Low pass butterworth filter

Thread Starter

noingwhat

Joined Jul 23, 2010
61
How do I design a low pass butterworth filter that cuts off around 50-300 hz, and has a pot for frequency adjustment? What kind of diagram do I need and how do I calculate the parts I will need?
 

Audioguru

Joined Dec 20, 2007
11,251
The second-order lowpass filter shown has a Butterworth response only at one setting of its level and frequency controls.
1) The filter is supposed to be fed directly from a very low impedance like the output of an opamp, not from a volume control.
2) The frequency is changed by adjusting two pots, not just one pot.
 

hgmjr

Joined Jan 28, 2005
9,029
How do I design a low pass butterworth filter that cuts off around 50-300 hz, and has a pot for frequency adjustment? What kind of diagram do I need and how do I calculate the parts I will need?
Do you know how sharply you would like the frequencies above 50Hz to be attenuated? That bit of info is need to determine how many poles the filter will require.

hgmjr
 

Thread Starter

noingwhat

Joined Jul 23, 2010
61
Do you know how sharply you would like the frequencies above 50Hz to be attenuated? That bit of info is need to determine how many poles the filter will require.

hgmjr
Not totally sure, but I just need something that would be an improvement over my current passive rc filter.
 
I would also like to ask, other than TL072, what other op amps are recommended to be used to design active low pass filters, high pass filters and band pass filters? I have OPA 2604 op amps but im considering using OPA 277 for designing a band pass filter by cascading a high pass filter with 20Hz cut off frequency with a low pass filter with a 500Hz frequency.
 

Audioguru

Joined Dec 20, 2007
11,251
The TL072 is old but it is still a good inexpensive dual audio opamp.
There are hundreds of other good audio opamps available.
 

Thread Starter

noingwhat

Joined Jul 23, 2010
61
What other options are there as far as opamps? It doesnt have to be a dual channel, mono would be perfectly fine, as I only need one channel.
 

Thread Starter

noingwhat

Joined Jul 23, 2010
61
Would something like the MC1741? I buy most of my parts from All-Electronics because it is a store nearby, and has so many things for cheap. Would any of these work? (op amps)
 

Audioguru

Joined Dec 20, 2007
11,251
A 741 opamp (LM741, TL741, uA741 and MC1741) is 43 years old and its performance is horrible:
1) At full output swing, its response drops above only 9kHz while most other opamps go to 100kHz.
2) It is very noisy (hiss) while many audio opamps are low noise.

Most opamps listed on the All-Electronics page are also at least 40 years old and are also horrible for audio. Maybe All-Electronics sells surplus old junk.
 

Thread Starter

noingwhat

Joined Jul 23, 2010
61
It is all surplus, im not sure where they get it, but it's usually cheap and atleast it works. I'm not out for audiophile quality right now, so it's not that big of a deal. Even with how old it is, would it work? Also what kind of stuff can you find at radioshack that would work better?
 

Audioguru

Joined Dec 20, 2007
11,251
RadioShack sells defective (seconds) or old parts at very high prices. Frequently they do not know who made them and do not have any spec's.
 

Thread Starter

noingwhat

Joined Jul 23, 2010
61
As I said, I am just looking for something that works. The quality is not that big of a deal, I just don't want to pay an arm and a leg for it, or have to wait weeks for shipping. Is there anything at all electronics or radioshack that will at least work, regardless of quality?
 

Audioguru

Joined Dec 20, 2007
11,251
I buy top quality electronic parts for a low competitive price from Digikey or Newark. When I order before 8:00PM then it is delivered the next morning and I live in a different country!
 

marshallf3

Joined Jul 26, 2010
2,358
As I said, I am just looking for something that works. The quality is not that big of a deal, I just don't want to pay an arm and a leg for it, or have to wait weeks for shipping. Is there anything at all electronics or radioshack that will at least work, regardless of quality?
What country are you in?

Radio Shack has some basics such as the TL082 and I doubt they're seconds or they wouldn't be allowed out with a brand manufacturer's name on them. They've taken their parts prices so high nowadays and the selection is so poor that it's rare I ever buy anything from them. They used to be fairly good, then over the years they declined almost week by week. They want something like $1.69 for an electrolytic you can buy for 0.10 in single quantities from any major supplier.
 

Thread Starter

noingwhat

Joined Jul 23, 2010
61
Wow, next day? Hmm... maybe I'll look into those two companies. How much does shipping usually cost you though?
 

Audioguru

Joined Dec 20, 2007
11,251
Since you don't show which planet you are on or which country you are in then we might simply guess.

Digikey is American but also ships to Canada.
Newark was recently bought by Farnell who have offices in many countries.
All shipping details are on their websites. I used to phone my orders to their local offices.
 
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