The bias is intended to be 2.5v isn't it? I'm confused.
5 amps. Should be well over enough.
Using the MIDI Shield, through the op amp circuit (10x gain & 4.7x gain, 5V supply), it has a raspy kind of distortion, but only on the glissando (I had to look that word up smiley ), not the individual notes.
Using a phone outputting a 900 Hz test signal though the op amp circuit (10x gain, 5V supply), there is no audible distortion. how high did you crank the volume on this test signal? Was it at least equal to the MIDI shield test?
Using an iPod to play music through the opamp circuit (10x gain, 5V supply), there is no audible distortion. How high did you crank the volume for this test? Was it at least equal to the MIDI shield test?
The MIDI shield, when hooked directly to the power amp, does not cause audible distortion.
Placing soft foam underneath the speakers does not improve the distortion, so it's not caused by the speaker rattling on something hard.
Just tested this again to confirm.- With the gain of the power amp set to 0 and 10x gain in the op amp circuit (1M/100k) distortion DOES occur near the upper limits of the signal volume.When I tested this before I must have had the gain of the power amp set above zero. I was able to make the tone painfully loud without any distortion.
According to my theory, setting the power amp gain above 0 would make the distortion occur at a lower volume on your phone, but about the same audio volume.
Just to be clear, the numbers you are using for the gain of the op amp, are those dB values or the volume setting values? I think they're dB.
Using the frequency generator app, use a moderate volume (no audible distortion) to establish a baseline for comparison. The input should be a clean sine wave, and the output should be a sine wave with a high frequency triangle wave superimposed on it, like what was in the video. Take note of the peak-to-peak value of the input & output, the maximum value of the input (especially if it's more than 3V), the gain setting of the power amp (in dB), and whether there is any clipping or other oddity on the output.
Do not use the op amp circuit for these tests.
dB. Assuming setting 60 = 30dB, setting 30 = 0dB etc.
The output certainly seems odd though there is no audible distortion, at any setting, without the op amp in the circuit.https://drive.google.com/file/d/0BxhYs4r5NIW6eHd1UGMtZ2ZQUTg/edit?usp=sharingBlue is the input, yellow the output.This is with max phone volume and max power amp gain. Decreasing the volume on the phone causes the ouput to more and more resemble a sine wave until about 60% where it looks perfect. Anything past that causes the flattening on the negative side though I don't notice any distortion aurally.The same sort of flattening on the lower side occurs when the midi shield is connected as well and maybe some on the top too.https://drive.google.com/folderview?id=0BxhYs4r5NIW6aDBaUmlNYVc2LTQ&usp=sharing
The Vpk-pk of the output of the power amp during the "glissando" , with the attack velocity at max and no op amp in the circuit (audible distortion) being ~13v leads me to believe that the output of the power amp is being over-driven. Though that doesn't explain why there would be distortion with the op amp in the circuit with a gain of 10, the attack velocity at 60, and the power amp gain at 0. I just measured this and the Vpk-pk of the output of the power amp is ~2.4v.
I'm thinking now that during some tests, the input of the power amp is being over-driven and during others, the output is being over-driven. I'll do a more detailed analysis of this scenario tomorrow.