About the product:
Roadstar RCR385 is a radio cassett recorder with a CD player. The product is from 1994.
i) Since the CD player could not play music CDs, I removed it altogether. Instead, I am using a portable mp3 player.
ii) The volume control started developing issues after about 2 years of use. The multiresistor pot was replaced with another one, but the latter is completely different. The original one was a tailor made component which seems to have been designed to make the product almost useless after about two years of use.
The Issue:
To power the mp3 player, I tried the internal built in power supply, 12V DC, with a linear voltage regulator to reduce the voltage to 3.7V DC, which is required by the mp3 player. This means, the common ground terminal of the radio cassett recorder merget with that of the mp3 player.
Playing audio tracks revealed that when the volume pot is set to minimum, that is, the signal output from the pot is shorted to ground by the volume pot, the actual audio volume does not go down to no-signal and an appreciable loadness is presented to the speakers.
Trying instead, an isolated DC power supply of 3.7V to power the mp3 player does not affect the volume pot with the latter duly presenting no signal when it is set to minimum.
The Question:
Can anyone explain why having a common merged ground, as is common practice in electronics, made the volume pot to fail to reduce the signal to zero (silence)? It seems, with a common ground the audio signal uses another path besides the legitimate one which uses the volume pot to achieve a truly zero signal.
Roadstar RCR385 is a radio cassett recorder with a CD player. The product is from 1994.
i) Since the CD player could not play music CDs, I removed it altogether. Instead, I am using a portable mp3 player.
ii) The volume control started developing issues after about 2 years of use. The multiresistor pot was replaced with another one, but the latter is completely different. The original one was a tailor made component which seems to have been designed to make the product almost useless after about two years of use.
The Issue:
To power the mp3 player, I tried the internal built in power supply, 12V DC, with a linear voltage regulator to reduce the voltage to 3.7V DC, which is required by the mp3 player. This means, the common ground terminal of the radio cassett recorder merget with that of the mp3 player.
Playing audio tracks revealed that when the volume pot is set to minimum, that is, the signal output from the pot is shorted to ground by the volume pot, the actual audio volume does not go down to no-signal and an appreciable loadness is presented to the speakers.
Trying instead, an isolated DC power supply of 3.7V to power the mp3 player does not affect the volume pot with the latter duly presenting no signal when it is set to minimum.
The Question:
Can anyone explain why having a common merged ground, as is common practice in electronics, made the volume pot to fail to reduce the signal to zero (silence)? It seems, with a common ground the audio signal uses another path besides the legitimate one which uses the volume pot to achieve a truly zero signal.
Quote --" I removed it (the CD player ) " .
It works okay with an "isolated " power supply --which I take to mean a bench power supply or a portable power supply.
Down to earth engineering logic would suggest the internal circuit of the radio had its "earth" tied to it or raised about ground zero as I cant find any schematic and that a sine wave signal passing through audio stages where one stage is controlled by a volume control and no longer being able to be "zeroed " would point to the fact the signal earth took a route through the CD player circuit .
In other words it was referenced via the part you removed.
Being part of an automobile system there can be several reasons for this including filters etc to stop ground induced noise being inputted to the radio.
Not having a schematic hinders "quick cures " .
It works okay with an "isolated " power supply --which I take to mean a bench power supply or a portable power supply.
Down to earth engineering logic would suggest the internal circuit of the radio had its "earth" tied to it or raised about ground zero as I cant find any schematic and that a sine wave signal passing through audio stages where one stage is controlled by a volume control and no longer being able to be "zeroed " would point to the fact the signal earth took a route through the CD player circuit .
In other words it was referenced via the part you removed.
Being part of an automobile system there can be several reasons for this including filters etc to stop ground induced noise being inputted to the radio.
Not having a schematic hinders "quick cures " .
Although, the brand name is ROADstar, this is not a radio-cassette recorder for cars, but for the home.
The "isolated power supply" is a power supply I built, and is the simplest possible one can get. It consists of a mains transformer, rectifier bridge, smoothing capacitor and a 6V regulator. An approximate value to the required 3.7V DC was obtained by using series of diode voltage drops.
The signals from the mp3 player are fed into the same points the CD player used. I very much doubt a basic error like a broken ground path is the cause. As soon as I can, I will check again whether the ground path is broken, but rest assured, it is not.
The signal path for the strange behaviour suggests a parasitic path which is not readily obvious. I attribute it to portable mp3 players not using ground referenced output terminations.
The "isolated power supply" is a power supply I built, and is the simplest possible one can get. It consists of a mains transformer, rectifier bridge, smoothing capacitor and a 6V regulator. An approximate value to the required 3.7V DC was obtained by using series of diode voltage drops.
The signals from the mp3 player are fed into the same points the CD player used. I very much doubt a basic error like a broken ground path is the cause. As soon as I can, I will check again whether the ground path is broken, but rest assured, it is not.
The signal path for the strange behaviour suggests a parasitic path which is not readily obvious. I attribute it to portable mp3 players not using ground referenced output terminations.