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.