Thanks for all the input. I knew that the request “constructive criticism” was dangerous, but sometimes you get exactly what you ask for. Which is exactly what I got…
Let’s see...
ejeffrey:
-Now that formula for cutoff frequency makes more sense. Even 250Hz would work for the human speaking voice, but 50Hz sounds like a good place to start.
- The DC is for an indicator light. This is connecting bussed intercom system that is common in the entertainment field. If someone isn’t listening to a channel for whatever reason pushing a button on your intercom pack will send the DC on the line and a bright light will light up on all the packs connected to that line to get their visual attention. Not the most high tech, but keep in mind this concept was designed in the 1960’s and has changed little since. Still runs many of the largest production shows in the world (and a particular one I work for in Las Vegas)
- Increasing C15 makes since. As well as R14/15.
- U3 was a big typo as I was transferring my design work in back and forth between Multisim to DipTrace.
- The LM384 is a little power amp chip that specially designed to run from a single sided supply without having to bias an input. All the diagrams and application notes I have been able to find ground out the non-inverting input.
- As the gain on the LM384 is hard set at +34dB (one thing I dislike about it) and the signal coming in from the audio lines is pretty hot, I doubt that I will need any amplification of the signal going into the LM384. More than likely I will need to attenuate the signal some. VR5 should give me the ability to pad about 25dB or so as needed, I think.
- When I remove C8-C11 from the Multisim simulation everything seemed to play nice. I now had various levels of DC present after the 10kohm mixing resistors, but C13 blocked those from getting to the TL071, no problem I guess. However, once I removed C13 and had VR1-VR4 at anything less than full tilt the final waveform was all messed up. I’m guessing that the DC bias on U3 was causing some type of rail clipping. I know Multisim isn’t the end-all and be-all, but it makes since. Maybe someone can clarify what I’m seeing in the simulation.
- Then would I be better to make C14 a 330nF non-pol as it is a DC blocker and not a filter cap?
JuKu:
- U3 was typo. A big typo, but a typo.
- The caps across R5-8 & 13. Are you suggesting 330nF’ish or more like 100nF’ish?
- Never actually measured the rise and fall times, but I image they are in the order of 250mS or so. I’m going to do some testing with the scope tomorrow I guess.
Gall:
- U3, I know, I know…
- See my above response to ejeffrey about my cap confusion.
- Ditto about the LM384.
- I was looking at some application notes about that earlier. I have some extra TL071 on the way. Worth a try to see if it adding it improves the performance any. Stay tuned...
- Interesting idea on the 7812 + 1824. But I’m not sure if that would work with the LM384 as it requires a single sided supply and it needs to be in the 20’ish range for the rails. Further research needed
- The TDA2030 looks a little over-beefy for my desktop application, but I can think of where that much power could be useful. I’ll need to put a couple on order to play with.
Vk6zgo:
I’m a little surprised of everyone’s niceness as well. Not that I don’t enjoy what I got back, because I REALLY do. Thanks all! See attached "Rev B".
- EM