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Hum in speakers with MAX9744 stereo audio amplifier
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Hum in speakers with MAX9744 stereo audio amplifier

by tom2021 on Mon Jul 04, 2022 5:05 am

The speakers have a buzz/hum when audio playing and when not playing, turning the volume (potentiometer) less than half makes the hum/buzz less noticeable. Potentially a ground loop isolator would fix it? However, since all the parts I connect are either via terminal screws or soldering I don't find any isolators that are without banana/rca plugs, any suggestions?

video of buzz/hum
image of wiring diagram

Parts:
Tsunami Super WAV Trigger (Qwiic)
Adafruit 20W Stereo Audio Amplifier - MAX9744
HiVi B3S 3" Speakers
Power Supply (QP-320D)
Adafruit Grand Central M4 Express

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Re: Hum in speakers with MAX9744 stereo audio amplifier

by tom2021 on Mon Jul 04, 2022 7:42 pm

I ended up trying another audio source, mobile and a laptop just connected from battery and played some audio and its clear, no hum/buzz. When connecting the AUX cable to the PC is when the hum/buzz starts so I thought because the PC is on a different wall outlet I'd connect it to the power board with the other devices using a three prong cable, that didn't work.

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Re: Hum in speakers with MAX9744 stereo audio amplifier

by adafruit_support_mike on Tue Jul 05, 2022 11:00 pm

Yeah, that's probably a ground loop picking up the noise from a switching voltage supply.

What are you connecting to AUX? That connection isn't shown in your diagram.

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Re: Hum in speakers with MAX9744 stereo audio amplifier

by tom2021 on Wed Jul 06, 2022 4:38 am

adafruit_support_mike wrote:Yeah, that's probably a ground loop picking up the noise from a switching voltage supply.

What are you connecting to AUX? That connection isn't shown in your diagram.


But then it would have an issue when connected to a laptop or mobile phone right? no issues when connected to these devices

Tried with a shielded 3.5mm to 3.5mm, buzz starts when close to PC aux jack and continues when plugged in.

Whoops, pink line is how the aux cable is connected.

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Re: Hum in speakers with MAX9744 stereo audio amplifier

by adafruit_support_mike on Wed Jul 06, 2022 8:36 pm

tom2021 wrote:But then it would have an issue when connected to a laptop or mobile phone right? no issues when connected to these devices

Not necessarily. If those sources run from battery power, there's no chance of getting a DC connection through the GND line.

tom2021 wrote:Tried with a shielded 3.5mm to 3.5mm, buzz starts when close to PC aux jack and continues when plugged in.

Shielding blocks magnetic and capacitive coupling but can't do anything about galvanic (directly wired or resistive) connections.

DC-to-DC converters work by storing energy temporarily in a magnetic field.. feeding energy in at one voltage/current, and pulling energy out at a different voltage/current. As long as the input power is the same as the output power, everything works for an ideal converter. Real converters always have some loss, so in practice a converter's input power will always be greater than its output power.. 5% to 10% more for good modern circuits, 30% to 40% more for older ones.

The process of charging and discharging the magnetic field puts a large intermittent load on a converter's input and output current. That creates voltage spikes that are all but impossible to filter out. The major driver of price/quality among power supplies is in the amount and quality of noise filtering. Computer power supplies are notoriously noisy because the whole machine is full of circuits that go from 0% to 100% as fast as possible. The main goal of noise control is simply to keep it from messing up the logic timing.

One interesting and effective trick in DC converters is to move the voltage noise to the GND rail. The VCC voltage is measured relative to that, and if they both have the same amount of noise, (DC voltage+noise) - (GND+noise) = DC voltage. As long as you stay in that 'VCC and GND have the same noise' realm, you get the effect of a clean supply without having to build an expensive filter.

A ground loop breaker provides a clean way to move signals out of that realm and into another. It's simply a transformer with the input signal-and-GND connected to the primary coil, and the output signal appearing on the secondary coil. The connections to the primary coil maintain the (signal+noise)-(GND+noise) relationship, so only the difference between the signal and GND makes it to the secondary coil.

You can make an adequate transformer by wrapping a couple pieces of wire around a nail, but a ferrite rod or toroid will transfer the signal more efficiently.

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Please be positive and constructive with your questions and comments.