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How to boost audio out signal from M4 GC?
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How to boost audio out signal from M4 GC?

by kgrove on Tue Mar 16, 2021 6:47 pm

I finally got some audio files to play after realizing I needed a capacitor (duh), but the signal isn't very loud.

I was hoping to make my own gain control output amplifier to run through some small speakers (2x4 ohm), but I'm not sure what components I will need.

I was looking at https://www.adafruit.com/product/2217 or https://www.adafruit.com/product/2341, but wasn't sure how to wire it up with my M4 GC and if it would be loud enough?

Any insight would be awesome. Thanks!

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Re: How to boost audio out signal from M4 GC?

by adafruit_support_mike on Tue Mar 16, 2021 11:28 pm

The FX Board doesn't process audio signals from somewhere else. It plays audio files that are stored on its own Flash memory chip.

The Grand Central's DAC isn't strong enough to drive speakers directly, so the first step would be to connect it to a power amplifier. The PAM8302 is good for small speakers:

https://www.adafruit.com/product/2130

To establish some basic theory, audio amplifiers are usually split into two groups: signal amplifiers (aka: preamps), and power amplifiers (aka: speaker drivers).

A preamp's job is to reproduce a small input signal (usually a few millivolts, down to a few microvolts for high-end microphones) at a higher voltage swing (usually 1V to 2V) while adding as little distortion as possible. At both ends, the signal tends to use very little current.. a couple of milliamps at most.

A power amp's job is to reproduce its input signal at the same voltage, but with a lot more current.

The split exists because the circuit techniques that make a good preamp also make a lousy power amp, and vice versa. Designing two circuts that do their parts well, then connecting them in series, is easier and produces better results.

The Grand Central's DAC output will already be in the 1V-to-22V range, but can only supply a few milliamps of current. Sending that signal through a power amp like the PAM8302 should give you a lot more audio volume for the same signal swing.

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Re: How to boost audio out signal from M4 GC?

by kgrove on Tue Mar 16, 2021 11:49 pm

Awesome! Thanks Mike!

I’m assuming the stereo version (https://www.adafruit.com/product/987) is similar, but for stereo audio files?

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Re: How to boost audio out signal from M4 GC?

by adafruit_support_mike on Wed Mar 17, 2021 4:13 pm

Yep. It can handle two audio channels, and a little more power.

It also has a 5-level fixed gain amplifier. That’s more for range setting than pure volume control.. if a signal source’s default ouput is a little too quiet, or a little loud, you can adjust it on the way to the speaker.

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Re: How to boost audio out signal from M4 GC?

by kgrove on Thu Apr 29, 2021 10:34 pm

So, I am just now getting around to the audio portion of my project. I've made the connections, but just realized I don't have a 5v 2A power source. I hooked it up to the 5v pinout on my Grand Central and it is still giving me a very weak signal. It is a tiny bit louder than what I had before, but still weak.

I'm unsure as to go about boosting the signal even more. I went ahead and bought a 5v 2A power cord, but I'm still doubting that this will give me enough gain.

I've got the https://www.adafruit.com/product/987 and a single https://www.adafruit.com/product/3351 speaker

At the moment I just have the right speaker hooked up, but I'm assuming it wouldn't make that much of a difference if I had them both hooked up. Will it be louder once I get the appropriate power supply?

I'm following along with the following lesson:
https://learn.adafruit.com/circuitpytho ... -audio-out

Just using the StreetChicken.wav portion of the lesson to generate audio

Combined with the single (right) schematic from this photo https://learn.adafruit.com/assets/3730

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Re: How to boost audio out signal from M4 GC?

by adafruit_support_mike on Sat May 01, 2021 2:11 am

Post a photo showing your hardware and connections and we'll take a look. 800x600 images usually work best.

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Re: How to boost audio out signal from M4 GC?

by kgrove on Mon May 03, 2021 10:35 am

Sorry about the mess. I'm still testing.

Note: I've tried each gain combo and the one that is the "loudest" is the 18dB setting.
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Re: How to boost audio out signal from M4 GC?

by adafruit_support_mike on Mon May 03, 2021 10:07 pm

It looks like you have the audio signal connected to the left channel's inputs, but the speaker is connected to the right channel's output.

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Re: How to boost audio out signal from M4 GC?

by kgrove on Tue May 04, 2021 1:13 am

But the schematic (https://learn.adafruit.com/assets/3730) shows that the right (ring) of the trs ¼” end is connected to the “L+” in the photo...

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Re: How to boost audio out signal from M4 GC?

by adafruit_support_mike on Tue May 04, 2021 10:19 pm

That diagram shows connections for a stereo jack. Both channels are connected.

Try moving the speaker to the same channel as the input signal and see what happens.

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Re: How to boost audio out signal from M4 GC?

by kgrove on Wed May 05, 2021 9:39 am

Here's a cleaned up photo. I made a jump between the L+ and R+ to distribute across both speakers, but it is still quite low. It is slightly louder than coming straight out of the GC pinout, but I'm not sure how much louder it's supposed to be.

Also, I tried hooking up a 5V 2amp power supply https://www.adafruit.com/product/276 (positive to VDD, and neutral to GND), but it is making an awful noise. I copied this diagram https://learn.adafruit.com/assets/3737. The photo I posted does not reflect using the power supply. I'm just using the 5V pinout from the GC
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Re: How to boost audio out signal from M4 GC?

by adafruit_support_mike on Thu May 06, 2021 1:35 am

Take the 10k potentiometer out of the signal path for now. Let's focus on getting the speakers working in general. Once you have good output volume, we can move on to volume control.

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Re: How to boost audio out signal from M4 GC?

by kgrove on Thu May 06, 2021 10:19 am

Yes, that is plenty loud. Do I need a new potentiometer? One that doesn't "weaken" the signal?

To be honest, I just bought the one that was less than the highest one I saw and more than the lowest.

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Re: How to boost audio out signal from M4 GC?

by adafruit_support_mike on Sun May 09, 2021 5:29 pm

10k is the traditional value for a volume control pot. That shouldn't be a problem.

Make sure you have the right signal connections. The pot connects between the audio signal and GND. Then connect GND to the MCP98306's L-/R- inputs, and the pot's wiper to the L+/R+ inputs.

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Please be positive and constructive with your questions and comments.