Need Help With Amplifier Output

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matthewmanten
 
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Need Help With Amplifier Output

Post by matthewmanten »

First of all, I'd like to say thanks in advance!

I purchased the adafruit MAX98306 3.7 watt stereo amplifier. I'm re purposing a vintage radio with this, however I only have a single speaker, not two. Adafruit's store page for this product and Adafruit's learn page give two conflicting views on what to do in my situation. The store page says connect the L- and R- to ground, and the other two to the speaker, while the learn page says just connect it to one output and leave the other since the outputs don't have a different signal. So which one do I do? And could you also tell me what ohmage/wattage my speaker should be? Thanks bunch.

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adafruit_support_mike
 
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Re: Need Help With Amplifier Output

Post by adafruit_support_mike »

R- and L- are the inputs. You connect those to GND if you're working with unbalanced (non-differential) signals.

The outputs are bridged -- one side goes high while the other goes low -- so you don't want to combine those.

If you only want to use one channel, short the + and - inputs of the other channel together and don't connect anything to the outputs.

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matthewmanten
 
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Re: Need Help With Amplifier Output

Post by matthewmanten »

wiringdiagram.png
wiringdiagram.png (206.51 KiB) Viewed 261 times
So if I do this it won't damage the amp, and it'll output both sides of the stereo signal into a single speaker?

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adafruit_support_mike
 
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Re: Need Help With Amplifier Output

Post by adafruit_support_mike »

Don't short the L+ and L- outputs. That will kill the amp.

If you want stereo input and mono output, things get complicated.

Everything will have to go through one channel of the amp, so short the L+ and L- inputs together and don't do anything to the L+ and L- outputs. Just let them float.

For the right channel, you'll need to mix the two signals into a single +/- pair. This circuit should be able to do that:
mixer.jpg
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matthewmanten
 
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Re: Need Help With Amplifier Output

Post by matthewmanten »

That makes more sense, just a couple questions :)

You said short the L+ and L- inputs together, does that mean just to connect them to ground? Because the store page says " if you don't have differential outputs, simply tie the R- and L- to ground".

Secondly, why do I need the capacitor on each input? Doesn't this amp have a 1μf capacitor on each input already?

And finally: I'm not exactly sure how to interpret the resistors on your diagram (I'm still relatively new to circuitry). Are there two resistors on each channel? I was under the impression that to make a stereo -> mono circuit you just needed one 10k resistor on each channel.

Thank you so much for your help, I'm learning a lot!

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adafruit_support_mike
 
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Re: Need Help With Amplifier Output

Post by adafruit_support_mike »

matthewmanten wrote:You said short the L+ and L- inputs together, does that mean just to connect them to ground?Because the store page says " if you don't have differential outputs, simply tie the R- and L- to ground".
Not related.

A differential (or 'balanced') signal has two signals that move in opposite directions:
differential.jpg
differential.jpg (14.12 KiB) Viewed 219 times
The blue lines represent a common reference voltage. It can be GND, VCC/2, or any other level. Since signals A and B are pushing opposite ways against the common reference, a receiver will see them as being equivalent to the signal A-B.

You don't want a DC connection between either A or B and the circuit that reads the input because both sides need to be free to move.

A nondifferential (or 'unbalanced') signal puts the whole signal on one line:
nondifferential.jpg
nondifferential.jpg (12.66 KiB) Viewed 219 times
You can make a DC connection between signal B and GND without causing any trouble.

The product page is talking about how to connect a single channel with an unbalanced signal. That's not what you're trying to do.
matthewmanten wrote:Secondly, why do I need the capacitor on each input? Doesn't this amp have a 1μf capacitor on each input already?
The caps aren't isolating the outputs from the amplifier's input. They're isolating the two inputs from each other.

You have two independent signals that you want to connect to a single point, but you can't short them together without causing problems. The capacitors give you a two-in/one-out connection without providing any DC path between the two inputs.
matthewmanten wrote:And finally: I'm not exactly sure how to interpret the resistors on your diagram (I'm still relatively new to circuitry). Are there two resistors on each channel?
Yes.

The center of the 10k-10k voltage divider corresponds to the blue line up above. It's the common reference voltage for the differential signals.

If we assume that both input channels are differential, Lin+ will pull the center of one voltage divider up while Lin- pulls the center of the other voltage divider down. The same is true for Rin+ and Rin-.

Both Lin+ and Rin+ connect to the same voltage divider, so that signal will rise by the sum of Lin+ and Rin+. At the same time, the other voltage divider will fall by the sum of Lin- and Rin-. If the two signals are differential, the sum of the signals will also be differential, so (Lin- + Rin-) and (Lin+ + Rin+) will push in equal but opposite directions around the commn reference voltage.

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