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Old 3rd July 2011, 10:22 PM   #1
johnr66 is offline johnr66  United States
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Default RF suppression in power chip amps

I am wondering where it is better to filter high frequencies. In the negative feedback loop or at the input. If one way is better than the other, then why?

In this case, I'm speaking of HF oscillations caused by connecting an AM radio to the amplifier and lots of noise is produced if the radio is near the amp or the volume is turned up.
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Old 4th July 2011, 04:14 PM   #2
AndrewT is offline AndrewT  Scotland
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Filter the input.
I suggest you double filter the input.
The main RF filter consisting of an R plus a C on the amp PCB giving an RC time constant between 300us and 1500us.
Fit an auxiliary filter right at the input socket. Try from 22pF to 100pF direct from socket barrel to socket signal. The resistance and the inductance of the interconnect and the Source create a 2pole RF filter that should not interfere with the audio signal.

The main filter should have as high an RC as you can manage without hearing any Audio Frequency attenuation (I use ~700us).
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Old 5th July 2011, 02:29 AM   #3
johnr66 is offline johnr66  United States
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Thanks Andrew. Can't wait to get the new amp project complete.
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Old 5th July 2011, 02:38 AM   #4
Previously: Kuei Yang Wang
 
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Hi,

Quote:
Originally Posted by johnr66 View Post
I am wondering where it is better to filter high frequencies. In the negative feedback loop or at the input. If one way is better than the other, then why?
You need to filter input and output, not the feedback loop.

The input stage of most chipamp's is bipolar, so it will demodulate Radio.

The ingress may come from the audio input, however I would consider speaker cables a more likely culprit (they have the right length to be good aerials for AM Radio).

If there is a capacitor across the feedback resistor, this RF will from the speaker cables will ride straight through to the differential input stage.


Quote:
Originally Posted by johnr66 View Post
In this case, I'm speaking of HF oscillations caused by connecting an AM radio to the amplifier and lots of noise is produced if the radio is near the amp or the volume is turned up.
Sounds like your Amplifier is oscillating. Best clip on a fast 'scope and have a look.

Ciao T
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Old 5th July 2011, 09:10 AM   #5
AndrewT is offline AndrewT  Scotland
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Oh bugger.
My post2 shows hundreds of us (microseconds) for the time constants. They should all read hundreds of ns (nanoseconds)
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Old 5th July 2011, 01:44 PM   #6
johnr66 is offline johnr66  United States
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The amp in question isn't even constructed yet, but my other amp has star grounding and .22 film supply decouplers as close the chip as possible. Speaker leads and input leads are kept separate and are twisted. It does not oscillate on its own (checked with scope).

OTOH, Connecting my Walkman set to AM and volume turned high enough when near the amp will cause oscillation. That amp has no RF filter. I wanted to correct that issue in the new project. I notice that some manufacturers show an RC high frequency cutoff in the neg. feedback loop (see TDA2003, 2030A and others). This is why I asked the question as input filtering seems better.
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Old 5th July 2011, 02:10 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by johnr66 View Post
I notice that some manufacturers show an RC high frequency cutoff in the neg. feedback loop (see TDA2003, 2030A and others).
This small cap in the NFB is part of the stability compensating system. It is used to adjust the stability margins, but misused can make the margins worse.
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Old 8th July 2011, 05:03 AM   #8
gootee is offline gootee  United States
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johnr66,

There are some ideas in Chapter 7 of ADI - Analog Dialogue | Op Amp Applications Handbook .

Cheers,

Tom
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Old 9th July 2011, 03:22 PM   #9
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Figures 7-111 and 7-112, specifically, with 7-112 being an X2Y filter. There are caps made specifically for that---the datasheet figures for the C0G parts in Johanson's X2Y series and Freescale's app notes on the IP provide a good introduction to the filter behavior. I think 7-111 has a typo, though; R1 and R2 on the left side should be 0.5R.

In practice I've found dominant pole compensation is most interesting as a lowpass option primarily in balanced designs. In audio those usually aren't implemented with instrumentation amplifiers so a standard X2Y makes them unstable. If you want any significant filtering for RF that means pulling the feedback pole down to 50kHzish. That's not uncommon for line outputs---if you have a look at TI's differential filter app notes and DAC data sheets/eval board schematics you'll see second order MFB lowpasses cornered just above the audio range are typical of better designed differential DAC outputs---and amp theory implies it should it should be fine for power amps as well.

However, a caution. I've anecdotally found audible differences between op amps with GBPs below 50MHz whereas with faster parts I can neither hear nor measure a difference. I've not seen any rigorous study on how moving the pole affects output sound so GPB may not actually be the issue---most chip amps are pretty slow anyway (the 3886 is one of the quicker ones at 8MHz GBP typ). But I'd suggest hedging your bets by laying out a board supporting different filter options for eval rather than comitting to any one filter type.
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Old 10th July 2011, 10:38 AM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by twest820 View Post
Figures 7-111 ................ I think 7-111 has a typo, though; R1 and R2 on the left side should be 0.5R.
in fig7-111, does the filtering C see both the resistors as parallel loads? i.e. 2r//2r = r and thus the formula for turn over frequency holds true F-3dB = 1/[2PiRC]
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