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Old 22nd March 2012, 04:52 PM   #11
Boscoe is offline Boscoe  United Kingdom
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Use a split supply and you can then use your nice simple design!
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Old 22nd March 2012, 05:19 PM   #12
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This has something to do with the biasing, Boscoe? I think i would like to try using a split supply in future projects, but for this one, ive gotten this far on a single sided supply, so I'd hate to rework the entire design this late in the game.

From what I've gleaned so far, there some significant advantages to using split power (especially in audio applications). So it's something I'm going to need to learn the in's and out's of.

-Chad
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Old 22nd March 2012, 05:56 PM   #13
DF96 is offline DF96  England
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Quote:
Originally Posted by scwhitely
To say I don't know what I'm doing is a bit of an understatement. (I do, however, intend to enroll in some electronics courses next semester). In the meantime, I will look into this biasing and this resistor pad you speak of.
Excellent! One of the marks of a wise man is that he realises the limits of his understanding.

An opamp needs a DC supply to bias each of its inputs. In the case of the unity gain follower, the - input gets its bias from the output. The + input needs to get it from the input, so if there is a coupling capacitor (which would be the normal situation) you need to include a resistor from the + input to a suitable voltage level (could be zero volts in many cases).
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Old 22nd March 2012, 07:40 PM   #14
Mooly is offline Mooly  United Kingdom
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Taking your circuit above and lets say it runs on a 9 volt battery.

1. You bias pin 3 with two equal value resistors in series across the supply to create a "half supply" reference voltage of 4.5 volts at the junction of the two resistors. Pin 3 connects to that 4.5 volt junction.
100K would be about right. That forces pin 3 to 4.5 volts and the output pin 6 follows suit.

2. You now MUST AC couple both input and output using small capacitors of say 10uf (microfarad). Make sure you get the polarity correct. Plus end of cap to pin 3 and same for pin 6.

Thats it... one opamp buffer.
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Old 23rd March 2012, 07:17 PM   #15
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Hmm, I had time today to give this a try, but I still seem to be doing something wrong, the signal is not showing up at the amplifier. Here are the configurations I tried. I tried both setups with and without the coupling cap on the output. For the record, the power supply is a 12V 3.2Ah SLA, which will end up being the power supply in the finalized project.

-Chad?
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Old 23rd March 2012, 07:23 PM   #16
Mooly is offline Mooly  United Kingdom
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The second one is 100% correct and will work. If it doesn't maybe the opamp is faulty. An SLA is pretty scary to work with.

DC voltages with no signal applied... check all the pins on the opamp.

Pin 3 should be at approx 6 volts and pin 6 should be the same.
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Old 23rd March 2012, 07:59 PM   #17
Mooly is offline Mooly  United Kingdom
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Another thought in trying to cover all bases... was or is the opamp from a genuine source ? They are so many fake parts around.

In your diagram pins 1 and 8 are (and should be) left floating and pin 6 is the output.

Measuring the DC current taken by the opamp might (might !) help show if there is a problem. It should be around 1 to 2.5 milliamps for the 741 opamp. Worth checking.

As to using an SLA (which I must confess I do all the time for some things), well it's worth staying safe and keeping what you are working on safe. For something like this a 100 ohm resistor in series with the positive supply might be advisable together with a small cap (10 to 100uf) across the "circuit supply end". So that means the caps across pins 4 and 7 on the opamp. The series resistor lets you measure the current too by just measuring the volt drop across it and using ohms law.

Better safe than sorry with SLA's the current capability is in the 100's of amps region.
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Old 23rd March 2012, 08:01 PM   #18
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Yep, you are right. I went back and double checked everything, which seemed to be in order.

However, what was happening was that for some reason, the TPA3110D2 amp chip had gone into shutdown mode when (and each time) I hooked up that battery (woops). After giving the amp a "gentle" reset, the 741 buffer amps (unity gain followers?) worked like a charm. Separate l/r channels for the VU meters, one mono channel for the amp. Beautiful. Thanks again for all your help!

As for the summing of the channels, as of right now I'm just hard wiring them together and it seems to sound pretty good (no thorough testing yet), but it couldn't hurt to look into these resistor pads if that's the correct route.

Once again, thank you so much. In the immortal words of the A-Team's Hannibal; I love it when a plan comes together.

-Chad
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Old 23rd March 2012, 08:04 PM   #19
Mooly is offline Mooly  United Kingdom
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Ah

Pleased it's all working for you. Yes the buffer is a unity gain amp. Without seeing the full circuit details it's hard to imagine how it's all hooked up but sounds like you are on the right track.

If you need gain from the opamps then that's easy, two resistors and a cap is all that's needed.

Excellent
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Old 23rd March 2012, 08:12 PM   #20
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Thanks for all the input Mooly, I should probably invest in a bona fide power supply for my work station. I've already spent plenty on a nice soldering iron, power-tools, and everything else, whats a little more dough? No pockets in the shroud.

I suppose the fact that my projects are almost exclusively battery powered has lead me toward a false sense of security with them. It probably wouldn't take much more than a minor mishap to change that.

As for the full circuit, I can confidently say it's the most complex project I've worked on to date; involving solar panels, class D amps, VU meters, on-board USB charging (Apple, Inc. didn't make that any easier on me), and now, nod to you, a buffer amp stage. I'm on the home stretch now!
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