Like in my other post, I am making a graphic equalizer. I was having trouble getting the op amp to work with the LM3915, but I fixed that. Now, having gotten a normal op amp to work, my bandpass filter won't work with the LM3915. Despite the infinite contempt this project seems to have for me, though, I think I'm actually learning quite a bit with this.
I decided to make a separate test circuit with a bandpass filter straight off the internet from here: http://sound.westhost.com/project63.htm
The only difference is that I didn't bother to put a bypass capacitor on the power supply, which is from 2 9V batteries put together to make a split power supply, and I didn't bother to use Resistor X (it was optional, so I just used a wire instead of the resistor. The op amp used is one from a LM248. I know it's a noisy chip, but I'm just trying to get the stupid thing to work right now. I can worry about sound distortion later. The input signal is directly from my electric guitar.
I used the standard LM3915 circuit from the datasheet (page 2 to be exact, and I'll include the datasheet with the post). The only difference is that I changed the resistor values. I know this part works because I can feed my unfiltered input signal directly from the source and it works fine.
Basically, the output of the filter SEEMS perfect. The sound is fine. I even hooked the output out to my laptop and used a free oscilloscope program. The AC signal looks fine. I know the LM3915 can take an AC input, so I'm sure that's not the problem (although it's only half the signal with AC). I compared it to the unfiltered signal straight from the source, and that had about the same power level (both were around 45mV for V eff according to the oscilloscope program). There did not seem to be any DC voltage (neither my multimeter nor the oscilloscope program showed any DC voltage). However, if I put a diode (1N4148, if I remember right) between the filter and the output, the oscilloscope program picked up a completely normal signal (unrectified) when oriented one way, and no signal whatsoever when oriented the other way. I assume this may have something to due with the low voltage of the signal, but I would have thought AC would merely be flipped more or less by changing the diode's orientation.
I've tried sticking a 10 uF capacitor between the output and the LM3915 input, but that didn't do anything.
Oh, and if anyone cares, the Velleman project kits are on clearance at Radioshack. I can confirm firsthand that it was at 2 different stores, so there's probably a good chance that they're on clearance at all the stores (but it could just be a weird coincidence). I scored a K1823RS power supply unit for $5 (normally $20, there were a few others with the same deal). So now's the time to get them if you want to give them to a kid for a present or something.
I decided to make a separate test circuit with a bandpass filter straight off the internet from here: http://sound.westhost.com/project63.htm
The only difference is that I didn't bother to put a bypass capacitor on the power supply, which is from 2 9V batteries put together to make a split power supply, and I didn't bother to use Resistor X (it was optional, so I just used a wire instead of the resistor. The op amp used is one from a LM248. I know it's a noisy chip, but I'm just trying to get the stupid thing to work right now. I can worry about sound distortion later. The input signal is directly from my electric guitar.
I used the standard LM3915 circuit from the datasheet (page 2 to be exact, and I'll include the datasheet with the post). The only difference is that I changed the resistor values. I know this part works because I can feed my unfiltered input signal directly from the source and it works fine.
Basically, the output of the filter SEEMS perfect. The sound is fine. I even hooked the output out to my laptop and used a free oscilloscope program. The AC signal looks fine. I know the LM3915 can take an AC input, so I'm sure that's not the problem (although it's only half the signal with AC). I compared it to the unfiltered signal straight from the source, and that had about the same power level (both were around 45mV for V eff according to the oscilloscope program). There did not seem to be any DC voltage (neither my multimeter nor the oscilloscope program showed any DC voltage). However, if I put a diode (1N4148, if I remember right) between the filter and the output, the oscilloscope program picked up a completely normal signal (unrectified) when oriented one way, and no signal whatsoever when oriented the other way. I assume this may have something to due with the low voltage of the signal, but I would have thought AC would merely be flipped more or less by changing the diode's orientation.
I've tried sticking a 10 uF capacitor between the output and the LM3915 input, but that didn't do anything.
Oh, and if anyone cares, the Velleman project kits are on clearance at Radioshack. I can confirm firsthand that it was at 2 different stores, so there's probably a good chance that they're on clearance at all the stores (but it could just be a weird coincidence). I scored a K1823RS power supply unit for $5 (normally $20, there were a few others with the same deal). So now's the time to get them if you want to give them to a kid for a present or something.
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