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bandpass filters?

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Neil Groves

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i Just built this circuit:

https://www.electro-tech-online.com/custompdfs/2012/01/2126272.pdf

well i have built it as far as the bass channel goes and thought i'd test it before i go any further, what i was expecting feeding the circuit with a sinewave was the bass channel L.E.D to start to glow at around 50Hz be fully bright at about 100Hz and then fade as i increased the frequency towards 200Hz, what i am getting in fact is the L.E.D starts to glow at 20Hz, is fully bright by 150Hz but then stays fully bright never dimming again as i increase the frequency well up to 6Khz and beyond, either i have an error in the way i have connected it on the breadboard, or there is an error in the schematic?

what am i supposed to see on Pin 7 of IC1b? i was expecting a dc level but i don't get that till after the diode, then it gets to 0.7v and stays there which i understand means the transistor has fully saturated?

can anyone give an insight to this circuit please?

Neil.
 
IC1b pin7 should match the input (at low frequency). IC1b is a filter. The output should not be "DC".
 
so the output of the filter should look the same as the input then at low frequency? then as the frquency rises what happens to the signal at pin7?

Neil.
 
Thanks Brevor, that was what i was expecting but it doesn't, it stays constant so i will have to check my connections again, i'll also change the cap values to see if that makes a difference.

Neil.
 
The cap values are determined by calculations, not guesses. The calculations are made with simple arithmatic and are shown in Google when you search for Multiple Feedback Bandpass Filter.

The circuit shown uses the input opamp to bias the input of the filter opamp at half the DC supply voltage. Then the output will also be at half the DC supply voltage so it can swing equally positive and negtive around it.
But you said your output has no DC so then the input must not be biased at half the supply voltage and it is a rectifier, not a filter.
 
As i increase the frequency, the output frequency increases by the same amount but doesn't attenuate as i expected it to, i'm going to put the circuit on stripboard to ensure good connections, it wouldn't be the first time a circuit has failed due to worn out breadboard connections.

Neil.
 
Breadboards ruin many circuits. I used a breadboard only one time and my circuit didn't work because of high stray capacitance, radio and TV pickup and intermittent connectors.
Then I soldered the circuit onto a compact stripboard and it worked perfectly like many of the stripboard circuits I made later.
 
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