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Old 17th February 2007, 08:01 PM   #1
jduffy is offline jduffy  United States
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Default Trying to remove RF

Any thoughts as to how to remove RF from a point-to-point chip amp?

I suddenly seem to be picking up a RF signal. It is ever so slight that one must put their ear up to my Fostex speakers to hear it.

Not being picky or anything. I'd just like to get rid of this. Other than that, the amp is dead quiet without a trace of hum. I'm using the LM3875 and have been really enjoying the sound.

Thanks for any suggestions.
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Old 17th February 2007, 08:09 PM   #2
BWRX is offline BWRX  United States
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National gives you one possible way in the LM3875 datasheet. Put a small (220pF) cap across the input pins. Another way is to use RC filters on the inputs.
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Old 17th February 2007, 09:53 PM   #3
sek is offline sek  Germany
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Yes, Siegfried Linkwitz shows it in his (classic) schematic. It filters RF in the input leads (but not the power wiring) from the amp chip.

The 220pF ceramic or film cap between the IC's input pins can easily be combined with this filter.

Another way is the compensation network in the feedback path as on page 5 of the LM3886.pdf datasheet.
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Old 18th February 2007, 06:41 AM   #4
gootee is offline gootee  United States
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Quote:
Originally posted by sek
Yes, Siegfried Linkwitz shows it in his (classic) schematic. It filters RF in the input leads (but not the power wiring) from the amp chip.

The 220pF ceramic or film cap between the IC's input pins can easily be combined with this filter.

Another way is the compensation network in the feedback path as on page 5 of the LM3886.pdf datasheet.
Also, if an input RF filter is desired for an inverting amplifier configuration, two resistors in series can/should be used, with the shunt filter capacitor's tap between the two resistors, so the cap is not connected directly to the inverting input.

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Old 18th February 2007, 01:21 PM   #5
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did you bypass the power supply pins ? use 100nF ceramics on Vcc and Vee as close to the power pins as possible.

a source of RF is the cables running from the power supply to the chip -- bypass at the filter capacitors as well.

you could place a ferrite bead on the input lead.

you could place a 30pF mica cap from the non-inverting input to ground.

RFI is something which I, as a ham radio operator, have had to deal with a lot -- mostly when I was starting out in the 1960's when my home-brew transmitter would cause havoc with the parent's television.
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Old 18th February 2007, 06:04 PM   #6
sek is offline sek  Germany
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Hi Tom,


Quote:
Also, if an input RF filter is desired for an inverting amplifier configuration, two resistors in series can/should be used
Good point, as this is also relevant for a fully balanced configuration, where common mode impedance mismatch between the inputs could be the cause.

Cheers,
Sebastian.

PS: Bad moment to mention that I never had success with toner transfering to any paper types I found in Germany, I know - but do you happen to know a european brand? Sorry to be OT!
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Old 19th February 2007, 08:17 AM   #7
gootee is offline gootee  United States
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Quote:
Originally posted by sek
Hi Tom,

Good point, as this is also relevant for a fully balanced configuration, where common mode impedance mismatch between the inputs could be the cause.

Cheers,
Sebastian.

PS: Bad moment to mention that I never had success with toner transfering to any paper types I found in Germany, I know - but do you happen to know a european brand? Sorry to be OT!
OT:

Hi Sebastian,

I don't happen to know of any specific available-in-Europe paper type that is known to work with the toner transfer method, as it is detailed on my website. Sorry.

But, if you email your shipping address to me, at TOMG (at) FULLNET.COM, I'll mail some of the Staples #471861 to you (That's the current Staples "part number" for the type that works well.). [Note: Please try to make sure that your email's subject line is "specific", because I get a LOT of spam, which usually makes it difficult to spot the "real" emails.]

At least, that way, you might be able to be more certain that your problem was only the types of paper that you tried. (And, of course, I hope that you'll then also be able to use the method to make some good boards, with the Staples paper.)

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

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Old 26th February 2007, 01:16 AM   #8
jduffy is offline jduffy  United States
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Quote:
Originally posted by jackinnj

a source of RF is the cables running from the power supply to the chip -- bypass at the filter capacitors as well.
Bingo! It was a combination of my rerouting those wires as well as running the power cable from the wall next to my audio input cables. The RF is gone.

Someone had also suggested that it might be a result of atmospherics as well. We've had some strange cloudy weather here that we don't typically have. Also, I live just downhill a ways from some very powerful radio antennas for some of the Los Angeles radio stations. These things might have had contributed to my problem.

Thanks for all the help.
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Old 26th February 2007, 11:36 AM   #9
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Quote:
Originally posted by jduffy


Bingo! It was a combination of my rerouting those wires as well as running the power cable from the wall next to my audio input cables. The RF is gone.

Someone had also suggested that it might be a result of atmospherics as well. We've had some strange cloudy weather here that we don't typically have. Also, I live just downhill a ways from some very powerful radio antennas for some of the Los Angeles radio stations. These things might have had contributed to my problem.

Thanks for all the help.
If there is a problem with RF getting in from a radio station it is (most likely) a result of a bad connection at your end -- a little oxidation can form a rectification circuit.

The worst radio frequency interference have experienced of late is driving south on 7th avenue in NYC through times square -- the operators of the light displays seem to pay no regard to fcc reg 15.
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