I have the base (non-Bose) audio system, and was wondering if I need to use LOCs if I am installing an amp with speaker-level inputs?

The stock HU sends differential signals to the speakers, will this work on standard amps with speaker-level inputs?

Thanks in advance...

 

Nope, you're good to go.

 

I have been researching the forum for a few days now. The sticky does indicate that you need a LOC for stock HU, but there are others who indicate that it is not required if you have an amp with speaker-level inputs. This has gotten me quite confused.

Anyone out there who has connected the speaker wires on their non-Bose HU directly to an amp that accepts speaker-level inputs? Would love to hear your experience.

 

Edit:

Okay, yeah. it looks like you will still need LOCs to prevent the amp from overloading the HU.

 

I have used my MTX amp which has speaker level inputs with the speaker level signals from the Bose amp.

I want to emphasize one thing and this is really important - at least 99% of speaker levels signals from headunits are differential. There is absolutely no reason not to think the same for the Non-Bose HU.

I don't have any direct evidence since I don't actually own the non-Bose HU, but hopefully the following will help convince you.

1. One of the most expensive things in a piece of electronics is the power supply. For aftermarket amps that need high voltages for high wattages, they need some sort of switching power supply to increase the rail voltages.

HU's don't have seperate power supplies. They use the standard 12V-14.4V battery voltage and then they "cheat" to boost the voltage/wattage. HU's use a technique called Bridge-Tied-Loads (or BTL for short). In such a system the "+" speaker signal has 0-12V signal on it and the "-" speaker signal has an opposite phase 0-12V signal on it. (Please note that there are no negative voltages at this point). The speaker itself is the summing junction. For loud positive music signals the "+" terminal of the speaker will have as much as 12V on it while the "-" terminal will have closer to zero. For loud negative music signals the "-" terminal will have nearly 12V on it while the "+" terminal will have almost "0". The net peak-to-peak voltage change is 24V from a supply voltage of only 12V!

This is also why you won't find any legitimate HU's that advertise more than 25 Watts RMS. For a sine wave, 24 Vp-p is about 8.5 Vrms using simple ohms law we have 8.5*8.5/4 = 18 Watts RMS (the power can go to close to 25Watts RMS when the vehicle is on and the battery voltage is higher.)

So to get build a HU that does more than 25 Watts RMS, you have to create some form of DC-DC switching power supply. That is expensive so manufacture's don't do it - so the output is thus capped to 25W RMS.

2. By mistake, one forum member was given a non-Bose replacement head-unit for his Bose system. So what happened when he plugged in his non-Bose headunit to his Bose system? It worked fine.

Why? Because normally the signals in/out of a the Bose AMP are differential "speaker level" signals and logically so are the non-Bose HU signals. I posted a picture of an oscilloscope capture of the signals going into the Bose AMP a while back (Here it is I hope it shows up, this is for the "+" signal note the roughly 4V DC offset and the plenty of signal to work with).


Bottom-line the signals are differiental with a DC offset just like speaker level signals. That is why the non-Bose HU worked in the forum member's Bose system.

3. If you don't believe me, you can test your HU yourself.

With a make the following measurements.

1. DC voltage between speaker "+" and chasis ground at low volume. The value should be about 6 - 7 Volts
2. DC voltage between speaker "-" and chasis ground. The value should also be about 6-7 Volts.
3. AC voltage between speaker "+" and chasis ground with a given loud piece music (perferably a test sine wave) Record that value here: _____
4. AC voltage between speaker "+" and speaker "-" with the same music. That voltage should be about twice that of the value measured in number 3.

If all of those measure the way they should you, it points to clear conclusion that you HU is BTL or differential.



I make these comments because they are important when it comes time to integrate an aftermarket system into your RX-8. The speaker-level inputs on your amplifier are designed for these type inputs wether it be the Bose signals before the amp, after amp or from the non-Bose HU, they are all the same type of signal. Now that doesn't mean you will get flawless results from any amplifier with speaker level inputs (some amps just suck) but you should get better results than from a LOC which will likely distort the lower frequencies (somewhat) or reduce the highs (somewhat).

-Mr. Wigggles

 

Thanks to all that replied. This is great info. It is much appreciated.

I have received my amp and will attempt to install it this weekend. Will post the results after the install.

 

I have just finished the install and all went fine. The setup worked great without LOCs. The amp (Polk/Momo C400.4) has speaker-level inputs of course.

I am having problems with noise, which I am having a hard time debugging. I had even re-routed the wiring such that power is on one side of the car and all speaker cables are on the other. What is worse is that even with the HU unit turned off, I still get noise from the speakers. (Remote turn-on is connected to the 12V accessory outlet.)

I will try to work on this more through the week, but it getting quite frustrating.

 

brtzrx8,

If you are getting noise then everything didn't go great. Is it alternator whine? Does it go up and down with frequency as you rev your engine?

Your likely cause is your amplifier ground. Make sure your ground point surface is large and sanded/filed/polished to a nice shine before you install the ground cable. Also gold-plated spade or ring terminals work very well. The rear seat belt anchor is an excellent place for a ground (make sure you torque the bolt very well when you reinstall it.)

Good luck,

-Mr. Wigggles

 

The noise is quite wierd. It does NOT vary with engine rev. It is just background static only noticeable at low volumes or when the HU is turned off (amp still on). It goes away if I disconnect the speaker-level inputs. Once I turn up the volume beyond 3 I can't hear it, or it gets drowned out by the music.

The noise also increases with the gain of the amp. I suspect this is noise coming from the speaker outputs from the HU for some reason.

I will try connecting the ground directly to the battery to see if this goes away.

Best regards.

Brett

 

More than likely this is a noise that is being induced through the amps built in high level inputs. MrWigggles and I agree and disagree sometimes but in the end the results are usually really good. Where I disagree with him this time is the bolt location for a ground point. This is one of the spot where a nut is spot welded onto the unibody of the vehicle for most cars, as such it is also a point where there can be a buildup of resistance (this is bad). Ensure that it is a good ground (and it may very well be - so I can agree here as well), by measuring the ground return resistance as shown in the grounding sticky.

 

That sounds like background noise from the actual speaker level signals (i.e. inherent noise).

Is this a sub install or a full system? Because if it is a full system, then your solution is actually somewhat obvious - turn down your amp's gain. For instance, if you never listen to music above "15" then turn down your amp's gain such that volume you used to get a "15" now comes at "30". You want to use the full dynamic range of the system. By using more signal and less gain, the noise floor becomes audibly lower. This is one of the reasons "4Volt" HUs became popular, they have about the same noise floor as other 1Volt HU's but they have 4 times more signal so less gain at the amp is needed and therefore the noise gets reduced by a factor of 4.

Good luck.

BTW, I don't know if the seat anchor is the best place on the RX-8 or not. I did not do my amp install on my RX-8 (it is quite extensive and beyond my expertise.) However, on my brother's Civic as well as many of my other previous cars, the seat-belt bolt has worked great. On the Civic it goes right into the frame. If the seatbelt bolt doesn't look like a good option, then you can also grind down the paint at any sheet metal point and do a self-tapping screw.

-Mr. Wigggles

 

You are right, turning down the gain does help. One question... Does turning down gains on amps mean that I am not using the full amplification power of the amp? (Sorry, I am a Mech Engineer by training.)

One last issue is that I get interference when I close the doors and the interior lights come on. It also buzzes when the lights start to dim. What is even wierder is that if either front or rear speaker inputs are removed, the sound goes away. I am using a y cable to hook the front spear inputs to both the front and rear channels of the amp. There must be some sort of interference between the two inputs, but I can't figure out what...

Best regards.

Brett

 

No, an amp makes the same amount of power regardless of where the gain is set. What did you wire the amplfiers remote on lead to in the car?

 

And where did you ground the amp?

 

I connected the remote turn-on to the 12V outlet in the back (as was described on the forum). On hindsight, I should have tapped something from the HU, since I already had it open.

Best regards.

Brett

 

brtz,

Sounds like you are now at an acceptable level of performance. If you can't hear system noise during normal driving or during quiter passages of music, then you are in good shape. Remember use plenty of HU volume and reduce the gain. JL AUDIO usually recommends setting the HU to "3/4" volume before doing adjustments. For the RX-8 I would say that level would be between 27-30 for you loudess music.

As far as the little noise sources you hear now and again when you operate controls, those things happen. In my system, I get some very minor noise when I adjust the driver's power mirror, but not the passenger's. Don't ask me why.

-Mr. Wigggles

 

Thank you all for the great input. It has been most helpful. I grounded the amp to a bolt I found in the trunk. There were two on the shock and another with some factory wiring grounded to it. I did not noticed a difference (electrically) with either so I grounded it to the factory ground point.

Here's where it gets interesting. When I drove my car today, I realized that I DID indeed have alternator whine. In a fit of frustration, I dropped by the local audio store and picked up a Audio Control LC6. I have just finished the install and the whine and interference is gone. There is still some background noise if the levels on the LC6 are turned up too high, but it works fine once it is turned down.

I am not sure that this tells me, but it is an interesting close to this thread. Does one technically need LOCs for an amp install? I believe the answer is "no", so long as your amp has speaker level inputs.

HOWEVER, what a high-end LOC could potentially do is reduce noise from interference, or at least that's what this experience tells me. Whether this is because line-level outputs are less susceptible to interference or there is some noise filtering circuitry, that is beyond me... ....

Will post some of my other findings during my install so that others may benefit.

Thanks a bunch, you guys have been a tremendous help.

Brett