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Old 31st January 2012, 12:26 PM   #1
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Default Match Your Output Filter to Your Speakers - All Boards Welcome

The LC+Zobel filter used on most Class-D boards discussed in this forum are independent of the amp they are on, and the effect on the sound they have is entirely dependent on your speakers. The filter on most boards is designed for speakers with DC Resistance and Voice Coil Inductance that are far different than the most common speakers. Matching just my zobel to my speakers has made a dramatic difference in my board's sound.

Here is what you do:
Measure the DC Resistance and Voice Coil Inductance of your tweeter (or look it up in the manuf docs).
Then choose one of the standard output inductors and use the following equation to see if the corner frequency of the filter will be acceptable. If your output is single ended, use the actual value of the inductor, If your output filter is BTL, divide the inductor value by two. Choose one of the higher inductor values, like 33uH, since most manufacturer's choose too low for the average speaker (or you can try your current inductor's value to see if you can keep it).

f=1000*(0.2251*Rs)/Lf
Remember to change the value of the inductor from uH to mH before putting it into this equation, then change it back to uH to get the final value. If the cutoff freq is more than 35,000Hz you can keep the inductor, and not too high (I don't know yet what too high is).


Then solve for C:
For Single ended filters:

Cse=1000000*(0.1125/(Rs*f))
For BTL filters:

Cbtl=2*1000000*(0.1125/(Rs*f))
This will give you the capacitor for the output filter (not the zobel, don't confuse the two).

Zobel:
For the zobel you can use any online zobel calculator such as:

http://www.carstereo.com/help/Articles.cfm?id=36
Just plug in your tweeter's DC Resistance and Inductance (in mH)
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Old 31st January 2012, 12:34 PM   #2
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If your tweeter is entirely resistive, no inductance like many planars, you should drop the zobel entirely.
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Old 31st January 2012, 01:15 PM   #3
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Once you have designed the output filter that matches your soeakers, you can move it from board to board, just remove the current output filter on the new board. I am going to put my output filter on a daughter board and design a way to easily attach it to new boards.
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Old 31st January 2012, 08:43 PM   #4
lenta is offline lenta  Norway
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Great thread jyoung!

http://content22-foto.inbox.lv/album...put-filter.jpg

This is the output filter of hifimediy T1 amp with sta510 chip. I dont understand what c3 and c4 caps are doing there? These are 0.1 ceramic caps!

Yes I will remove zobel for planar B&G NEO3 tweeter.

Can I remove zobel for mid driver too?
Its low inductance Vifa NE149w driver.
Click the image to open in full size.
Le: 0.14 mH
Re: 3,64 Ohm

Speakers is tri-amped. Midrange driver and planar tweter conected directly to amps output without passive crossover.
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Old 31st January 2012, 10:54 PM   #5
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About C3 & C4, I don't fully understand all of output filter design. Hazarding a guess, I would say they are rolling off the highs. Probably because the output filter and zobel cause a peak. If you are going to design your own output filter, definitely remove them.

About the mids, I haven't given enough thought to all the ramifications of output filters for mids. Again, hazarding a guess, the effect of the output filter is high enough that it probably won't have a great effect on your results. Also, if you are using a second order or greater analog low pass, or greater, on your mid, and if you moved the filter to immediately after the amp, you should be able to get rid of the entire output filter and zobel, since all the output filter does is get rid of the switching frequency and the mid, and woofer too, low pass will do that for you. You don't want the switching frequency running down the speaker wires, hence the move of the filter. If you are using an active system, I think it might be best to leave the output filter in since the switching frequency messes with electronics.
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Old 1st February 2012, 08:39 AM   #6
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Quote:
Originally Posted by jyoung070848 View Post
If your tweeter is entirely resistive, no inductance like many planars, you should drop the zobel entirely.
With my humble respect, please be careful if you do this. From the TP2050 datasheet: An RC network, or “Zobel” (RZ, CZ) should be placed at the filter output to control the impedance “seen” by the TP2050 when not attached to a speaker load.
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Old 1st February 2012, 09:12 AM   #7
bonalux is offline bonalux  Italy
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Hi jyoung, can I measure the tweeter inductance just with an inductance meter or it needs to be measured at a particular frequency (eg. 10kHz)? Also, in your formula for the output inductor, Lf is the inductance of the coil and 0.2251 a fixed value?

Thank you very much.
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Old 1st February 2012, 09:49 AM   #8
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Darn, I forgot that, thanks! I retract my statement about removing the zobel entirely. I will have to rethink and see if I can find a way to minimize the effect of the "zobel" when the tweeter is purely resistive.


Quote:
Originally Posted by kristleifur View Post
With my humble respect, please be careful if you do this. From the TP2050 datasheet: An RC network, or “Zobel” (RZ, CZ) should be placed at the filter output to control the impedance “seen” by the TP2050 when not attached to a speaker load.
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Old 1st February 2012, 09:54 AM   #9
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An inductance meter is fine. The inductance of voice coils changes little over the audible range, it is the resistance that changes with frequency as a result of the inductance and that is what the zobel corrects.

Yes, 0.2251 is a fixed value. It is actually a combination of PI and other constants.

Quote:
Originally Posted by bonalux View Post
Hi jyoung, can I measure the tweeter inductance just with an inductance meter or it needs to be measured at a particular frequency (eg. 10kHz)? Also, in your formula for the output inductor, Lf is the inductance of the coil and 0.2251 a fixed value?

Thank you very much.
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Old 1st February 2012, 11:32 AM   #10
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Class-D on a purely resistive load. We can reduce the impact of the zobel by increasing the value of the resistor. It would be easy to test the effect of the zobel simply by removing it while the speaker is connected and listening to the difference. We could also reduce the value of the capacitor, though I wouldn't go to an extreme.
What do you think kristleifur?
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