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Infinity Amplifiers in Chrysler Cars

by Jeremy Schrag

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Back in the eighties, no American auto makers were really taking audio seriously. There were a few premium head unit options, with a couple of half-hearted stabs at adding things like booster amplifiers and co-axial speakers, but for the most part car audio was an afterthought. Then, as the aftermarket car audio industry really began to get going, all of the Big Three decided to try to give consumers better sounding audio systems so that they would be less tempted to go aftermarket. (Perhaps they were motivated by the squeaks and rattles and ancillary breakdowns caused by amateur installations, and unskilled "professional" retrofits.)

GM was the first to dip a toe into the waters by teaming up with Bose in 1983 or so. Few cars got Bose speakers, but the name Bose alone attracted buyers. Ford saw what GM was doing, and in 1985 was the next to capitalize on the name brand speaker idea. They contracted with Harman-Motive for the 1986 Lincoln Continental to bring in some JBL speakers.

Naturally, Chrysler didn't want to be left behind. But what brand to choose? Harman-Motive again had the answer to that question, and we saw our first Chrysler/Infinity sound systems in the 1987 Lancer, LeBaron GTS, and New Yorker.

While Ford and Chrysler were both working with Harmon, this does not mean that their Infinity and JBL components will interchange. The systems were different in many ways, one of which was amplification. Ford has always used an outboard amplifier module; while Chrysler systems were designed so that the system head unit powered the tweeters, while the system woofers were powered by individual booster amps on the speakers themselves. If any one amplifier failed, the rest of the system would still work until the affected speaker was replaced. That advantage was outweighed by several drawbacks.

With the amplifier bolted to the speaker, replacing speakers got expensive if that amplifier failed, and fail they did. Dash speaker amps developed cold solder joints from the heat cycling of the sunshine coming in through the windshield. Door and liftgate speakers got slammed around, developing bad solder joints despite the use of potting compound to make them more durable. As a result, speakers would suddenly start blasting static, go completely silent, or play distorted music. This added up to a bunch of irritated consumers, who then turned and ran to the aftermarket audio companies when they found out how much a new speaker would cost.

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In 1990, Chrysler and Harmon decided to go ahead and finally try the dedicated amp module approach. First found on the 1991 AY body (Imperial and Fifth Avenue) cars as part of the Reference Standard audio system, these dedicated amplifiers then became used more and more often until they eventually took over. To my knowledge, Chrysler is no longer using the amp on speaker idea, though this approach persisted into the mid 2000's.

Now, I'm going to open up and show you a pair of these dedicated amplifiers. Above, you see two of them. The one on the left is one of the originals... a Reference Standard amplifier I took out of my 1992 Imperial. The one on the right is a more recent unit, found in a 2000 300M.

We'll start with the Imperial's amplifier, but first some basic information. All Chrysler name brand audio systems operate much the same way. The head unit outputs speaker level signals from amplifiers inside. The amplifiers then convert this signal back into line levels to be processed and fed into the actual name brand amplifiers. This is done to maintain compatibility with Chrysler's non-name-brand stuff. A head unit had to work as well with those terrible Oxford speakers as it did with the Infinity ones.

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The name brand amplifiers were, until recently, modestly powered. For the most part, 20-30 watts per speaker was enough to get the job done well at reasonable volumes. That kind of power doesn't go very far when you're trying to set SPL records at your local sound-off, but for most of Chrysler's target consumers it was enough.

Before we go on, observe the above picture. You can see three small screws in this shot, and six big ones. The three small ones hold on the black plastic strip that surrounds the unit on three sides. You will have to remove these to get inside the amp. The six big ones will then have to come out to access the electronics. More on that later. This is a very easy amplifier to get into, so I haven't bothered with my usual red arrows. As long as you keep track of where the different sizes of screw go, you'll be ok.

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Here's the connector end of the amplifier. From left to right, we have the speaker level input and power antenna feed from the head unit on the smallest connector, the speaker outputs on the biggest connector, and then the power and ground connector on the right. All pins are labeled for troubleshooting purposes, though you could also use the labels to install this amp into any vehicle you wanted to. That said, the internal speaker crossovers are vehicle specific - I cannot vouch for sound quality if you try this thing in your Rolls-Royce or something.

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Before getting into the amp, we have to remove the mounting plate by removing all the screws arrowed in red. Use a Torx T-15 to do it.

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Now, it's time to remove the plastic wrap-around piece. Remove the three small screws I mentioned earlier. There will be another three on the back of the amp. This piece will just pull right off now. Note that this piece does hold the circuit board straight inside the unit... watch out for that when you put it back together.

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Here's the amp without that wrap-around piece. Now, it's time to take out those bigger screws on top.

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The one metal side of the amp is the side all the internal amplifier chips are bolted to. The metal housing acts as the heatsink for these chips. Remove these screws to continue disassembly.

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Now we can pull the mainboard out of the amp. This may require a little force to separate the insulating tape over the amp chips from the housing.

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This is the solder side of the mainboard. Make sure you touch up any suspicious joints you find. This one's in great shape for its age.

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Four Toshiba TA8210AH chips act as power amplifiers in this beast. These are the same chips found in many Chrysler head units, and are capable of roughly 15 watts of clean power per channel at 4 ohms. Multiply that by eight channels, and you get about 120 watts for the whole amp. Each amp chip is fed signal by two MC34074P op-amps.

And yes, this is really an eight channel amp. This amplifier basically takes over the job of crossover from the speakers too. In the Imperial, the rear speakers have four wires each. Woofers and tweeters each get their very own amplifier channel, with the amp itself determining which frequencies to send to them. Dash and front door speakers also get their own amplifier channels.

All this crossover stuff makes it a little tricky to upgrade speakers on these cars. Wire them to the low pass output from the amp, all you get is bass. Wire them to the high pass, you get no bass and all treble. My advice on dealing with these systems? If you're upgrading, just remove the amp and all of the speakers. If you're going aftermarket, you might as well go all the way. Wire new speaker wires right up to your new head unit's wiring harness, bypassing all of the factory wiring so you can re-install the factory system at a later time. You can hack into the factory harness, as I did for the Imperial, but it looks ugly and unprofessional at best. Besides, if you mess something up and an exposed speaker wire hits bare metal, your new $500 deck isn't going to be a happy camper.

Jim Z. wrote, "Aftermarket car radios use the same 4-channel BTL power amp ICs that the factory ones do; they're internally protected from shorts between speaker wires and from shorts between speaker wire and ground, and speaker wire and battery." Jeremy replied, "Modern factory decks do use chips that have short protection, but that wasn't always the case. I'd rather err on the side of caution."
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Moving on, it's time to have a look at the 300M amplifier to see what's changed. First things first, there are six screws to remove so we can pull off that mounting bracket.

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While this amplifier looks to be constructed the same way as the older one, there is in fact a major difference in how it goes together. On this one, you have to remove all visible screws to get it apart. You will need a Torx T-10 in addition to the T-15. The black plastic wrap-around piece is bolted to the circuit board, so the way you get this one apart is to pull on the big metal clamshell until the two pieces separate.

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The connectors are no longer labeled as to their function, so you now need a wiring diagram to know which pin does what.

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A look at the topside without the mounting plate attached.

And now the underside. Remember, all screws have to come out to get this apart. Once again, there is insulating tape on the metal clamshell that will fight you - I ended up having to pry gently on one end to get it to move.

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With the guts exposed, you can now remove the wrap-around piece. Remove the screws indicated in red.

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In some ways, this amp is very similar to the older one, in other ways not so much.

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The op-amp chips used in the preamp are now found underneath the mainboard as surface mount components. They are BA14741F parts, and there are ten of them.

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This amplifier is in fact a ten channel affair using five Toshiba TA8233H parts. In fact, only nine channels are actually used in the car this came from, so one of these chips is sitting there doing only half the work its buddies are. Specs are about the same as the older amp, with one major difference... these chips are 2 ohm stable, meaning as much as 20-25 clean watts per channel could be produced. It is unknown to me what impedance the speakers in this car were actually running, but it's pretty safe to call this a 150 watt amplifier.

Most of the Chrysler/Infinity sound systems using an outboard amplifier module should be using amps similar to these two. There will be component differences in the crossover sections of these, but the basic components should all be similar. Note that this article does not apply to the Boston Acoustics, Kicker, or the new Beats by Dr. Dre systems, just the Infinity ones. I hope to get my hands on some of those other brand components one day.

General Chrysler-related radio and stereo articles at Allpar:
CD and DVD systems (stereos have a three-letter code on the face plate)
Tape and tape/CD systems
From here to Infinity
CD changers
Classic systems (before tape decks)

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