# Setting amp crossover/gain and head unit RCA output



## Whitworth (Aug 11, 2011)

It seems theres a wealth of information and knowledgeable people here, so I guessed it would be a good place to ask my question...

I've recently bought a Mk3 Golf to use as a daily and have started to upgrade the audio. At the moment its running factory door and dash speakers, a sony MEX-BT3900U head unit and a Mutant MTSBP12 bass pack (not great I know).

The amp on the bass pack is a single channel one - I was wondering how you set this up with the sub. It seems you can change crossover frequency and the gain on the amp.

If I understand what i've read correctly, the crossover determines the frequency cut off, so if I set it at say 50Hz then nothing above that frequency will be output? If it were higher then how would the characteristic of the sound change? Also, what is considered to be the optimum crossover frequency? I believe the sub has a frequency response of 28Hz - 400Hz.

Moving on to the head unit... It has Front and Rear/Sub RCA outputs. How is the signal likely to change when the output is changed from Rear to Sub? Does it change the head units crossover freqency for these outputs? The installation manual for the head unit //pdf.crse.com/manuals/4199773111.pdf) says to select Sub-OUT for a subwoofer or Rear-OUT for a power amplifier... I assume you still use SUB-OUT even though its going through an amp? I've read online about setting the amp gain with a 50Hz sine wave and multimeter - how does the gain you can apply with your head unit effect this - does the gain just up the RCA output voltage?

Is it viable to set a crossover frequency with a 50Hz sine wave recording and turn the crossover from 0Hz up until the amp only just starts to reproduce the wave?

Sorry for such a lot of questions in one go, I'm just quite keen to understand everything properly. Thanks in advance


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## minbari (Mar 3, 2011)

welcome to the party 

you are right the crossover set the freq that the sub will play. most crossovers are 12dB/oct meaning it will not instantly stop playing right at 50 hz, but that is where the output starts going down. IMHO, 50 hz is WAY too low. set it for about 75-90 hz depending on how well it plays. do not use the 0hz turn up method you mentioned. you will hear the sub WELL before you hit 50 hz on the crossover and it will be set WAY to low.
for setting gain, if you are technically savy, then go through the process of setting the gain with a test signal and multimeter. if not, then read in your HU manual how much voltage the RCAs put out and set the gain for that in the amplifier. you can make small adjustment later to suit your tastes and blend in with the rest of the system.

as for your HU. if it has a sub output, then by all means use it. does the HU have a crossover in it as well or does it simple have the ability to change sub output level? if it just has a level setting, then it is just there to help adjust bass to suit your tastes. generally I leave it set to the lowest setting. (too high a setting will just make distortion)


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## jcollin76 (Oct 26, 2010)

Start with using the sub output from the hu. The stub output is non fading, meaning when you adjust the fade from front to rear, the sub output will not change.

A good place to Start with a sub crossover (on amp) is 80Hz. This can be tuned up, or down, depending on your taste and how it sounds in your vehicle. At around 100Hz, the sub will start to be localized pulling your stage backwards. As you progress up in Hz, your sub will start to reproduces midbass frequencies. This could sound 'muddy', or okay depending on the sub and setup of your frontstage.

As far a setting your gain, I would just use your ears and an appropriate test tone. Adjust your gain slowly starting all the way down, up until you start to hear distortion... Then back it down a little. Also make sure all bass boost settings are off, or turned down during setting. Adding a little boost after for your taste is okay, just not much. Keep in mind too much boost can also push into clipping sooner.

Have your hu volume At roughly 75% max volume. This is a guess for when your source might start to clip. It will also give you some head room for poorly recorded material.

Hope this helps some...


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## Whitworth (Aug 11, 2011)

Thank you for your replies. I think the head unit just has the ability to change level, not crossover.

You say aim for 75-90 hz... how can I actually measure the crossover frequency at the output of the amp? The dial on it is useless and doesn't have any markings.

What voltage should I be aiming for with the multmeter method? It doesn't say in the manual //pdf.crse.com/manuals/4199773111.pdf) the output.


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## minbari (Mar 3, 2011)

wow, it really doesn't say, weird. I would aim for setting the gain somewhere in the 2-4V range.

without a o-scope it is really, just best guess.

you can do it if you have a test cd with freqs from 20-100hz and a multimeter.
if you want to set it for, say, 80 hz. turn the crossover all the way up. then play a 80hz test signal. 
measure the signal level with multimeter (set on AC of course)
take that number and multiply by .707
turn down the crossover until it reads that voltage number for the 80 hz test track. should be close.


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## Whitworth (Aug 11, 2011)

I've got a good multimeter so I could do that. Whats the significance of the number 0.707, what bit of maths does it come from?

Thanks


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## minbari (Mar 3, 2011)

Whitworth said:


> I've got a good multimeter so I could do that. Whats the significance of the number 0.707, what bit of maths does it come from?
> 
> Thanks


its the 3dB down point. I can dig up the formula if you want


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## Whitworth (Aug 11, 2011)

I think I see how it works... by turning the crossover down from say 200Hz all I'm doing is translating the low pass filter curve in the negative X direction? This means that -3 dB is where the filter really starts to take effect and will be in position over 80Hz. This assumes the crossover filter in my amp is a butterworth one though right?


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## Whitworth (Aug 11, 2011)

Also, is the relationship between dB and voltage out of the amp linear?


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## minbari (Mar 3, 2011)

these are all good questions. that is why i said, it will be close. 

you are right on the relationship of the -3db vs freq. since you are playing an 80hz wave (in this example) once you start to see the voltage go down you are getting close to the cutoff freq. I just chose -3db because it is the half power point and will be closer to the tuned point. 

as for amplifier linearity, your guess is as good as mine with out a frequency analyzer. no amplifier is perfectly linear, but I think we can assume it is linear enough. also remember this is starting point. once you have set where you think it will sound good, mark it on the dial. then if you want a little higher bass, little lower bass, you can adjust it. but at least you will have a good reference to put it back later. your own cars response, speaker response, etc will determine it in the end. dont be afraid to play with it.


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## minbari (Mar 3, 2011)

if you care, this is where the .707 comes from

Frequency Response


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

Personally, a DMM isn't my ears... so I don't set up with one.... 

I prefer 0 or -5db test tones... 

60Hz and 1kHz for a 2way system.. 

Set your crossover, exact is NOT necessary.. 

All gains at zero, all boosts/eq/loud ect off/zero... 

unplug the RCAs from the sub amp.. play the 1khz tone through the mid/high... 

Increase volume on the HU until a distinct change is herd in the tone... back off, come up to that point, back off a little and use as "HU max"

Now, play the same tone at HU max, move to the amp, bring the gain up until you hear a distinct change in sound, back off, hit the mark again, back off slightly and consider this AMP max 

Now, unplug the RCAs from that amp and plug the sub amp RCA back and run the same procedure for the 60hz tone on the subs... 

THIS GETS YOU IN THE BALLPARK.... let's say 1-2nd base... lol... 

Now it's time to start listening to music you know well and make adjustments...


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## Whitworth (Aug 11, 2011)

Thank you both  I've got it sorted now and can make some tweaks without worrying about damaging anything. Learning lots already


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## minbari (Mar 3, 2011)

good to hear it..................


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## PottersField (Mar 18, 2011)

I don't mean to thread jack but I have a related-ish question regarding setting the gains that may even help the OP. Maybe, maybe not 

So I've been running my components on the head unit up until today and this afternoon I'm wrapping up the wiring and switching them over to the amp. I know that at around 45 on the head unit I start to hear a bit of distortion. Is it safe to say that I can skip the test tones (I don't have a test CD), use that number as my max for the head unit and then set my amp gains off of that?


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## minbari (Mar 3, 2011)

Mr. T. said:


> I don't mean to thread jack but I have a related-ish question regarding setting the gains that may even help the OP. Maybe, maybe not
> 
> So I've been running my components on the head unit up until today and this afternoon I'm wrapping up the wiring and switching them over to the amp. I know that at around 45 on the head unit I start to hear a bit of distortion. Is it safe to say that I can skip the test tones (I don't have a test CD), use that number as my max for the head unit and then set my amp gains off of that?


you were hearing distortion on your HU when it had the speakers connected to the internal amp on the HU, right? that has nothing to do with where the RCAs will clip (or distort)


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## PottersField (Mar 18, 2011)

Correct. I don't have a DMM or any of the fancy **** the pro's use. I wish I did but I have to rely on my ears


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## minbari (Mar 3, 2011)

well I have found that 80% of the time this works.

1) turn up HU to 2/3-3/4 max volume.
2) play test tone CD or music with alot of dynamic content.
3) turn up gains until they start to distort then turn it back a bit.

always do this setup with bass boost off.


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