Car audio amplifier and multiple speaker outputs

hello!

my knowledge of electronics is fairly small, i'm not sure what i need
i have some ideas but don't know what theyre called

what i have is a car radio; i'd rather not tamper with it

what i want is a decent aux hookup, so i bought one of these; amazon.co.uk/dp/B00H1O4ZOA

the advice im seeking is how to hook this up to all 4 speakers at the same time as the radio

at first i thought of a way to switch the speaker inputs from amp to radio and back, but an 8 channel switch seemed crazy and i couldnt find it

the smaller method seems to be to hook both systems up as constant inputs, but have something in place so that the amp output cannot reach the stereo, and vice versa
please help me out with what this is called, and if you know any small cheap circuits with this feature

thanks!

You bought a Chinese amplifier. We do not know if the IC is a REAL TDA7297 or a poor Chinese copy.
Its datasheet says it is made for use in a TV or a portable radio, not in a car. Its speaker is supposed to be 8 ohms, a 4 ohms speaker which is common in cars might burn out the amplifier.

You need a car stereo FM transmitter that is commonly sold to feed an MP3 player or phone to a car radio. Most work very well but cheap ones have horrible sound.

thanks Audioguru
while those are definitely other paths that could be taken
theyre not topics on which i'm currently seeking advice for

you dont want to hook that up to the speakers at the same time as the radio - it would take more parts and time then its worth and probably not sound very good. Audioguru is right - you want to feed your aux into the head unit and let the head unit power the speakers. There are wireless fm transmitters and wired ones that will plug in through the antenna connection. Not sure if its an option but an aftermarket head unit with an aux input could be a fairly cheap and simple option ( and better sounding then fm transmitter)

It is rated for 4~8 Ohms so wiring speakers in parallel series to match within this range and proper polarity or phase is required.
Do you know each speaker impedance?
can you do this?

The TDA7297 datasheet of its inventor ST Micro does not mention anywhere a 4 ohm speaker that is commonly used in cars. The Chinese company that made the module probably did not read the English or Italian datasheet so guessed wrongly that it will work with 4-8 ohms speakers. If you try to use it then its outputs cannot be connected to the outputs of your car radio and each of its 4 outputs have two signal wires so you will need a huge relay with 8 changeover switches, or 2 or 4 relays.

It can drive 4 Ohms, but you will need to put a Larger Heat Sink on it.
And considering this IC is good for Up to 20 Volts, it should be OK for in a car.
But be sure to FUSE It with a 2 Amp Fuse.

Audioguru said:

its outputs cannot be connected to the outputs of your car radio and each of its 4 outputs have two signal wires so you will need a huge relay with 8 changeover switches, or 2 or 4 relays.

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C1500 said:

you dont want to hook that up to the speakers at the same time as the radio - it would take more parts and time then its worth and probably not sound very good

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yeah im trying to figure how complicated it would be, or what the things are called that would make the connection not interfere with the other device

the other option would be a switch between, but i have never seen an 8 channel switch, and have no idea what to search for when looking for something similar

chemelec said:

But be sure to FUSE It with a 2 Amp Fuse.

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hadn't thought of that yet, thanks!

for reference the chip on it says Singapore, it's just really hard to see that on amazon

The TDA7297 amplifier IC for TVs and portable radios is not rated for driving 4 ohm speakers because its maximum output current is only 2A. Its output power is rated with a 16.5V supply and gives 15W into an 8 ohms speaker with 10% distortion. The voltage across the speaker is about (10% distortion is a very bad sinewave) 10.8V RMS which has a peak of 10.8V x 1.414= 15.3V then the peak current is 15.3V/8 ohms= 1.9A which is close to the limit of 2A.

The old TDA1554 bridged stereo amplifier is similar with the same maximum supply voltage of 18V, the datasheet mentions car radios but its max output current is 4A therefore it is rated to drive 4 ohm speakers.

The TRUE output power is just before clipping. If an amplifier in a TDA7297 IC is limiting the current to a peak of 2A into a 4 ohm speaker then the peak power is 2A squared x 4 ohms= 16W and the RMS power per channel is 8W. Then the total output of all 4 channels is 32W. Its efficiency with 8 ohm speakers is 60% so with 4 ohms speakers it might be 50% then its total power from the battery is 64W. Then the fuse for a new 4 channels amplifier should be at least 64W/14.4V= 4.44A.
Edited.

thanks for the tech background

but back to topic in hand here please guys...

i can very easily figure out an audio quality issue with my own ears, and go and purchase a better amplifier within about 20 mins on an online shop

but the thing i cannot do is magically understand what electrical components i need to switch or insulate one device from the other

i have come on here asking your help with this latter problem

please stop getting distracted

I think you may be getting distracted from what everyone is trying to tell you - that your not going to get the results you want from that little amp.

What you need to isolate the amp from the head unit are relays - the big problem is you have 4 channels of amplification from the head unit and only 2 channels from that small amp - this means you would have to use relays to break open the connection from the head unit and connect it to the amp.

lets say to make things easy you bought two of those little amps (this will give us 4 channels of amplification) then you would need a dpdt relay per speaker - connect the common contact to the speaker, and the normally closed contact to the head unit, the amp would connect to the normally open contact. when the relay is not energized the head unit will be connected as normal - when you energize the relay it will break the connection from the head unit and connect the speaker to the amplifier.

This is the simplified version - please do lots of research before you start hacking apart the wiring in your car. As an automotive electronics tech I have seen a lot of damage done to car electrical systems from people trying to modify them. (if you blow up a circuit on your test bench its not that big of a deal - if something goes wrong wiring up your car the damage and cost can be enormous - I've seen it too many times - you may not just be out the radio but the car might not work at all)

Lastly - the cost of those relays are probably around $5 each (guessing) you will need at least 4 plus the two amps plus all the little extras you will need - you will probably budget $60 for this and end up spending more(not to mention your time). I was in the department store the other day and they had brand name aftermarket head units with and aux input on sale for $80 - spend the extra few dollars up front and you will get something that is much better quality with less head aches.

C1500 said:

a dpdt relay per speaker

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thanks for that
do i need to be careful of what type? not sure what kind of current a speaker uses tbh

Your existing car radio might be destroyed by oscillation if your new relays disconnect the speakers from it when it is still turned on.
I had a car that came with 2 ohm speakers. That would seriously cripple your Chinese amplifier that cannot drive 4 ohms or 2 ohms. It would also require double the current rating of the contacts on the new relays.

Will your aux input device provide volume controls and tone controls for the new Chinese amplifiers?
Does your aux input device have 4 channels?
Did you borrow a friend's FM transmitter for car radios to hear how good it sounds and how easy it is to use? My son bought a cheap Chinese one online. The word STEREO embossed on it was spelled wrong and it was mono, not stereo. It sounded awful but it was very cheap.

current radio never had back speakers, it understands when things are unplugged

this is my backup plan https://www.amazon.co.uk/dp/B00O8D4CEK
but it's a lot bigger, but i'm unsure how cheap the parts used are
i was gonna try one tiny amp, and buy a second if the quality was reasonable
if these cheap things break no ones going to cry bout it

Audioguru said:

Your existing car radio might be destroyed by oscillation if your new relays disconnect the speakers from it when it is still turned on.

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I used to be afraid of that (terrified actually) when I first started installing bluetooth systems, most of them were basically a little box of relays to break open all the speakers and play the telephone through the front speakers - the rear speakers were left open. I asked the reps and tech support and coworkers about it and everyone told me it was ok to do. I didn't know to call it oscillation but figured it wasn't good to just disconnect a speaker from an amp when it was playing - but Ive done hundreds if not thousands of the bluetooth units and never had a problem with it damaging the radio. When a car has more speakers than the module can accommodate there is an output that can be used to open extra speakers with a standard bosch automotive relay - never had a problem. maybe the modern car radios have enough protection to deal with it?

They do have protection. I call them Snubbers or a simple power > 50kHz LPF , RC load to prevent full power oscillation.

The new no-name-brand cheap stereo amplifier has a power input of 12V at 2A which is 24W. The output power will be the speaker output power plus heating so the output will be no more than about 7W per channel plus 5W of heating per channel. Your car radio probably produces double that small amount of power.

Another problem: At first you said the car has 4 speakers but now you say, "It never had back speakers". Then what are the two extra speakers? Tweeters? Tweeters need the crossover networks that might be inside the original radio.

Audioguru said:

crossover networks that might be inside the original radio.

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i see this as unlikely, though maybe i'll go back and inspect the wiring to double check, kinda thought it was only running on 4 speaker wires previously

First the car had 4 speakers, then it did not have rear speakers and now it has only 4 speaker wires?
If the tweeters are very cheap then they use a crossover capacitor at each one.

Maybe the car has two woofers and two tweeters for a total of only 2 channels?? Who knows, because we are simply guessing about a lot of things?
A little FM transmitter to add an aux input is looking better with each post about the car radio.

it's a vw caddy, it has 8 wires after i put two speakers in the back doors
originally two in the front doors and 2 tweeters in the dashboard

i'm gonna take apart the hands free kit that i have, just to have a look at it,
sounds like it will have these dpdt relays in, and a real cheap amplifier which is the cause for the terrible quality i've been getting