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Old 17th June 2011, 03:15 PM   #1
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Location: Christchurch, New Zealand
Default Boosting line out and adding low-pass

Hi all, I've been out of electronics for a long long time, but my daughter and I just built a crystal set together and thats kind of got me interested again. However, I digress...

I've got a cheap but ok-sounding head unit in my car which has a single line out (part of the rear speaker circuit, not a dedicated sub out), and a cheap 4 x 35W bridgeable Sony amp, and a couple of 10" Infinity subs I got cheap (Getting the idea? I am cheap - well, actually I have a wife and daughter, so I'm cheap because I have to be, not because I want to be ). The car has 4 x 6.5" speakers, which I am running from the 4 speaker outs on the head unit, and the line out is run to the amp. I have both channels bridged in the amp (the booklet for the amp only documents one as bridgeable... but experimentation has led me to discover they both are - and nothing has blown up yet ), and each bridged channel runs one sub.

This is fine, and it makes the right noise when wound up - but at low levels I can't get enough bass from the subs to really even know they are there - even with every bass control wound fully up, and the input level on max on the amp. So, I'd like to put a pre-amp in-line with the amp to boost the signal so the subs should be more audible. I'm assuming that doubling the level would suffice as a max gain? And while I am doing this, I'd like to add some sort of low-pass filter to remove unwanted high frequencies (the amp has a low pass filter for one channel, but not the other).

So, I went looking and found lots of pre-amp circuits... but they all need +/-15VDC or so... which is a problem in a car without a DC-DC converter. I've had a look around on here and discovered the LM4562 which I could possibly use with a simple voltage divider and run it on +/-6ish volts.

I would love some input on design and problems I will face - will this IC work? Will the voltage fluctuation from the car supply be a problem? Can I fix that with - say - 5.1V zeners and run the opamp on +/-5.1V? Could I include an adjustable cutoff for the low-pass? And does anyone have a suitable circuit I could use as a basis for the design (or one already made?). As I said, I am a long time out of electronics as a hobby, but keen to get back in... and although the brain is a little rusty, its keen to try

Cheers in advance for any help

Steve
Earthquake City (Christchurch, New Zealand)
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Old 17th June 2011, 03:27 PM   #2
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single supply subwoofer preamp with low pass, works on a single supply..

here you go ...

Car Subwoofer Driver - RED - Page142
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Old 17th June 2011, 05:52 PM   #3
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Location: Louisiana
That circuit doesn't have any way to isolate the audio ground from the chassis ground which is likely to cause engine noise unless it's used with a ground loop isolator.

15 can probably be taken from within the sub amp if it has a switching power supply.

What model is the sony amp?
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Old 17th June 2011, 08:51 PM   #4
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Yes it doesn't have isolation, will cause whine noise, you need to isolate both via an 1:1 transformer.. I've used a 1.1Kohm transformer to isolate..works flawlessly..
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Old 18th June 2011, 02:32 AM   #5
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Quote:
Originally Posted by dragonxwas View Post
single supply subwoofer preamp with low pass, works on a single supply..

here you go ...

Car Subwoofer Driver - RED - Page142
Many thanks for that circuit... I'll have a crack at it.

Quote:
Originally Posted by Perry Babin View Post
That circuit doesn't have any way to isolate the audio ground from the chassis ground which is likely to cause engine noise unless it's used with a ground loop isolator.
I've got a 600R:600R isolating tx here somewhere - I'll try using that.

Quote:
Originally Posted by Perry Babin View Post
15 can probably be taken from within the sub amp if it has a switching power supply.

What model is the sony amp?
I totally didn't even think about that... I could probably even build the booster into the amp itself (although I was going to put it in the dash so I could adjust it at will.

Thanks for the ideas guys, I'll give it a go...

Steve
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