Is it possible to keep the stock head unit in my car but replace the stock amp and/or speakers with aftermarket equipment?

YES.

Upgrading sound system while maintaining stock PREMIUM HU:
The stock premium head unit has no internal amplifier. Instead, an amplifier is located in the trunk, under the left black support brace.

Other than amplifying the audio signals coming from the stock HU, the amplifier in the premium system also adds heavy equalization to the output signal going to the speakers.

Because the external amplifier adds the amplification and equalization to the audio signal, the audio signals leaving the stock HU can be treated as a very low voltage preamp signal. By very low voltage I mean that the audio signals as they leave the stock HU and enter the stock amp in the trunk are < 2V.

In addition to the low voltage of the signals leaving the stock HU, the signals are also not your standard type of signal. Unlike conventional single-ended (that's what they are called) signals, the G35 sound system utilizes differential-balanced (that's what they are called) signals. Without going into deep explaination, differential-balanced signals do not connect one side of the audio signal to chassis ground. Because of this, balanced-differential signals often times help to eliminate any voltage sensitivity between one input terminal and ground and greatly reduces the incidence of alternator whine in a typical car audio installation.

Most aftermarket amplifiers expect RCA-style connectors/connections on the input jacks. When bypassing the stock amplifier to connect an aftermarket amplifier, you will quickly realize that you are working with speaker wires, not RCAs. There are two ways to convert the speaker wires located in the trunk (where you are bypassing the stock amp) to RCAs (to connect to your aftermarket amp's input jacks).

1. Line Output Converter (LOC). This device simply has speaker wires coming out one side and RCA jacks on the other. After splicing into the appropriate input wires coming from the stock HU, you will be able to connect the RCA jacks on the LOC to the RCA input jacks on your amp using a standard RCA interconnect cable. NOTE: If your amp does NOT accept differential-balanced input signals on its input jacks, you MUST get a LOC that is capable of converting the differential-balanced signals in the G35 to the standard single-ended signals the aftermarket amp is expecting on its input jacks. Soundgate makes an amp that does this.

2. Splicing braided/twisted RCA cables (PREFERRED IF YOUR AMP ACCEPTS DIFFERENTIAL-BALANCED INPUTS!!!). If your amp accepts differential-balanced inputs on its RCA input jacks, all you must accomplish to connect your amp is to convert the speaker wires in the G35 to RCA-style connectors that your amp requires. While a LOC will accomplish this task, it is overkill and adds unnecessary complexity and possible noise into your system. If you have ever cut open a standard RCA cable, you will notice that there are two conductors, one of which is often VERY tiny. Splicing such a tiny conductor to a speaker wire is very difficult and does not yield quality sound for your inputs (which the entire system is dependant upon!) Look for twisted RCA cables, or braided RCA cables. These use thicker gauge wiring for both conductors. INOXS makes cables that work well for this.

Upgrading sound system while maintaining stock STANDARD (Non-Premium) HU:

The stock standard head unit incorporates an internal amplifier which powers the car's speakers. To upgrade the standard sound system while keeping the stock standard HU, there are two popular options:

1. Choose an amplifier which accepts both speaker level inputs and differential-balanced signals (on the inputs).
2. Use a speaker-level-to-preamp converter such as this Scosche. Use LOCs or twisted/braided RCAs (if your amp supports low voltage inputs) to convert speaker wire in trunk to RCA for amp inputs. Your amp would still need to support differential-balanced input signals, unless the LOC converts them to standard single-ended input signals.

 

a few quick questions:

1. on bose headunits, even though its the amp that does the eqing, what is its freq response? I have seem graphs of similar permium HU's output, and most of them is not that flat and heavily trails off at the low end.

2. what its signal to noise, i know that within limits, these are a huge deal, but if we are talking about a difference of 15-20db sn ratio difference?

3. on the bose system, coming out of the stock headunit, how many pairs of balanced outputs? just one? or a pair for each speaker? if thats the case, say i have the stock bose, and want to add a full aftermarket system, but want to keep completely fading control? and if there is separate balanced outputs coming out for each pair of sepakres, are they crossed over in anyway by the HU or that purely done (if at all) by the amp?

4. on base system, does the HU itself then do the eqing of hte sound related to volume?

great info! just wondering about htese

thanks!

Bing

 

Good questions. Unfortunately I cannot claim any ownership over the following research, work or answers. You can thank Geeman for supplying his 2003.5 Sedan to El_duderino to run some analysis on. Here is a link to the thread that answers your questions.

https://g35driver.com/forums/showpos...08&postcount=1

I have also placed these pictures and el_duderino's comments on another server, in case the one linked to in that post goes down.

The head unit has a + and - pair of wires for front left, front right, rear left and rear right channels. 4 channels total. From these the Bose amp creates additional channels when necessary (such as the for the sail speakers in the premium coupe and the sub channel in the premium sedan). The front "components" (term used loosely for the stock speaker crap) are seen as a single channel & load to the Bose amp.

I don't know if the Bose amp actually sets up any crossover points, but instead places its own predetermined EQing on each channel it outputs- theoretically making each set of speakers sound "optimum" in the car. The front "component" sets are not actually crossed over either. Instead, a "full range" goes down one pair of wires to the door, where the wire splits to the tweeter and woofer (hence why the stock Bose amp seems them as a single channel/load). The tweeter has a resistor on the back that acts as a bass blocker.

 

someone needs to sticky this...

 

That's the entire purpose I did it Godzilla said he would. I'm going to make some more stickies over the next few days. Just wait in anxious anticipation!

 

16, many of those graphs are from JL and are not quite straight.

The worst graph is from VWs with Monsoon.

There are 4 OPs, F and R, L and R. All are identical.

The FR is flat above setting 15 or so on the volume **** and below that it has an auto loudness feature with a slight bass bump (around 3db at its worst, IIRC) that is hardly an issue based on Fletcher Munson hearing studies.

The S/N of ANY CD player is better than anything else in the signal chain. It's not that great in my experience, it's kind of lame for an OEM HU (low signal level), BUT it's NOT bad enough to not use - it can do quite well. While amthar used NO LOC, I tend to use a Soundgate LOCPREA or an Iso Max CL2RR if I can. (Hey! My analyzer will measure THD +N and we have an input on geeman's HU for an aux in from the sig generator! Hm...)

On the non-Bose HU, I have not had one in to RTA yet (still an open invite...). But I suspect it is not very dynamically EQd and if anything it's probably that auto loudness again.

 

that answers my questions perfectly thanks!

basically i know now more or less what to recommend precisely to customers

b

 

That's my role... helping out the pros...

 

So is it safe to say that the stock Bose HU is a good Hu to have, and that an aftermarket headunit would not make much of a difference sound wise?

 

Yes. Please read.

 

I think that you can get a small SQ improvement in going from OEM to aftermarket, IF you have a very good set of front speakers, a very good sub in a box, and a very good (not neccesarily very BIG, just good quality) amp - and you've deadened the doors as amthar has talked about.

Better front speakers and sub, better sub box, and sound damping in the door will ALL make bigger SQ improvements than the HU itself will make for you. (Features are another thing entirely).

 

Agree 100% with the above. Did all of the above in a 03.5 BOSE coupe, by concidence in the order listed, the HU (IMO) did not make as big a difference (except for the "features" ) as the upgrades mentioned first.

 

bump... going to bump it every day until an admin stickies it.

 

I want to replace the speakers on the rear deck, and possibly the amp. Any suggestions?

 

This isn't the thread for that discussion. You can use the instructions in this thread for how to replace the amp. If you are looking for brand recommendations or removal instructions for the rear deck speakers and/or amp, you should really many a separate thread asking those questions.