10-09-2015, 11:14 AM
Wow - that's hard to read! Do you have a higher resolution copy?
It looks like they aren't actively switching the audio signals between the radio and the cassette pre-amp - rather, they are joined together, and power to either the head preamp or the radio receiver is switched according to mode. Hard to be sure from the schematic though.
From there, the signals travel via a transistor pre-amp before being applied to the volume control and hence the power amps.
Now, it would be nice to know what the signal levels are. It might be that the iPod doesn't require the extra gain afforded by the preamp, in which case you'd inject the signal at the top of the volume control. However, the headphone output of a portable device is often a fairly low voltage, so perhaps the preamp is useful? Impossible to say without the thing in front of me. If the schematic was clearer it might be possible to determine the "gain structure", but as I can't read any of the component values...
Having determined where to put it, I can't imagine why the 4-pole switch would be required. Obviously I'm aware of this type of configuration, but it's only used when separation of 80dB or more is required. Just think about the amount of signal switching in a sound desk with 48 channels and 8 groups, 2 mains, 6 auxes, 50+ insert points, etc, etc. Total isolation between channels is required, and that is usually the case. With conventional single-pole switching. Read some of Douglas Self's writings about this, where he deals with the subject comprehensively.
So, given that we don't need anything like 80dB of isolation in a car, I suspect that a previous case of breakthrough was caused by some other undiagnosed factor (leaky switch, perhaps?), but since a 4-pole switch cured it, it's become a routine.
Personally, I would rely on the double-throw contacts within a 3.5mm socket to do the switching. You can get sockets that have a pair of contacts that are separate from the input connections, which give you the flexibility to switch signals at any level, or to switch rails to disable the radio and the tape preamp. Lots of options, and no separate switch needed.
In answer to the original question, no, you don't need resistors for the usual reason (to stop the output stage of the portable device "fighting"). If the signal levels seem about right to you, then you don't need resistors to attenuate the signal. I assume you are going in at the same place where the signals merge?
However, I would recommend the following:
As the signal comes in, I would put resistors to ground. This is to ensure that any output capacitor the device has is able to charge to the expected DC voltage. Some use bootstrapping that requires a DC path to ground. Something like 1k would be OK.
Then I'd put a series resistance from there. This is to protect the input of amplifier, should a DC transient appear (like when the device is initially plugged in). Something between 1k and 10k is a good starting point. If you go too high, you'll start to lose level (which you might want anyway) - only you can determine this.
After this series resistance, I'd add a cap to ground. This is to keep out RFI. It would be a few hundred pF, perhaps more depending on the resistance "seen" by it. Choose it to have a turnover frequency of 15 to 20 kHz. If you omit this, your set might suddenly become much more prone to cell-phone interference and similar problems. To stop re-radiation within the set, keep the connections between the input and this cap as short as possible - otherwise a cap in parallel with the input socket contacts might be needed.
Next, you'll need a series capacitor to couple the audio in to the base of the audio preamp, just like the radio and tape signals do. Probably sensible to make this the same as those already used by Radiomobile.
Next comes the switch. I presume you'll be putting this in series with the base of the preamp, after the junction of the two electrolytic capacitors?
If you're going in at the volume control (rather than into the base of the preceding transistor), then you can omit the series coupling cap, but I'd still include the other components.
Does this make sense?
It looks like they aren't actively switching the audio signals between the radio and the cassette pre-amp - rather, they are joined together, and power to either the head preamp or the radio receiver is switched according to mode. Hard to be sure from the schematic though.
From there, the signals travel via a transistor pre-amp before being applied to the volume control and hence the power amps.
Now, it would be nice to know what the signal levels are. It might be that the iPod doesn't require the extra gain afforded by the preamp, in which case you'd inject the signal at the top of the volume control. However, the headphone output of a portable device is often a fairly low voltage, so perhaps the preamp is useful? Impossible to say without the thing in front of me. If the schematic was clearer it might be possible to determine the "gain structure", but as I can't read any of the component values...
Having determined where to put it, I can't imagine why the 4-pole switch would be required. Obviously I'm aware of this type of configuration, but it's only used when separation of 80dB or more is required. Just think about the amount of signal switching in a sound desk with 48 channels and 8 groups, 2 mains, 6 auxes, 50+ insert points, etc, etc. Total isolation between channels is required, and that is usually the case. With conventional single-pole switching. Read some of Douglas Self's writings about this, where he deals with the subject comprehensively.
So, given that we don't need anything like 80dB of isolation in a car, I suspect that a previous case of breakthrough was caused by some other undiagnosed factor (leaky switch, perhaps?), but since a 4-pole switch cured it, it's become a routine.
Personally, I would rely on the double-throw contacts within a 3.5mm socket to do the switching. You can get sockets that have a pair of contacts that are separate from the input connections, which give you the flexibility to switch signals at any level, or to switch rails to disable the radio and the tape preamp. Lots of options, and no separate switch needed.
In answer to the original question, no, you don't need resistors for the usual reason (to stop the output stage of the portable device "fighting"). If the signal levels seem about right to you, then you don't need resistors to attenuate the signal. I assume you are going in at the same place where the signals merge?
However, I would recommend the following:
As the signal comes in, I would put resistors to ground. This is to ensure that any output capacitor the device has is able to charge to the expected DC voltage. Some use bootstrapping that requires a DC path to ground. Something like 1k would be OK.
Then I'd put a series resistance from there. This is to protect the input of amplifier, should a DC transient appear (like when the device is initially plugged in). Something between 1k and 10k is a good starting point. If you go too high, you'll start to lose level (which you might want anyway) - only you can determine this.
After this series resistance, I'd add a cap to ground. This is to keep out RFI. It would be a few hundred pF, perhaps more depending on the resistance "seen" by it. Choose it to have a turnover frequency of 15 to 20 kHz. If you omit this, your set might suddenly become much more prone to cell-phone interference and similar problems. To stop re-radiation within the set, keep the connections between the input and this cap as short as possible - otherwise a cap in parallel with the input socket contacts might be needed.
Next, you'll need a series capacitor to couple the audio in to the base of the audio preamp, just like the radio and tape signals do. Probably sensible to make this the same as those already used by Radiomobile.
Next comes the switch. I presume you'll be putting this in series with the base of the preamp, after the junction of the two electrolytic capacitors?
If you're going in at the volume control (rather than into the base of the preceding transistor), then you can omit the series coupling cap, but I'd still include the other components.
Does this make sense?
