Should i use a line driver for my carputer?

**amirharoun** · 07-02-2007, 08:45 PM

Hi, I am building my first carputer and I have a few questions about the audio. (I am using a non amplified sound card; signal amplitude about 0.3V)

1. Do i need to put a pre amplifier between the BASS output on the soundcard and the subwoofer amplifier?

2. if so, would the pre-amplified signal connect to the high level or the low level input of the subwoofer amplifier?

3. I built a circuit board to "pre amplify" signals from 0.3V to 2.5V; Low THD% sounds good overall.
My Speaker Amplifier can take an input of up to 6V.
Would my homemade circuit be sufficient or should i buy a professional line driver?

4. Does it make a difference in distortion if I am at the high end or low end of the allowable input signal (Range 0V to 6V)

I have searched many online resources for the last 3 days and experemented with various circuit boards and even went to the local car audio shop, no one seems to know!!!!!!!

**ironfinger** · 07-02-2007, 08:59 PM

1. no you don't HAVE to, but you can.
2. you connect it to the low level input.
3. 2.5v is average. Good enough for the BASS
4. It depends. Usually the higher the better, but there are diminishing returns as it will start to distort..perhaps even clip

**amirharoun** · 07-03-2007, 03:27 AM

I have been reading online that a pre-amplified signal should go to the high level input, because low level input will get damaged from too much voltage.
Is that true??

**ironfinger** · 07-03-2007, 04:16 AM

yeah if there is too much.. just stay at 6 or under and you should be fine.
Hi levels are for getting a signal from a speaker wire. The application is usually for people who have factory headunits without RCA outs.. So they connect their speaker wires coming from the head unit straight into the hi-levels inputs on the amp.. as you can imagine, the signal on those speakers wires are going to be high.

Usually the hi-level inputs on the amp will not give you the best quality, so you want to use the low-level inputs.

**gibroni** · 07-03-2007, 10:30 AM

Interesting topic. I was thinking about getting the audio control unit. My amp is old and I no longer have the manual so I can't check there to see what a safe voltage is to input. I guess we try it without first then see if it's needed.

**durwood** · 07-03-2007, 10:33 AM

Originally Posted by gibroni

Interesting topic. I was thinking about getting the audio control unit. My amp is old and I no longer have the manual so I can't check there to see what a safe voltage is to input. I guess we try it without first then see if it's needed.

Exactly!

line drivers are usually not needed unles the amp's input gain isn't sensitive enough to deliver full power with the amount of signal level injected into it. 99% of the time you do not need a line driver. Either way, if you could not properly match you amp gain to the signal level and acheive the output you desire then that is where a line driver comes into play.

**turbocad6** · 07-03-2007, 11:23 AM

another factor is how many amps you have on the line, 2.5v to one amp will be a whole lot better than 2.5v to 3 amps.

**durwood** · 07-03-2007, 11:30 AM

I don't know if I would agree with that. The input impedance on an amp is usually high enough it won't make any difference and voltage in parallel is still the same voltage level seen at all the amps connected.

**turbocad6** · 07-03-2007, 11:36 AM

hmm, this I won't argue as fact, but from my experience I've found this to be true, a bank of amps will require a higher line voltage if there all run off the same line is the impression I've always been under.

**durwood** · 07-03-2007, 11:58 AM

I won't argue with experience but I was just curious where that came from. I have never really ran a bank of amps so I don't know, but anything is possible if the source unit can't deliver the current if it's heavily loaded down. Similar if you split the signal a bunch of times. I have only ever used a 6 channel line driver to multiple amps.

I still stick with my statement of most of the time you won't need a line driver, but it does more than just increase the signal voltage

i.e.

-lower output impedance of the source = more current to input signal of amp
-clean up ground loops if it is properly designed (or add ground loop noise if it's improperly designed)
-can help lower the noise floor IF installed properly.