Altec 601-8D A Puzzlement--HELP!

[soundguy64]

Since a tweeter stopped working on one of my 601-8D "Duplex" speakers a few weeks ago, I've been doing some digging on the puppy. A couple of issues and interesting facts have surfaced:

1) The crossover issued with the speaker is the N-3000-E, which I knew already. This was billed as a 3 kHz two-way crossover. As I found out, it isn't. It's a 3 kHz high-pass filter with a level control for the tweeter. The woofer is fed straight to the speaker terminals; no filter at all. Want upper mid-range? You got it! (Too much of it, in fact.)

2) There is a center-tapped coil used in the tweeter filter, but only half of the coil is connected across the tweeter terminals (via the level control as part of the filter). The other coil half is wired in series with the tweeter! Yup, this "super tweeter"--supposedly having a useful response up to 22 kHz--has a high-cut filter on it.

3) The tweeter is wired out-of-phase with the woofer. Seems strange, since the tweeter is mounted right on the front of the woofer's center pole piece in what should be an almost perfect time-aligned position with the drivers in phase.

As mentioned, one of these tweeters is no longer working. As measured with a VOM, there is no continuity across its terminals at all, which tells me the driver may not be fried, but may just have a broken lead underneath the center dust cover. I can only hope that's what it is, because I've had no success finding a source of Altec 3000A tweeter elements. All this leaves me with questions.

1) Is there a good source of replacement elements for the tweeter, or am I gonna hafta replace the whole tweeter (assuming it is fried)?

2) I'm assuming that "3000A" is the variant used in the 601-8D. Am I correct?

3) What is the nominal impedance of the 3000A (or whatever tweeter is used in the 601-8D)?

4) Why is the tweeter wired out-of-phase with the woofer?

5) Why is that pesky half-coil wired in series with the tweeter? Somebody at Altec allergic to highs in the 1960s? (I mean sonic highs.) I am--you should pardon the expression--baffled at this.

Any pertinent insights offered will be much appreciated. Many thanx to all who read this.

[grumpy]

http://www.audioheritage.org/vbullet...ll=1#post70051

I think most of your questions are answered in the above article.

...
3) What is the nominal impedance of the 3000A (or whatever tweeter is used in the 601-8D)?

4) Why is the tweeter wired out-of-phase with the woofer?

5) Why is that pesky half-coil wired in series with the tweeter?

30 ohms

with 6dB/oct (woofer effectively) and 18dB/oct (tweeter), the signal reaching the drivers at the crossover frequency is out of phase. Flipping the polarity on one driver corrects this.

Not being used primarily as an inductor, but as a transformer (autoformer)... the “tapped inductors” I assume are wound on the same former/iron slug. The winding to ground picks up the input voltage and then supplies it to the tweeter (higher voltage) via the series element.

[soundguy64]

http://www.audioheritage.org/vbullet...ll=1#post70051

I think most of your questions are answered in the above article.

Hello, grumpy! Many thanks for the reply. I suspected the 3000A tweeter had an impedance greater than 8 ohms; you've confirmed that.

The Altec article from that link is for the 601A, which uses the N-3000A crossover; a completely different device than the N-3000-E. The N-3000A appears to have a low-pass filter for the woofer, the N-3000-E has no low-cut filter at all. This can be seen in the schematic for the N-3000-E drawn by Nibeskov farther up that thread. I had already read the Altec article, and verified Nibeskov's drawing against a N-3000-E that I have in hand.

It's significant that the N-3000A used with the 601A incorporates a low-pass filter in its design. The 601-8D woofer needs one as well. I've done free-air measurements on the 601-8D woofer, and it has a significant output as high as 8kHz. It doesn't roll off enough on its own at the high end. Without a low-pas filter rolling off at 3kHz, there is way too much overlap of the woofer and tweeter. The result is that the 601-8D has an upper mid-range peak, and significant upper mid-range "beaming" to go with it, since the excess output between 3kHz and about 7kHz is coming from a woofer-size diaphram. Since we first began using a pair of 601-8Ds in the early '70s, I've always had to use a lot of EQ with them, but even a 10-band graphic equalizer doesn't take care of the problem completely. Even if one does manage to pull down the excess mid-range, the "beaming" remains, leaving me with inadequate mid-range off-axis.

Imo, the solution is to roll off the woofer properly, thereby treating the disease (excess, beamed mid-range from the woofer) instead of the symptom. It should be no problem to build 2nd-order Butterworth low-pass filters to feed these 601-8D woofers. I just hope I can get that one inoperative tweeter fixed.

[soundguy64]

Hello, grumpy! Many thanks for the reply. I suspected the 3000A tweeter had an impedance greater than 8 ohms; you've confirmed that.

The Altec article from that link is for the 601A, which uses the N-3000A crossover; a completely different device than the N-3000-E. The N-3000A appears to have a low-pass filter for the woofer, the N-3000-E has no low-cut filter at all.

That last part should read "the N-3000-E has no low-pass filter at all."

Sheesh. Musta been a rough day at work.