# Preouts are cleaner than speaker level signals



## Maylar (Dec 6, 2012)

Conventional wisdom says to use RCA preouts to drive our amps because they're "cleaner" than speaker outputs. In the research I've done on this I have not been able to find quanitative evidence to support that statement, and I question the wisdom behind it.

Modern bridged HU speaker outputs are differential, high voltage, and have very low source impedance. In terms of noise immunity they're far superior to preouts. You also get ground isolation and freedom from the dreaded ground loop.

So then what makes preouts "cleaner"? Distortion? HU preouts are rated for some max voltage, but I never see THD numbers in the specs. CEA compliant HUs show xx watts at 1% THD, but what is the THD if you back down a couple clicks from max? And what is it if you run no load and only use them as signal lines? Nobody knows.

While reading specs on 12V chip amps I found one that said 25W @ 5% THD, 18W @ 1% and 10W @ 0.15%THD, so it seems that if you reduce the load you also reduce distortion. Why should we shy away from using them as signal sources?

Some day I'll own a distortion meter and check it for myself. Meanwhile I'm feeding my front stage with speaker level inputs because it was convenient wiring.


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## ChrisB (Jul 3, 2008)

You are missing the other side of the spectrum though. Many times, the built-in amplifiers, while more powerful, are rated to reproduce that signal over the 20 Hz to 20 kHz spectrum at + or - 3 dB. That is a HUGE swing whereas the pre-amp voltage is generally a tighter tolerance across the 20 Hz to 20 kHz audible range.


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## ATOMICTECH62 (Jan 24, 2009)

The THD on the op amps for the RCA's are around 0.009 to 0.000003%.The frequency response is normally about 1hz to 200,000hz.Signal to noise ratio is on the order of 100 to 110 db.Slew rates on the order of 5 to 100 volts per micro second,and most have a unity gain band width of 10-100 megahertz.
Head units use all different models of op amps so the specs will vary from unit to unit.
Most can drive loads down to 600 ohms at 6 volts.
There are no power IC's that can compete with high end op amps.
Check out the LM4562 on alldatasheet for one example.


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## glastron (Jan 18, 2009)

cajunner said:


> I think the point is that we *can* use the speaker outputs without serious issues, whenever the solution is more difficult to go the RCA way.
> 
> It's done in home audio often as well, most powered subs have speaker-level inputs along with their RCA jacks.
> 
> A mostly clean signal is still "good enough" for most people's hearing requirements.


Distortion in the sub frequencies is much less noticeable than the rest of the spectrum though. The 70% BL that many reputable manufacturers rate xmax at is around the point where they hit 10% distortion.


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## Maylar (Dec 6, 2012)

ATOMICTECH62 said:


> The THD on the op amps for the RCA's are around 0.009 to 0.000003%.The frequency response is normally about 1hz to 200,000hz.Signal to noise ratio is on the order of 100 to 110 db.Slew rates on the order of 5 to 100 volts per micro second,and most have a unity gain band width of 10-100 megahertz.
> Head units use all different models of op amps so the specs will vary from unit to unit.
> Most can drive loads down to 600 ohms at 6 volts.
> There are no power IC's that can compete with high end op amps.
> Check out the LM4562 on alldatasheet for one example.


I totally agree that the specs on preout driver opamps are way better than a chip amp. But the final result is as much dependant on circuit topography and the upstream signals as the chip itself. And it's odd to me that HU specs don't show THD on the preouts, even on the high end models I've checked.

Point taken, however. Thanks for the input.


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## Sine Swept (Sep 3, 2010)

I use speaker level inputs for my front stage - noise floor is buried in the floor. I didn't do it on purpose though. I had an unfortunate incident with a live speaker wire to dead short. It ended up frying a transistor in my head unit (The rear preouts do however still work). I never had the heart to tear it out for repair, so I made do with what still does work. I run the source unit flat and volume at 27-29 out of 35 then there is still some gain adjustment in my hi/lo adapter. If there is music content I am missing, I certainly can't tell its missing.


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## dobslob (Sep 19, 2011)

There are pros and cons to using speaker level as signal input to a system. If you are using an aftermarket head unit, you will likely find that the speaker outputs clip much earlier on the numerical volume level than the RCAs will. This simply means that you have to know the limits to where you can adjust the volume. On lower priced head units the RCAs will often be under 2 volts. While this may be enough to drive your system to full output, it can leave you with a signal that is very susceptible to induced noise. Meanwhile the speaker level will be much higher and much less likely to allow noise into the system. If your head unit has a clean 4-5 volts on the RCAs I would recommend using that as your signal, if not I would look into using the speaker level outputs.

In many cases speaker level is your only option while integrating into a factory system, so there are very capable signal processors available to take that signal and manipulate into a fantastic signal to send to your amplifiers. Currently, 65% of the new cars in Europe have head units that cannot be replaced, so integration has become absolutely vital, and it can be done with fantastic results.


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

dobslob said:


> There are pros and cons to using speaker level as signal input to a system. If you are using an aftermarket head unit, you will likely find that the speaker outputs clip much earlier on the numerical volume level than the RCAs will. This simply means that you have to know the limits to where you can adjust the volume. On lower priced head units the RCAs will often be under 2 volts. While this may be enough to drive your system to full output, it can leave you with a signal that is very susceptible to induced noise. Meanwhile the speaker level will be much higher and much less likely to allow noise into the system. If your head unit has a clean 4-5 volts on the RCAs I would recommend using that as your signal, if not I would look into using the speaker level outputs.
> 
> In many cases speaker level is your only option while integrating into a factory system, so there are very capable signal processors available to take that signal and manipulate into a fantastic signal to send to your amplifiers. Currently, 65% of the new cars in Europe have head units that cannot be replaced, so integration has become absolutely vital, and it can be done with fantastic results.


So you understand that EVEN in a 4-5v "system" you aren't even coming CLOSE to 4-5v untill the very end of the volume range... right... 

So most times you're running milivolts


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## GreggG (Jan 1, 2013)

Hi guys, I'm getting ready to add a powered sub in my 2012 Chevy Volt. I've read a few forums trying to gain as much knowledge as possible before I actually make the connections. The Volt has a Bose system, and the bass is weak. Anyway I have read various things, this thread included. On another thread on this forum I got some very good feedback that was helpful. I did find another thought from someone on a Mazda forum regarding the Bose systems. I'm posting it below and I want to know if what this guy is saying makes sense, as far as hooking the hi level Bose amp outs to my amplified sub low input. I don't wish to blow anything and since you guys are obviously very much in the know, I would appreciate your thoughts. It seems to kind of play into what you guys are talking about.

Anyway, here it is:
"The first go was a total letdown. Everything, routed, and hidden, yet I barely got any “thump” out of my Bazooka. Based on my RX-7 install of the identical tube, I KNEW it could put out way more than I was getting. Finally, after a little more research on the Bazooka site, I found a ray of hope… a FAQ. The page can be found at: http://www.bazooka.com/tech/faqs/Amp...ass Tubes/24 and reads as follows:

“Q. How do I add an amplified Bazooka to my Nissan/Delco Bose system?

DON'T PANIC! This is easily accomplished with any Amplified Bazooka.
What makes this application a little tricky is that Bose uses low impedance speakers (0.5 ohms) on their amplifiers (located inside the enclosure of each speaker) that use relatively small amounts of power to generate decent output volume from their speakers.
Also the signal from the Bose head unit to their amplifiers is not what one would typically considered speaker level or line level. It actually falls below what is normally considered to be line level. This makes the signal a little difficult to use to drive the inputs of most amplifiers
on the market.
The output from the Bose amplifiers however, is in the 3 Volt range. That puts it at the very bottom of the Amplified Bazooka speaker level input sensitivity which if used, may not leave you with much adjustment on the LEVEL control.
To avoid this potential problem we recommend that you drive the low level inputs of the Amplified Bazooka with the high level outputs of the Bose amplifiers.
This is accomplished by cutting the RCA connectors off of the Amplified Bazooka's low level input leads, and using the Blue wires as the positive leads and the White wires as negative leads. Tie into the high level leads of one right and one left Bose speaker. These high level
leads are located inside each enclosure just behind the speaker. Make your connections to these wires and run them out of each enclosure to the Amplified Bazooka inputs.
This configuration will allow you to have much greater control over the bass level adjustment and still take full advantage of the Amplified Bazooka's balanced input circuitry.”

I figured since my 3 has a Bose that the same thing could possibly work. But cut the wires??? I said a small prayer, took a deep breath, and grabbed the cutters. Two snips later I was ready to give it a whirl.
It worked like a charm! Nice booming bass like it should be."

Well that's it, let me know your thoughts. Thanks in advance.


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## blazeplacid (May 19, 2008)

1. you could of easily taped into the wires and not had to cut anything.

2. make your own post??


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## 14642 (May 19, 2008)

Maylar said:


> Conventional wisdom says to use RCA preouts to drive our amps because they're "cleaner" than speaker outputs. In the research I've done on this I have not been able to find quanitative evidence to support that statement, and I question the wisdom behind it.
> 
> Modern bridged HU speaker outputs are differential, high voltage, and have very low source impedance. In terms of noise immunity they're far superior to preouts. You also get ground isolation and freedom from the dreaded ground loop.
> 
> ...


If only head unit volume controls were more often scaled in software so that they don't clip, the benefits of the isolation from ground and the higher voltage and lower output impedance would be far superior to the crapy RCA connections we all seem to love. Furthermore, there would be no more market for 300 dollar RCA cords.


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## chad (Jun 30, 2005)

bingo


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## dobslob (Sep 19, 2011)

Aaron'z 2.5RS/WRX said:


> So you understand that EVEN in a 4-5v "system" you aren't even coming CLOSE to 4-5v untill the very end of the volume range... right...
> 
> So most times you're running milivolts


You are absolutely correct, although most people are not likely going to be seeing milivolts "most" of the time. The scale is definitely weighted pretty heavily on the high end of the scale.

This is why we try to get as many people as possible to use the DRC when it comes to using our processors.


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