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Topic: How do I convert speaker out to RCA? (Read 32049 times) previous topic - next topic
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How do I convert speaker out to RCA?

I want to hook up a stock car radio to an amplifier, but most of these do not have any kind of AUX output available (and i'm not going to make a soldered solution).

I've read that you can use an adaptor to convert the speaker line out signal to RCA, but also that the quality differs.

1. What are the actual difference between the two signals and which devices do the best job at converting it to RCA?
2. Can it be converted without any loss?
Can't wait for a HD-AAC encoder :P

How do I convert speaker out to RCA?

Reply #1 –
Quote from: odyssey on
I want to hook up a stock car radio to an amplifier, but most of these do not have any kind of AUX output available (and i'm not going to make a soldered solution).

I've read that you can use an adaptor to convert the speaker line out signal to RCA, but also that the quality differs.

1. What are the actual difference between the two signals and which devices do the best job at converting it to RCA?
2. Can it be converted without any loss?

I guess my main concern about using a speaker output for a line input would be loading. The speaker output expects to be driving a resistive load, more or less, and depending on the circuit design might not work very well if there is no speaker load. You might want to place a 8 ohm resistor on each output, but make sure its wattage rating is high enough, and preferably don't use a wirewound resistor unless it is specifically non-inductive. You also might want to attenuate the signal so as not to overload the line input.

As far as sound quality, you will be picking up whatever noise and distortion are added by the output stage of the amplifier, which will probably not be noticeable as long as you set the output volume of the radio to be not so high that you increase distortion, and not so low that you increase noise. If you are going to be listening to it in a car then you will probably be OK.

How do I convert speaker out to RCA?

Reply #2 –
Quote from: pdq on
Quote from: odyssey on

I want to hook up a stock car radio to an amplifier, but most of these do not have any kind of AUX output available (and i'm not going to make a soldered solution).

I've read that you can use an adaptor to convert the speaker line out signal to RCA, but also that the quality differs.

1. What are the actual difference between the two signals and which devices do the best job at converting it to RCA?
2. Can it be converted without any loss?

I guess my main concern about using a speaker output for a line input would be loading. The speaker output expects to be driving a resistive load, more or less, and depending on the circuit design might not work very well if there is no speaker load. You might want to place a 8 ohm resistor on each output, but make sure its wattage rating is high enough, and preferably don't use a wirewound resistor unless it is specifically non-inductive. You also might want to attenuate the signal so as not to overload the line input.


With most small amplifiers there is no need to provide much of a dummy load - using an 8 ohm resistor would generate a fair amount of heat (which is a very real problem) but likely wouldn't affect the amplifier signal.

I would recommend that you build a simple voltage divider in order to approximate a line level signal. Most stock car radios are roughly 5 watts into 8 ohms -> around 6 VRMS. So a good starting point would be a divider that reduces this to 1 VRMS. This can easily be done with standard 1/2 watt resistors of modest value.

Example: make a divider from 470 ohm and 100 ohm resistors in series. You will derive the attenuated signal from the middle of the divider, like so:

Amp hot -> 470 ohm -> output -> 100 ohm -> ground

Power dissipation will be very small (about 0.07 watts) and you will have isolation and signal attenuation. A power amp is probably going to be a bit noisy, but it should work.

How do I convert speaker out to RCA?

Reply #3 –
Quote from: odyssey on
I want to hook up a stock car radio to an amplifier, but most of these do not have any kind of AUX output available (and i'm not going to make a soldered solution).


Car audio shops sell devices for that purpose.

Or you can build one with couple of resistors.  I have used these instructions at home audio to connect speaker out to subwoofer line in.
Teemu

How do I convert speaker out to RCA?

Reply #4 –
This is common problem, and you should be able to buy an adapter at a car stereo shop. Here's one, but with the heatsinks, it looks like overkill to me.  Like Brad, I don't see a reason to put a "power load" (low resistance) on the stereo.

If the amplifer has a level/gain control, you can probably get-away with connecting it directly.  I guess I'd be somewhat careful/cautious if the amp is really expensive...  A low-power car stereo's output voltage isn't much higher than line-level.  ...Line level is about 1V...  A "simple" amplifier running directly off a 14.4V car battery can output 14.4V peak-to-peak (5V RMS) maximum... It will be less with the volume turned-down.    If the power amp has a level control, it can handle that.  And, the amp's high input resistance (10K ohms or more) means you are not pumping any significant power or current into the amp.   

Another issue - Many "high power" car stereo's (~ more than 10 watts) have balanced speaker outputs (sometimes called "bridged" or "push-pull").  With that type of connection, there are two "hot" wires going to the speaker (no ground).    That type of set-up could get tricky, and you might need something more advanced than a resistor-network attenuator.  Again, if the amp is expensive you might want to take it to a professional if you've got that type of car stereo...

Quote
1. What are the actual difference between the two signals...
  Mostly the voltage level.  Speaker outputs have much higher current capability than line outputs.  You get more power from the speaker outputs because of the current capability combined with the low resistance of the speaker.  This current capability means that you can "suck more power" from a speaker output than from a line-level output.

If you connect a speaker output to a line-level input, the high resistance means you don't pull much current (or power) from the speaker output (no problem).

If you connect a speaker to a line output, the speaker will "pull down" the voltage (to almost zero), and it might burn-up the electronics (big problem). 


------------------------------------------------------------------------------------------
Here are some numbers & formulas for you -

This is Ohm's law:  Current = Voltage/Resistance  (lower resistance = higher current)
With algebra you also get: Voltage = Current x Resistance
And: Resistance = Voltage/Current

This is the power formula:  Power = Current x Voltage
With algebra and the above Ohm's law formulas: Power = (Current^2) x Resistance (current squared x resistance)
And: Power = (Voltage^2)/resistance 
- 10 volts (RMS) into 4 ohms gives you 25 watts
- 20 volts into 4 ohms gives you 100 watts
- 40 volts into 4 ohms gives you 400 watts

- 10 volts into 8 ohms gives you 12.5 watts
- 20 volts into 8 ohms gives you 50 watts
- 40 volts into 8 ohms gives you 200 watts



NOTE - Amplifiers (and preamps) are like batteries or "wall current".  The amplifier voltage is "constant" with a constant input.  The current (and power) are then determined by the load (resistance).  This is why you get more power with a 4-ohm speaker than with an 8-ohm speaker.

In other words, current is the result (with "normal" amplifiers).  You can't directly "push" more more current...  You can "push" more current with higher voltage, or you can "pull" more current with lower resistance.  ...Just like you can't "push" more than 100 watts into a 100-watt light bulb (unless you increase the voltage).