Hi. I'm trying to build a vu-meter with low-pass filter for line level signal, single power supply from 5 to 12 V and i need to set some kind of gain. In the attached schematic, when running some tests, OP1 was 1/4 LM324,everything works beautifully when replaced with TL071 all leds remain light up as there was no signal connected to the vumeter. What am i missing? TL071 can't work in single supply mode?
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P2 does nothing. And C1 is wrong placed. I believe P2 is in between out and (-) input, and this way adjust gain. And C1 must be at the P1 to to block DC input. And if the case, there must be another C in the middle of P1 to input (+) and a voltage divider about VCC/2 to bias the (+) input.
That just doesnt look right.
Even if you look at the common configuration for a non-inverting op-amp.
Even if you look at the common configuration for a non-inverting op-amp.
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I tried to reproduce in my own way this schematic. Could someone propose a corect schematic for low pass filter with adjusstable gain, single supply?
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I cant recall, is the LM324 a bi-polar Op-amp. The TL072 is definitely FET.
That might explain how this poor circuit might just work with the original chip.
That might explain how this poor circuit might just work with the original chip.
Don't try to build variable gain.
If you use a variable pot in the feedback loop you are asking for noise and trouble.
Simply attenuate the input or the output and use a fixed gain amplifier.
Place the Low-Pass filter at the input and use a simple resistor chain to give you a balanced supply.
If you use a variable pot in the feedback loop you are asking for noise and trouble.
Simply attenuate the input or the output and use a fixed gain amplifier.
Place the Low-Pass filter at the input and use a simple resistor chain to give you a balanced supply.
I just skimmed through all this...
The circuit in post #7 is OK but NOT for a single supply. It also needs "Vin" to be correctly biased.
For a VU meter noisy pots don't really matter do they ? If all you are doing is setting gain.
If running a single supply then the opamp has to be biased accordingly... that's easy, but is it going to do what you want ? The output will be at half supply. This all means the input and output has to be AC coupled unless it's incorporated into somthing that provides the appropriate bias and works with the output at midpoint.
The circuit in post #7 is OK but NOT for a single supply. It also needs "Vin" to be correctly biased.
For a VU meter noisy pots don't really matter do they ? If all you are doing is setting gain.
If running a single supply then the opamp has to be biased accordingly... that's easy, but is it going to do what you want ? The output will be at half supply. This all means the input and output has to be AC coupled unless it's incorporated into somthing that provides the appropriate bias and works with the output at midpoint.
Question
Does it matter if the opamp stage inverts ?
If just driving a VU meter, then it doesn't and it perhaps makes the design maybe a little easier.
Does it matter if the opamp stage inverts ?
If just driving a VU meter, then it doesn't and it perhaps makes the design maybe a little easier.
i just want to have a cut-off freq at about 150-200hz, to adjust some gain, and a single power supply, the rest is up to you. Thank you all.
Why ? 🙂
Active volume controls use that approach. With a FET opamp there is no real DC bias current flowing.
One feature of the '324 is that the common-mode range includes the negative supply rail. (Maybe even a few mV below the negative rail.) The TL071 isn't guaranteed to work with common-mode voltages any closer than 4 volts from the rails.
At the very least, you need some variant of a 2-resistor "rail splitter" on the non-inverting input.
Dale
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