CactusGeorge

I replaced the Bose amps and speakers with a Soundstream amp and Focal speakers. The Soundstream was connected to the signals at the input of the Bose amp.

After a few weeks of operation, the system stopped working, and it seems likely that the HU outputs were compromised by connecting directly to the Soundstream inputs. The Soundstream Edge 5700 has floating ground compatible RCA-level inputs with adjustable sensitivity. It seems that RCA-level amps should be connected through a line output converter.

I just talked to David Navone about the FMS/Bose setup and how to tap into the system.

Unfortunately, it looks like taking the signal directly from the HU to the Soundstream amp probably over-taxed the HU outputs and caused them to fail
Anyone have recommendations about where to get the HU repaired, or where to buy a new one?

David recommends his N774V four channel converter tapped into the OUTPUT of the rear-deck Bose amp. His recommendation is based on:
1. The HU output is probably about 2 Volts, whereas the Bose amp is at about 10 Volts--this provides better S/N at the Soundstream.
2. The Bose amp does not apply any Bose-specific equalization because it's cheaper to keep the amp linear.

I know some sub installs tried tapping in before and after the Bose amp, and at least one person commented that the sound was better before the Bose. I don't know what to make of this conflicting info.

Rotarygod, Overload, Mr Wiggles, XeRo or any other technical/experienced types care to expand on the pros and cons of line converters and where they should be placed in the RX-8 audio chain?

CactusGeorge

Soundgate made the following line converter model recommendation:
"I would suggest the LOC4.4 for adding a new amplifier to this system. This
is the four channel version. Thanks CHET"

mdw33333

Cactus, do you have a pic of the harness that plugs into the back of the radio? I've been researching and I think I found an interface that will work with the Mazda Bose head unit. PAC (Pacific Accessories Corp.) makes a plug and play interface that allows you to plug in (at the HU) and tap 4 channel signal, and it provides a remote turn on lead.

I know you've had your HU out and I need to see the factory harness that plugs into it. PAC has not tried the interface on the RX8, but they told me if the plugs are the same a Mazda Bose head units in the past, then it will work.

Please post a pic if you have one, because I'd rather not pull my HU if i don't have to.

Thanks,
Mike

CactusGeorge

MDW, here's the pic...

CactusGeorge

From the "wire" side...

mdw33333

Cactus, I think this interface will work. That is the current standard Mazda radio plug, but it's configured for the Bose system. I'm going to order this interface and check it out. The part numbers needed are:

OEM1, ADDMAZ, and AMPRCA

Those three parts make up the interface.

EDIT:

After talking to the tech at PAC, I found out some different things. The harness that plugs into the Bose HU in the RX8 is not compatable with the harness listed above (ADDMAZ). I had the tech hop on this forum and look at Cactus's pic and he told me it was not a match. However their interface can still easily be used by hardwiring it in right at the plug. Cactus has made it easy, because he has identified the signal leads at the plug. Assuming his findings are correct, the OEM1 can be tied right in, and the interface will provide you with a clean 4 channel preout right there.

Also, to avoid "turn-on" problems or "turn-on thumps," we can use another interface called the TR4. We tie it into the Bose amp in the rear. By tapping the amplified signals to the rear speakers, and 12v hot and ground. This unit senses signals to the rear speakers from the Bose amp and provides a delayed soft turn-on for any amp we might add.

mdw33333

For those who aren't "getting" what we're discussing, this means that with these parts, you can now convert the Bose "preamp" signal into a usable "preamp" signal for use with any aftermarket amps or signal proccessors. Thus, no more line level converters! And using the TR4 (mentioned above), there is no need to tie into an ignition hot lead for amp turn on! IMO, this is great!

Tamas

iTrader: (1)

Sounds great...
Any chance that such a solution would work with the non-Bose system? I suppose yes, since the head unit is the same, but we have no Bose amp.

But I'm still struggling trying to understand what exactly needs to be done to:
- tap a real 'remote turn-on' wire for an amp
- get that real 'preamp' signal to drive an amp.

Could you please elaborate for someone who has no idea about electrical engineering? :p

mdw33333

Sure, first, we are taking the Bose preamp signal, tapping it, and converting it to a usable preamp signal for our own amp.

Second we are installing a module which senses signals going to the rear speakers and generates a 12v, 1 amp turn-on feed for our amp.

Many on this forum have installed amps using line level converters which tap in to the amplified signals at the rear speakers. That isn't the cleanest signal to tap, and line level converters are known for having signal loss.

Also, people are tapping a 12v feed that powers up when you turn on the ignition to turn their amp on. The Bose amp turns on by sensing a signal from the head unit, not with a 12v feed, so one isn't available to power up an added amp. We are "making" one by using this module. It's simple, this module senses audio signals from the head unit and it trips an internal relay which provides a 12v output to turn on our added amp.

Mods like these have been available for quite some time, but for some reason they've never been discussed on this forum (as far as I know). Most people just went the common easy route of the line level converter and ignition turn on, because they didn't know any better.

CactusGeorge

Just to clarify:

At this point, it is my belief that the "preamp" connections at the HU are differential in nature.

Although I don't have an oscilloscope to verify the point, it looks like the HU outputs are driven by NJM4556A op amps.

After clearing the issue with Soundstream, I connected these "preamp" (notice the quotes--I don't believe that these are the traditional preamp type outputs) to the aftermarket amp. End result, the HU outputs stopped working. I've verified that the Soundstream is working, so the failed HU is pretty much verified.

Moving forward, I would either use a David Navone or Soundgate line level converter to isolate the differential outputs from the Soundstream. -or- I would use only the positive signal from the HU and tie the shield connection to ground. (i.e. LF+ and Gnd).

I've asked OverLoad to take some measurements at the HU outputs on the assumption that he has access to an oscope, but the results are not in yet...

For me, it looks like I'll be buying a new HU

mdw33333

Hey Cactus, that's exactly what the PAC OEM1 does. It converts the differential preamp out puts of the Bose and isolates them (converts them) to usable preamp outputs.

CactusGeorge

MDW:

The OEM2 from PAC includes a line output converter and a remote turn on for the amp. Couldn't see much info on the web site about the OEM1, but if the "preamp" outputs from the HU are differential, that unit will need to isolate the signals from the aftermarket amp...

http://www.pac-audio.com/products/oem1.htm

CactusGeorge

MDW:

I think the PAC is marketing a line output converter in its OEM1 package.

If this is true, it would be the same principle as using a Navone or Soundgate LOC--but I've had zero feedback about how different LOC's compare in quality...

Peripheral also makes a remote turn on assembly that can be activated by a preamp signal (although having both the LOC and remote functions in one package can be very convenient.) Again, I have no idea how the quality of LOC's from various manufacturers compare...

CactusGeorge

More info on converters and installation:

I've noticed that many converters are housed in a plastic case, which can be a problem for low-level signals. If you're tapping into the output of the Bose amp, this is probably less of an issue.

AudioLink (http://www.crutchfield.com/S-z5JZW8...Group.asp?g=721) and SoundGate (http://www.logjamelectronics.com/sounlocbosli.html) both have converter models in aluminum cases.

Once I have my replacement HU tuner module, I'll do a bit of experimenting to see what sounds best.

For turning on the amp, companies like Peripheral (and PAC) sell a trigger device that supposedly generates an amp remote signal from a speaker output wire (Model #LVT2, available from SoundDomain...)

As a side note, to reduce noise in the aftermarket system, the grounding point for the amp is very important, and may require experimentation to find a low-noise ground path. As car accessories are turned on and off, the ground level at different points on the chasis can change, thereby causing noise.

CactusGeorge

Answer to a question about differential inputs:

In an aftermarket HU preamp signal, there is a + and a GND (shield).

The RX-8 FMS HU has a + and a - (not ground) signal that are 180 degrees out of phase--this is great for noise immunity, but aftermarket amps, in general, are not expecting this type of signal.

Something that I have not tried that may have good results, is to use the + signal from the FMS HU and ignore the - signal (so the center connection for the RCA cable is + and the shield is tied to GND).

Otherwise you have to use a line output convertor (or possibly speaker level inputs of an aftermarket amp) to utilize the differential signals.

OdinGuru

Hmmm... intersting to hear that the pre-amplified outputs on the Bose are differential. This actualy make a LOT of sense in the noisy car environment.

For those not familier with the concept, basicly the idea of differential outputs is that "noise" can be rejected. This is because noise picked up by both the + and - wires will be the "same" and the actual audio signal will be "opposite". So by subtracting the two the noise is canceled.

For use with standard inputs, the diffential signal will have to be converted back to a single ended (signal + ground) setup. Since the differential signal has inherrent noise rejection properties it would be best to do the conversion just before the amp input (after the long run) rather than near the head unit output. The best kind of cable to use for the run would be twisted pair, possibly with an additional exterior shield (which should only be connected at the receiver side). This should reuslt in much better quality than single ended RCA cables. To convert from differential back to single ended, either a solid state converter (like the OEM2 discussed above) or a 1:1 audio transformer can be used. The key here is getting high "common mode rejection", to maximise noise immunity, and low insertion losses/distortion.

Please, DON'T connect the - terminal to ground (outer part of an RCA jack). That basicly shorts one of the op amp outputs (unfortunatly as discoverd by the original poster). Connecting just the + to the center RCA pin probably will work (and leaving the - unconnected), as long as the amp input can handle a DC offset due to possibly different ground levels (which is likely the case). Though I would perfer to test this assumption before trying it. The biggest disadvantage of this setup is that it doesn't provide ANY of the noise immunity advantages that differential signaling provides. Also those with the Bose option should already have apropriate twisted pair cabling installed back to the trunk amp.

mdw33333

Well, I did some experimenting over the weekend. Unfortunately, it didn't go as well as I would have liked. The PAC OEM1 that I've been referring to is currently on back order through my supplier, so I attemped to use a similar piece made by Peripheral. It too supposedly has the ability to tie into "pre-amp" Bose signals or speaker level signals and convert them to usable preamp signals for an aftermarket amp. It also generates a 2 amp turn-on feed for powering up the amp.

I first tied this unit into the "pre-amp" inputs leading from the head unit into the Bose amp in the rear. I got a somewhat clean signal by doing so, but these leads were to low in voltage to allow my Peripheral converter to trip its 12V turn on. The converter I'm using has to see a input signal of at least 5V on the positve side of the inputs to generate a turn-on.

The second step I took was to tie in directly to the speaker level outputs of the Bose amp for the rear speakers. These leads did test over 5V on the positive side, actually they were at 6.5V or so. I could not tell any audible difference between this signal and the signal going into the Bose amp, and this signal was strong enough to generate the turn on from my converter.

All "seemed" good at this point as I thought I had a clean signal and a good turn-on lead for my new amp. I proceded to install my test system. It consisted of a single 10" Kicker L7 woofer and a Kicker 1200.1 amplifier. Some may ask why so much power for just a single sub? I've always believed in having "head room" when it comes to amplification. Not using an aftermarket head unit, I new the signal I was tapping wasn't best from a sound quality standpoint. So, I compensate by using plenty of power to keep my gains at a minimum.

I was sadly disappointed with things at this point. Like I said, during my tests I tapped signal before and after the Bose amp. Both signals seemed to come through my converter very weakly. Could it have been the converter? Definitly possible, however I wasn't sure, as the signal seemed to almost "fade" in and out at times. I couldn't quite pin point what the problem might have been, one thing's for sure, this Bose system isn't friendly to aftermarket upgrades. Another problem, was that the converter would turn-on the amp, but wouldn't turn it off. I would turn off the radio using the **** (leaving the ignition on), and the amp would stay on without a signal. This was definitely the fault of the converter, because as soon as the speaker signal stops the amp should shut off. Instead, it stayed on until i turned off the ignition. This I didn't understand. Anyway, still trouble shooting that.

with my gain just over half way up, i got the dreaded "bass hum" when turning off the ignition, because the amp wasn't powering off correctly. I could turn the gain down even more to stop it, but then, I would lose my desired output.

I'll be posting more, as I go.

Bob The Eskimo

So far I have a difficult time believing that this is a differential head unit signal. The head unit that the RX8 comes with is identical for the stock and the bose systems, the point of change is at the Bose amp.

This head unit is also the same head unit that came in the Mazda 6 for the past two years. On the Mazda 6 forum, they have had these same discussions that we are having and ended up hooking up amps like eXcentric has done. Since this was over a year ago, and no one has updated with a disaster story, I think we are safe in assuming (although assumption is the mother of all F*#@ ups) that we have preamp signal.

If you connect a differnetial head unit up to the speakers directly, the only thing that you should hear is noise; as the music would experience phase cancelation. Just use a low amp fuse in the mix so that the fuse blows instead of the head or the voice coils if we are wrong. Even if it is preamp level you should stil be able to hear something.

Has anyone tried a phase inverter on the negative signal, and then attach it to a speaker or graphical eq to see if it is differential?

How about a voltage meter?

Also, since there are several people who have used this preamp signal directly into amps(+side), and grounded them (-side) to the shield of the RCA input on the amp, the signal should have shorted to protection mode immediately. If nothing else, this should create a massive amount of noise in their systems.

I am not disagreeing with you that it is not differential, I am just saying that we truly don't know either way at this point. There is a large void in the information we have amassed (or slightly piled if size is relative).

One question for the stock (non bose) stereo users out there. Does the rx8 come with an external amp for the rear 6x9's?

Please don't flame too bad, Just healthy discussion to get to the bottom of the issue.

bob

mohrt

For convenience to the readers, compare the PAC OEM-1 and OEM-2 here:

http://www.pac-audio.com/products/oem1.htm

And price shop with this froogle search:

http://froogle.google.com/froogle?q=PAC+OEM-2

Tamas

iTrader: (1)

No, it doesn't. The base stereo feeds the rear speakers directly off the head unit. The Bose amp location (on the underside of the rear deck, in the middle) is covered with a metal plate instead if I remember correctly.

Bob The Eskimo

So if anything, this tells us that the signal from the head will not be differential, but instead a standard speaker level signal. To confirm, does anyone know the colors of the wires used for the speakers in the non bose 6x9's?

If the are the same, then we have a winner, if not then we are still on the drawing board.

Tamas

iTrader: (1)

They ARE the same
Well, at least the wire colors that go to the speakers themselves are the same in the Bose- and non-Bose systems. Whether the wires going from the head unit to the rear Bose amp are the same or not is not clear since I don't have my factory shop manual CD with me.
However, I think there is no reason to think that they would use a different wire color from the head unit to the Bose amp then switch to the same wire colors like in the base system when going from the amp to the speakers.

So, simply put, I'm pretty sure the signal coming from the head unit to the Bose amp is a speaker-level signal, which is the same that powers the speakers in the base stereo.

Bob The Eskimo

This is good news and strange news. Now we know that the HU puts out a speaker level signal. So how has eXcentrics, and G8Rboys, and several others amps not fried themselves??????

No RCA should be able to handle 25 watts of peak power at 12v~14v. they are rated at 200mv to 5v. The resistance should cook it.

The colors of the wires are earlier in this post, can you check if they are the same in these photos as they are in your car Tam.

Bob The Eskimo

For those who have hooked up the supposed preamp signal into the RCA's already, please tell us where your gain is set at. Does the control feel normal, or is it way too sensitive. If they are speaker line signals and haven't cooked the RCA input for some miracle of chance, then the gain control should be stratospheric, and probably reach distortion at a low level.

Any info?

Tamas

iTrader: (1)

Hm... honestly, it's very possible that I'm wrong in assuming that the Bose amp receives the same speaker level signals then. One thing is certain though: according to the wiring diagram, the wire colors going to the rear speakers are the same in the base stereo like in the Bose system.
My car is 500 miles away from me, unfortunately. Am stuck with a dumb rental Taurus
So I can not compare the wire colors until the weekend, sorry.

But why do you think that the amps hooked up to those wires should have been fried? I thought they were able to handle speaker level signals... does that not mean that they can deal with the 25 watts peak?
I'm not experienced with this stuff.