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.

So — here are the schematics I used. The basic deal.

Well..I feel a lot better that I got this stuff working. I was sorta nuts last week trying to make that mixer and just having everything go bonkers. I think maybe it was important to just get back to basics and build up from basics rather than using someone else’s circuit design and hoping for the best.

What this helped me do is figure out what I want the special weird mixer thing to do. It needs two microphone inputs — so two preamps for mic-level signals from the two microphones for capturing ambient audio. It should have two stereo inputs for two different sources like a music player and something else, like a phone. The mic inputs should be able to pan from left to right. I think all of these signals should also be able to go through an “effects send” to do fun things to them — maybe even phase them a bit to do some experiments with spatialization. I think there should be one other mono input for signal generation — you know..for annunciators and maybe synthesized voice or something. So — that’s three mono inputs and two stereo inputs. The stereo inputs should also be able to have their balance adjusted, I think.

I think that’s the rough spec. Onward.

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