speaker isolation

[Ivan Sedneff]

Hello,

I made a portable enclosure for a car radio that has a negative ground
but the left and right speaker outputs have isolated grounds from each
other and the chassis.

I needed to common out the (-) from the left and right speakers in
order to attach them to the common connection on the headphone jack. I
successfully connected each of the (-) outputs from the left and right
speakers using non-polarized 47 uf electrolytic capacitors and joined
the ends of the caps to the headphone jack.

All is well when the jack is floating and un mounted, but when I mount
the jack to the chassis, even though the audio output is great, I get
annoying loud "clicks" and "pops" when I wiggle the headphone
plug/jack, most likely because of the caps discharging/charging.

Other than figuring a way to physically isolate the headphone jack
from the chassis, is there a way to drain the charge using resistors,
and if so, what values do you suggest?


Thank you for your help,

Ivan

 

[no_one]

are they really isolated? do you have enough circuit info to show that
connecting the returns together will upset the stereo amp(s). many (older)
car stereos would actually use the car chassis as a common return for the
speakers. This was only a problem when you had a car with a positive ground
such as the british used. I would expect that you can connect the returns
together and not use the cap.

 

[SimonLW]

Hello,

I made a portable enclosure for a car radio that has a negative ground
but the left and right speaker outputs have isolated grounds from each
other and the chassis.

I needed to common out the (-) from the left and right speakers in
order to attach them to the common connection on the headphone jack. I
successfully connected each of the (-) outputs from the left and right
speakers using non-polarized 47 uf electrolytic capacitors and joined
the ends of the caps to the headphone jack.

All is well when the jack is floating and un mounted, but when I mount
the jack to the chassis, even though the audio output is great, I get
annoying loud "clicks" and "pops" when I wiggle the headphone
plug/jack, most likely because of the caps discharging/charging.

Other than figuring a way to physically isolate the headphone jack
from the chassis, is there a way to drain the charge using resistors,
and if so, what values do you suggest?


Thank you for your help,

Ivan

let me get this straight. You connected the - outputs together as a ground
through the caps for the common of the head phone jack? If so, very bad! You
are essentially shorting higher frequency signals not equal to each other
through the amplifier's outputs. To derive more power from 12volt auto
supply, many car stereo's use what is known a bridge amplifier. It is two
single ended amplifiers outputs connected together through the load
(speaker). One of the amps is fed the signal 180 Deg out of phase. This
allows for double the voltage swing on the output, thus double the current
which means a potential for quadruple the power output as compared to a
single ended amplifier.

So, connecting one negative terminal to the other channels negative terminal
(not really a negative line, just used for proper phasing between the
channels) means you are shorting high frequency signals through one side of
the bridge amp to one side in the other channel. This could likely
eventually damage your car stereo's output amplifier.

The proper way is to take either the + or - lead (as long as the same for
both channels), put a 220 ohm 1/2 watt resistor in series with a 100 uf cap
from each channel. Connect to proper channel on the jack. The ground of the
jack goes to the chassis ground of the car, near the ground of the stereo to
prevent possible ground loop noise.

The capacitor removes the DC component as each side of the bridge amp is
biased a 1/2 the supply voltage. The resister limits the current so the
voltage across the headphones is reduced so you don't blast your ears with
full output from the amp.
Simon