Audio and amps - building a preamp.

**claylong** · 03-22-2002, 11:54 AM

Since my last post mentioning my need for an amp and a preamp for my car, i have found something interesting. My car has a premium sound system, and to the contrary of what the dealership believes, has an external amp in the trunk. I have decided to try and run a signal from my computer (4 channels) to the stock amp, to the stock speakers. I have been very satisfied with the system, but now the problem of getting the amp to work with my sound card is upon me.

Ok, here it is. I think i need a preamp. I whipped out my hand ociliscope, and plugged the outs of my stock headunit (to be removed) to get an idea of the profile of the voltage going into the amp. (Because the ociliscope is so old, I was not able to determine EXACT numbers, but I believe these to be fairly accurate. ) The resulting pattern is as desscribed (do not know the terms for all of this) - With the head unit on, and no sound coming out (vol at min) the ocili- reads a flat .5v. When the radio is on, the wave sits at .5v, fluxuating up and down with higher amplitude corresponding to higher volume. At a realativly high volume (%75 of max for a stock unit) the amplitude from the center .5V is probably less than .08 volts.

I then hooked my computer's outs the in ociliscope and found soemthing a little different. The flat wave of the computer out appears to be sitting at 0v, with any sound or volume generating amplitudes just above and below 0v. The amplitude is very very small, and is difficult to measure. Less than .05 v.

The problem. I know that i need some sort of preamp to get the computer's audio signal to a higher level, in order to match with the input of the amp. BUT, how do i do this?? I am searching for a car preamp, but all i can seem to find are eq's w/ preamps, which the eq is unneded. (Sound card has nice eq). NOT TO MENTION, i do not even know if these preamps will input or output my needed levels.

The question. Is it possible to build a preamp for my application. I looked around various electronics sites, and found small IC audio amps and such, but what is really needed to get the ins and outs to match? Not just 1 IC per channel, right?

I know there are power gurus that can help me with this one... Just let me know what you think. Now i must go do the 'guru dance', in hopes one or more of you will read this and know what the hell i am talking about.

THANKS!!!!!
Clay

**Rob** · 03-22-2002, 01:25 PM

I don't think this is a PSU, but I'm willing to give you a hand, purely so I can be called a Guru.

Start with have a look at www.blackwidowaudio.com

They're a pretty good resource for such circuits.

I managed to find this one which does pretty much what you want by the look of things.

As you only want the pre amp stage you won't need IC3 & IC4 or ony of there supporting junk.

This will only solve the difference in output levels, to get around the .5v offset you should be able to use a ground isolator avalible from car audio stores or alternatively apply a voltage offset to IC2.

Well that's a starter for 10, as they say.

Rob (MP3CAR GURU)

**claylong** · 03-22-2002, 02:21 PM

Looks good so far. I will have to sit down and analyze the diagram, and the site you posted, in order for it to make sense. Thank you.

**Mav** · 03-23-2002, 02:29 PM

Those ic's are they 747 chips, just wondering,

Mav,

**claylong** · 03-25-2002, 01:44 PM

Anyone have any other ideas? I have seen VERY FEW preamps in my search, and most all of them have EQs. As mentioned, I do not need an EQ. ALTHOUGH, the more i think about it, the more i realize that i will need a volume controll. Now i am looking around for circuits w/ vol control.

Lemme know what you think.

**mp3z24** · 03-25-2002, 01:53 PM

you might want to check out a "headphone amp", they are quite common in the DIY scene. and most, if not all will have a volume control. one example out of many on the web.

also, take a look at www.national.com, they have some audio amplifier chips, and on their data sheets they shown several examples of configurations easily made with thier chips. included among the configurations are usually: fewest components, standard operation, as well as increased, or decreased gain.

just my $0.02

~mike

**mp3z24** · 03-25-2002, 02:05 PM

after a little more searching on the net:

DIY pre-/power amp combo

go the th left side, under projects. you will find the link for the combo amp.

might be of some use to you, but to me it looks a little scary, plus it is set up for AC, not DC. i will keep looking

~mike

edit:

pasive preamp
also, take a look at this web-board

**joeb0b** · 03-25-2002, 02:34 PM

hey clay, I'm curious what kinda car do you have? It may be important to determine what kind of premium stock sound you have.

-joebob

**claylong** · 03-25-2002, 02:54 PM

z24, the sites you gave me look good. I, like you, are scared of some of the stuff they are doing on that second site.

I have a 96 Chrysler LHS. I do not know much about the headunit or the amp, other than the amp has high freq and low freq outs, going to the tweeters and the larger speakers in the car. I also know what i mentioned above, regarding the voltages and stuff.

What else can i tell you?>

**Rob** · 03-25-2002, 03:12 PM

Originally posted by Mav
Those ic's are they 747 chips, just wondering,

Mav,

No they're OPA2604 chips, oppamps like 747 chips, but alot better quality, especially when it comes to audio.

Originally posted by claylong
z24, the sites you gave me look good. I, like you, are scared of some of the stuff they are doing on that second site.

If you look at it for awhile, and then hack off all the bit's you don't need (multiple inputs, bass boost, headphone amp) it's not too bad, only one chip and a handfull (depending on size of hand) of support components for mono, and X2 for stero.

Rob

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