Outputting from car audio head unit to home theater receiver

**mach5** · 05-26-2007, 11:40 PM

Okay, I know at this point everyone is scratching their heads going, "Why on earth?" I would explain the long and the short of why I need to connect a car audio head unit to a standard home theater receiver, but I really don't want to waste the time typing that info up.

So let's skip that part and get on to the how.

I'm running into two distinct problems running off of the RCA preamp outputs on the deck to any of the inputs on the receiver (VCR, CD, Tape, all give the same results).

The first is a 60hz hum. If it doesn't go away with a solution to the second problem, then I'll throw a ground loop isolator into the mix. But if it's a symptom of whatever is causing the second problem, I figured it was worth mentioning.

The second is extremely low audio levels. I have to turn the receiver's volume level up to "11" just to come close to matching the volume coming out of the head unit-driven speakers, and as a result, the audio signal is getting over-amplified and washed out.

I've hooked up a half a dozen other devices into this receiver, from iPods to PCs, on the same run of RCA cable, and there's no volume issue. So the problem is obviously with the output strength of the preamp outputs on this head unit.

I know someone else who has a similar configuration of components and is experiencing the same problems. He's somewhat resolved the levels issue with a line driver, but I haven't heard his setup and wonder about audio quality issues that might crop up by adding another stage of amplification.

I'm hoping for a solution that balances out sound quality and expense, but at this point, I'd just like to have some options to start investigating before I even worry about feasibility.

My basic knowledge of the differences in car audio and home audio lead me to believe the problem is related to the output voltage of the signal from the head unit. How much home audio line level voltage differs from car audio line level voltage, though, I have no idea.

**malkamus** · 05-27-2007, 01:03 AM

From what I have found searching, home receivers line out voltage is like .775 volts. Car receivers can be up to 8 volts. Possible that your preamp or whatever you are hooking up to is circuit protecting the input to save it from the high voltage it sensed from the car receiver. Try a cheapo 1 volt car stereo and see if the volume output is better.

**mach5** · 05-27-2007, 01:16 AM

Well, the line is going directly from the car stereo to the receiver. Are you suggesting the car stereo might be dropping the levels to protect the receiver, or the receiver itself is dropping the levels?

**malkamus** · 05-27-2007, 01:28 AM

I am a little confused now, Are you plugging this into a receiver that has a FM radio already in it? Or are you plugging it into a preamp to replace your receiver? The preamp of the home stereo would be the one tripping circuit protection. If it reads a high voltage it would think it was a surge and protect the circuitry. If you are plugging this into an all in one system where your preamp, amp, receiver is all one it may never work the way you want. If you are going into a preamp then like I said see what the preout voltage is on your car receiver and find someone with as low a voltage as you can find.
Remember car audio uses more line in voltage and lower ohms out because it has to make up for having to use low power to make music(from what I was taught, I may be wrong and someone more knowledgable can correct and inturn teach me correctly.) If you take a high voltage unit and plug it into a low votage unit andif it has builtin protection, it will cut the unit off and give you what you describe.

**mach5** · 05-27-2007, 01:40 AM

Basically I need to pipe unamplified audio (FM, XM, CD, etc) from the head unit to the receiver to put out over the attached speakers. The receiver is a Harmon Kardon 3250 if that helps at all.

I'm connecting the front RCA preamp outputs on the head unit to the receiver's RCA inputs. I've tried it on CD-in, VCR-in, etc, to the same result on each input.

Sorry if I wasn't clear about how things were being connected.

**malkamus** · 05-27-2007, 01:59 AM

PREAMPLIFIER SECTION
Signal-to-Noise Ratio (ref. 1 Volt, A-Wtd)
CD: 92dB
Video 83dB
Input Sensitivity/Impedance
Video, CD, Tape: 150mV/22kW
Tone Control Range
Bass @ 50Hz: ±10dB
Treble @ 10kHz: ±10dB

They are using a 1 volt signal to rate noise so I bet that the car unit has too high of a line voltage. What head unit are you trying to connect to the receiver. It is also an all in one unit, I have not seen anyone hook up a different receiver unit of any kind to one so I do not know if it is possible to even get another home receiver let alone a car unit to work through it. But first the car unit's voltage needs to be found out.

**mach5** · 05-27-2007, 02:02 AM

I'll look into that and get back with that info. I don't have the head unit's info handy.

Thanks for giving me somewhere to start!

**durwood** · 05-31-2007, 10:01 AM

Ground the case of your headunit to the case of the receiver and see if the 60Hz hum goes away.
YOur other problems sounds like a level matching problem although i still think the preamp should be enough to keep up with the HU amp driven speakers. Maybe try a line out converter on your speaker or high level ouputs instead.

**mach5** · 05-31-2007, 11:20 AM

Well, I checked, the Clarion head unit's preouts are 2v. It seems odd that the home receiver would have any issues with the output from this stereo. I'm going to try hooking the pre-outs into the line in on a computer and seeing if I have any issues with the signal there versus a comparison device. If the volume difference between the head unit output and, say, an iPod, is different depending on whether it's hooked up to the receiver or a computer, then we'll know there's something going on with the way the receiver is taking in the audio.

Durwood, what do you mean by a line out converter? And by high level outputs, do you mean the amplified outputs from the HU?

**durwood** · 05-31-2007, 12:20 PM

google "line output converter" or also refered to as a "LOC" and you will find plenty of info. It's basically a transformer that steps down or attenuates the high level (amplified speaker outptus) to low level (pre-amp level) and then matchs the impedance that makes it proper for running into an amplifier. They can come in all shapes an variety. I like the PAC ones. You can select different attenuation ratios and they are also adjustable. You can pick them up from any Bestbuy/Circuit city/etc car audio shop.

It's possible that your Clarion pre-amp signal level voltage is too low at volumes that work for the speakers connected to the headunits amplifer section. A headunit will not usually reach their max (a 2V signal in your case) with a pure 0db sinewave until it's maxed out. Car amps have a gain adjustment that allows you to match the level, a home amp/receiver you have to rely on the volume knob even though a gain knob on a car amp is not a volume knob.

Maybe disconnect the speakers connected to the HU and then turn up the volume on the HU to see if you can get more volume out of your receiver.