Hi everyone,
At the moment I'm building a portable speaker as a project for some friend. It's a simple second hand passive hi-fi speaker that I rigged with a Chinese class-D amp powered from a 12v lead-acid battery through a DC step-up module that raises the voltage to 28 volts (I think). Everything here is working fine, I can recharge it and hooking up a music player or phone works great.
However, I also want to be able to use a microphone with it and while that does work, the signal is too low (pretty understandable as it's much lower than a phone). So I thought I'd build a simple preamp from a tl072 and use the spare op-amp circuit as a mixer or as a small secondary amplification stage if the mixing resistor turned out to attenuate the signal too much.
This has been giving me huge issues, however. For some reason, I can't get this (or other opamps I've tried recently as was getting desperate) to work properly and it's only functioning around 10-20% of the time. I've hardly been able to get to the mixing stage of the project.
What's been causing most frustration is that it sometimes does seem work on both my breadboard or when I solder it together afterwards. I made a first iteration that seemed to be working pretty well, but as soon as I actually tried to put everything together permanently there suddenly was a huge amount of distortion. After tinkering with it some more, it won't work anymore. I've been trying multiple different chips, component values and even breadboards to get it to work. I've reassembled everything a million times. Sometimes it works, sometimes it doesn't. The same circuit seems to give different results every other 5 minutes.
I've attached the schematic I've been using as my basic idea. I've tried biasing the inverting input, building a virtual ground with two resistors, but the only way it has actually worked is without either of those.
I'm not sure what to troubleshoot, replace, rebuild, scope out, check the connection of, anymore.
Any help would be greatly appreciated.
Kind regards,
Wouter
At the moment I'm building a portable speaker as a project for some friend. It's a simple second hand passive hi-fi speaker that I rigged with a Chinese class-D amp powered from a 12v lead-acid battery through a DC step-up module that raises the voltage to 28 volts (I think). Everything here is working fine, I can recharge it and hooking up a music player or phone works great.
However, I also want to be able to use a microphone with it and while that does work, the signal is too low (pretty understandable as it's much lower than a phone). So I thought I'd build a simple preamp from a tl072 and use the spare op-amp circuit as a mixer or as a small secondary amplification stage if the mixing resistor turned out to attenuate the signal too much.
This has been giving me huge issues, however. For some reason, I can't get this (or other opamps I've tried recently as was getting desperate) to work properly and it's only functioning around 10-20% of the time. I've hardly been able to get to the mixing stage of the project.
What's been causing most frustration is that it sometimes does seem work on both my breadboard or when I solder it together afterwards. I made a first iteration that seemed to be working pretty well, but as soon as I actually tried to put everything together permanently there suddenly was a huge amount of distortion. After tinkering with it some more, it won't work anymore. I've been trying multiple different chips, component values and even breadboards to get it to work. I've reassembled everything a million times. Sometimes it works, sometimes it doesn't. The same circuit seems to give different results every other 5 minutes.
I've attached the schematic I've been using as my basic idea. I've tried biasing the inverting input, building a virtual ground with two resistors, but the only way it has actually worked is without either of those.
I'm not sure what to troubleshoot, replace, rebuild, scope out, check the connection of, anymore.
Any help would be greatly appreciated.
Kind regards,
Wouter
Attachments
The first stage looks OK.
But the mixer stage has some issues.
First of all it will work as the very simple non-powered "mixers" guitarists used to use 40 years ago😉
But that in itself would work.
Next - and this may very well be your problem: the mixer stage does not have a DC bias for the non-inverting input: the signal from the microphone stage goes through a capacitor (C2) and in most cases the output stage from the other input source would also have a capacitor in series.
What you experience with the maybe 5 minutes working time would be the extremely low bias current in a tl072 slowly charging the beforementioned capacitors.
Allthough I would recommend to change the second stage to a real mixer: try to add a resistor from ground to the non-inverting input on the second opamp.
Somewhere between 47K and 1M (this is only for testing)
Cheers
Martin
But the mixer stage has some issues.
First of all it will work as the very simple non-powered "mixers" guitarists used to use 40 years ago😉
But that in itself would work.
Next - and this may very well be your problem: the mixer stage does not have a DC bias for the non-inverting input: the signal from the microphone stage goes through a capacitor (C2) and in most cases the output stage from the other input source would also have a capacitor in series.
What you experience with the maybe 5 minutes working time would be the extremely low bias current in a tl072 slowly charging the beforementioned capacitors.
Allthough I would recommend to change the second stage to a real mixer: try to add a resistor from ground to the non-inverting input on the second opamp.
Somewhere between 47K and 1M (this is only for testing)
Cheers
Martin
Last edited:
Hi,
Thanks for your reply.
Sorry, I should've been a little bit more clear. Even the first stage doesn't seem to work properly most of the time. If it works even remotely, the sound is robotic and doesn't sound like clipping or anything, But (as I mentioned) sometimes it does seem to work and sounds perfect.
Up to now the mixing stage has been an afterthought, that I've built before but hasn't been my main issue. Getting the preamp stage to work consistently has been giving me most headaches.
Your response suggests to me that it's me putting everything together that is the main issue, but, as I said, I really don't know anymore where to go from here.
Wouter
Thanks for your reply.
Sorry, I should've been a little bit more clear. Even the first stage doesn't seem to work properly most of the time. If it works even remotely, the sound is robotic and doesn't sound like clipping or anything, But (as I mentioned) sometimes it does seem to work and sounds perfect.
Up to now the mixing stage has been an afterthought, that I've built before but hasn't been my main issue. Getting the preamp stage to work consistently has been giving me most headaches.
Your response suggests to me that it's me putting everything together that is the main issue, but, as I said, I really don't know anymore where to go from here.
Wouter
Last edited:
.........if you use a single supply you need a voltage divider at the input (to get halve supply voltage, e.g. a 100k res from point c1-r2 to supply voltage vcc......)
Ups - overlooked that, you are absolutely spot on, MJF.
I would add though that there should also be added a capacitor in series with R5 - otherwise the output would stay firmly at VCC.
I would add though that there should also be added a capacitor in series with R5 - otherwise the output would stay firmly at VCC.
Hmmm... interesting. As I mentioned briefly, I tried biasing the input with different values resistors but that doesn't seem to work. I'm still not getting an output.
I had never tried adding the capacitor with c5, i've never seen it mentioned that that one is absolutely necessary. Trying a 10uf cap doesn't really matter. Still no output. On either of the circuit I have laying around now.
Any more help would be greatly appreciated...
I had never tried adding the capacitor with c5, i've never seen it mentioned that that one is absolutely necessary. Trying a 10uf cap doesn't really matter. Still no output. On either of the circuit I have laying around now.
Any more help would be greatly appreciated...
