LPF,HPF,BPF design

[Jamesmeister]

im working with the design given specifications required for us to have.
300 Hz (Low) -LPF
1 kHz (Mid) - BPF
4 kHz (High) - HPF

Cut Boost (Av dB) ±12 dB

Ive done my computations in LPF and HPF and i can show a graph using pspice that shows my computations are correct but INDIVIDUALLY. Meaning, i haven't used a summing amp yet to combine all of them. so here is a picture of the circuit of LPF and HPF (Individual)

with the picture of the graph

But if i connect both of them to a summing amp.

graph looks like this:

the gain should only be about 12dB. as far as i know Rf=Ri so the amp will become a buffer amp which is what i did in the summing amp. i dont know why it attenuated. help please

what i want to happen is for example, if i have 3 inputs of different frequencies, low frequency signal will enter the LPF, the freq bet 300 and 4k will enter the BPF, and the freq above 4k will enter the high pass filter. a graph like this:

so what i did was to add a summing amp at the outputs of the 3 filters. Its like the purpose of the summing amp is to delay the signals so that they won't fight with each other.

 

[Electrobrains]

First glance shows you have connected the outputs together! That's for sure wrong (and destructive).

 

[duke37]

I am not clear as to what you ae trying to do.
There is a filter design, which I can not find at the moment which gives out low, medium and high frequencies.

There is a problem with your circuit in that the two amplifiers have their outputs connected together so they will be fighting each other. You need a resistor from each to your buffer output.

Edit. Pipped at the post!

 

[Jamesmeister]

ive edited my post explaining what i want to do. help please.

 

[(*steve*)]

It's best not to edit your post like that because it's hard for people to see what's changed.

Apart from including the images, this seems to be the major addition:

what i want to happen is for example, if i have 3 inputs of different frequencies, low frequency signal will enter the LPF, the freq bet 300 and 4k will enter the BPF, and the freq above 4k will enter the high pass filter. a graph like this:

so what i did was to add a summing amp at the outputs of the 3 filters. Its like the purpose of the summing amp is to delay the signals so that they won't fight with each other.

The summing amp has nothing to do with delaying the signal. It simply combines them together.

Google summing amplifier and you'll see what you've done wrong.

If that graph you show is what you want to get out, then the practical effect is to simply reproduce the input. However you've done only th red and yellow bits, not the blue bit.

 

[Jamesmeister]

i just showed what the links look like, for others to easily see the images.

If that graph you show is what you want to get out, then the practical effect is to simply reproduce the input. However you've done only th red and yellow bits, not the blue bit.

yes, i still haven't done the BPF yet(blue). so its not possible to have the output like that even if i input 3 signals of diff frequencies at the same time? because we are required to have a pre-amp (3 mic inputs 2 line inputs) and as far as i know, all of them are connected to a summing amp so they are of different frequencies right?

 

[(*steve*)]

because we are required to have a pre-amp (3 mic inputs 2 line inputs) and as far as i know, all of them are connected to a summing amp so they are of different frequencies right?

I see no reason why they would be different frequencies.

I still really don't understand what you're trying to do.

At first I thought you might want to adjust bass, treble, and mid-range individually. Now you say you simply want a preamp with 3 inputs that are mixed. No need for filters in that description.

 

[davenn]

............... we are required to have a pre-amp (3 mic inputs 2 line inputs) and as far as i know, all of them are connected to a summing amp so they are of different frequencies right?

why would they be ? ... sounds like you are just making a 5 channel mixer ... yes?

That doesnt explain the need for filtering as you have done

Were you really wanting to filter each of the 3 microphones in a different way ?
if so, why ? ... it seems quite pointless, specially when you are combining the results from each mic and filter anyway

Dave

 

[Jamesmeister]

this is what we are tasked to do:

>Mixer section
Three (3) high impedance microphone-level inputs
(unbalanced), two (2) high impedance line-level inputs

Microphone input section
- Input impedance (Zin): 800 
- Level gain (Av dB): 42 dBu
Line input section
- Input impedance (Zin): 20 k
- Level gain (Av dB): 25 dBu


>Tone control section
Three-band tone control
Cutoff frequencies
300 Hz (Low)
1 kHz (Mid)
4 kHz (High)
Cut Boost (Av dB) ±12 dB

sorry if i lack knowledge about this. when we had a short discussion on tone control using james baxandall circuit (treble bass), what i understand is that it only function as a filter. i've seen james baxandall circuit with mid range but we are required to show COMPUTATIONS that's why i ended up planning to design with lpf, hpf, bpf. what do you guys suggest should i do? thanks for all your help!!

 

[(*steve*)]

Why don't you look at the first part first. Do the mixer first.

Then worry about the tone controls

 

[davenn]

this is what we are tasked to do:
sorry if i lack knowledge about this. when we had a short discussion on tone control using james baxandall circuit (treble bass), what i understand is that it only function as a filter. i've seen james baxandall circuit with mid range but we are required to show COMPUTATIONS that's why i ended up planning to design with lpf, hpf, bpf. what do you guys suggest should i do? thanks for all your help!!

OK

As steve said ... work on the preamp stages for each of the 5 inputs ..... 3 mics and 2 lines

Then look at tone control ideas

The point you really missed in your tone control filters is that they are not variable
so they really are not very useful
in a normal sound system there are 3 styles of tone controls commonly used
1) a single control swinging between bass and treble,
2) two controls, one doing bass boost/cut and one doing treble boost/cut
3) and finally multi-band controls, anything from 3 ... bass, mid-range and treble
then right through to a full multi band graphic equaliser. I have seen some with up to 20 bands on professional mixing desks

Dave

 

[Jamesmeister]

i cant find any reference with computations on it. but ill try to work on mixer circuit tom.