amp on/off light for TPA3255 build


2020-01-16 3:36 pm
Hi All, newbie here so not looking to take up much of anyone's time. I built a pair of MTM speakers (Dayton RS180's) and am now looking to build an amp for them (rather than my old Cambridge A5 amp that is giving up the ghost!) using TPA3255 and LRS350-48. My question is on a power switch for the new amp I am hoping to use a push button switch but need a light to go with it to show it's on. As I am a relative beginner in electronics I am hoping to avoid a low voltage light switch and the relay necessary for it on the 240v side so will either use a 240v 16amp illuminated toggle switch which looks a bit ugly or a push button that then needs a separate light. As I'm trying to keep it simple can I somehow run a LED from the low voltage side of the power supply with a resistor to give me the 'on' light?? I hope this makes sense. Thanks for any help
If you are using a pre-made board for the 3255 chip, it may have a built in provision for delayed turn on and standby. If it doesn't you will get a canon-like thump out of your speakers when you click on the AC.

There is a problem with SMPS and Class D boards. When you first apply AC there is a huge inrush of current that can damage the rectifiers in the power supply, little by little over time, until one day it fails. It's not going to happen today, but it will shorten the life of your power supply. To avoid this it is best to keep the whole thing powered on all the time but to put the amplifier (3255) into standby mode.

Usually this just amounts to grounding the delay turn on capacitor with a switch. On the better modules it is brought out to a connector or solder points you can hook up to. On the cheaper ones you may need to do a little exploring to find it.

I recommend implementing this method instead of turning the AC on and off all the time. Your SMPS will thank you :)
The TPA3255 data sheet is HERE

You will need to study the application examples, but IIRC the way to put the amp into powered-on standby with the outputs turned off is by holding the RESET pin low.

(The others here likely have more experience with this chip than I do and hopefully one or two will chime in to help.)

The lamp circuit itself is pretty simple... See the thumbnail.


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Glad to help...

The switch circuit I gave you cheats ... when it's ON it allows the reset pin on the chip to go high, enabling the amp and turning on the LED. When it's OFF it holds the chip in reset and turns off the LED until you turn it on again. It's pretty simple.

I do hope one of the others with more savvy on this chip will chime in to help...


2020-01-16 3:36 pm
So as a quick follow up if I am understanding this - By using the switch circuit this will be my main on/off switch for the amp. I was going to put it before the PS but this will be after the PS. So if I understand correctly I will not need a switch before the PS so there will be a 'permanent' current through the PU while the amp if off - this I take it is not an issue and if my reading is correct it is better for the amp to 'power down' via the amp rather than on/off before the PS. Hope that makes sense.
On most computers the power button is actually a software button that is monitored by the power supply. Pushing this button only enables the secondary in the power supply and of course starts the motherboard reset cycle. The primary side of the PSU is on and those big bulk capacitors are charged constantly.

Repeatedly turning off the "babysitter" switch on the back of the computer has been known to substantially shorten the lifetime of the power supply. Eventually it burns out the AC rectifiers.

This happens because of a huge inrush of current recharging the bulk capacitors as soon as the AC is turned on. In the PC case this can reach 30+ amps for a brief moment and it is this inrush that kills the power supply. Thus the solution is to simply leave it turned on all the time.

The same recurring problem has been noted in laptop power supplies, televisions, home theatre, and various other devices powered by SMPS. To avoid this, like the PC, these devices are never truly off, just in a deep standby state.

In the case of a DIY project it is probably best to simply leave an SMPS on all the time and put the amplifier (etc) into a standby state. This avoids the large inrush of current in the power supply and in the amplifier itself as it recharges it's own bulk supplies.

With the amplifier outputs off there will be very little load on the power supply, probably no more than a couple of watts. Basically just enough to keep the bulk supplies active.

My little switch trick simply holds the amplifier chip in standby mode with it's outputs in high impedance state and the PWM oscillator shut off. Thus a very low drain on the supply.

Yes you could put an AC switch in there as well. But you should treat it as a "babysitter switch" much after the fashion of the one on your PC. Leave it on all the time unless you have good reason to turn it off (for example, when your kids are with a babysitter and you don't want the system to be messed with). Use the standby switch for normal on-off activity.