Bluetooth Speaker as circuit audio monitor

Thread Starter

RobinGriffiths

Joined Aug 16, 2011
20
I'm currently getting a setup together for building and testing audio circuits (synth modules). So I've got PSU, scope and function generator. I thought it would be nice to actually hear what's going on. To this end I bought a bluetooth speaker (with 3.5" audio input socket).

Now I'm not sure what the input impedence is, and what voltage range the amplifier expects. I would expect to be working with waveforms maybe up to +/- 5V peak to peak, with a possible offset if I connected the function generator to the bluetooth speaker.

I presume that I'd need to add a small capacitor to decouple any dc component going into the amplifier. I'm not sure what I need to do so that the amplifier doesn't get overdriven or damaged by too large an input. A simple resistor, a voltage divider, or some other attenuator of some sort ?
 

wayneh

Joined Sep 9, 2010
17,168
I'm currently getting a setup together for building and testing audio circuits (synth modules). So I've got PSU, scope and function generator. I thought it would be nice to actually hear what's going on. To this end I bought a bluetooth speaker (with 3.5" audio input socket).

Now I'm not sure what the input impedence is, and what voltage range the amplifier expects. I would expect to be working with waveforms maybe up to +/- 5V peak to peak, with a possible offset if I connected the function generator to the bluetooth speaker.

I presume that I'd need to add a small capacitor to decouple any dc component going into the amplifier. I'm not sure what I need to do so that the amplifier doesn't get overdriven or damaged by too large an input. A simple resistor, a voltage divider, or some other attenuator of some sort ?
So the fact that it’s Bluetooth is just coincidental? It’s an obvious solution, to use a BT link instead of a wire.

I think the AUX input on your speaker is designed to accept a headphone level input as opposed to a line level. So I suspect it will be fine with no further protection. A 0.1 to 1 microfarad capacitor to block DC wouldn’t hurt and should be all you might need.
 

Reloadron

Joined Jan 15, 2015
6,593
Over the Holidays a few years ago I ended up with a few of these OontZ Angle Solo - Bluetooth Portable Speaker systems. Finally decided to open the boxes and they actually work quite well, I use them with my phone but anyway they do have a 3.5mm audio in jack and here is what they say about it. AUX IN Jack connect from TVs and non-Bluetooth devices with a 3.5mm Line-In cable; so I assume normal Line In voltages would apply and I have used them just connecting to the headphone out of my phone. Line Levels are defined here and those numbers seem right.

Ron
 

wayneh

Joined Sep 9, 2010
17,168
Thanks for that. Assuming 10k nominal input impedance, maybe a 20k to 47k resistor in series with .1u cap should do the trick.
You could certainly start with that and see if you get enough volume. If not, up the capacitor a little and/or drop the resistance a little until you do.
 
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