impedance of audio outputs and inputs

hello all

being somewhat if not mostly deaf I'm not by any stretch of imagination, an audiophile. But I find myself needing some guidance on speakers and signals.

I am building a gsm gate system for a friend whose gate is half a km from the house and whose daughter was recently traumatised by an unwanted visitor.

the intercom system (gate and mic) is by videx, has an amp inside, an electret mic and an 8Ohm speaker. It takes a single speaker input.

the gsm800L module I have exposes only the SPK1 pins; which are a differential output and can, so the datasheet says, directly drive a 32Ohm speaker.

I have tried directly connecting the module and the speaker, without the amp, and get a very very quiet signal - sometimes. Most often I get an under or voltage warning on the board and it closes down. I imagine because too much current is being sourced from the speaker pins.

I have also tried connecting the SPK1P pin (and the N pin sometimes) to the speaker input of the amp - and just get garbage noise and the occasional under/over voltage warning from the module as it shuts down.

I have read that with differential speaker output it is feasible just to ignore one of the outputs (leave it floating) and for the mic, tie one to ground. this was my intent.

Am I on a hiding to nothing trying to get the amp driving an 8Ohm speaker to accept input destined for a 32Ohm speaker?

If so, can someone point me to a potted primer on the dangers/necessities of impedance matching? or otherwise a pointer to a minimal-component amp circuit to take differential input intended for a 32ohm output and push into a speaker loud enough to be sensibly audible outside on a road?

I don't mind replacing the speaker with a 32Ohm equivalently sized alternative. it would be an annoyance but not fatal.

many thanks in advance for your collective wisdom.

Justin

I imagine because too much current is being sourced from the speaker pins.

Exactly!

A 32 Ohm speaker would be very rare,and id be surprised if you can find one. 4 & 8 Ohms speakers are common. 32 Ohms is common for headphones, so that’s probably supposed to be a headphone output.

Technically, you don’t “match” the impedance, but the amplifier has to be rated do drive the load.

So, you’ll need an amplifier which you can either build or buy.

Thanks

I can source 32ohm speakers. Seem to be loads on Amazon and eBay. Yes probably is intended to be a headphone output. Although it can do “hands free” too.

Re the amp circuit. The current amplifier is a tba820ml and works fine with an output from an arduino playing a way file. But literally zilch from the gsm module.

Any thoughts on what to check to debug the why of this?

I can source 32ohm speakers. Seem to be loads on Amazon and eBay. Yes probably is intended to be a headphone output. Although it can do "hands free" too.

Headphones are not loud unless they are right next to your ear. ;)

A 32 Ohm speaker will draw 1/4th the current (so 1/4th the power) of an 8 Ohm speaker, assuming the same applied voltage. Without knowing the efficiency/sensitivity of either speaker we can't be sure, but it will probably be quieter than an 8 Ohm speaker.

What's the sound? If it's a high-frequency beep/tone you can use a piezo speaker/transducer. These are higher impedance (basically a capacitor) and at higher frequencies they can be rather loud with very little power.

Re the amp circuit. The current amplifier is a tba820ml and works fine with an output from an arduino playing a way file. But literally zilch from the gsm module.

Any thoughts on what to check to debug the why of this?

The datasheet I found says 8 Ohms. Maybe the GSM module together with the amplifier is killing your power supply. Do you have a multimeter to make sure your power supply is holding up?

The power supply for the amplifier is a 12V 80Ah lead acid battery. Unlikely to be suffering issues and I assume the regulators on the amp circuit are properly rated.

The power supply for the gsmodule is rated at 3A and is a standard switch mode buck regulator also from the battery. It's tuned to 4V. I will test it to see whether the current is going mad. But my suspicion is that the over/under voltage warnings are being thrown by the module for any excess current draw on an output pin.

If the amp is designed for 8 ohm does that mean it is also designed for an 8ohm input impedance? If so it may be trying to draw too much current from the speaker pins of the module.

The sim800 chip also has an 8Ohm output with a built in class ab amp but unfortunately these have not been exposed on the module. Possibly I could get to the pads with a very fine drill bit and some delicate micrometer work but it would be hit and miss.

So is there a way to amplify a 32ohm rated output and use it sensibly in an 8ohm output ? Surely like plugging an external speaker into a headphone output from your phone?

Thanks Justin

P.s. the.sounds will be voice traffic for the gate intercom.

For audio, usually amplifiers are designed to have a fairly high input impedance, and to be able to drive low impedance loads.

jpadie: So is there a way to amplify a 32ohm rated output and use it sensibly in an 8ohm output ?

In these modern times, normally another amplifier would be used to do this. You'd feed the output signal from your amplifier which is limited to 32 ohms minimum into another amplifier. That other amp will likely have an input impedance much more than 32 ohms, but usually amplifiers work fine driving higher impedance (lighter loads) or no load at all. Of course, for the 2nd amp, you'd choose one capable of driving your 8 ohm speaker.

As recently as a few decades ago, transformers where commonly used for this purpose. A transformer with a 2:1 turns ratio will output half the voltage you feed in, and the impedance seen on the primary side will be 4X the load you connect on the secondary (output) side. In the days before high current transistors existed and when adding lots of extra transistors (or tubes) for a differential input and feedback loop was prohibitively expensive, often a single transistor was used at higher voltage and relatively low current and then a transformer provided the ability to match that circuit to lower voltage, higher current requirements. But of course transistors have become capable of high currents and very inexpensive in modern times, while the cost of copper wire, laminated ferrous metal cores, insulating tape and a labor intensive manufacturing process has remained fairly constant. Transformers designed for audio band signals today are quite rare.

Thanks Paul. I’m not sure I have any suitable transformers in my bits box but I could always wind one.

I’m at a bit of a loss to work out why the amp is killing the module (fried the third one over the weekend plugging the spkP output into the amp input). The amp is a tba820ml. I spent some hours trying to draw out the circuit but there are a lot of intervening PNP and NPN transistors in the way of the traces that does not make it straightforward. But I can see that the signal goes indirectly to the Vi pin of the chip only passing by a pot which is labeled as gain. Which is odd as gain is handled by another pin. So I suspect the pot is just acting as a voltage divider. It is currently set at the midpoint (1.2kOhms) so I guess is dividing the voltage. There is no current limiting resistor in series with the signal though.