Passive RLC Notch Filter

[Chummy]

Dear Forum,

I have built a micropnone that incorporates a low-z mic capsule the specs are as follows: frequency response: 100Hz to 10kHz , impedance: 600 ohms +/- 30% , sensitivity: -78dB +/- 3dB. I step it up with a 1:10 impedance transformer so it is high-z. I am not pleased with the mid outputs and would like to create a low pass passive RLC notch filter to alleviate the trebble in the mids and to allow for more bass to come through. This is a very simple curcuit which consists of only three components not including the mic capsule and transformer however I do not know where to start as I am electronically handicapped.

I thought about making a little board with a variable conductor, inductor and resitor to find the tone I desire and measure the values however once again how large or small must I go - we are talking micro henrys for the inductor and microfarads for the capacitor. I do not wish to make it too tight as I think it will be too low Q so a wide notch is what I wish to have if this can in fact take that terrible trebble out of the sound it is currently producing.

I read the FAQ and I just could not find any other way than to ask the pros outright. Any help with this would be greatly appreciated!!!!

Thanks,
Chummy

 

[ericgibbs]

Dear Forum,

I have built a micropnone that incorporates a low-z mic capsule the specs are as follows: frequency response: 100Hz to 10kHz , impedance: 600 ohms +/- 30% , sensitivity: -78dB +/- 3dB. I step it up with a 1:10 impedance transformer so it is high-z. I am not pleased with the mid outputs and would like to create a low pass passive RLC notch filter to alleviate the trebble in the mids and to allow for more bass to come through. This is a very simple curcuit which consists of only three components not including the mic capsule and transformer however I do not know where to start as I am electronically handicapped.

I thought about making a little board with a variable conductor, inductor and resitor to find the tone I desire and measure the values however once again how large or small must I go - we are talking micro henrys for the inductor and microfarads for the capacitor. I do not wish to make it too tight as I think it will be too low Q so a wide notch is what I wish to have if this can in fact take that terrible trebble out of the sound it is currently producing.

I read the FAQ and I just could not find any other way than to ask the pros outright. Any help with this would be greatly appreciated!!!!

Thanks,
Chummy

hi,
If you can construct a simple filter project, download free from the web TI Filter Pro its a filter calculator.
here:
http://focus.ti.com/docs/toolsw/folders/print/filterpro.html

 

[Chummy]

Hello Eric,

Thank you very much for your quick reply. I have downloaded and looked at this program and it seems very good however in my case most of it is like reading chinese. As I stated I am very handicapped or almost a complete cripple when it comes to these things. I can put them together no problem as I really like to make things. This kind of calculating is way out of my league. I find it very fun and interesting but am not an electronics guru at all.

If I wish to to create a passive low pass notch filter with three components the program has only one or the other to choose from: either a notch filter of low pass filter. What would be the best application in my case? Do I have enough data in in order to do this? I have used the search engine but am unsure as to what I require. Where do I start? Scratch, scratch. I probably need someone to walk me through it once or twice to get it nailed down.

How are things up there in Hampshire? Any ash fallout? I work inLive and work in Germany and everything is real quiet not a plane to be heard. It is quite strange.

-Chummy

 

[ericgibbs]

Hello Eric,

Thank you very much for your quick reply. I have downloaded and looked at this program and it seems very good however in my case most of it is like reading chinese. As I stated I am very handicapped or almost a complete cripple when it comes to these things. I can put them together no problem as I really like to make things. This kind of calculating is way out of my league. I find it very fun and interesting but am not an electronics guru at all.

If I wish to to create a passive low pass notch filter with three components the program has only one or the other to choose from: either a notch filter of low pass filter. What would be the best application in my case? Do I have enough data in in order to do this? I have used the search engine but am unsure as to what I require. Where do I start? Scratch, scratch. I probably need someone to walk me through it once or twice to get it nailed down.

How are things up there in Hampshire? Any ash fallout? I work inLive and work in Germany and everything is real quiet not a plane to be heard. It is quite strange.

-Chummy

hi,
I will try to post a simple filter, later today.

There is ash in the air in Hampshire, its a bit like a mist over the sea, also its definitely on the cars.

 

[Chummy]

Thank you so much Eric.

As I write the first commercial aircraft in three days takes off.

-Chummy

 

[ericgibbs]

Thank you so much Eric.

As I write the first commercial aircraft in three days takes off.

-Chummy

hi,
This is a simple R/C passive filter, you can see that you will get a reduction in overall response amplitude.
I would suggest a active filter will be required.

 

[Nigel Goodwin]

What are you trying to do with the mike?, it's far better to get it right in the first place than try and process it later.

 

[Chummy]

Thanks Eric!!! I cannot use an active filter as the micro is rather small and cannot incorporate a battery source. I also asked an a techi friend of mine in the states what he would do as he builds effect pedals and amplifiers specifically for harmonica. Yes the ol' tin sandwich. That is what I am attempting to do Nigel make a decent mic sound better. In other words I want it to sound Fat. The low freqs sound ok however the mids are a little too trebbley.

This is my friends suggestion:

A notch filter will notch or lower the tuned frequency, might I suggest a low pass filter?

A simple experiment will suffice, install a 47pf cap across the secondary of your transformer.

Secondary impedance = 6000 (600 ohms x 10)
Cutoff freq = 567hz

567hz is the 3db drop off point, above this freq losses will be noticeable and this should flatten out your overall freq response and balance the mids to highs. If this helps you can fine tune it by varying the values of the cap.

I do not know how he came up with this solution. What do you all think? Feedback is welcome! I am going to ask him as I would like to do more of this. I build cigar box guitars as well. I place one or two simple piezo transducers in them as a pickup and they are very prone to high freq. pickup thus causing unwanted feedback and poor tone.

This is a very nice forum.

 

[ericgibbs]

hi chummy,
As the tonal quality of music is subjective, I would do as your friend suggests, add low value caps until the 'top cut' matches your taste.
I would have thought a 47pF cap is a little on the low side.