Safety First

[pacificblue]

Originally posted by star882
With the smaller amplifiers, there actually is very little danger apart from the high voltage side of the power supply. So use a wall wart to power your first amplifier and there will be almost nothing to worry about.

Small is a very unsharp definition. When a wall-wart is sufficient, the amplifier must indeed be small. The trouble with wall-warts is that very few of them provide split power supply.

However I don't agree with your assessment. A capacitor that blows up right into your face is just as dangerous in a small amplifier as it is in a big one. Even a small amplifier can produce enough DC at the output to damage speakers. And this thread does not inspire a lot of confidence in wall-warts either, which brings us back to post #5.

[pacificblue]

”Voltages above 50 V are life-endangering“

So simple does an old textbook of mine put it. The threshold for DC is a bit higher, but because there must be a safety margin, internationally agreed regulations have it that AC voltages above 25 V and DC voltages above 60 V require protection against direct touching. “What does that have to do with my chip amp?” you may say.

Well, obviously there is a higher voltage present until the transformer primary, but you knew that of course and you have that under control, don’t you? Okay, let us assume you do.

Now let us have a look at some datasheets:

LM3875/LM3886
“Supply Voltage |Va| a |Vb| (No Signal) 94V
Supply Voltage |Va| a |Vb| (Input Signal) 84V”

TDA7293
“120V - 100W DMOS AUDIO AMPLIFIER”

Those voltages are obviously above the thresholds. So, what does that mean for the common chip amp? It could be as simple as reducing the supply voltage so that the rail voltages remain below 60 V DC and the output swing remains below 25 V AC. The latter means the rail voltages should not exceed 25 V x 1,41 = 25,25 V
…ever.

25,25 V rails including transformer regulation and mains fluctuation will bring you into the range of a 2x15 V transformer and nominal ±20 V rails. According to the Overture Design Guide V1.5 you get an 18,5 W into 8 Ohm or 36,5 W into 4 Ohm amplifier, when you use the LM3886 with those rails, assuming you have big capacitors and your PSU will not sag too much under load. But weren’t you after the 50 W into 8 Ohm or even 68 W into 4 Ohm the datasheet promises?

Many chip amps will exceed the thresholds that require protection against direct touching. No need to be scared! All you have to do is to make sure the speaker terminals and wires are isolated.
- Don’t use non-isolated binding posts, neither on your amplifier nor on your speakers.
- Don’t strip off too much of the isolation from your speaker wires.
- Use only isolated lugs and sleeves.
- If you use banana plugs, use isolated ones. There are versions with retractable isolation available for speaker terminals that don’t accept the plastic sleeve around the plug.
- If you use open baffle speakers, make sure that the driver terminals and wires are isolated, covered or out of reach.