Author Topic: Tech information about the Gen 2 factory amplifier & speakers.  (Read 2152 times)

Shovel

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I'll edit this later to have further information,  but to get us started:

DC Resistance on the dash speakers is 2.8 ohms.     Pretty typical of a 3 ohm impedance driver.

The factory amplifier is located in its own little kingdom below the back floor,  for reasons unknown to man or beast. 



It's not difficult to remove, peel up the rear carpet and remove a bunch of 10mm screws. 





The factory Infinity amp contains no inverter and no DC-DC power supply so it's just 12V DC rail voltage with a divider, an offset and three dual channel BTL devices.    If you don't know what that means it's OK there will be more on that later.

This is also a six channel amplifier (there are eight speakers in the Infinity sound system,  six amplifier channels) .   There does not appear to be a time offset or phase correction circuit, nor is one expected in this kind of amplifier.   




If you care the three devices are Toshiba TA8233H which are rated 30W at 2 ohms, 14.4v and 10% THD (ha!)  - or 19W at 4 ohms, same voltage and THD.      The white paper for that device doesn't list an output specification for 3 ohms but we can assume somewhere in the vicinity of 21 watts at 13.2V at 10% THD.   

 


One noteworthy thing that becomes obvious upon study of the white paper is that asking more than 10 watts per channel at any impedance seems to send distortion through the roof on these so while they're pretty clean at low levels, the top half of their output loses its composure pretty quick.   

In the real world this amplifier makes the exact same power into a 4 ohm speaker as an aftermarket head unit does,  so if you've got an aftermarket head unit and aftermarket speakers you'd do well to bypass this amplifier.   

The other thing this amp does, other than simply amplifying is a little bit of filtering and probably EQ for the OEM speakers.      There are seven BA14741 op-amps - these are general purpose four path devices that are almost certainly being used for band-pass filtering (or large Q equalization.. ),  and there is also a single MC3403P op-amp which is likely being used as a low pass filter for one of the channels.

This is another reason you'll want to bypass this amp if you're using an aftermarket head unit with aftermarket speakers. 


If you're keeping factory speakers you might want to keep this amp because it clearly has filters (crossovers) specific to the factory speakers.

Also this amplifier uses optical couplers on each pair of channels,  though I have not tried to determine their exact function yet.   
   
They may be used in a feedback noise reduction circuit that reduces output when clipping is detected but that's speculation.
« Last Edit: July 23, 2017, 11:05:48 PM by Shovel »

Shovel

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Re: Tech information about the Gen 2 factory amplifier & speakers.
« Reply #1 on: July 24, 2017, 11:32:16 PM »
More stuff:

So you have 8 speakers and a 6 channel amp.  How's that work?

The lower dash speakers and the sail panel tweeters are parallel,  with a passive filter (a capacitor) mounted on the sail panel tweeter so it doesn't get any low frequency content, and a crossover slope of 6dB/octave.    The lower dash mid speaker does not have a filter which means they're 90 degrees out of phase with the sail panel tweeters - a fact that doesn't really matter much since there is probably a really high crossover point on those tweeters.     It's crude but obviously it works.   There is a high pass filter in the amplifier as well. 

Both of those crossover frequencies can be determined at a later date as I study this more - will edit the info in as I learn it.     A similar Infinity system installed in Chrysler vehicles of the same era filtered the front mid/tweeters at around 240hz and I would expect something not too different from that here.

The front door speakers get their own amp channels, and these I would expect to be the ones filtered by that MC3403P op amp.    Low pass for bass only and probably a crossover point around 150hz .    The reason we would expect a big gap between the low pass frequency for these and the high pass frequency for the mid/tweeters is to cancel out cabin resonance.     Reduces the "muddy bass" / "mumbling male vocalist" sort of sound that can happen in a carelessly installed car audio system - remember you're not just playing speakers you're playing the room they're in.

The rear 6x9 speakers also get their own amp channel and I would expect this channel to be wide range, maybe even full range but possibly with some EQ to remove some midrange from back there.    Nobody likes "inside a subway" sound. 










Justice

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Re: Tech information about the Gen 2 factory amplifier & speakers.
« Reply #2 on: October 22, 2017, 05:56:01 PM »
Opinions on best/easiest way to add and Aux. input? The cassette adapter doesn't work for me. Go in from the CD changer cable?

Shovel

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Re: Tech information about the Gen 2 factory amplifier & speakers.
« Reply #3 on: October 22, 2017, 11:49:06 PM »
. Go in from the CD changer cable?

It's been a long time but I think the only way to make that work was to use the CD changer cable but it was necessary to put a CD in the changer and play it,  so that the deck would think it's playing a disc.     The audio is unbalanced line level so aux ("headphone" 3 conductor common ground) works with it from an electrical perspective.

DICE makes a media bridge that provides Aux & bluetooth and works through FM but it's not like one of those shitty FM transmitter things, it intercepts the antenna wire and works extremely well with very little loss of fidelity (my employer used to sell a rebranded version of it called Civita, which sounds like a good name for a pill..)  but that device costs upwards of $150,  which buys a pretty kickass aftermarket head unit instead.   


Justice

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Re: Tech information about the Gen 2 factory amplifier & speakers.
« Reply #4 on: October 25, 2017, 09:22:38 PM »
I've seen some online hacks, that go in at the changer plug and they play a CD in the changer with a "silent MP3" track to trick the changer controls.

I don't really care about retaining use of the changer, but I'll keep it there if need be. Seems kinda like a lot of trouble. I think I could  cut off the changer audio input and tap in the Aux downstream a while keeping  the control wiring hooked up to allow the changer to think its working? then I could just leave any old CD in there and not worry about playing a silent track?

I like the look and feel of the old stereo in this 99 for some weird reason. So I want to try and retain it. Its the one with a CD slot below the radio. The last 99 I had, I put in an aftermarket double DIN that fit nicely, but the lighting and controls just seemed out of place in a 99 and it was hard to get it to sound right with factory amp. My drivers side  door speaker is blown too, so need to find a suitable replacement for those that will work with the factory unit. I dread taking the door panel off but, I should get over it and just do it. Funny I can pull the engine down to the block but I'm scared to remove door panels...

JohnnyBfromPeoria

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Re: Tech information about the Gen 2 factory amplifier & speakers.
« Reply #5 on: October 26, 2017, 05:32:22 AM »
Even I can replace the door panels easily. Not sure how much the little sail panel/tweeters get in the way on a gen 2.5, since I have a 95, but the pop-out clips seem pretty tough and reusable, so that just leaves the two weird pop-up clip things near the front edge of the panel (center pulls up to make it release), then the two screws behind plugs in the grab handle and removal of the trim behind the opener pull handle. Start at the bottom and un-pop the clips and move the door panel up and over the door lock button, disconnect the wiring and there it is. Clean stuff after you get it apart. I like to hit the window wiper with silicone spray so it doesn't grab the window as it goes up and down. Kind of funny how you can do a timing belt in your sleep but don't like touching these things, as you mentioned.

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concealer404

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Re: Tech information about the Gen 2 factory amplifier & speakers.
« Reply #6 on: October 26, 2017, 07:47:09 AM »
Hrmmm... wish i had found the factory amplifier before i installed my new one.  I would have liked to put the new one in that hole maybe.  Maybe a new fun afternoon project.
1998 Montero Winter Package (Mine)
2003 20th Anniversary Montero (Wife's)

Shovel

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Re: Tech information about the Gen 2 factory amplifier & speakers.
« Reply #7 on: October 26, 2017, 08:42:39 AM »
Grom Audio makes an aux adapter that supposedly works on 98+ Montero factory stereos to fake whatever signal tells the head unit that the CD changer is running, and provides a 3.5mm input.   About $65 - I don't have any experience with it.  But if it works as advertised then that's probably the cleanest way to do aux into a factory head unit.

For replacing door speakers on a factory premium system when you are keeping that factory amp remember that the factory "premium" door speakers are just used as woofers and they are around 3 ohms, as well they have their own unique efficiency within the band they are playing so you'll want to find a junkyard oem speaker to match it.    Or find an aftermarket woofer that is approximately equivalent and replace both.   A very high efficiency 4 ohm woofer can be a good call here, potentially even improve your snr a fraction.

Charly

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Re: Tech information about the Gen 2 factory amplifier & speakers.
« Reply #8 on: December 26, 2017, 03:04:06 PM »
Thanks for this.

I have a new head unit, rear 6 x 9s and a Metra 70-7002 harness enroute. Will be bypassing the factory amp. Possible external amp in the future.

i have a Sony CD head unit for sale if anyone is interested. comes with the harness to use the factory amp.
K9ADV

Chautauqua

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Re: Tech information about the Gen 2 factory amplifier & speakers.
« Reply #9 on: January 05, 2018, 12:46:54 PM »
So I'm removing the factory head unit and replacing 4 speakers.  The head unit I'm using is the double din Pioneer MVH-S600BS.  My plan is to bypass the factory amp and I'm only swapping out the door speakers and the 6x9's in the rear.  My question is there are a few metra connectors listed. Which metra connector adapter should I buy? 

Thanks,

Ross
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rodmankn3

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Re: Tech information about the Gen 2 factory amplifier & speakers.
« Reply #10 on: October 01, 2018, 07:47:46 AM »
is it only me? I can't see the pictures!!!

Shovel

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Re: Tech information about the Gen 2 factory amplifier & speakers.
« Reply #11 on: October 01, 2018, 10:24:41 PM »
is it only me? I can't see the pictures!!!

In late 2017 there was a malicious code attack on the forum and a lot of stuff was destroyed.   Thankfully most of the text content was preserved but images didn't make it  :(