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Adding Tweeters - should everyone? wiring/crossover?

11 January 2021
Ontario, Canada
I've got some tweeters that go in the front "sails" of the doors that I want to install. Unfortunately I don't have the specs on the tweeters other than that they sound great when I connect a sound source to them. I'm hoping this will help with the lack of the high end in my NSX. I'm figuring to this in the winter while I have the door apart doing the Hugo window fix.

My search turned up empty. I'm wondering how to wire the tweeters in? Do I need specific crossovers coming out of the BOSE amps? I assume I need more than just a capacitor in series? Have there been any threads on this, preferably with pictures? Or is the strong recommendation to add smaller tweeters to the stock BOSE units? I've seen some older threads on this, but looking at it, it seems it would make more sense to have the tweeters in the door sails pointing at the occupants than down by my knees.

Thanks in advance for input on it.
I assume your speakers are still original? In that case you would have to attach them to the actual speaker pins inside the speaker box. The Bose system utilizes active speakers that are supplied with line level signals and power - compared to passive speakers used in most vehicles.

A capacitor in series (to create a high pass) is a good idea - you'll probably have to experiment a little and check how it sounds.
The actual speakers in the box are 2 Ohm, which is unusually low - just to keep in mind when calculating filters.
You are entering terra incognito! The Bose amp is a class D switching amplifier that uses internal feedback for signal conditioning which is not really conducive to having its output circuits messed with.

On a class D, the speaker impedance can be a critical component of the switching circuit on the H bridge output. This may be less of an issue with the Bose because it is an older design and still has those external inductor coils on the board in series in the speaker circuit. I have a class D guitar amplifier with no inductors in the output bridge and relies totally on the inductance of the speaker voice coil. Mess with the speaker and you are seriously messing with the amps output circuit.

If the Bose amp feedback circuit is tolerant of changes to the impedance - frequency profile of the speaker, you need (or should have) a crossover and speaker combination that still presents the same impedance to the amplifier as a single speaker. That is one of the functions of the crossover. If the original Bose speaker is 2 ohms you want the crossover to look like 2 ohms when both speakers are connected to it. If the Bose speaker is 2 ohms and the tweeter is 2 ohms and you just connect them in parallel it will look like 1 ohm which can create problems for the amp (dead output FETs). A properly designed crossover will allow you to attach 2 speakers of the same or different impedances and present a 2 ohm impedance to the amplifier.

A tweeter connected in series with a capacitor is just a low pass blocking (oops - meant high pass ) filter (a filter that you don't know the crossover point on). If you connect that in parallel with the existing speaker you still run into the total impedance miss match problem. To connect a proper crossover, you need to disconnect the Bose speaker from the amp terminals and run the amp output directly into the crossover input. Then one output of the crossover goes to the Bose speaker and the other goes to the tweeter. If you scrounge around on the web you can find guides on how to design a passive crossover to get your impedance matches and desired frequency roll off. Be aware that passive crossovers are not efficient. Because you are starting with a very low impedance Bose speaker you will need to add parasitic elements in the cross over to get the total impedance back into the acceptable range so you may experience a slight loss in volume.

All that said, there may be one easy out if you can find a high impedance tweeter such as 8 ohms (16 ohms would be even better). If you connect a 16 ohm speaker directly across the terminals of the existing Bose speaker the net impedance to the amp is 1.78 ohms. The Bose amp may be OK with that as long as you don't crank the volume up to max. You can add a capacitor in series with the tweeter as a high pass filter to block low frequencies to the speaker; but, unless you have the impedance versus frequency profile for the tweeter you cannot calculate the 3 db roll off for the tweeter circuit.
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Thanks @Oldguy, that was much more elaborated than my response ;)

As far as I can tell, the central speaker is attached via a capacitor onto basically the same amplifier design and output coils, therefore I wouldn't be too concerned about the Bose amps getting damaged - but I might be wrong.
I agree that as long as the tweeter is not really low impedance the risk of damage to the amp is probably minimal.

I see that some automotive tweeters are sold pre packaged with what they call a cross over network. However, since it is a single input and output device it cannot be a crossover and is likely a high pass filter. If you get a tweeter with an included 'cross over' the vendor has likely matched the 3db point on the filter to the response of the tweeter which eliminates the guess work associated with trying to specify an input filter capacitor. A tweeter with a matched high pass input filter increases the impedance (of the tweeter) as seen by the amplifier at low frequencies. Since high power at low frequencies is usually when the amp output stage takes it on the chin, that probably reduces the risk of cooking the output FETs to a couple of places on the right side of the decimal point.

So, if @Wild Turkey has tweeters with matched crossovers / input filters then he is probably good to go. As you note, just connect the tweeters to the actual amp output terminals inside the enclosure.
No crossovers, so perhaps I need to abandon the idea of using these old tweeters, which were used in an NSX before (thus being built in to front "sails",) since I don't know how they were hooked up to the stock NSX speaker enclosures.

Suggestions welcome for kits to add tweeters either to some stock front sails, or mount behind the material with the stock speaker enclosures in the door when I do the Hugo window repairs.
Are they built in as in 'that sucker is never coming out' or can the tweeter actually be removed from the housing? If the tweeter can be removed, the safe would way be to find a new tweeter of the same dimensions sold with a matching filter. If that is a no-go, audio vendors do sell input filters for tweeters. Crutchfield has some which they more accurately refer to as 'blockers'
The Crutchfield things look suspiciously like a capacitor covered with heat shrink. Search around and you may be able to find something a little more sophisticated.
Following up - should everyone add accessory tweeters if they have the OEM stereo?
SPECTACULAR! I can hardly believe how much better the stereo sounds with much clearer high midrange and high end. The difference is less dramatic with FM, but with the bluetooth input off from the GROM, what a difference! (Probably no difference with cassettes, especially with Dolby NR activated...)

I had a look at the way the tweeters were wired originally. Tweeters seem to be higher impedance, so less effect on the total impedance if added in parallel, as @Heineken thought. I measured 6 Ohms, for a total of 1.5 Ohms using Ohm's law, but I don't know how to add the (unknown) capacitor into the equation. The tweeters were previously wired in parallel inside the Bose speaker enclosure, as you both hypothesised, with a capacitor in series with the tweeter, 1/2 way between the Bose enclosure and the tweeter. No dedicated crossover. 6 Ohms seems to be in the ball park. Much higher and they would put out much lower sound levels than the Bose drivers.

I wired them the same way, but with the Crutchfield capacitors located inside the Bose enclosures. (As @Old Guy surmised, it's just a capacitor in series on one of the wires.)

Looking back, old news. (There was also another thread about tweeters added to the interior door panel but I can't seem to find it any more):

Some jewels here:
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