It's been sitting in my needs troubleshooting box for over two years, and finally I decided to stare the dragon straight in its eyes and figure out what exactly was going on, or in this case not going on. Many years ago I had built a Really Cheap Compressor and I liked how cheap and effective it was, so when I saw the Univox Uni-comp I decided this would be a piece of cake! Little did I know, initially, it was more like a pile of crap. After this little circuit failed to work at all I went to the search engines and found that, what looks like a very simple circuit on paper, seems to be notorious for not being able to get it right. There are multiple schematics and layouts that all differ, with very poor success rates from what I'm reading as well. I'm not a very smart man, but I am tenacious!
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| The original layout I used |
With legends being passed down from generation to generation that this circuit is indeed viable, I decided to sit down and compare all the schematics and layouts I could find. I made 100% sure that mine was exactly like the layout I used (above), and it was. From there I started tracing things back and forth between all of the layouts and schematics, and I noticed a few things. On the layout I used, the 3.3M is going through a jumper to pin 3, as where all the others went to pin 2. I desoldered the jumper at pin 3, stuffed it through the cut trace beside pin 2, and soldered it where I attached the socket beneath. Then I traced both 1M resistors at pin 3 and realized neither went to ground. The lower of the two went precisely where it was meant to go, so the upper required me to move the top leg down one space, which now gave me the correct voltages on the LM741. I also noticed the LED anode and it's components weren't isolated, so I made a cut to do that.
Once those changes were done, I finally had audio, but just barely. The LED was also behaving like it was possessed. Sometimes stuck on, sometimes refusing to light at all, making me think I’d cooked it. Between the weak output and the LED acting like a mood ring, I was stumped. I checked every schematic and layout again, and everything looked right. That’s when I turned to AI (big ups to Copilot!). I showed it the layout, explained the fixes I’d already made, and after some thinking it told me the LDR needed to connect to Q1’s collector. I scoffed. In none of the schematics or layouts does the LDR go to the collector of Q1. In mocking robot voice: “Error, error, cannot compute.” But the AI was persistent. It suggested disconnecting the LED and LDR entirely to see if the volume came back. Wouldn’t you know it, it did! Not only that, but with the Comp and Level maxed, the thing actually turned into a surprisingly cool little overdrive. That’s a project for another day, but it proved the sidechain was the culprit.
I questioned my use of an MPSA05 instead of an 06, and AI assured me the only thing that needs to happen is the LDR needed to connect to Q1's collector. Fine, we'll try it your way. *eye roll* Well, yet again I was wrong. Now I'm not saying it's perfect, but it actually works. The LED flickers into the LDR, it compresses, but it is gritty. Could this be the 1N4148s? Who knows. The reality is this little circuit now works, again probably not perfectly. What I had written off over two years ago now actually works. So if you're one of the people who had issues with the Univox Uni-comp, here are the changes to make to the layout above to get you started.
- Add a cut at E12 to isolate the LED and it's components
- Move the Jumper from I6 to H6, connecting to Pin 2
- Move the 1M top leg from D4 to E4, giving the opamp proper voltages
- Connect the top leg of the LDR to C11, connecting to Q1's collector
