Amplifier Interference Issue

[mtripoli]

You've got a whole list of things to run down...

Where is your power coming from? A battery, wall-wart, PC? If it's anything other than a battery you need to see how quiet the supply is. Wall-warts are incredibly noisy and unless you take care you are going to get noise. What kind of decoupling do you have on the supply? You're going to need something fat and something skinny. Start with 100uF-470uF electrolytic on the rail. Pair that with a 0.01-0.1uF ceramic (maybe both). Skipping this and you might as well forget it. You can go further and use a series resistor in the supply but start with the caps.

How is the input connected? AC-couple (use a cap on the input to the amp). Something like a 0.47uF should do it. Then it doesn't matter if the PIC is an output or an input (the cap blocks DC. Regardless if it's an output high or low nothing is showing up at the amp. The only time the amp sees signal is when you are outputing "sound" as an AC waveform). Use a high value resistor to ground on the input of the amp (on the amp side of the cap). Something like a 1M ohm (the data sheet shows a 10K but this is to match the output from a preceeding pre-amp or line level. You don't need that coming from a PIC). This keeps the input at ground unless there is a signal.

Are your grounds tied together at one point (preferably at the supply)? If you have multiple ground paths then you're going to get into trouble. Tie the supply leads together at the supply. Use a single wire from the output of the PIC to the input cap.

Looking at the data sheet you should run this in BTL (bridge-tied load) configuration. This uses both amps in the package and gets rid of the big caps on the output (the 470's). This part uses a lot of caps, make sure you are using them. Keep the leads short and if you are using a "plug-strip" protoboard get rid of it and solder the stuff together. Flying leads between components are far better than proto-boards.

There are amps out there that have an enable pin. You can get one of those and turn the amp on and off as you need. If something is battery powered this saves on power. This only masks the problem however; the hum and noise are still present unless you get rid of it.

Mike Tripoli

[Art]

Ok, that gives me a lot to think about.
Unfortunately the protoboard has to stay (this is a one off).
I am using a plugpack, but the interference sound seems vary according to what the pic/s are doing, so I don't know what that means.
I don't think i've ever tested this particular project with a battery though,
so that might be interesting, thanks

I guess I'll revive this if I run into trouble.

[Archangel]

. . .but the interference sound seems vary according to what the pic/s are doing, so I don't know what that means.

That could be a variation in leakage across the diode's junction due to changes in load . . . Used to see that a lot in car alternators.

[Art]

Is that a rectifier diode in the plugpack power supply ?

Hey mtripoli,
How would you like to make a PCB for this:


Give me a bit and I'll whip up a schematic with the transistors the wrong way round
Nah just kidding

I'm not actually using the 7805s at the moment because I have a 2 amp 5 Volt regulated wall supply.
The plugpack is a charger for a Sony PSP, and I would have thought the caps would be in that.

So it's a talking clock with a display now, but I also want to add an FM radio that I'm making with a Philips FM radio IC tonight.

There should be some circuit to switch the input of the amplifier chip
between the pic and the radio chip.
Should I use signal transistors?

[mtripoli]

I don't know what the output of PSP supply looks like. I have never seen what a "plug-pack" (what we derisively call a wall-wart) looks like for the PSP. I would be very surprised if it has any kind of filtering at all. Usually these things are a transformer, a bridge (if you're lucky) and MAYBE a single cap (it could be a universal input switching supply as well. If so the thing most likely has tons of high frequency component on the DC. Some ceramic caps strategically placed may be all you need. LM78xx devices don't respond at these higher frequencies and will let a lot of this through). The manufacturer places the caps and related circuitry in the device being powered (for lots of reasons). So first, whenever you are using a "plugpack" put some big caps on the input of the supply to the board. 470uF-1000uF is not unusual. Use some ceramics as well. Watch the voltage; don't use a 6.3V cap on a "5V" supply (higher voltage never hurts). Typically voltage rating is under load, so a "5V" supply may actually be much higher than that.

Mike Tripoli

[Byte_Butcher]

hmmm.... using a wall wart for power.....

Many "modern" wall warts now contain switching supplies instead of the traditional xformer/rectifier setup in order to meet minimum efficiency standards
that now apply in many areas.
That may well be the source of your noise. Like Mike said... an electrolytic cap of a few hundred uF, with 1 or more small (.01-.1uF) ceramic caps in parallel may help a lot.


steve

[Art]

Thanks Guys,
a 1000uF cap across the supply almost eliminates the noise
That's a bit big, but I'm sure I'll find a combination that works well.