More Audio Experiments

Well — about half the day..actually more! Spent trying to get a rudimentary single op-amp circuit functioning. I mean…maybe I’m a bit distracted and maybe in a rush. Either way — this should be going easier.

I ended up stripping everything off the breadboard and starting fresh. I even went back to an LM741. My biggest confusion through all this was the split-rail supply and how to hook it up. I made a simple circuit using a TLE2426. But then..I was a little flumuxed about what to do with the VGND signal. But I figured it out in between bouts of frustation and lots of shrugging. Basically, the -V of the TLE2426 circuit goes to the “GND” or negative supply of the op-amp. Turns out — the VGND of the TLE2426 goes to “ground” of the rest of the circuit. The +V and -V from the TLE2426 goes straight to the supply terminals of whatever op-amp you’re using.


Well — I sorted it out. Just stayed a bit late Friday after helping make a little film. Once things quieted down I could focus on debugging the circuit. I started from scratch. I basically wanted to go back to the rudiments. Build an inverting amplifier; a non-inverting amplifier; and a circuit I could use as a pre-amp for a microphone or something. And then just put signals up on the scope to see what sort of power I’d need in order to support mixing and amplifying signals without distortion.

I tried with two different op-amps — the LM741 and the LM348. I don’t know which is better than the other. They look like they perform similarly. The upside of the LM348 is that it comes dual in an 8 pin package. I tried them both in inverting and non-inverting set ups.

I’m working with all this on a breadboard cause, basically I’m rewiring stuff left and right and I haven’t worked with op-amps since college and I’ve been mucking up bad enough that I didn’t feel comfortable at all spinning a PCB even though these circuits are stupid simple.

Here’s a comment.

The chip in the middle of the breadboard up there is the TLE2426 which is providing the split-supply for the op-amps. Without that, the circuit will clip any signal that falls below the supply rail. That was something I wanted to test a bit empirically, so I tried to see under what parameters they would clip a signal.

In the image below I found that a 4.08v signal peek-to-peek, the op-amp would clip the signal if the power rail was 6.4v or less. That 4.08v signal was produced from the maximum power output on a MacBook Pro with the signal being a 440Hz sine wave.

Here’s that signal run through the LM358 set up non-inverting. See? It’s not inverted.

And above — the LM741 inverted.

Then I tried the LM358 with a x1000 factor amplification to see how well it’d do as a microphone pre-amp. Curiously — none of the ragged old little lavaliere microphones I had around seemed to work at all. So..I used my hipster headphones as a sorta microphone by “listening” to the signal from the headphone jack with the headphone draped over a crap speaker I use to listen to music in the studio.