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Registered Member #1563
Joined: Wed Jun 25 2008, 03:55AM
Location: Wimer Oregon, Wewt for sticks!
Posts: 30
I replaced my head unit in my car a while back ago with a different model that was stock in the same car. I added my own line in on the old unit as it didn't even have a tape deck in it. As you can see, the signal came in from the bottom brown cable, and then immediately jumped around the amp board. I just patched in through one of the jumpers. I want to do the same thing with my new (new to me) unit, however things are a little different. The amp is designed differently as it has a 5 channel equalizer ( yes, it has the sliders =D) and a fairly well done bass boost. As you can see, the signal comes in through the red highlighted wires and instead of jumping around, it goes though some filters. I figured this was good because if I was to plug my mp3 player in (I plan on making a dock for charging and line out) I would have to worry about DC offset. Here's the problem, when probing to find the incoming signal, every time I hooked up an external source the audio would pop. I had the source at an extremely low volume so I was expecting a little click or small pop, but this would be devastating if it was to go through a second amp (as it will be). Granted the mp3 player will be in line out mode so its DC offset should be minimal, but if I hook anything else up while the amps on, kaboom. What I'm wondering is, is there something I can add to the all ready installed filter caps (highlighted in green) so that the difference in voltage when hooking up other devices isn't as drastic. Should I put some metal film resistors in so the caps charge slowly? How will this effect my sound?
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
If you could find some audio isolation/impedance transformers you could use those. Radio shack sells some but I'm sure the quality of those is lacking.
If you could find a commercial "direct box" meant for hooking high impedance devices (electric guitars) up to a low-z microphone input on profesisonal mixing board. That would certainly be higher in quality, and might not be that costly if you were to find two (stereo ;) ) for cheap from a pawn shop or thrift store who happens to have them.
Registered Member #1563
Joined: Wed Jun 25 2008, 03:55AM
Location: Wimer Oregon, Wewt for sticks!
Posts: 30
The problem is a high DC spike during initial connection of an external device, that would still pass though the isolation transformer. I guess I should go look how DC offset is handled in other devices.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If there are already DC block caps in the signal path, it shouldn't pop. Unless the head unit designer forgot to put resistors to ground, to set the initial conditions on those caps.
So, I suggest you try putting a 100k resistor from each terminal of the aux input jack to ground. Metal film, whatever, doesn't matter, won't affect the sound.
If this doesn't change the pop, or makes it worse, you know the problem is something more involved. Maybe the aux input jack has switch contacts that connect the signal from the head unit through to the power amps when the plug is removed, and maybe that signal has a DC offset. So when the plug is inserted, the DC offset actually goes away.
Registered Member #1563
Joined: Wed Jun 25 2008, 03:55AM
Location: Wimer Oregon, Wewt for sticks!
Posts: 30
I'm going to try the resistors. I can't imagine those caps doing anything else, the tuner board must just turn on more gracefully, so they didn't think the resistors were needed. Thanks guys, may be a few days till I get back to it.
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