Power supply for Chinese audio amp board TPA6120A, marked AC15-0-15V

[Triode]

So I have this board I'd like to use, it looks to be exactly the same as the one [**broken link removed**]

My best guess is that I would want a small toroid transformer that supplies 2 outputs at 15 VAC. The amp chip is rated at 1.5W and the smallest torroid I can find in 3.2VA so even if I need the full power of the amp I think it will work.

The data sheet for the TPA amp IC says the two 15V inputs can be tied together. Do I really need two, or does the separate power supply really do anything? Again it's kind of a moot point because they all seem to have two taps anyway. I don't know if tying them together into one 15V supply is a problem.

I found [this transformer], it's cheap, but seems like it would work
"Power:3.2VA
Input:2 x 115V
Output 2x 15V"

Just to be sure, I wire it with 3 and 5 together into one AC supply pole, and 4 and 6 together into the other? Then to my amp (I think) I would wire 12 and 14 to 0V input, 13 to one of the 15V inputs and 11 to the other? I don't think there is any reason I would want them to be out of phase since that would supply 30V, but maybe for all I know the board is meant to work that way.

Thanks for helping. Sorry if any of these are stupid questions, I just want to be sure I don't blow up anything.

 

[Tony Stewart]

split supplies allows DC coupling on outputs. Parallel 120Vac input or series 240V input .
outputs 12V are jointed at center for bipolar AC output as required by weblink spec. 15~0-0~15 gives 15Vac-0-15ac input as req'd. so pin 13,12 are joined... as well as 3-5, 4-6

Diode bridges use pulse current so transformer VA rating must be increased 30-40% over load power in W, so this seems to be a good match.

 

[Tony Stewart]

- Use UPC1237 and Hongfa relay output delay for protection
- The two red switches corresponding channel gain switch

Basic parameters:
- Board size: 104MM * 88MM
- Operating Voltage: AC 15V-0-15V
- Applicable Headphone Impedance: 32 Ohm -600 Ohm
- Frequency response: 20HZ-20KHZ

 

[Triode]

I'm glad you mentioned the bipolar output. I didn't want to just guess on that. Just be sure I have you right here's how I think it should be wired according to what you're saying:

Transformer pin -->
3,5 --> AC Line
4,6 --> AC Neutral
15 --> amp 15V input
12,13 --> amp 0V
11 --> amp other 15V input

Do I need to add any additional protection from inductive kickback when the transformer turns off? It looks like it has it for the speakers.

Thanks again for the help!

 

[Tony Stewart]

That should do it.

It's 32 Ohm impedance for headphones not 8 ohm speakers

 

[Triode]

Yep, I'm planning to use it with 32 ohm headphones.

Thanks for the help!

 

[KeepItSimpleStupid]

I'm slightly confused with the mention of 15 and 15-0-15. Just one comment, the dots mean something. Think of them as either a plus or a minus sign.

 

[audioguru]

I think the 15VAC-0-15VAC transformer when rectified will destroy the TPA6120A IC because its maximum total allowed supply voltage is 33VDC.
15VAC has a peak of 21.2V and the rectifiers will drop it to 20.2V. Then the supply will be +20.2V and -20.2V for a total of 40.4V. It will be higher when the transformer is not with its rated load. A transformer rated at 12VAC-0-12VAC is safe but its total is 31.9VDC. See what happens when you buy cheap Chinese junk from a Chinese importer? They know nothing about electronics.

 

[Triode]

Good catch. I was able to change it to a +-12 volt before it shipped. I thought it might be using regulators, but upon taking a close look at the board there were none.

As a reference I found this design which used a very similar transformer at 15V, but he has a pair of voltage regulators to bring it down. A 7812 and 7912 to be specific.

I also found this design, which uses a 12V transformer and adjustable regulators.

I wonder if this approach would provide a smoother power supply. Do linear regulators with a slight drop do much to reduce ripple? It seems odd that my board has no apparent regulation, but maybe I'm missing it.

 

[audioguru]

Audio power amplifiers do not have and do not need voltage regulators because they have excellent supply ripple rejection. Besides, the current for a power amplifier is much too high for an IC regulator but is fine for a flea-power headphones amplifier where you can hear the hum without a regulator.

 

[spec]

So I have this board I'd like to use, it looks to be exactly the same as the one [**broken link removed**]

My best guess is that I would want a small toroid transformer that supplies 2 outputs at 15 VAC. The amp chip is rated at 1.5W and the smallest torroid I can find in 3.2VA so even if I need the full power of the amp I think it will work.

The data sheet for the TPA amp IC says the two 15V inputs can be tied together. Do I really need two, or does the separate power supply really do anything? Again it's kind of a moot point because they all seem to have two taps anyway. I don't know if tying them together into one 15V supply is a problem.

I found [this transformer], it's cheap, but seems like it would work
"Power:3.2VA
Input:2 x 115V
Output 2x 15V"

Just to be sure, I wire it with 3 and 5 together into one AC supply pole, and 4 and 6 together into the other? Then to my amp (I think) I would wire 12 and 14 to 0V input, 13 to one of the 15V inputs and 11 to the other? I don't think there is any reason I would want them to be out of phase since that would supply 30V, but maybe for all I know the board is meant to work that way.

Thanks for helping. Sorry if any of these are stupid questions, I just want to be sure I don't blow up anything.

Hy Triode,

Your TPA headphone amplifier board requires an AC input of 15V RMS 0V and 15V RMS, as stated on the AliExpress web site.

I am afraid that the 12V transformer that you have ordered will not have sufficient voltage to power the headphone amplifier board. Still it will be a nice little transformer for other projects.

Just a word about transformer power: it is not necessary, or even desirable, to match the power of a mains transformer to the power consumed by a circuit. All you need to do is to make sure that the voltages are correct and that the transformer will provide sufficient power and, as Tony said in post #2, if a transformer is feeding a rectifier you need to increase the power of the transformer by a factor. So, for example, in the case of this headphone amplifier, a 15V-0-15V RMS AC 1000 Watt transformer would be suitable electrically, but perhaps not practically.

Apart from higher power output, a higher power transformer has a lower output impedance and, in the case of your headphone amplifier, I would advise buying the highest power torodial transformer that will physically fit in your equipment. A 100ma transformer would not be advisable, especially as the current consumption of the headphone amplifier board is not stated.

The two AC inputs to the headphone amplifier board will be converted to +19.2V DC and -19.2V DC by the four rectifier diodes (positive and negative full wave rectifiers) and the two large reservoir capacitors on the board. These two supply lines will have noise and a ripple voltage, depending on the transformer used, the diodes, and the capacitance, and other characteristics of the reservoir capacitors: the higher the current drain on the reservoir capacitors, the higher the ripple voltage.

These two raw DC supply lines will then be stabilized to +15V and -15V by the two, three terminal voltage regulators on the board to produce two nice clean supply lines for the NE5534 pre amps and the TPA6120A2 stereo headphone amplifier. Stabilized supply lines will not only minimize hum and noise but also reduce distortion in the headphone signal. Hum, even small amounts, can be extremely disturbing with headphones.

You will need to orient the mains transformer in relation to the headphone amplifier board to minimize induced hum. The board would be best placed in a metal case.

The transformer connections in your post #4 are correct. Keep the wire going from pin 12/13 separate, but twist together the other two wires going from the transformer to the headphone amplifier board.

You will not need to worry about any power-on surge current when you turn on such a small torodial transformer. Current surges are normally reduced by connecting a Negative Thermal Resistance (NTC) thermistor in series with the transformer primary. You will need to fit a fuse though.

The headphone amplifier has headphone protection and only turns the headphones on when the circuit has settled down after switch-on. This eliminates any power-on thump in the headphones.

The headphone amplifier board you have selected appears to be well designed and constructed. I bought a DAC headphone amp from Hong Kong via ebay seven years ago. The construction, workmanship, components, and electrical design are excellent. And the sound quality is superb. I would expect your headphone amplifier to sound good too.

spec