ApexJr Ribbon Tweeter

Steve at ApexJr has come across some ribbon tweeters he is selling for $5 each.

I have a set of these running in a set of mini-monitors with just a 3.3 mF cap in series (and an inductor on the woofer). They took a couple days to loosen up, but as they did, the more I listened to them the more amazed I was at how good these tweeters are. I am using them with the bezel & grill removed.

From the efficiency rating of the woofer, they are in the 88-90 dB 1 watt range.

The magnet structure must be inherently shielded since i can bring the ribbon element within a couple inches of my monitor before i get any effect. I get no effect when it is right up against the back or side of my monitor.

The bezel & grill are fairly easily removed by scraping off the melted part of six tabs on the back. Getting the ribbon elemnt out of the plastic faceplate is somewhat more challenging as it is glued in.

If someone gets more measurements I'll happily post them. Just e-me.

Initial reaction: If anyone wants a really decent sounding tweeter that you can use in an AV setup for almost no cost, this is a good one.

More Info:

tweeter as received

front bezel & grill removed

ribbon element

3 3/8" D x 3/4" deep

3 3/8" D x 5/8" deep

1.5" D x 7/16" deep

please excuse the picture quality -- taken with my cheesy, but free digital camera

exploded diagram

Tom Woods comments:

CARTRIDGE CONSTRUCTION:
Magnet sandwich with thin plastic membrane and spiral "voice coil" at the center. There is a spiral attached to either side of a plastic membrane. The two sides are connected together in the middle and clamped on either side at the outer edge by the plus & minus terminals. Membrane diaphragm is held within paper rings forming the primary spacer between the magnets (about 1.5 mm spacer thickness). Solder terminals (one is marked in red) emerge out the side of the diaphragm. The whole sandwich is held together by a frame of formed aluminum. There is no magnetic shield.

Two ceramic magnets pressed together so that 'positive' poles face and magnetically oppose each other and squeeze the diaphragm sandwich. Magnets are each about 4 mm thick and 38 mm in diameter. The forward facing magnet has nine 5 mm holes through it to port the sound. The rear magnet has no holes defining a monopole construction. (The rear magnet has a paper cover to protect it from physical abuse.) The front hole pattern has one at the center and eight forming a circle of 22 mm diameter c-to-c.

MOUNTING FLANGE OR BEZEL:
The cartridge is glued into a recess in the back of an 85 mm circular plastic mounting plate (the bezel) which has four mounting holes at the rim. The front of the bezel has a 5 mm 'bump-out' which forms the acoustic lens and contains the perforated metal protection screen.

IMPEDANCE MEASUREMENT:
A limited impedance measurement on one sample cartridge indicates a 6.7 W resistance in series with a 17.5 microhenry inductance. This means an added inductive reactance of 1.1 ohms at 10000 hz. For the purpose of calculating crossover component values, a nominal 7 ohm resistive load is suggested.

impedance chart (measured Re=7 W)

measured by Andrew Zeitoun using Speaker Workshop
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