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Old 14th February 2013, 10:27 PM   #1
sfasf is offline sfasf  United Kingdom
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Default caps for 4/8ohm with TA2020

TA2020 datasheet (7.1) says output caps Co and Cz should be 0.47uF for 4ohm speakers and 0.22uF for 8ohm speakers. What does this affect? and how important is it?

I ask as I am making a board that I want to be suitable for use with either 4 or 8 ohm speakers, and having to include 2x the output caps and switching won't be trivial. Should I just use the 4ohm values? (most boards seem to do this and I haven't heard of problems with anyone with 8 ohm speakers). Or use something inbetween? or ??

[As an aside; the datasheet actually says Cz should be 0.22uF for 4ohm, but changed to 0.22uF for 8ohm. I take it this is an error, and it should be 0.47uF for a 4ohm load?; this is also how it is shown on the evaluation board's datasheet (7.03). Are there any other areas where I should stray from the datasheets?]
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Old 14th February 2013, 10:30 PM   #2
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The capacitor/inductor/speaker are a filter so altering any one or more will affect the breakpoint of the filter.
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Old 14th February 2013, 11:07 PM   #3
sfasf is offline sfasf  United Kingdom
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right.. thanks.. so what would be the effect of using the 0.47uF caps with 8 ohm speakers be compared to the recommended settings; would it cut off the high frequencies or let higher frequencies pass through? (or am i misunderstanding?)

What would be the negative results from allowing higher-than-specced frequencies through?
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Old 14th February 2013, 11:16 PM   #4
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The larger the component the lower the cut off frequency.
So using a larger capacitor value would lower the cut off frequency.
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Old 14th February 2013, 11:32 PM   #5
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So what would be the negative consequences of running with a higher cut off frequency? (eg 0.22uF caps with 4ohm speakers). Would it reduce the power at a specified frequency? increase distortion? or just draw more power/reduce power efficiency?
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Old 14th February 2013, 11:37 PM   #6
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Running at a lower frequency would mean you started to lose some top.
Running at a higher frequency would mean more carrier getting through to the speaker and radiating.
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Old 14th February 2013, 11:53 PM   #7
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Thanks. What would be the issue if more HF got through and radiated? Since it is presumably above the audible range? Might it cause damage to tweeters?

Do you have a opinion of what values to use? (it's hard to know where the default cutoff lies to know how much headroom exists.. would it be the gentle roll off that starts at 6kHz [around -0.5db @ 18kHz], or would the cutoff be much more immediate where it occurs?)
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Old 14th February 2013, 11:58 PM   #8
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HF radiating could interfere with other equipment.

You could try using a capacitor in between the two values and compromise.

You really need audio to be cut off above 30KHz so you dont lose top to the tweeters.

My 2092 based class d amp has about 10 volts of carrier left to the speaker and that's using the app note circuit as a basis for my own design.
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Old 15th February 2013, 10:20 AM   #9
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You will either have a peak or a dip around the 20k region depending on the non-optimal R/C combo being used. If I had to choose one, I would go with the peak.
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Old 16th February 2013, 04:13 PM   #10
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Default setting: 0.47, Use it.
Safe, easy and any losses will be Wayyy beyond your Human hearing capabilities (unless 12 yrs old ?) in any event.
These are supposed to be simple/stoopid gizmos Why complicate it?
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