amplifier supply and ground

Audioguru

Joined Dec 20, 2007
11,248
Without the output capacitor the output of the opamp averages half the supply voltage. If it gets higher than about +5.7V then the input of your AD converter might be destroyed.
But the minimum supply for the TL071 is 7V then its max output is +6.3V which is too high for your AD converter.
 

Thread Starter

alarassi

Joined Feb 6, 2010
52
Without the output capacitor the output of the opamp averages half the supply voltage. .
i tested the circuit and it worked well but now i want to have a bandpass filter after the pream (HPF with fc=30 followed by LPF with fc=1000) but without affecting the value of the reference voltage which the half of supply voltage!
in other word, i want my final output after the filters to averages half the supply voltage!
i don't mind using either passive or active filters!
how can it be done?

thank you
 

Audioguru

Joined Dec 20, 2007
11,248
i tested the circuit and it worked well but now i want to have a bandpass filter after the pream (HPF with fc=30 followed by LPF with fc=1000) but without affecting the value of the reference voltage which the half of supply voltage!
in other word, i want my final output after the filters to averages half the supply voltage!
i don't mind using either passive or active filters!
The preamp has three coupling capacitors that create a very droopy passive highpass filter. They can be calculated for 30Hz but then 30Hz will be -9dB instead of -3dB. A Sallen and Key lowpass active filter can be added to the output of the preamp. If the output capacitor of the preamp is removed then the preamp's output can bias the input of the active lowpass filter at half the supply voltage.
 

Thread Starter

alarassi

Joined Feb 6, 2010
52
The preamp has three coupling capacitors that create a very droopy passive highpass filter. They can be calculated for 30Hz but then 30Hz will be -9dB instead of -3dB. A Sallen and Key lowpass active filter can be added to the output of the preamp. If the output capacitor of the preamp is removed then the preamp's output can bias the input of the active lowpass filter at half the supply voltage.
i just tested the circuit on a simulator with sallen key LPF and it worked well (thank you ;))

i really don't understand well the filters' frequency responses matters but as i know it's better to be -3db! so is it ok if use here -9db or it's going to affect the performance!? what's the best choice i can do for the HPF ?
if the choice is to calculate the coupling capacitors to have HPF of fc 30Hz, how can that be done!

thank you Audioguru, you really help me and i learn a lot from you
 

Audioguru

Joined Dec 20, 2007
11,248
A cutoff frequency is -3dB (0.707 times), not as low as -9dB (0.35 times).

Audio is from 20Hz and up so a 30Hz highpass filter will just stop very low frequency rumbling sounds from heartbeats and earthquakes. I would use my mic preamp circuit with a 0.22uF input capacitor that produces a cutoff frequency of 14.6Hz. Its feedback capacitor to ground is too big at 22uF but a 10uf capacitor will produce a cutoff frequency of 7.3Hz. The total of both cutoff frequencies is about 25Hz at -3dB.
 
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