TL092 currently, but origianlly I had 358s in there, same as the ones you're using. I attached a schematic of my project for you to look at if you want. It's DigitalAmplifier.pdf. The main volume control stage is slightly different than the one I gave you, but the end result is the same.
A description is still not as good as a schematic drawing.
Just to be sure, I have attached the setup I want you to make as 2nd Schematic.png. If this does not boost the volume significantly, please post a picture of your setup so we can verify the wirings.
Impressive, I can tell you're an engineer. I hope to get to that level some day.
I've attached an old drawing that I never posted. This shows how I had the pot wired.
I cleaned up the wiring the best that I could for a breadboard (which reduced the noise, but not quite far enough) and took some hi-res photos. You'll see in the pictures that I used a 220k and 47k resistors rather than a single 270k (it's what I had on hand). I also shot some 10sec videos, one with the op amp and one with the midi shield connected directly to the power amp for comparison. I did the best that I could in a short amount of time to keep the camera in the exact same place for each video. If you compare the two short videos at a given volume you'll hear that the op amp set up does increase the volume but you'll also hear the noise that it introduces as well.The pictures and clips can be found at: https://drive.google.com/folderview?id=0BxhYs4r5NIW6dmZVei1acE9tRlU&usp=sharingAt this point, I'm skeptical that putting this on copper clad would reduce the noise sufficiently. I'm open to suggestions for different hardware as well.
Actually I'm still just a student. I graduate this September.
Is the noise you are referring to that high pitched squeal in the second video?
It could be the MIDI signal. My project has an LCD screen, and every time it updates the speaker "chirps" a little bit. It's not that audible when music is playing, but it is when it's turned all the way down. This is caused by the digital ground not being isolated from the analog ground. This is just my guess, but it seems to be the most probably solution. I confess that I don't know anything at all about MIDI. Is your Teensy constantly sending data to the shield? If so, what is the baud rate (if such a thing is applicable to MIDI signals)? That could be causing your squeal.
Try increasing the gain on your op amp configuration (change the 270k ohm to 1M ohm) and decreasing the digital gain of the power amp to about +20 dB, instead of the max +30 dB (46 or thereabouts on the volume scale). If my estimation is correct, this should give about the same volume (maybe a tiny bit louder) and might have less noise.
Larger resistors are more suseptable to noise. You could try reducing the impedance of the op amp's feedback network. Change the 100k and 1M into 1k and 10k, and change the 0.1uF to a 10uF. You'll probably have to use an electrolytic, so hook the - lead to the MIDI shield. It has the lower bias voltage. This will keep the gain and high-pass corner frequency the same, but should have less noise if it truly is picking up EM interference.
Also, so far you've done this with the MIDI shield hooked up. Have you tried it with your alternate source (I think your iPod, if I remember from the other thread).
Unplug the MIDI shield completely: power, signal, audio, everything. Completely remove it from the circuit. Take the sides of the capacitors that were formerly connected to the blue and orange wires (which I shall call the input sides) and ground them. This should remove any input signal from the op amp. Power the circuit back up and listen to the output: is the squeal still there?
If I'm reading the specs of your scope properly, you should be able to save the waveforms as an image to USB. If so, please post the images for each point.
I tried disconnecting the MIDI shield completely and powering up everything else. The noise was still there though much quieter. It had some of the same characteristics of the noise in the full setup as well. About every 5 seconds there's a sort of bleep sound that occurs with and without the midi shield connected.
When you did this, did you leave the shield's power plugged in, or did you truly unplug everything?
The problem with most of the scope photos you took is that the horizontal setting is 200 ms / div. The squeal is likely at a couple of kHz, so you need to set the scope to about 500 us / div, possibly lower in order to see the squeal waveform.
What does it sound like when you use the iPod as the input to the opamp circuit?
And according to this product page on Amazon (http://www.amazon.com/Rigol-DS1102E-Oscilloscope-Channels-Sampling/dp/B001VKCJ0M), your scope should be able to save *.bmp files.
Do you have two probes for your scope, or just one? If you have two, you can use AC coupling to monitor the signal at both the input and the output at the same time and see at what level the clipping happens.
Can you download a frequency generator app onto your iPod that is able to create a pure sine wave as a test tone? I know android has a few of them, I'm sure iOS must have some too. That should help with troubleshooting to be able to set the frequency and amplitude of the test tone.
- I super-imposed the input of the op amp on top of the output of the op amp looking for any obvious flattening of the signal, which is what I assume the clipping would look like. I didn't see anything dramatic but,
- I measured the peak voltage from the output in DC coupling mode to be 3.62v (10k/1k) and 3.72v (1M/100k). Which leads me to believe it is in fact reaching the maximum output of the op amp.
- Oddly I measured the peak to peak voltage of the input to only be 34mv. Shouldn't the output of the op amp be only ~2.2v to ~2.8v with a gain of 10?
I'd like to try increasing the supply voltage of the op amp but I have a question first:Do I leave the 10k/10k voltage divider on both channels connected to 5v? I'm thinking yes, because the 2.5v from this is what sets DC bias but I'm not 100% on that.
My iPod cannot, but my android phone can. I'll wire up a 3.5mm jack for that after this post.
What was the minimum voltage?
I have PA Tone on my phone.
Have you double-checked your resistor values with a multimeter? Can you post a photo of your current setup?
Just to be clear, when you measured the noisy 900 Hz signal with the power amp on, did the sound coming out of it sound scratchy or raspy like it does for the MIDI shield?
I just noticed that my speaker makes a bit of a rattling too when I overdrive it a certain way. This isn't distortion of the audio signal, it's actual physical rattling of the speaker against my hard glass desk. Maybe put something like a folded up T-shirt under your speakers and see if that helps. I doubt it will, since your oscilloscope is pretty definitively showing electrical noise, but it's something to try.
Maybe it's caused by overdriving the input stage. It's only biased at 1.5V, so its output can only swing that far low before it hits ground. It's set up in the schematic for a 1x gain, and you're currently feeding it with a much more massive signal than that. Though that doesn't make sense for the iPod which, at max volume, should be overdriving it even more.
Try halving the size of the feedback resistor (use a 4.7 k instead of 10 k) and seeing if that improves the scratchy sound from the MIDI shield. Power the op amp and bias voltage divider from 5V again.
What is the current rating on your 12V supply?
I noticed you left the Mute pin unconnected. It doesn't have a resistor pulldown on the PCB. Ground it, see if that helps, though I doubt it will. I'm kind of grasping at straws here.