A few months back I added an amp+sub combo to my '06 MCS (stock, boost sound system). Over time I noticed that the amount of bass varied – it never completely cut out, but got so quiet that it was inaudible unless you had your ear on the sub. This all happened seemingly at random, with no obvious cause + effect. Sometimes it'd be earth-pounding (as a 500w RMS amp + 12" 500w RMS sub should be), and sometimes... wimpy.

A few days ago I asked one of my audiophile friends about this. Today, he gets back to me with an idea. A BMW friend of his, who works at Bavarian Sound Werks (www.bavariansoundwerks.com) said that certain BMWs have "balanced output" headunits.

This was explained to me as follows: Normally, the sound (sine waves) travel through the positive wire, with the negative simply being a ground. However, with a balanced output HU, the positive still has the sine wave, but the negative has a mirror image, "180 degrees off sine wave," and this is all used for noise cancelation purposes.

Now, if you have a balanced output HU, but an amp that cannot recieve balanced input, then you'll get variable sound levels. Additionally, it can fry the amp. I'm just using a cheap and reliable Profile amp.

My first question is... Can anyone confirm all of this? And secondly, has anyone else had this problem? It doesn't seem to be a wiring issue, and it's not faulty equipment. I'm at a loss otherwise.

The solution is either a new amp or a new head unit. HU is obviously the way to go, but I dread finding one that'll look nice and play nice with steering wheel controls / etc.

 

If the amp has RCA level inputs as well as speaker level, you could try an external level converter - get one which is happy with balanced inputs. Less expensive than a new HU or amp.

 

Depends on how you have the amp wired. If you are using the speaker outputs then none of this matters as speaker signals are almost always balanced. Most adaptors and amps are designed to work with this right out of the box. The balanced output he was talking about would only matter with low level signals going from the head unit to a factory amp, and even then most cars do not use this type of setup.

Need to know how you have your amp wired for a better diagnosis.

 

if you have balenced ouputs but not inputs, a good system will give you half of what you ask for. The "-" line just goes to ground. (This is where the amp blowing can come from, but most units are protected from this).

But whatever the wiring issue, the fact that the behaviour changes "at random" lead me to believe you have a connection issue.

But I could be wrong here (god knows that happens pretty often). Try driving it with a iPod or the like and see if you have the same issue!

Matt

 

Most HUs have balanced speaker outputs. RCA jack outputs are, of course, unbalanced. Many line level converters take only the + speaker output, DC decouple it and attenuate the signal to come up with the line level output. Better still would be an active converter that takes the + and - outputs from the HU and use a differential-style op amp to convert the output to line level.

Conversely, most bigass amps are not balanced output, since they use a bipolar internal switciong supply to power the output stages. Balanced outputs tend only to be used where the supply voltage is limited (12 volts in a head unit, for example) and there is no room for the rather considerable size of a voltage converter/switching supply. In the case of the head unit amp, it is balanced simply to give the maximum power output into a 4 ohm load given a 12-14 volt power supply. By driving each amplifier output 180 degrees out of phase, the maximum voltage swing (in differential mode) would be as close to the 12-14 volt power supply as practical. If the speaker output would have one side at ground, the available output voltage swing would only be 6-7 volts, resulting in one-fourth total power than would be had with balanced/differential outputs (Ohm's law).

Balanced outputs in ths case is not done for noise, but power output. Balanced outputs are inherently noise suppressing, since any common mode noise is in the same phase on both outputs and is cancelled out (there is no 'differential' between them). Balanced line levels are used in noisy situations such as studios and live venues to combat line induced noise. They can also be used in cars, but are very rare, ironically.

 

Yes, so currently I am using a line level converter near the HU to go from the speaker level to RCA patch cable.

And I did recently buy a headphone -> RCA cord so that I could play the subwoofer directly with the iPod. Problem is, is that I haven't had an opportunity yet where it was playing very quietly thru the HU. So I haven't been able to determine whether the iPod set-up delivers full bore or has same problem.

And until I figure out problem, I'm not using sub.. just in case. So more help plz :P

 

So nobody can give me a qualitative yes or no whether the boost head unit sends out balanced sine waves?

Or actually, Greg S basically put this fear to rest – nearly all speaker level is balanced. I'm currently using a line level converter. And if my amp was going to blow, it would have blown by now.

I've been connecting and disconnecting almost everything lately, between autocrosses and hauling some winter tires around. I also replaced a fuse that I accidently blew, that went from battery -> amp, that was beginning to corrode sorta (old wiring kit). I have not noticed wild fluctuations in loudness of the sub lately, but am still on the look-out. Suppose it could have been simply some poor connection somewhere. I hate it when the most obvious answer is true... We'll see I guess. Nobody has any other suggestions?

.
Off the subject a bit, I was also wondering on the need for a capacitor.. How readily does the MINI provide high levels of wattage? I notice some distortion at higher levels, although the sub doesn't seem close to maximum travel..

 

just look at the voltages into the amp at the power in lug. If it's fluctuating with demand (put on something with big bass hits), then there's too much resistance in your power feeds and caps will help. If the voltage looks nice and steady, you're fine, and the distortion is from something else...

Matt

 

I think you just had a loose connection somewhere that was causing the problem. This is the most common reason for phantom, or occasional, problems with sound systems.

As for the need for a capacitor, an easy way to check is to turn your headlights, engine running, facing a wall then turn up your music and see if the headlights dim when the bass hits. If they do try it with the engine revved up, as this gives more power to the amp. If they still dim then yes you need a capacitor, or you need thicker power lines. You should have at least 4-ga line if you amp is a true 500 watt RMS amp, if you're running less than that then no capacitor will help as you would be starving the amps power supply from the beginning.

 

I'm using 6-gauge wire since the amp is so close to the battery.. Less than a foot all told... Still possible prob? I could snag some 4 off of a friend to see what happens. Hm.

 

6-ga should be fine for that. Is the ground wire the same, and where is it grounded? I'd just do the headlight test and then see what happens. If that is your only amp you really shouldn't have much of a problem with power draw.

 

if you're going to the trouble of installing a high end stereo, and are doing it yourself, get a volt meter! They can be had for cheap, and you get numbers out of them.... For example, I don't know if the headlight trick works for Xenons (so if you have them, use the high beams for the light test...).

Matt

 

i thought balanced outputs are 3 conductors.... +, -, and ground... like XLR
or tip/ring/sleeve 1/4" headphone jacks used for mono apps.

 

I have a voltmeter.. Needed one back in the days of cars that drained their batteries overnight. I'll check, been sorta busy. The ol' headlight thing doesn't show me much, although I do have xenons and I haven't look closely at how they act (don't act) with high beams on.

 

They are for low voltage applications like stereo interconnects where you can run a risk of interference and need a shield. for higher voltage applications like speaker outputs you do not need the shield as you almost never get interference at those levels so they only use the positive and negative.

 

Car off, voltage is 12.5 with no music, and with bass up drops to 12 or slightly lower.

With car running, voltage is 14 with no music, and varying wildly between 12.5 and 13v with bass up.

 

both larger gauge wires and caps! Also, for high current levels, I've found it's always a good idea to solder your lugs after you crimp them....

Matt

 

Definitely go to 4-ga wire for both power and ground, make sure you scrape away the paint where ever you do the ground, if it's not directly to the battery, and use either high quality screw lock ring terminals or if you are using crimp terminals then solder after crimping. I'd probably wait on the cap until after doing this just to see what happens, but plan on getting one.

 

Thanks for the responses guys. I have double-checked all of the connections, but still fairly certain that output is varying somewhat. I can snag some 4-gauge off of a friend and then get serious with the wiring. We'll see what happens!