Having had a read of the Ti document on LC filter design I think I understand that they are designed such to filter out the PWM carrier whilst allowing through the audible range.
So take an example such that I want power a stereo pair 2 way speakers with 4 tda8932 boards. Could I implement lc filters on 2 of the boards for tweeter between say 3000 and 25000hz and on the other pair of boards for the mid/bass a filter to cover those lower registers?
I understand at some point the physical size of the caps and coils will become prohibitive to implement, especially on small 8932 boards. So let's say we take the LC filter off board. Now we can have the filters as large as we need.
Haven't I now just got a conventional speaker crossover and an 'active'/bi amp system ? I may as well then just put the lc filters in the speaker enclosures?
Is it all one and the same thing?
Thanka
So take an example such that I want power a stereo pair 2 way speakers with 4 tda8932 boards. Could I implement lc filters on 2 of the boards for tweeter between say 3000 and 25000hz and on the other pair of boards for the mid/bass a filter to cover those lower registers?
I understand at some point the physical size of the caps and coils will become prohibitive to implement, especially on small 8932 boards. So let's say we take the LC filter off board. Now we can have the filters as large as we need.
Haven't I now just got a conventional speaker crossover and an 'active'/bi amp system ? I may as well then just put the lc filters in the speaker enclosures?
Is it all one and the same thing?
Thanka
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Good and creative thinking!
So take an example such that I want power a stereo pair 2 way speakers with 4 tda8932 boards. Could I implement lc filters on 2 of the boards for tweeter between say 3000 and 25000hz and on the other pair of boards for the mid/bass a filter to cover those lower registers?
You can for sure lower the cut-off frequency of the LC low-pass filter as long as you keep the filter "light" without too large filter ripple currents at the carrier frequency and still good damping at the resonance frequency by the speaker load.
A band-pass filter will evidently need a different design probably similar to the passive cross-over filters used in the speakers. Again, you will need to ensure good damping of the filter by the speaker load.
You may need to do some simulation of the filter designs.
I understand at some point the physical size of the caps and coils will become prohibitive to implement, especially on small 8932 boards. So let's say we take the LC filter off board. Now we can have the filters as large as we need.
I do that but with short leads to the filter.
Haven't I now just got a conventional speaker crossover and an 'active'/bi amp system ? I may as well then just put the lc filters in the speaker enclosures?
If you start having longer cables between the class D amplifier and the filter your system will emit a lot of noise to the surroundings. A less good idea.
Is it all one and the same thing?
Your ideas are very interesting.
So take an example such that I want power a stereo pair 2 way speakers with 4 tda8932 boards. Could I implement lc filters on 2 of the boards for tweeter between say 3000 and 25000hz and on the other pair of boards for the mid/bass a filter to cover those lower registers?
You can for sure lower the cut-off frequency of the LC low-pass filter as long as you keep the filter "light" without too large filter ripple currents at the carrier frequency and still good damping at the resonance frequency by the speaker load.
A band-pass filter will evidently need a different design probably similar to the passive cross-over filters used in the speakers. Again, you will need to ensure good damping of the filter by the speaker load.
You may need to do some simulation of the filter designs.
I understand at some point the physical size of the caps and coils will become prohibitive to implement, especially on small 8932 boards. So let's say we take the LC filter off board. Now we can have the filters as large as we need.
I do that but with short leads to the filter.
Haven't I now just got a conventional speaker crossover and an 'active'/bi amp system ? I may as well then just put the lc filters in the speaker enclosures?
If you start having longer cables between the class D amplifier and the filter your system will emit a lot of noise to the surroundings. A less good idea.
Is it all one and the same thing?
Your ideas are very interesting.
Thinking of the tweeter high-pass filter, you could build that with a gapped ferrite transformer fed direct from the TDA8932 via a suitably calculated capacitor. The 'shunt' inductance is the trafo's primary magnetizing inductance and with a suitable winding topology the trafo's leakage inductance could provide the low-pass function. The shunt capacitance of the trafo might be an issue though - perhaps it could be mitigated with ferrite beads?
Great example of holistic approach. But what will all lovers of those superfast, MHz-bandwidth amplifiers say about it? 😛
You seem to be asking whether a filter at the output of a Class D amp can do double duty: filter out the 'carrier', and act as crossover in an active speaker system. Two reasons why this is a bad idea:
1. If you have two amps driving two speakers (i.e. one amp for woofer, other for tweeter) you want the crossover at the input of the amps, not the output. There is no real advantage in having two amps driving two drive units if both amps have to handle the full frequency range.
2. A filter will only work effectively and block certain frequencies over a limited range of frequencies. The role of a Class D output filter is to stop the 'carrier' from radiating and causing interference to neighbours etc. To do this effectively it has to be designed for the task. A filter also acting as an audio range crossover will probably have inappropriate components for RF filtering.
1. If you have two amps driving two speakers (i.e. one amp for woofer, other for tweeter) you want the crossover at the input of the amps, not the output. There is no real advantage in having two amps driving two drive units if both amps have to handle the full frequency range.
2. A filter will only work effectively and block certain frequencies over a limited range of frequencies. The role of a Class D output filter is to stop the 'carrier' from radiating and causing interference to neighbours etc. To do this effectively it has to be designed for the task. A filter also acting as an audio range crossover will probably have inappropriate components for RF filtering.
Thanks for the helpful replies. It seems I am asking questions that have complicated answers, so I think I will leave this idea for those with more experience.
Can' t see what is the goal of this?
Ususlly, driving a passive speaker by Class D you have:
1. Low pass to filter out the carrier and sidebands (within the amp)
2. Low pass for the woofer
3 high pass for the tweeter.
Combining these to one filter leads to the following stages:
1. Low pass for the woofer
2. high pass for the tweeter
3. Low pass for the tweeter to filter out the carrier and sidebands.
PLUS: You got the unfiltered wire radiating tons of RF noise between amp and speaker. Not so great.
Where is the advantage / savings??
Ususlly, driving a passive speaker by Class D you have:
1. Low pass to filter out the carrier and sidebands (within the amp)
2. Low pass for the woofer
3 high pass for the tweeter.
Combining these to one filter leads to the following stages:
1. Low pass for the woofer
2. high pass for the tweeter
3. Low pass for the tweeter to filter out the carrier and sidebands.
PLUS: You got the unfiltered wire radiating tons of RF noise between amp and speaker. Not so great.
Where is the advantage / savings??
Good evening Tom.
I have enthusiasm for my new found hobby of diy HiFi but little experience and knowledge. A lot of what I am learning is very deep. At least to me!
Having read the Ti LC filter papers I downloaded the Ti spreadsheet for designing the filters. I saw that I could input any frequency, so I thought 'what if' one was to aim for a speaker level cut off frequency. But knowledgeable people like yourself are pointing out that it isn't that simple and doesn't work like that.....which is fine...I'm learning everyday.....hey I just asked on the pre amp forum how to measure dc offset....so you maybe can understand my level here...(beginner!)
I have enthusiasm for my new found hobby of diy HiFi but little experience and knowledge. A lot of what I am learning is very deep. At least to me!
Having read the Ti LC filter papers I downloaded the Ti spreadsheet for designing the filters. I saw that I could input any frequency, so I thought 'what if' one was to aim for a speaker level cut off frequency. But knowledgeable people like yourself are pointing out that it isn't that simple and doesn't work like that.....which is fine...I'm learning everyday.....hey I just asked on the pre amp forum how to measure dc offset....so you maybe can understand my level here...(beginner!)
ALL OK and great that you are willing to learn! Always good to see some guys start to learn about audio / electronics / Class D .. etc..
About class D amps I am personally learning life-long (with some decent progress over the years as you will see soon, when I ll be ready to show my new project here.)
About class D amps I am personally learning life-long (with some decent progress over the years as you will see soon, when I ll be ready to show my new project here.)
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