T-350 impedance matching

[CaptnBob]

I'm working on an elderly Electro-Voice Patrician IV (the king-sized Klipschorn, not the 30" woofer one.) Unfortunatly someone somewhere in the last half century or so managed to knock out the tweeter. The unit is a T-350, which looks like a T-35 hit with a colossal ray. This thing is huge!

The problem is, although it takes the same diaphragm as a stock T-35, it needs a 16 ohm voice coil, which I understand is no longer available. So the question is - do I

A: Put in an 8 ohm voice coil and pretend nothing has happened. (This was the path suggested by Klipsch when the same problem came up many years ago with one of my Shorthorns. It worked fine, thank you.)

B: Put a 4 - 6 ohm resistor in series with the tweeter. (This is what's suggested by the speaker repair guy.)

C: Take a 16 ohm voice coil out of an extra T-35 and hope it doesn't get damaged in the transplant process.

D: Buy another, working T-350 on ebay and use the bad one as a striking, chrome-plated paper weight. (I don't really want to do this, as the last 16 ohm T-350 sold for close to $400 and I don't feel that prosperous.)

Any thoughts or suggestions would be welcomed.

Thanks,

Capt'n Bob

[Al Klappenberger]

Bob,

I have been thinking about your problem. It's an interesting one! I think I would pick option 1, stick in the 8 Ohm diaphragm and not tell anybody! I would definitely NOT use the resistor. That would cut down the sensitivity of the tweeter. High sensitivity is the reason that tweeter was used. Chances are the network is not critical enough to be messed up by the 8 OHm load. That's my two pennies worth anyhow.

Al K.

[TBrennan]

Bob---The Patrician crossed to the T-350 at 3500hz. If the impedance at the crossover point is only half that the network was designed for the tweeter won't turn on until 7000hz leaving you with a big hole in the response, especially as EV phenolic mid drivers aren't noted for strong upper range response. Are you sure EV can't supply you with a 16 ohm diaphragm? have you tried the online prosound shops like Jammin Jersey?

www.chicagohornspeakerclub.org

[djk]

A couple of thoughts here.

You could get away saying the 8 ohm replacement coils are 16 ohms for most intent and purpose.

Although they only measure about 6 ohm DC the AC impedance is quite a bit higher.

How high?

As much as 12 ohms in the top octave, as much as 20 ohms in the crossover region.

The inductance of the voice coil and top plate cause the impedance to rise at high frequencies, mouth reflections cause impedance peaks in the crossover region.

My father has an original '63 vintage pair of the Electro-Voice model VI: a kind of mini Patrician with the 1/2" thick foam coned 18" woofer, 8" midbass, 4" X 10" midhorn, and T35 tweeter. The system impedance stated on the input panel is "16 ohms", the tweeter measures 6 ohms DC.

The old Atlas mid drivers start to run out of gas above 4Khz. The type A, B, E networks all used just a single 2µF cap feeding the tweeter. If you assume a 20 ohm impedance in the 4Khz region this would work out to a 4Khz crossover with the 2µF cap. If you assume an 8 ohm impedance for the tweeter the crossover works out to be 10Khz for the 2µF cap.

Considering that there is not a big hole between 4Khz and 10Khz in any of these speakers, the impedance of the tweeter must be closer to 20 ohms that it is to 8 ohms.

One final thought.

The JBL2426H is considered an 8 ohm driver. The DC resistance is 3,2 ohms. On a 2370 horn the minimum impedance in the 5Khz region is about 6 ohms. Near its 630hz suggested minimum crossover frequency it is about 30 ohms.

[BerndH]

Bob

there are two auctions on ebay with 16 Ohm tweeters. One auction is for your T 350 tweeters, the other for T 35 tweeters. See

http://cgi.ebay.de/ws/eBayISAPI.dll?ViewItem&item=1395629583

http://cgi.ebay.de/ws/eBayISAPI.dll?ViewItem&item=1395473519

Don't know, if the diaphragma of the T 35 tweeter would fit in the T 350.

[k2dx]

Great Plains Audio can help you. (and reasonably!)

http://greatplainsaudio.50megs.com/

You should not change the 16 ohm impedance.

They have the tooling to make 16 ohm EV diaphgrams...

[Manuel Delaflor]

This thread made me think about my own tweeter. I have the K77M wich, apparently, uses the T35. What about the parameters of the Klipsch Heresy crossover? Will I run in to some problems if I buy the EV diaphragm?

[CaptnBob]

I thought it might be fun to post my empirical (and highly subjective) results, now that I've had a chance to put everything back together and try it out.

The 16 ohm K-77 extended lower than the the 8 ohm version when used in the Patrician, although this was much less apparent when connected after the "L" pad. When a 25 watt 7.5 ohm resistor was inserted in series with the 8 ohm K-77, its frequency response seemed quite similar to that of the 16 ohm driver. There was a slight, but noticeable drop in overall output, but this could more than be made up for by adjusting the aforementioned "L" pad.

The T-350 exhibited the same characteristics as the 8 ohm K-77, except, of course, at a somewhat higher level, including the lower frequency response with the 7.5 ohm resistor.

These tests were all done with white noise supplied by my trusty Advent 400 table radio (It's very impressive to have a radio the size of a small box of Kleenex drive the 300 pound Patrician to sound levels usually associated with above ground nuclear testing, but I digress.)

When I listened to music, with the driver actually in place in the system, I couldn't tell much difference. In fact, I believe I preferred it without the resistor, and the higher frequency response cut off. This may be a reflection on the irregularities of the T-25 midrange, but I thought I'd mention it.

One final note. The other Patrician (a Mark II, I'm told,) has the old style narrow throat no phasing plug T-35 in it, which I was able to replace through the wonders of ebay. What a nasty sounding thing! It's got a serious peak, I'm guessing, between 7 and 8 k, and not a whole lot above that. I can certainly see why PWK opted for the University tweeter in his early production.

Thanks to everyone for your help!

Captain Bob

[Malcolm]

PWK apparently didn't think much of the University 4401. It was just the best he had available at the time.