I am working on an amplifier for a car radio for a friend of mine. He is away for few weeks so I can't measure the radio output.

That's why I am asking here, does anyone know, what's the peak-to-peak voltage from car radio, that then goes to the amplifier?

Is it universal, if not, how do these amplifiers usually do their preamplifiying?


Is this +-voltage or only positive?


2 Answers 2


This depends on whether this "radio" is meant to drive speakers directly, or meant to drive a separate power amp that then drives the speakers.

The typical non-power audio signal for interfacing to other equipment is about 1 V RMS with 600 Ω impedance or less. That is sometimes called line audio or a line level signal. A separate stand-alone power amp would most likely expect such a signal as input.

If this radio drives speakers directly, then it is really a combination of a radio receiver, and power amp all in one. If the receiver and power amp were separate boxes, then you'd expect the signal between them to be line level. However, in this case that is totally internal to the box, so is probably not accessible, and may not even exist in that form internally anywhere. All that comes out are signals intended to be connected directly to the speakers.

Car speakers are usually 4 Ω. From that and knowing the output power you can compute the voltage. For example, let's say the radio can drive each of the speakers with up to 15 W of sustained power.

P = V²/R

When V is the EMF in volts, R the resistance in Ohms, then P is the power in Watts. Working this backwards to solve for V yields V = sqrt(15 W * 4 Ω) = 7.7 V RMS. For sine waves, the peaks are sqrt(2) higher, or 11 V. Peak to peak would therefore be 22 V.


Just some info that may help, stock radios for older vehicles usually only have speaker level outputs. You can buy speaker to line level converters that are just voltage dividers inside. Or make your own.

Sometimes on fancier stock systems you'll find that there is an external amp somewhere, possibly in the trunk (the Volkswagen Jetta comes to mind). In that case there is probably line level outputs from the radio to the amp.

Aftermarket radios usually come with one or more pre-amp outputs which are line level usually a little less than 1V peak to peak. This is what your typical aftermarket amp is expecting. But it's not really an enforces standard. A quick look at a common car amp datasheet shows it can take (RCA Input): 0.15 - 4.0 V. It also has a speaker level input :)

So depends what your friend has, if he bought a new radio and wishes he had an amp chances are you'll have some line levels to play with.

  • \$\begingroup\$ So, do you know how do they to their preamplifying? Since different voltages can bt present, there cant be a fixed gain/attenuation. \$\endgroup\$
    – Golaž
    Dec 25, 2014 at 10:27
  • \$\begingroup\$ Well you could do fixed gain because the signal level will vary with the volume knob, but most amps I've seen have a little potentiometer knob to adjust to the input level range. \$\endgroup\$ Dec 25, 2014 at 13:36
  • \$\begingroup\$ Ohh, ok. Another question, is the output voltage only positive, or negative-positive? \$\endgroup\$
    – Golaž
    Dec 25, 2014 at 17:07

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