resistors, capacitors and home made filters

[Alex JB 10+ year member]

Hi,

I'm sure this has been covered but I searched and couldn't find quite was I was looking for so...

I just posted a big rant over on the Ford F150 forum about my set up so far,

F150 post

and something I'd like to fix is having a high pass and low pass filter on the channels to my rear doors.

I had a set up many years ago on a Citroen where the rear pillar speakers were only 3" cones and struggled with any kind of bass.

Someone gave me a tip to get a resistor of a certain rating to put inline and it would work as a HPF to cut off all the bass.

I read up on wikepedia and got to ---

1/(2*Pi*R*C) r= resistance c = capacitance gives the frequency cuttof of 70% of the voltage.

Or I could just punch in the fequency required and a resistor like 1KOhm and find the capacitor you need into this calculator...

http://www.opamplabs.com/rfc.htm

so in theory, a 1uF capacitor in + line and a 150Ohm resistor across the + and - would give you about a 1000Hz HPF...

Anyone done this recently?

Or do I need to get myself to radio shack.

I plan to do this and use the LPF (already at 100Hz ish for the front door speakers) in my 5 channel amp to cut off the high end - voila... a mid bass driver from a standard speaker, hopefully that doesn't pop and crackle like it does when you feed it a full range signal.

I think I'm on the right track, but please correct me, plus if you did this, let me know the resistor Ohms you chose and what results you got.

I basically can't afford to change the standard speakers out right now and I have what I have as far as H/U, amp and sub so I'm trying to make the best out of some not so brilliant OEM speakers.

Click to read more...

 

[helotaxi 5,000+ posts]

You can also use the resistance of the speaker in the formula and just use a cap. That's a basic first order filter. If you add a resistor, you're going to affect the impedance that the amp sees.

 

[thch 10+ year member]

Hi,I'm sure this has been covered but I searched and couldn't find quite was I was looking for so...

I just posted a big rant over on the Ford F150 forum about my set up so far,

F150 post

and something I'd like to fix is having a high pass and low pass filter on the channels to my rear doors.

I had a set up many years ago on a Citroen where the rear pillar speakers were only 3" cones and struggled with any kind of bass.

Someone gave me a tip to get a resistor of a certain rating to put inline and it would work as a HPF to cut off all the bass.

I read up on wikepedia and got to ---

1/(2*Pi*R*C) r= resistance c = capacitance gives the frequency cuttof of 70% of the voltage.

Or I could just punch in the fequency required and a resistor like 1KOhm and find the capacitor you need into this calculator...

http://www.opamplabs.com/rfc.htm

so in theory, a 1uF capacitor in + line and a 150Ohm resistor across the + and - would give you about a 1000Hz HPF...

Anyone done this recently?

Or do I need to get myself to radio shack.

I plan to do this and use the LPF (already at 100Hz ish for the front door speakers) in my 5 channel amp to cut off the high end - voila... a mid bass driver from a standard speaker, hopefully that doesn't pop and crackle like it does when you feed it a full range signal.

I think I'm on the right track, but please correct me, plus if you did this, let me know the resistor Ohms you chose and what results you got.

I basically can't afford to change the standard speakers out right now and I have what I have as far as H/U, amp and sub so I'm trying to make the best out of some not so brilliant OEM speakers.

keep in mind that the speaker's impedance factors in as well. in your example, the 150R resistor would likely be in parallel with the driver, giving you a resistance close to 4ohm. I've actually seen data sheets of slightly more complex circuits that make similar mistakes.

some designs may add a resistor in parallel with the driver to limit the driver's impedance to something lowish. more often an inductor is added to make a second order filter.

keep in mind that the RC or RL circuits are not meant to be strong filters. -6dB/oct.

now you can build passive filters in line with the amplifier. the amp might have a 10k+ input impedance, which is more manageable.

 

[Alex JB 10+ year member]

You can also use the resistance of the speaker in the formula and just use a cap. That's a basic first order filter. If you add a resistor, you're going to affect the impedance that the amp sees.

Ah, yep, a school boy error //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

I'll measure the resistance over the speaker and work from there.

I'm only a mere mechanical engineer, this electrical stuff is not my forte'.

Thanks.

 

[helotaxi 5,000+ posts]

It is thch's forte'. Take his advice. Either build a proper 2nd order filter or build a filter for the pre-amp.

 

[Alex JB 10+ year member]

Ok, thanks.

I measured 4.8Ohm so 400Uf woudl be about 80Hz, I'll give it a shot but if it's not too effective I'll just have to abandon the rears and just wait untill I can sneak a $200 6 1/2" front set past the wife into the garage for front door istalation - ditch all hopes of making OEM base model speakers sound half decent. //content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif

 

[Linkz 10+ year member]

Try this website. They have lots of tools to use as well.