car stereo

[mpj111]

I have a basic question which is confusing me.
I have a car radio whi is working with a speaker set.
I am going to install CD player to the same speakers .

Can I directly connect out puts from both set to the same speaker set.
What will happend when both sets are on.
Does is overload the speakes.
I can use one at a time, but is it bad if I turn on both sets a t the same time.

Much appreciate your answers

 

[MikeMl]

You cannot tie outputs from different amplifiers together; they fight each other, even without speakers connected. If you want to share speakers, use a multi-pole relay or switch to select which unit drives the speakers. This might require up to an 8-pole switch/relay.

 

[Jon Wilder]

It's not that they "fight" each other per se. If you look at each amplifier as you would a power supply, the amp that has its output voltage set lower than the other will draw current from the amplifier that has its volume control set in a way that makes its output voltage higher than the other. It would be like connecting two batteries of non-identical voltages in parallel...not a good idea.

 

[MikeMl]

Well, if you want a technical explanation, the amplifiers have a very low output impedance (due to the negative feedback intrinsic in their design). If one amp has music, and the other is just sitting there quiet, the one carrying the program is effectively operating into a dead short.

 

[Jon Wilder]

Hence the "one drawing current from the other".

I try not to explain things in terms of "high impedance/low impedance" as it confuses newcomers to electronics. Most of the time when the term "impedance" is used they haven't yet gained the understanding what impedance is so I try to stay away from that.

 

[mpj111]

I got the idea, thank you all.

 

[audioguru]

Even if one amplifier is turned off (not being powered) and its output is connected to the output of the other amplifier that is playing then the outputs of both amplifiers will probably be damaged.

Try towing a car that is in running in REVERSE from another car that is trying to go FORWARD and see what happens.

 

[Jon Wilder]

Even if one amplifier is turned off (not being powered) and its output is connected to the output of the other amplifier that is playing then the outputs of both amplifiers will probably be damaged.

Actually...no. The collector-emitter circuit (or the drain-source circuit if they're MOSFETs) is off when the amp is off, which puts them in high impedance state where they cannot source nor sink current. And since the power supplies in the amp would be off as well, there's no way they could draw from the amp that is on.

 

[audioguru]

The outputs of most car amplifiers are emitter-followers with the emitters at the output. The reverse-bias breakdown voltage of the emitter-base junction of most transistors is only 5V to 6V so the unpowered amplifier will have its ouput transistors with avalanche breakdown with probably a high unlimited current.

The output protection diodes on the unpowered amplifier will conduct from the signal of the playing amplifier which will power the unpowered amplifier. Then the "unpowered" amplifier's output will try to be a dead short.