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Old 10th August 2014, 10:34 PM   #1
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Default How to tune class D output filter by ear?

I want to tune my output filter so that the higher frequencies sounds right.
The speaker manufacturer hasn't published frequency/ohm specs either. I've tried some 15uH and 1uF caps / 22uH and 0.680uF but nothing sounds just right. Is there anyway to tune it by how it sounds? For example more/less timbre or something like that?
I am modding tpa3116 boards which sounds good, but the upper treble isn't as lifelike as I know it could be.. If you have any experience on this matter please share it!
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Old 13th August 2014, 09:02 PM   #2
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Anyone who has changed their output filter please comment on how it changed the sound..
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Old 13th August 2014, 09:48 PM   #3
danzz is offline danzz  Spain
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Have you tried ferrite beads!?, If you are not using long length cables, could be the easiest and cheaper solution.
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Old 13th August 2014, 09:52 PM   #4
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The output filter is a compromise between being fast enough to allow audio through but also being low enough to stop carrier signal through.
The higher the carrier frequency the easier it is to get good top end on the audio without allowing carrier to get through.
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Old 14th August 2014, 03:18 AM   #5
kp93300 is offline kp93300  Malaysia
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Hi
In my experience with TPA3123 board, the nature of the high frequency is greatly influenced by placing low value eg 102/ 101 ceramic caps as close to the power pins of the TPA chip as possible.
My set up is TDA1543 dac, keantokean buffer and markaudio12p.
Most of my friends comment very positively of the natural high freq reproduction..
cheers
kp93300
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Old 14th August 2014, 09:59 PM   #6
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I have come to the same conclusions too but I wan't to try to custom tune the output LC filter also.. I use long cables so ferrite beads are out of the question.. I use aluminium electrolytics as power caps which remove most of the noise and they improve the sound very much. But I think the upper treble could be even better!
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Old 14th August 2014, 11:26 PM   #7
gmarsh is offline gmarsh  Canada
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If you've got a true RMS detecting DMM, oscilloscope or other means of measuring the voltage at the amp output, drive the amp with a frequency generator and measure the amp's frequency response with the load connected.

If it's peaky, rolls off too quick, etc... then that gives you some idea of what you have to change.
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Old 17th August 2014, 01:08 AM   #8
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Thats one way to do it.. But the whole purpose is to make the amp sound good.. If the output filter doesn't impact hearable differences, what's the point of tuning it??
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Old 18th August 2014, 07:28 PM   #9
stoc005 is offline stoc005  United States
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My take on this, based on some LTSpice simulations........ Choose 2 order low pass with butterworth response at 80KHz or so at the rated impedance. But recall that speakers are anything but resistive and they do vary vs. frequency. Lower impedance than the design impedance causes the output to roll off sooner. A filter designed for 8 ohms will roll off some 20KHz (or less) if being driven into 4 ohms or less. All of engineering is a compromise.
Another goal of Class D filters is to reduce EMI/RFI from all that switching garbage as well.
By ear??? No way.
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Old 18th August 2014, 08:21 PM   #10
xrk971 is online now xrk971  United States
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Quote:
Originally Posted by stoc005 View Post
My take on this, based on some LTSpice simulations........ Choose 2 order low pass with butterworth response at 80KHz or so at the rated impedance. But recall that speakers are anything but resistive and they do vary vs. frequency. Lower impedance than the design impedance causes the output to roll off sooner. A filter designed for 8 ohms will roll off some 20KHz (or less) if being driven into 4 ohms or less. All of engineering is a compromise.
Another goal of Class D filters is to reduce EMI/RFI from all that switching garbage as well.
By ear??? No way.
Yes, very easy to sim in SPICE(I use TINA, the free spice sim program from TI) Here is LC filter and typical reactive driver as represented by lumped TS elements. This is for TPa3116d2 with 10uH and 680nF caps and snubber circuit. The gain and phase with reactive load is actually better than pure resistive load.

Why crossover in the 1-4khz range?
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