External speakers -- Audio Mezzanine board?

mezzanine

#1

Hello-

I want to use the 410c board to drive external speakers. These are two, 4 ohm speakers (10W each).

AFAIK, the Audio Mezzanine board doesn’t do much for this as it doesn’t have an amplifier.

I am searching for a solution which will drive these speakers have a similar or smaller form factor as the 410c board (like the audio mezz board), and doesn’t require a second power supply.

Any recommendations for this?

Thx!


#2

Hi @kcdev

I have not seen a mezzanine board with an audio amplifier on the board, but it would be possible to design one. In the absence of a 96Boards form factor board, you can use something off the shelf, although it will require you to connect wires to the 410c, and to the amplifier.

There are many little amplifier boards available so you have to pick one. There is power available for your amplifier on the low-speed connector, either 5V (5Watts), or 12V (7Watts).

An example amplifier that you could use is something like this: https://www.adafruit.com/product/1552 You would need to connect the amplifier outputs to your speakers, and the amplifier inputs to the Audio connector on the 410c board. Power for the amplifier would come from the +5VDC pins on the low-speed connector. This specific amplifier is only 2.1Watts per channel, but this should be enough to drive your speakers to quite acceptable volume levels.

Please let us know what you end up doing and post a picture of your solution.

Full Disclosure: I am an employee of Qualcomm Canada, any opinions I may have expressed in this or any other post may not reflect the opinions of my employer.


#3

Thx. I was thinking of this one: https://www.adafruit.com/product/1752


#4

Hi @kcdev

I did see the 10W+10W amplifier, but I did not recommend it for a reason.

If you use a 20W (total) amplifier you will require an adequate power supply. This particular amplifier requires a 12V DC supply voltage, and 12V is available from the Low-speed connector. Assuming an efficiency or 88%, in order to drive 20Watts out to the speakers, you would need just under 24Watts of input power.

However the 96Boards specification limits the power draw from the Low-Speed connector to 7Watts. The ‘reason’ for the 7Watt limit is the fact that the connector is only rated at 0.5Amps per pin, there are two pins, so a maximum of 1 Amp. When the voltage at the SYS-DCIN pins is at the minimum or 8Volts then the draw on each pin would be 0.44Amps which is in spec.

If you never drive the amplifiers beyond 3Watts per channel, then you will remain under the 7Watt limit. Of course limiting your output to 3Watts means you should have simply used a less expensive 3Watt amplifier.

The most common power supply provided with the 410c is 2Amps at 12V, or a total of 24 Watts, but the 410C needs power and the peripherals (USB, HDMI) also require power, hence there is only 12Watts available to the mezzanine board (7Watts on the 12V pins, and 5 Watts on the 5V pins). If you want to drive the full 20Watts to the speakers you will need a larger power supply.

Since the available power output on the low-speed connector is limited, you will need separate wiring from the (larger) power supply to each of the 410c and the power amplifier (assuming you are using the 10+10W power amplifier).

Realistically a speaker rated at 10W power, reaches about 10% distortion at 10 Watts. The amplifier provides another 10% distortion at full output, so you probably not want to listen to it at full power for very long. Remember 2.5Watts is only 6dB lower volume (and much lower distortion) than 10Watts and probably loud enough for most uses.

Full Disclosure: I am an employee of Qualcomm Canada, any opinions I may have expressed in this or any other post may not reflect the opinions of my employer.


#5

Thanks again for these detailed replies. They are very helpful. I purchased a 2 x 15w amp board that can drive the speakers in a stand-alone manner (eg, with an mp3 player). I’ll evaluate the speakers and the PA needs using that and then follow one of the paths you have outlined in your posts.