Some "Gotcha" with my 902s

[ckdf]

Just wanted to share this experince, even though it makes me look a bit stupid:

I am experimenting for quite a while now with some Altec gear (515-gs, 902s, 511s and, er, some "other" 2405s). I bought this stuff several years back from an Altec guy. When he sold me the 902s they were part of some custom kits that had in a box a 902 and a very short horn like the ones used in the 604 duplex drivers.
As I wanted to go for 511s he kept those short horns and just sold me the 902s. In my experiments with them I was always a bit disappointed by the response of the 902 in the 511s. I didn?t get a good low end response no matter how much optimization I tried in the xovers.
So I finally decided that it makes no sense crossing them lower than 800 Hz or even higher.
This weekend I decided to measure the impedance curve of the 902s as I wanted to see whether they would have some resonance at the low end calling for a higher order crossover. When I saw the results I was puzzled:
Impedance was rising below 1kHz up to infinite for low frequencies. That left only one explanation: There must be a capacitor inside the driver! I quickly took an ohm meter and indeed there was no DC path trough the drivers. I opened them and found a 6uF bipolar electrolytic soldered into one lead.
So obviously those kits had drivers with built in 1st order crossovers. As they are 16 Ohm that is a 1600Hz xover! (maybe some of the eductated Altec guys on this forum know what these kits were for?)

Don?t know which feeling was stronger: The embarrassment that I had not detected this earlier or the sheer happiness of recognizing that I had never discovered the full potential of the 902/511 combo so far!

So should you have a 902 or 909 of unknown source with an unsitisfactory low end: better take out your ohm meter to make sure it doesn?t have this little "bug" inside.

Claus

[David Deratany]

When I was bidding on a pair of 902A MR -16s on Ebay I learned the MR was for the small mantaray horn mounted with the driver, and that they were used for extended hf on pa type systems, and that is likely why the capacitor and lackluster low end. If you lose the capacitor you end up with a standard 902.

It may be Jim Dickenson who posted a pic on this site, I'm guessing about six months ago.