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Op Amps with battery circuits

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dpixel

New Member
Hello all!
I'm just pondering op amps in small circuits. Say a circuit running 9v or 12v dc. I recently built a LM3915 VU meter for a synth with a TL072 op amp. Is the only way this can be done is with a dual power supply or a conventional power supply(transformer + rectification, etc)? Right now I'm running the op amp off a 9v battery with split rails with a virtual ground and the rest of the circuit off a 12v wall wart. Everything works great.
I understand what a dc load will do to a split rail supply. Does this mean op amps are generally not used in single 9v battery circuits? Or am I just using the wrong op amp.
Ultimately, I'd like to run this whole synth project off a 24v 3amp wall wart, splitting it into +9v, -9v, and +5v(or 4.5v).

And what if I wanted to build a small circuit with just a LM3915 and some small amplification to boost low level audio signals so the LM3915 will work correctly with just a 9v battery.

I'm kinda new to op amps, have worked with TL072, LM386, and LM324.

Thanks for looking.
 

audioguru

Well-Known Member
Most Helpful Member
In my Sound Level Indicator project that has an LM3915 I use an MC33172 dual opamp designed with inputs that go all the way down to ground (a TL072 input fails if it is within a few volts from its negative supply voltage) and it works when the supply drops to 3V (the TL072 fails if its supply is less than 7V). Therefore my selected opamp does not need a negative supply and it still works when the battery voltage is very low. The MC33172 is similar to the LM358 (a dual version of an LM324 quad) but the LM has crossover distortion and poor performance above 2kHz and the MC has no crossover distortion and good response to 35kHz. Any of the new rail-to-rail opamps can also use only a positive supply.
I added an automatic gain selector so the LM3915 shows a range of 50dB instead of 30dB. My project is normally powered from a 9VDC wall wart but it also has an 8.4V (called a 9V) Ni-MH battery that is charged from the 9VDC.
 

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dpixel

New Member
Yeah, I'm not aware of the differences in op amps. I'm still learning how to read opamp datasheets. So I can assume it's not so much about the power supply, but the right opamp for the project?

And say your building some else s circuit that calls for pos and neg supply to the opamps, is a dual supply required without tweaking the circuit(not knowing how they powered it)? I mean, are there any stable virtual ground circuits?

When you refer to 7v supply of the TL072, are you referring to the positive and negative voltage combined? That's pretty much right where I am now. If I connect a weak 9v battery, the signal seems to drop somewhat on the LM3915 outputs.
I tried just reading the positive side of my synth signal, but sometimes the wave form goes from 0v and lower, so it wasn't til I supplied a negative voltage to the TL072 that the Lm3915 saw it.
I guess my next question would be...what are the new rail to rail opamps? Can these amplify the full ac signal?

That looks like a nice circuit audioguru. I wanted to try an LM3915 with an electret mic as well at some point. Can I assume Q2, C8, and R13 make up the peak detector, or is the second opamp part of it to?

And thanks for the sticky Nigel.
 

ci139

Active Member
if i recall right then for the j-Fet input Op Amp the minimum Tot supply is 9.6V e.g. ±4.8V -- there it starts to respond as dif-amp but not yet operate . . .
most of op-amp.-s work best at specified voltages -- there with the full : -speed / -sensitivity / -slew rate / -e.c
the Spice test may be 10-s of times over-optimistic but for comparison it somewhat will do
 

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