MAX9744 speaker impedance

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JFQ
 
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MAX9744 speaker impedance

Post by JFQ »

MAX9744 speaker impedance

Good day.
Perhaps someone can help me.
I am attempting to modify a motorcycle helmet headset, Sena SMH10 https://www.sena.com/us-en/product/smh10 , this Bluetooth headset uses 32 Ohm 40mm speakers which I want to replace. From the speaker leads, of the SMH10, to the amplifier and then to 500 Ohm 50mm speakers https://www.aliexpress.com/item/1005004 ... TX5S0&mp=1 . To take advantage of their wide frequency range. For loud, quality HD sound.

I have the Adafruit MAX9744 installed inside the helmet foam with a 14v Li-Ion rechargeable battery power pack attached on the outside of the helmet. MAX8744 set up for ‘Analog’ use.

I am simply wondering if it would be adequate. Writing on the tiny board does mention 4 Ohm or 8 Ohm speaker tolerance. 500 Ohm are quite the monster to power. Very heavy and very, very powerful magnet. The maximum voltage that can applied to this amplifier is 14v… Could any external addons be used to fully power those speakers? Anyone have any ideas?

Thank you.

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barshatriplee
 
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Re: MAX9744 speaker impedance

Post by barshatriplee »

The Adafruit MAX9744 amplifier you have installed in your motorcycle helmet is designed for driving low impedance speakers, typically 4 Ohms or 8 Ohms. The 500 Ohm speakers you mentioned are significantly higher in impedance, which means they will draw much less current from the amplifier.

Using the MAX9744 amplifier with 500 Ohm speakers may not provide sufficient power output, resulting in lower volume levels and potentially unsatisfactory audio quality. It is important to match the speaker impedance with the amplifier's recommended impedance range for optimal performance.

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adafruit_support_bill
 
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Re: MAX9744 speaker impedance

Post by adafruit_support_bill »

At 500 Ohms, the amp won't be able to deliver the rated 20 Watts. But you don't need (or want) anywhere near 20 watts of power blasting millimeters from your ears. Given the high sensitivity of those drivers, you will only need a few milliwatts of power for adequate sound levels in a headphone type application.

A different concern however, is how well these will perform with a 'filterless' Class D amplifier output. Class D amplifiers like the MAX9744 use high frequency PWM to modulate the audio signal. The output is then run through a low-pass filter to remove the PWM switching noise.

Low-pass filters typically involve bulky inductors. 'Filterless' amplifier modules are designed to use the speaker coil itself as the inductor, resulting in a more compact module. It is not clear how well the inductance of your drivers will work in that filtering role. You can try them and see how they sound.

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JFQ
 
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Re: MAX9744 speaker impedance

Post by JFQ »

Thank you for the response.

You are correct, you don't necessarily want 20 watt that close to your ear. However at 60 or 70 km/h, the half helmets I prefer to use drown out the output of those speakers. Notably the alarms I receive from a Diabetic Glucose Monitoring app (Abbott FreeStyle Libre 2) installed on my Smart Phone. Important that I receive those alarms.
But thank you. I will keep looking into this.

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JFQ
 
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Re: MAX9744 speaker impedance

Post by JFQ »

The MAX9744 has an output impedance for 4 or 8 Ohms depending on the speakers used.
The Sena SMH10 I use, on its own, has outputs for 32 Ohms drivers/speakers. Any higher impedance drains the SMH10 battery too quickly.
I am not certain, but when the SMH10 speakers lead signals go to the input of the MAX9744, does it not become 4 or 8 Ohms going out, depending on the speaker impedance used at the MAX9744 output?

Hence 4 Ohm drivers/speakers should be able to deliver 20 or 22 watts, 8 Ohm about 13 watts, with a 14v DC power pack as a power supply?
Class AB amplifier PCB boards (often 1 per channel) are significantly larger, require greater voltage power supplies (often 1 per channel) which makes them cumbersome to attach and deliver way more watts then necessary, so dismissed.
Any ideas?
All I would need, if this is so, would be a pair of quality 50mm or so, HiFi 8 Ohm drivers/speakers. Yes???
¡Muchas gracias!

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adafruit_support_bill
 
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Re: MAX9744 speaker impedance

Post by adafruit_support_bill »

The MAX9744 has an output impedance for 4 or 8 Ohms depending on the speakers used.
The MAX9744 data sheet does not specify the output impedance of the amp. But the amplifier outputs are optimized for use with speakers in the 4 to 8 ohm range.
The Sena SMH10 I use, on its own, has outputs for 32 Ohms drivers/speakers. Any higher impedance drains the SMH10 battery too quickly.
That is odd. Have you verified this behavior? A higher load impedance reduces the amount of power that the amplifier can push through the load. That generally means it will be drawing less power from its power source too.

Ohms & Watts is only part of the story. You have to consider the sensitivity of the speakers as well. All else being equal, if you have two 4 Ohm speakers and one is rated for 97dB/W and the other for 100dB/W, the 100dB/W speaker will be twice as loud.

You probably need less power than you think. Your drivers have a rated sensitivity or 109 dB/W which is very good. That means 1 watt of power would give you 109 dB at one meter distance. Since sound follows the inverse square law with distance, that translates to 143 dB at 2 cm which is above the threshold of pain. To get 100 dB at a 2 cm distance, you would need just 78 milliwatts.

Do you have any specs for the Sena amplifier? What power source does it use?

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JFQ
 
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Re: MAX9744 speaker impedance

Post by JFQ »

Very grateful for your interest. I am in contact with Sena hoping to receive detailed specs for the Sena SMH10-11 Bluetooth headset device. My guess; as far as the power supply goes, the device is charged through USB so, probably 5v, 1000 or 2000mAh, 12 hours constant functions, I'll get details.
Merci.

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