do I required a pre amp

[antony james]

Hello
I am integrating a gps unit into the audio system of my car.
I intend to use the headphone out audio singal from the gps unit, run it to two amplifiers, but also in parallel I want to implement an audio signal activated switch to mute my stereo.

Should a use a preamp? I want to split one output into two, plus one for the mute switch.

Kind regards

Antony

 

[Harald Kapp]

Hi Antony, welcome to our forum.

The headphone outputs should have a suffciently low impedance so you can drive 2 or 3 amplifier inputs from them without an additional amplifier.
As for the audio activated switch, it depends on the desiogn of the switch. What is the input impedance of this switch? If it is well above 1 kOhm, it should be o.k. to parallel it with the amplifer inputs.

Harald

 

[antony james]

Hi Harald
Thank you for your reply.

This is the switch I was looking at: http://www.quasarelectronics.co.uk/Item/cebek-pm-11-audio-signal-controlled-relay-switch-module

Basically When the gps gives a voice command or a telephone call comes through, I would like for the stereo in th car to mute, and for the sound of the gps commands to be heard over the front car speakers.

Thank you once again

Antony

 

[Harald Kapp]

This module has an input impedance of 1kOhm, that should suffice for paralleling it to two audio amplifiers.

The relay on the module is a SPDT switch, which means singe pole, double throw. It can switch over one signla between the normally open contact and the normally close contact. This would be suitable for mono applications. For stereo you would want to switch both left and right channel in and out of the signal path. This requires a DPDT (double pole, double throw) relay or 2 SPDT relays.
If you don't find such a module, you can use this module plus an external DPDT relay where the module's relay controls the coils side of the external relay and the contact side of the external relay is used for switching audio.

 

[antony james]

Hi
Excellent that switch will do nicely, I as going to use the relay to ground the telephone mute wire on the existing stereo.

I just noticed on the wiring diagram for my vehicle, that the input for the amps that I was hoping to splice into, also powers two 1 inch tweeters.

I am thinking that two amp inputs, an input to an audio signalled switch, plus inputs to two tweeters maybe too much for the headphone out circuits?

Maybe a pre amp would be advisable to protect the gps unit audio circuits?

Many thanks

Antony

 

[Harald Kapp]

I just noticed on the wiring diagram for my vehicle, that the input for the amps that I was hoping to splice into, also powers two 1 inch tweeters.

I don't understand this. How is the input of an amplifier supposed to power tweeters? Can you scan this part of the wiring diagram and post it here?

Maybe a pre amp would be advisable to protect the gps unit audio circuits?

I still don't think it is necessary. For a first try, put a 100Ohm resistor in series with the audio output of the GPS unit. This will protect the unit from too high a current. The volume of the sound will be a bit reduced. Once everything works fine, remove the resistor to pump up th evolume - or leave it in place as a permanent protection if the volume is good enough for you.

The audio output of your GPS unit should be able to drive as little as 8 Ohm, since it is designed for headphones. The manual of the unit should state the minimum resistance (impedance) for the load on the audio output.

 

[antony james]

Hi again

Sorry for the delay, I have attached an image showing the wires that I intend to splice into. I got a little confused with the last post with the tweeters.

The only other question is will I require diodes to protect the stereo when the gps is outputting an audio command? The stereo will be muted at this point.

Then when the stereo is unmuted and the gps is silent will I need to use a diode to prevent feedback into the gps unit from the stereo?

Kind regards

Antony

 

[Harald Kapp]

Sorry, that image is too small to decipher. Please post an image with higher resolution - you don't need the white space below the diagram .

Diodes are not only useless, they are detrimental. The audio is an AC signal which, when rectified by diodes, will sound terrible.

 

[antony james]

Hi
Sorry for that.
The circled wires out the output from the stereo and input into the amplifier.
I intend to splice into these wires with the output from my gps unit.
Could this damage the gps or stereo when the other unit is outputting audio? For example if the gps is quiet or muted, and the stereo is playing, could this damage the gps unit?

Thanks again

Antony

 

[BobK]

Yes, you cannot simply wire different outputs together since both are trying to put out some voltage. Put a resistor between each output and the input. 1K would be a good value to try.

Bob