Hey guys so I got my GMOs 04 all hooked up to my pioneer nex and I noticed right off the bat; during high sound it distorts a lot and the bass sounds horrible.
my volume on the pioneer goes from 0-40 and I played with the gmos04 needle and tried setting it but if I lower the gain a lot, then it will stay clear but the volume isn’t loud enough at 100, while if I raise it then it destorte at 30-35 volume.
right now I got it set so that it’s decently loud at volume 30 and clear but when I gst to 37 I starts to destort. I am positive with my old cd changer that up to max volume I would never get bass distortions or even audio quality but then again I haven’t used the audio in my hummer for music listening in years.


is this just a case of the factory bose getting old and therefore distort and stuff? Or do you think it’s a wiring issue.

I asues butt butt connectors and crimped them, and everything powered on perfectly the first time,


i just want it to be clear at max volume too but seems like the only way I can do that is lowering the gmos gain but then the max volume isn’t really loud.


thanks

 

Intersting you should mention this, I too have a Pioneer HU.

There are two sound controls on the GMOS-04, one for the "Chimes Audio" Level and one for general "Audio Level"

Recently I adjusted my chimes as it was too loud, but I adjusted the "audio level" by mistake (I forgot there was a seperate Chimes pot)

Guess what? At high HU levels the sound is distorted and the bass awful just like yours, whereas before it sounded great. So when I get the chance I will put it back as before.

So for you its probably not bad wiring, or an aging Bose unit but more likely a case of matching the levels.

HU's are likely to distort at top ends so it would be better to operate around 75% of that or less.

The GMOS-04 is probably acts like an attenuator and mixer, so at low settings the HU will need to overdrive just to reach an acceptable volume. Conversely at higher settings the HU will be barely driving and the GMOS-04 will be boosting the signal plus any background noise.

The GMOS-04 data sheet says:-

"Distorted Audio Output: When using the GMOS-04 with a radio that has a high pre-out voltage, usually greater than 2 volts, the audio signal may become distorted"

So I think its a case of finding the "sweet-spot", I'd be tempted to set the GMOS-04 to half-way and work from there (which is what I will do)

My Pioneer used to be set around 50% or less to achieve high volumes (too loud) and would be distortion free. The HU manual quotes 3V outputs so 50% would be 1.5V well inside the 2V limit.

 

thanks a ton for the reply! did you set your gmos half way? i lowered my gmos compared to before because i had a static/hiss noise when it was higher when no music was playing but now when i lowered my gmos about 1/4 turn, i lost alot of my louder top end volume so ill probably revert the change because i want it to be loud like how it used to be before

 

Yes, originally it was at half-way.

When I next re-visit my system I'll measure the levels with my scope and try and establish the best settings.

As you say Volume Vs Hiss.

 

Oh ok! So you think its probably not my ground, and with the gmos and bose we will always have to balance hiss vs volume? Because I was thinking of trying a diff ground but this seems to just be a gmos and bose issue which we have to balance.

 

Its not just the GMOS and the Bose. That is something you have to balance on any system with an external amplifier. Typically you are adjusting the gain on the amp itself. In this scenario you are adjusting the gain on the low level signal to the amp which has the same affect. Crank the gain up on an amp and you gain volume at the expense of clarity, distortion, and audible hissing.

One thing I have not experimented with though. Using a Tech2 I know I have seen the volume level of the Bose amp. I need to see if that changes when using the volume on my Kenwood (also through a GMOS) or if the Bose is basically at the level it is at and our volume is solely based on the level of the input we feed it. If the Bose volume does not show as changing on the Tech2 there is a remote possibility that the Bose amp could simply be stuck forever at whatever the volume was set at when the stock head unit was disconnected. Seems unlikely though, but if that is the case it might be possible to temporarily plug the stock head unit back in, power up, crank the volume to the max, kill the power then reconnect the aftermarket unit. If that is the case it would also make finding the perfect balance even more complicated as you then need to adjust for the Bose amps level, the GMOS level, and the head units level. lol

 

Looked for the BOSE volume level but could only find "Rear Seat Audio" controls which read only. As I have no stock radio or navigation units installed I couldn't access those options.

It's an intereseting point regarding the BOSE levels, one can only assume that the default is max volume. I've read many threads on HU installation but no one has mentioned it to the best of my knowledge.

 

I just checked on mine too! Different results though. I do have a rear seat audio and as you said, nothing to really do there, particularly since I have any aftermarket head unit. But.... I also have a Bose Amplifier choice to look in.

 

Looking in the Bose amplifier section there is nothing that can be adjusted. But the data display is intriguing...... I did try changing the volume on my Kenwood head unit but nothing changed and the volume stayed at the 78.8%.

So yes, now this has me even more curious. Unfortunately I'm not going to have time to pull her into the bay, pull the dash apart, etc to test any further anytime very soon. Things I'll be curious about are for one, would changing the level on the GMOS change the Bose amp volume? Second and regardless of the GMOS, would reconnecting the stock radio and changing the settings on that let me adjust the fader to zero, volume to max, etc then kill power and those settings be retained in the amplifier? My guess is they would as the stereo always starts up at the last volume level, something that has likely caught everyone off guard at some point. Like having the stereo blasting when you last shut it off and forgetting, start in the morning and jump out of your skin when it surprises you because it is suddenly blasting. lol On a side note, my Kenwood is good about that..... be listening quite loud, phone rings, and audio is automatically muted, have a nice long conversation, hang up not remembering it was blasting when the call came in.

Anyways so there it is. Mine is apparently stuck at 78.8%, fader is stuck at -7%, bass and treble reasonably well centered, chime volume... had no idea that was even adjustable. I seem to remember that was adjustable via the GMOS so another thing to test, does adjusting the chime level on the GMOS change it on the Bose amplifier? One I did not check was if I adjust the fader on the Kenwood does that -7% change on the Bose? Based on the volume not changing I suspect not. If setting these things on the factory head unit before disconnecting it does retain than that basically means a whole different level of tweaking settings is possible, albeit somewhat tedious and time consuming and of course would require still having a working stock head unit and a lot of plugging and unplugging. Of course you would also need a Tech2 to verify if any of the settings are retained as well for certain but it could be verified without by, for example, connect stock HU, turn volume to zero, disconnect and then connect aftermarket HU, test, and then repeat this at a few different levels.

 

Interesting, I have an entirely different display to yours, maybe my Tech2 software is different, older ? (also my truck is not CAN) I see your display has Function Keys whereas mine has a Scroll List, curious??

The GMOS has no provision for Serial Class2 / CAN that I can find, so is doesn't communicate with the BOSE (I think) Also my HU is not connected to the vehicle bus, so it does look like the BOSE amp is acting alone (78% as you say)

The Pioneer HU does have provision for controlling the HVAC and Parking Sensors via the vehicle bus, but I figured this was probably only a CAN option and would integrate fully, besides I have no parking sensors (yet!)

The GMOS has a seperate pot to adjust the Chimes, this was what I meant to adjust last time I was there, but stupidly set the audio pot by mistake (such a dummy)

Your theory on tweaking may well be true, though you would think someone would have encountered that by now, who knows. I'd love to take a GMOS unit apart and see whats in there, same goes for the ASWC-1