Floating Differential Signal

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I was just thinking about this idea of sending the PWM signal down the twisted pair and just power amplifying it at the other end... is there some way I could adjust the volume from the amplifier end? it doesn't seem like I would be able to to much with it at that end other than perhaps change power supply voltage, but that may just make distortion if it isn't high enough. I think for my outdoor speakers, it will not matter, because I would want them all at the same level anyway, but for indoor speakers inside rooms, I may want to adjust just one pair to be different than the rest. Perhaps I can use my balanced line level signal for indoor runs and the pwm signal for the long outdoor runs.
 
Here is a circuit board I came up with for the opto isolated power buffer idea. It's a solid ground plane on the top side, mostly a +V plane on the bottom side. Going to order some parts and some blank copper clad boards and give it a try.
 

Attachments

  • mcp1407 both.jpg
    mcp1407 both.jpg
    114.3 KB · Views: 96
  • mcp1407 bot.jpg
    mcp1407 bot.jpg
    118.4 KB · Views: 85
  • mcp1407 top.jpg
    mcp1407 top.jpg
    98.1 KB · Views: 90
  • mcp1407 sch.jpg
    mcp1407 sch.jpg
    165.9 KB · Views: 92
Last edited:
Will the opamps work with the floating signal?

perhaps I could just use a dual output op-amp like the LT1016 previously mentioned which would give me both the inverted and non inverted output with an 8pin DIP. I was trying to stay away from op amps because of the split supply. is there a way I could make it work properly with a single supply?
I guess I could use a +/- 15V supply and just not hook up ground to the TPA2122D2 so it would get 30V, and capacitor couple the inputs, still a single supply would be a little more elegant

I think I am going to use the original version with balanced line level inputs for my indoor speakers, (once I figure out the floating signal problem) and try the pwm method for my outdoor speakers.
 
Last edited:
Opamps will work with your floating signal.
Since you are using a single-ended power supply, you will need to create a phantom center tap. You can do this by using a couple of resistors and a capacitor to remove the power supply noise.
When creating the phantom center tap, you must use coupling capacitors on the outputs of the Opamps.
 

Attachments

  • Balanced IO.jpg
    Balanced IO.jpg
    46.7 KB · Views: 45
Thanks for the info.

Looks like that LT1016 I was looking at is really a comparator, not an op amp, I don't know if that would really work in an analog application, and they are quite expensive as well, I don't know why, but they are over $5 each at Digi-Key.

Any recommendation on a good op amp for this? I happen to have some TL084s, I can probably use those for a prototype. Is there a better quality opamp I should look at?
 
Here is a circuit board I came up with for the opto isolated power buffer idea. It's a solid ground plane on the top side, mostly a +V plane on the bottom side. Going to order some parts and some blank copper clad boards and give it a try.

I'd strongly recommend using more powerfull diodes, perfectly schottky ones like 1N5822

P.S. Have you had a look at TC4422 driver? It seems better.
 
Last edited:
Your original circuit would work with the simple addition of a couple of Opamps on the receive end.
One opamp to receive the balanced audio and another to invert the phase.

The common mode range of such circuits is very limited. They are not intended to work with fully floating signals.
Practically they are only good for ground loop eliminators or so-called DI-boxes, where all signals are referenced to different ground potentials.
 
The common mode range of such circuits is very limited. They are not intended to work with fully floating signals.
Practically they are only good for ground loop eliminators or so-called DI-boxes, where all signals are referenced to different ground potentials.

Sorry darkrenriz. You are completely wrong.
Opamps have been used for decades as balanced microphone and line level input stages.
They perform very well when driven by floating, balanced audio signals.
 
Sort of. If you connect a microphone to such an input stage it will perform flawlessly, but the microphone isn't perfectly floating anymore.
Connect one microphone to several such stages placed in several physically distant locations (which is I believe a merit of this very thread) referenced to different "grounds" or lack of them and you get a nice mess.
 
Yes, galvanic isolation is the safest way, tranformer being the simpliest and generally best way to do it.
Optocoupler working on the discrete representation of the signal, which I suggested is second best.
Differential amplifier solution is fine as long as the common mode range of "floating" is within some limits. Period.
 
I'd strongly recommend using more powerfull diodes, perfectly schottky ones like 1N5822

P.S. Have you had a look at TC4422 driver? It seems better.

Thanks for the commendation on the diodes, do you think I should use these just on the output and keep the smaller 1N4148s on the input?

I did not see the TC4422, looks like it would be worth a try. I had a look at the data sheet, the description at the top states power supply of 20V but then the charts list max power supply of 18V... strange


Sort of. If you connect a microphone to such an input stage it will perform flawlessly, but the microphone isn't perfectly floating anymore.
Connect one microphone to several such stages placed in several physically distant locations (which is I believe a merit of this very thread) referenced to different "grounds" or lack of them and you get a nice mess.
If all the receivers were powered by floating power supplies, wouldn't the signal on the line still be floating?

Yes, my intention with this project is to send a balanced signal down a long run of UTP cable, and have many receivers along the way. I am trying to create something like a central audio system, that extends to outdoors and into other buildings as well. since I want many receivers, I am trying to keep the cost down per receiver.. When you start multiplying, costs go up fast. which is another reason I like the PWM method, the cost per unit will be fairly low.

For the indoor part, I may still use the analog signal, but the runs will be short, and if I can't get something simple to work, I can use small audio transformers, there won't be anywhere near as many indoor units as outdoor.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.