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Class D Switching Power Amplifiers and Power D/A conversion

TPA3118 volume control
TPA3118 volume control
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Old 28th November 2017, 12:58 PM   #1
jackstraw is offline jackstraw  United States
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Default TPA3118 volume control

Hi guys. I'm building a bluetooth speaker and need some advice on volume control. This is a single-speaker amp, so I chose THIS mono TPA3118 based amp. The bluetooth module i'm using is THIS ONE. I'm planning to power them both with a 12v (ish) li-ion pack, or a 12.6v power supply when the unit is plugged in for charging. I'm currently using a 4ohm 5" full-range speaker... but I have a question about that as well that i'll ask later.

When testing last night, the volume seems very high. That is, with my phone connected via bluetooth (or wire) and on its lowest possible volume setting, the volume is higher than i would want my minimum volume to be. Going past half-way on my phone's volume results in the amp being way too loud and beginning to distort.

So i'm thinking i should wire up a potentiometer. I know I need a logarithmic pot, but i don't know how to choose the pot's value. Can someone tell me what value pot would appropriate for this project?

Also, the pot should go between the bluetooth's output and the amp's input, right? Like this:

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And finally, concerning the speaker: If battery life is a concern, would an 8 ohm speaker be a better choice?

Thank you for your time and advice!
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Old 28th November 2017, 01:44 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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I would use a line level pot of anything between 10 and 50k. Lin/log; your choice but no preference as it doesn't matter.
Loosely speaking, an 8R speaker needs more voltage for the same output as a 4R speaker. A 4R speaker will draw twice the current than an 8R speaker for the same sound output but the power is the same for the same volume.
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Old 28th November 2017, 01:58 PM   #3
jackstraw is offline jackstraw  United States
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Jon, thanks! I'll look for something between those values. Is my wiring correct?

Ok, so it sounds like the speaker ohm doesn't really matter when it comes to battery life... the same power draw for the same volume. Thanks!
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Old 3rd December 2017, 11:18 PM   #4
FauxFrench is offline FauxFrench  France
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According to my understanding the Bluetooth module is the source (of sound) and the amplifier is the "consumer" for the signal. As I see it, your diagram is not the way it is normally done. The upper-most pin on the potentiometer (left-most seen from the front) is the GND pin that has to be connected to ground on the Bluetooth module and ground of the amplifier input.
Then, as you apparently want to turn a Bluetooth stereo signal into mono, you connect two 4K7 resistors from each of the Bluetooth outputs (stereo) to the lower-most pin on the potentiometer (opposite of the GND pin). The middle pin of the potentiometer has to be connected to the input (signal input) of the amplifier.
Use a 10K potentiometer with the two 4K7 resistors.
Just summing two stereo outputs by a short circuit (as you indicate) may distort the signal (use resistive summing instead).
Good luck with it!
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Old 4th December 2017, 09:46 AM   #5
ICG is offline ICG  Germany
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Quote:
Originally Posted by JonSnell Electronic View Post
I would use a line level pot of anything between 10 and 50k. Lin/log; your choice but no preference as it doesn't matter.
What a nonsense! Ofcourse does it matter! That's an audio signal, not a DC voltage! If you use a linear pot, you'll end up with 90% of the way it doing next to nothing and the whole range of the rest of the volume is within 2mm and extremely fiddly to adjust. And if it was a DC voltage, you'd have the same problem with a log pot!

@jackstraw: You need a log pot, 50k is usually a good choice, everything from 10k-100k will work though. FauxFrench already said, it's better to limit the influence of the mono-'short' from the BT audio output since it got a low output impedance (<600 Ohm). 4,7k is fine, if you don't have these at hand, you can go up to 10 or 15k if you got a pot of 20k or 50k Ohm. Just keep the resistor values below ca. 1/2 of the resistance of the potentiometer.
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Old 5th December 2017, 10:34 PM   #6
jackstraw is offline jackstraw  United States
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Quote:
Originally Posted by ICG View Post
And if it was a DC voltage, you'd have the same problem with a log pot!

@jackstraw: You need a log pot, 50k is usually a good choice, everything from 10k-100k will work though.
ICG, thanks so much for the reply. This is a DC voltage amp.. aprox. 12v. Is log still going to be ok?
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Old 5th December 2017, 10:36 PM   #7
jackstraw is offline jackstraw  United States
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Quote:
Originally Posted by FauxFrench View Post
Bluetooth outputs (stereo) to the lower-most pin on the potentiometer (opposite of the GND pin). The middle pin of the potentiometer has to be connected to the input (signal input) of the amplifier.
Use a 10K potentiometer with the two 4K7 resistors.
Thanks Faux. I will add the resisters as you say. If i read that correctly, you are saying i need to reverse the 2nd and 3rd pins (the non-grounds) in my diagram?
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Old 6th December 2017, 06:52 AM   #8
FauxFrench is offline FauxFrench  France
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Yes, swap the wiring to the 2nd and 3rd pins and insert the resistors. The black wire is difficult to see near the potentiometer but you probably mean it right.
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