Suggestions for audio noise mitigation?

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Make sure your audio ground is isolated from digital ground. It doesn't take many synchronized digital power surges to make an audible racket. Watch out which ground is used by which bypass caps. Try to tie them together at one place, to minimize any common impedances.

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I'd try wrapping a coil of wire around the audio cable in hook it in series with a capacitor. Maybe if you're lucky it will create a tank circuit with an inverse frequency, dampening the noise field. It might make it worse and it may not do a dang thing.

The amplifier chip is made for battery-operated equipment - there is little information on its power supply rejection ratio in the data sheet.

The chip seems to be somewhat susceptible to RF inputs, as it likes to play an AM radio. I'd put small capacitors directly across the amplifier inputs, with maybe some series resistance.

There are separate signal and power ground pins on the chip. If the noise comes in due to a ground loop, it's tempting to separate them and take the signal ground along the input cable from the same source. To test for a ground loop, I'd disconnect the input and ground it to the signal ground, being careful not to disturb the DC level on the input.

A second test would be leaving the circuit as it is now, but feed the power from a clean supply.

You do have both recommended bypass capacitors? Philips' data sheet recommends the usual combination of a ceramic RF bypass and a sturdy electrolytic for audio bypassing.

HTH

Tauno Voipio tauno voipio @ iki fi

At least simple LCDs produce an awful amount of noise due to their structure. The whole panel is a big capacitor fed with a largish square wave. I do not know whether this applies to all LCDs, but this may be the noise source. What is the frequency of the noise? The noise produced this way is a clean series of sharp spikes (high-passed square).

RC or LC (with a ferrite) filter in the input might be worh trying. Decreasing the shunt resistor does decrese the impedance of the line. However, the impedance is likely to be rather low already, as it is driven by an output. A filter needs at least two components, one in series and one parallel (shunt).

The parallel cap is not extremely effective against noise without a series inductance. If your noise frequency is lowish (in the kilohertz region), it is rather unlikely you get it through the power supply, as the PSRR of the amplifier is probably good enough in low frequencies.

:)

The same effect could be obtained by a common mode choke near the amp. If you can make the audio line differential, then you could use the following filter (which can be built on the PCB):

- ferrite beads in series in the line (both conductors) - a capacitor across the line near the amp - two capacitors between the conductors and GND

The ferrites are not very effective in low frequencies, so you might tey to replace them with suitable resistors. You can tailor the common mode and differential filters separately, as they use different capacitors.

You can try doing something like this even with a non-differential signal. Just pretend it is differential and use separate conductors for the power supply ground and the signal ground. The common-mode range of the amplifier may not reach the possibly negative input voltages during spikes, but you can make a voltage divider to some virtual ground to avoid that problem.

EMI/EMC is a nightmare. Why doesn't the signal obey the circuit diagram...

- Ville

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Ville Voipio, Dr.Tech., M.Sc. (EE)

Now I found it. Your problem did sound somehow familiar. Something quite similar was in a TI spam I just got.

Have a look at TI TPA6211 datasheet, figures 30 and 32. It is a differential audio aplifier, but can be used in single-ended systems as well (figure 30). The input filter is shown in figure 32.

The part is designed for cellular handsets, i.e. for bad EMI environment. I haven't tried it myself, but according to the datasheet the part should behave much better than the Philips part. The price seems to be $0.55 in 1 ku. Of course, you'll need two of them for stereo, but the QFN package is not exactly large. (There is also an EVM available, which may save some time and trouble.)

Other manufacturers may have similar parts. And EMC/EMI is full of "should"s. But maybe this helps.

- Ville

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Ville Voipio, Dr.Tech., M.Sc. (EE)

TI never sent me any thing until I subscribed :)

I think your power supply suggestions should do the trick since the OP said that an externally amplified speaker set didn't have a noise problem.

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Best Regards,
Mike

Your problem is the internal amplifier as you've already determined. If you look at the Philips spec sheet:

you'll see that the part is intended for mobile applications using a battery which has a much stiffer, cleaner, supply voltage than a conventional power supply. Especially if you're using a switching supply.

The spec to look for is SVRR, Supply Voltage Ripply Rejection which is only 34 db. If you absolutely must use the part then clean up the supply. First I'd try a linear supply if you now use a switcher. Or even a wall-wart. Then a nice hefty inductor of say a henry with an equally hefty electrolytic of maybe 10,000 ufd between the choke and the amplifier.

This is a bit of black magic since we can't see your layout or shielding nor do we have any idea of what other digital noise abounds. First overkill to find the noise threshold then back off on the filtering.

A last resort might be to add some ferrous material between the LCD and the amplifier as close as possible to the LCD. Ferrite beads are useless at audio frequencies, you need mass.

-- Regards, Albert

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Hi Albert,

The only reason we chose this is because it worked well on the breadboard and it's both cheap and the circuit is very very simple. This subassembly is assembled by hand, so simplicity is important. Actually we were originally using the TDA7053, but it seems to be impossible to buy that part, so I respun it for the 7053A.

The amp isn't always necessary. We have two sources of SBCs. One source uses an LM4863 "Boomer" amp directly on the SBC, so an ext amp isn't needed. When supply fluctuates, we use a different SBC that doesn't have an on-board power amp.

Our appliance has an internal open-frame SMPS, Astec SA40-1313, with

+12 and +5 rails. We can't change this. I guess I could put say a 7809 on the 12V rail and power the amp from that +9V.

The whole thing is black magic really :) But overkill works for me.

The LCD is separated from the amp and other electronics by a 3mm thick steel wall. The appliance is built around a subchassis. The LCD and its inverter are screwed to one side of this wall, the electronics are screwed to the other side, and a two-piece cosmetic painted metal can surrounds the resultant assembly.

The source of the noise appears to be the LCD controller board, rather than the LCD itself (this determined by selectively powering things down and disconnecting cables).

Thanks for the reply!

I saved my ass once by amplifying some of the ground noise and feeding it back inverted into the input of the audio amp. It worked amazingly well.

There is historical precedent for this technique. Most old tube amps feed a little 60Hz from the filament transformer back into the input to cancel out the hum.

Hi Dilton,

You mean, take say 20 turns of wire-wrap wire, wrap it around the audio cable, and solder both ends to a cap?

Hi Tauno,

Check, I'll try this.

I'm not sure if this will achieve much. "Signal ground" on the SBC goes straight to chassis ground, which is physically screwed to the chassis. The LCD controller board's digital ground is also in direct contact with this chassis ground. There is no isolation.

? I'm not sure what you mean by the latter part of this sentence. I've capacitor-coupled the inputs to the amp, so there should be no DC component. There certainly is a significant DC component on the output from the SBC.

I tried this, and it makes some small difference but not a whole lot.

There is subtle trickery afoot here. If I touch a metal object to random spots around the chassis, I can make the noise much worse (just discovered that this morning).

Yep.

Thanks for the suggestions.

First, have a look at your grounding arrangement. Make sure you keep the different grounds separate. In particular, you should have a chassis ground, which is separate from analog (audio) ground, which is separate from digital ground. They should all be connected together, but in *one* place only. This might be a convenient spot close to the power supply.

Connect all cable shields on external cable connections to chassis ground, as close as possible near the cable entry into the chassis. Do not connect it to signal ground! If you need a signal ground connection, run a separate wire for it inside the cable. If this is done properly, it keeps the radio stations out of your box.

Watch the layout of your amplifier PCB. Are there any wiring loops that could pick up stray magnetic fields? Minimize the area of those loops. Magnetic fields can not easily be shielded off (unless you use expensive mumetal shields).

If the audio connection from the SBC to the Amp creates a ground loop, try to break it. If that's impossible, put a differential amplifier in front of your power amp (that's a simple OpAmp circuit). Connect the ground from the SBC to the negative input of the diff-amp, not the signal ground of the power amp. Alternatively use an input transformer.

Use a common mode choke on the input of the power amp. Use shielded wiring from the SBC to the power amp. Connect the shield to chassis ground on both ends.

Moral: You need to make sure noise currents of whatever origin *cannot* flow in the audio amplifier's signal ground wiring, as this tends to create noise voltages along the ground wiring which will be in series to the input signal and hence get amplified with it. A common error is to confuse signal ground and chassis ground. They serve different purposes and need to be kept apart.

Good luck!

Cheers Stefan

The chassis is not equipotential at this level of signals we are after. There are surprisingly many different buzzes around the chassis. I mean that the signal ground should be connected to ground point very near the circuit genereting the signal.

That's OK. I mean that the input pins must not be connected directly to signal ground, but via the coupling capacitors.

That's good - you do not need to make big/expensive supply filtering.

HTH

Tauno Voipio tauno voipio @ iki fi

series

with

Yeah, tell me if it works! LOL

If you're using a fixed amplifier gain then the amount that you're amplifying the noise will be fixed also, and it's probably fixed high. The noise level will be high even when you feed a low level signal from the sbc. So you have a low level signal and high level noise, i.e., a poor signal to noise ratio. The reduced complexity of the amplifier gain circuit may not have been the best option.

Dan

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I just noticed that the older TDA7053 (no A) was described as having differential input stages already. This suggests that you could break the connection between power ground and signal ground on the power amp PCB. The data sheet of the TDA7053A is different in this respect. It is not clear whether the separation of grounds is ok, or how much potential difference is allowed between them. Still, I think it is worth trying.

If this can be done, connect the Amp's signal ground to the ground of the audio circuitry on the SBC, using a wire in the cable that runs the audio signals from SBC to Amp.

Tell us whether and when you've cured it.

Cheers Stefan

This suggests the amp chip/circuit is picking up RF interference out of the air. A metal shield would then help, OTOH an audio amp really shouldn't be THAT sensitive. Call Phillips and ask for an FAE to help you solve your problem. They'll want to hear that you're thinking of ordering really large quantities of this chip. With such sensitivity, it might be oscillating or on the verge of oscillation. Look at various points with an oscilloscope. Try putting a small inductor in series with the input pin, as shown in the TI/Burr Brown INA103 and INA163 datasheets.

Make sure all such caps are as physically as close to the chip as possible, with traces as short as possible.

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