How and why: Installing a 12V trigger in a cheap TPA3251 based amplifier from Audiophonics

[XaVierDK]

Having bought the Audiophonics-branded TPA3251 amplifier for outboard amplification of additional channels, I was a bit disappointed that my usual scheme of using a power-saving extension-cord would result in very audible and worrying "pop" from the connected speakers on power-off.

Turns out, the front-switch on the amp isn't actually a power-switch but a standby-switch which controls a soft power-on/off circuit. If power is cut it stops working and the speakers pop.

So I had to find a way to turn it on and off similar to what the switch was doing, and a NC/NO relay with a 12V control voltage turned out to be the ticket.

It sadly doesn't have an optocoupler for galvanic separation, but at least there's a diode for safety from loop back current from the coil.
A small jumper runs the 12V trigger voltage to power the transistor as well.

When the trigger-signal is not present, the NC output from the relay ensures that the switch on the front works as normal, routing the power back to the middle pin of the switch, allowing it to control the on/off function.
I had to desolder and shorten the middle pin and route the current from there to the relay. Apart from that and the hole bored for the minijack in, no modification to the original amp is needed.

When the trigger-signal occurs, it switches the relay to its NO output, which runs to the power-on leg of the switch, turning on the amp while cutting power to the middle pin, rendering the switch at the front non-functional.

The most harrowing part was running the leads from the switch legs. Not an immediate problem, but I had to use rather heavy gauge wire to not have to worry about resistance, and soldering that is always annoying.

I turned the relay module upside down to avoid the PCBs interfering with each other.

Happy to say it worked perfectly the first time I tested it, and with a minijack in drilled on the back I think it turned out pretty discrete.

This mod also should work with the Aiyima A07 and similar, seeing as they use the same kind of circuitry and standby/power-switch.

 

[lap]

Awesome build! I really need to do something similar for my old JVC receiver. Can you do a simple wiring diagram sketch of it? Thank you.

 

[XaVierDK]

Awesome build! I really need to do something similar for my old JVC receiver. Can you do a simple wiring diagram sketch of it? Thank you.

I can try. I just winged it.

 

[Bobesku]

I was thinking of building an external box with this kind of relay module. I need to control an Aiyima A07 amp by using an USB connection from an old Yamaha AVR.

So my idea would be not to butcher the interior as I am not skilled with soldering and also my model has a very large heatskink leaving almost no room inside , but instead add this thing in-between the Power Brick and the Amp. I would just use 2 DC connectors, 18AWG wires, a 5V relay module and sacrifice an old USB cable to get the control power out of the AVR. All nicely put inside some sort of plastic enclosure to keep things safe.

So my main question would be, is this a good idea? Would it hurt the AMP as it is not a Soft-Power ON/OFF method?

 

[XaVierDK]

I was thinking of building an external box with this kind of relay module. I need to control an Aiyima A07 amp by using an USB connection from an old Yamaha AVR.

So my idea would be not to butcher the interior as I am not skilled with soldering and also my model has a very large heatskink leaving almost no room inside , but instead add this thing in-between the Power Brick and the Amp. I would just use 2 DC connectors, 18AWG wires, a 5V relay module and sacrifice an old USB cable to get the control power out of the AVR. All nicely put inside some sort of plastic enclosure to keep things safe.

So my main question would be, is this a good idea? Would it hurt the AMP as it is not a Soft-Power ON/OFF method?

So what I think you'd end up doing is similar to having a USB-controlled extension-cord, that would switch off all but the main outlet (here that would be your AVR) when the AVR powers off/USB power is removed.
So the amp would power off completely, meaning the anti-pop circuitry would not work as it's intended.

 

[Bobesku]

Thanks for the reply. Well, to be honest I don't have such a big pop... I mean, I can barely hear it now. I had a ground-loop issue that I fixed by grounding the AMP chassis to the AVR chassis, and that seems to have also solved the pop thing and some hiss in the speakers as well.

So, my actual question would be more like - do I shorten the AMP life by doing this? Any issues with the big capacitors within the AMP, that would be switched ON/OFF like 4-5 times a day?

Also, you got me thinking of getting a USB-controlled extension cord, BUT.... then I remembered that people say on the forums that powering ON/OFF these power bricks (the ones of the AMPs) is going to kill them. So that is why I am thinking of building this little device that would actually stand in-between the power brick and AMP, leaving the power brick ON all the time, but keeping the AMP OFF whenever there is no trigger signal.

 

[XaVierDK]

I mean... They're just laptop power supplies. They shouldn't really be dying just from being turned off at the socket, that's how you'd usually turn them off I guess? Anyway, I don't think the amp would be damaged by this, but maybe on a long enough time line, who knows

 

[Bobesku]

So, after a bit of thinking I realized this method presented here is the best option.
Therefore, I have done the same mods but using a 5 volt relay module to work with USB voltage and all is working perfectly.
It was a bit annoying to solder the heavier wire at the switch, but even for a newbe like me, it took 10 mins. Also, I used small cutting tool in order to remove the middle leg on the switch, which also took several minutes, as I didn't want to destroy the switch. And also used electric screwdriver and several drilling spirals to make a nice hole at the back.
Thank you XaVierDK for this post, you made my life easier with this mod.