While probing a "live" car stereo pins, I also probed the speaker output pins but left the meter in dc volts mode and got a reading of 6v, wich I think is odd, as I asume the output should be considered AC.

I want to know why would the meter show that reading? Is it the meter or could be the radio?

I had the black probe connected to vehicle ground and was probing all the unused pins with the red one (trying to find is the radio had a 12v pin that where "hot" when the radio was ON, like the connection used to turn on external amps...) The back of the radio has three connectors but only one is used in my car (I know one is for external CD changer and the other for rear speakers) however, I could not find the exact information for my unit but for a similar one, that's why I probed all the pins, in case some of them didn't match the description I had.

It was the same reading regardless of how much I Increase the volume, but if I turned the radio off, the voltage reading would slowly drop to zero (taking somewhere beetween 30 seconds to a minute to get to zero). Any of the speaker pins would give out that reading, the marked as negatives included.

I do not think the radio puts a DC voltage through it output as it would make the speaker cones go up (or down) and "stay there" or oscilate around that offset position, wich it doesn't. It also does not pop or click the speakers when powering on or off.

The radio is a factory unit made by Panasonic Automotive Systems (Thailand), (Model no UN36 66 9CO, CQ-LM8282TA) and is labeled to use 4Ohm speakers.

  • \$\begingroup\$ the speaker is connected only to the output of the stereo , not to the output and ground ... measure across the speaker, not between speaker and vehicle frame \$\endgroup\$
    – jsotola
    Jan 17, 2020 at 17:13

2 Answers 2


Your car radio's output amplifier is working in "bridge mode".

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Figure 1. A typical bridge mode configuration.

Bridge mode amplifiers are popular in car audio systems. The output of each amplifier is biased to half-supply. (Since both sides of the speaker are at the same voltage in the quiescent state, no current will flow through the speaker.) One amplifier is fed directly with the signal while the other is driven with an inverted version. The output then "see-saws" about the mid-supply voltage.

The advantage of this arrangement is that the peak to peak voltage across the speaker is almost double that of when one end of the speaker is grounded. Since P = V2/R doubling the voltage gives four times the power for the same loudspeaker. Elimination of the DC decoupling capacitor (which you were expecting) eliminates a bulky component and eliminates the bass roll-off that the capacitor would introduce.

  • \$\begingroup\$ But why 6 Vdc? I’m thinking class D half bridge. \$\endgroup\$
    – winny
    Jan 17, 2020 at 19:30
  • \$\begingroup\$ As explained, The output of each amplifier is biased to half-supply. 12 V / 2 = 6 V. If it's a half bridge then wouldn't you expect one side of the speaker to be grounded? \$\endgroup\$
    – Transistor
    Jan 17, 2020 at 19:35
  • \$\begingroup\$ Oh! Thanks. I’m thinking half bridge, capacitor voltage divider and measure that output to ground. 6 Vdc. \$\endgroup\$
    – winny
    Jan 17, 2020 at 19:45

Not odd at all. Car stereos don't necessarily have ground and positive for the speaker, they can drive both wires in a H-bridge configuration. Both speaker wires idle at about 6V, and there would be 0V over the speaker, and this way it is possible to drive up to +12V to -12V over the speaker. They usually say this in manuals too that speaker negative is not ground.

  • 1
    \$\begingroup\$ That makes me understand a little bit better why many aftermarket car stereos state in the instalation manual that one should not connect speaker negatives to ground nor join them together. \$\endgroup\$
    – Jahaziel
    Jan 17, 2020 at 17:34

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