Active low pass/ active cut filter

Thread Starter

noingwhat

Joined Jul 23, 2010
61
I would like to build some sort of circuit so that it will pass freqs < 80hz at normal volume, and cut out freqs (-30db atleast) > 300hz. What kind of circuit would I use to achieve this? Low pass or high cut? What is the difference between the two? And would you suggest that I make it an active or passive (rc) filter?
 
Last edited:

Ron H

Joined Apr 14, 2005
7,063
A fourth order Butterworth lowpass filter with 120Hz cutoff will do what you want.
What sort of load are you driving?
 

Thread Starter

noingwhat

Joined Jul 23, 2010
61
I feel like this is a silly question, but since there are only 3 solder spot on the circuit diagram, but there are 8 pins on most of the opamps I am looking at, where do I solder the three points on the diagram?

Another small question, would the values of the caps and resistors be dependent on which op amp I decide to use?
 

Ron H

Joined Apr 14, 2005
7,063
I feel like this is a silly question, but since there are only 3 solder spot on the circuit diagram, but there are 8 pins on most of the opamps I am looking at, where do I solder the three points on the diagram?
Another small question, would the values of the caps and resistors be dependent on which op amp I decide to use?
I haven't seen the circuit diagram you are looking at. I assume you are talking about an op amp. Answer my previous question:
What sort of load are you driving?
Also, where are you getting your input signal, and what is the maximum amplitude of the audio, if you know.
Do you have a DC power supply voltage(s) in mind?
Knowing the answers to all these questions (there are 4 of them), we should be able to provide you with a proper schematic, including the answer to your question about all those extra pins.
 

Thread Starter

noingwhat

Joined Jul 23, 2010
61
I am trying to build a cheapy subwoofer using an old driver I found lying around, so this circuit is going to have the input from the TV, and output to the subwoofer amp. Not completely sure of the input voltage and current, but from what I understand it should not be a lot because it is just the RCA cables, and I am pretty sure there are only a few volts in there.
 

thatoneguy

Joined Feb 19, 2009
6,359
I am trying to build a cheapy subwoofer using an old driver I found lying around, so this circuit is going to have the input from the TV, and output to the subwoofer amp. Not completely sure of the input voltage and current, but from what I understand it should not be a lot because it is just the RCA cables, and I am pretty sure there are only a few volts in there.
Do you have a power amplifier for the subwoofer? Or is that what you are trying to build with this thread?

There are two parts;
1) Active Crossover/pre-amp (Your desired low pass filter with op-amp above)
2) Power amplifier

If you don't have the power amplifier, things get... difficult.

You may want to read some of these Excellent introductions from ESP to understand what you want a bit better.
 

Ron H

Joined Apr 14, 2005
7,063
I am trying to build a cheapy subwoofer using an old driver I found lying around, so this circuit is going to have the input from the TV, and output to the subwoofer amp. Not completely sure of the input voltage and current, but from what I understand it should not be a lot because it is just the RCA cables, and I am pretty sure there are only a few volts in there.
You didn't answer the question about the power supply(s).

@Thatoneguy:
He mentioned "output to the subwoofer amp". I assume that means one already exists. As you said, if it doesn't have one, "things get... difficult".:)
 

thatoneguy

Joined Feb 19, 2009
6,359
@Thatoneguy:
He mentioned "output to the subwoofer amp". I assume that means one already exists. As you said, if it doesn't have one, "things get... difficult".:)
My concern is that he may want to use an old receiver as his "sub amp" or something similar. Need the information, tried to re-word some of your questions, I guess.

I guess we'll find out, knowing the make/model of the power amp will answer most all of the questions/concerns.
 

Thread Starter

noingwhat

Joined Jul 23, 2010
61
Yes, it's and old reciever as an amp, all I really need is the crossover. I had used a passive rc filter that I built for the speaker that I have used for a few months, but it never sounded all too great. It leaks too much up into the higher freqs (I messed up and made the Fc 300hz and for an RC filter, this doesnt provide much of a "subwoofer") and I am looking to get a bit better sound quality out of it. I'm not looking for much, but it sounds pretty good considering I only spent about $1 on it.
 
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