was mono. What I noticed was that, when I set the CH3 knob on the top to 50%, the master to 50% and aux level to approximately 80% (pointer was horizontal), my headphones connected to amp's PHONES out had left and right balanced. Also, at these knob-orientations, the volume was 'usable' for my specific application. (I haven't tried the 'Guitar' example yet.)
In that experiment, I also noticed that at very low volume from the shield, the intensity on PHONES-left was slightly higher than that on PHONES-right. However, at extremely low volume from the shield, it was the opposite; right was slightly higher than left. The exact happened when I swapped left and right from the shield (shield-left -> CH3 right; shield-right -> CH3 left). In my opinion, this imbalance is because of the way the amp internally treats CH3 and AUX IN differently. Since CH3 (and CH2, CH1) is meant to be used with musical instruments (keyboards in this case), while aux is for audio players (phone/laptop), the subtle high harmonics are more linearly mapped in CH3. I'd prefer to use CH3 for my application, as the high harmonics matter there.
Question: If I was to use the headphone out of the shield with CH3 of this amp, what value in sgtl5000 volume() would closest resemble line level, which CH3 expects? I'm guessing 0.5 is too high.
