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Old 8th August 2005, 03:50 AM   #1
LNeilB2 is offline LNeilB2  United States
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Question Rear firing Seas 27 TDFC

I'm going to post this on multiple forums to see what everyone thinks. I have an extra pair of Seas 27TDFCs. I'd like to add them to my current 2 ways as defeatable rear firing tweets (toggle switch) with their own dedicated crossovers. What frequency would you recommend bringing them in at? I'm thinking pretty high, 10-12k. Just looking for added ambiance at times.
Could you recommend a circuit? I have no idea what I'm doing.
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Old 8th August 2005, 04:06 AM   #2
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Do you have a Zobel on your existing tweeters?
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Old 8th August 2005, 04:12 AM   #3
LNeilB2 is offline LNeilB2  United States
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Default No

I don't. Why?
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Old 8th August 2005, 04:26 AM   #4
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Mmm. This might be a good time to add one. Here's how:

1) Calculate the zobel for the Seas tweeter
2) Add it in parallel with the 27TDFC
3) Calculate the zobel for your front panel tweeters. Likely the resistor will be close to the impedance of the Seas tweeters. That's good, because you'll use the Seas with the Seas zobel as the resistive element in the zobel for the front tweeters.

That's it! The zobel capacitor will cross in the back tweeter at a highish frequency around 10 or 12 kHz, at pretty well the point where the front tweeter series inductance starts rolling off its power response. The power response drop is not obvious on an on-axis frequency response graph since tweeters also get more directional with increasing frequency.

The advantage of this technique is that you have a 10-12 kHz first-order crossover on the back tweeter, exactly what you want for ambience, but you also have the main tweeter crossover keeping the midrange away, which is important since a 1st order would only be 12 dB down at 2.5 kHz, 18 db down at 1.25 kHz, and 24 dB down at 625 kHz. That might sound reasonable, but power also increases in the midrange so the poor thing would be working hard for no reason. This method avoids that problem.


Cheers,
Francois.
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Old 8th August 2005, 04:30 AM   #5
LNeilB2 is offline LNeilB2  United States
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Default Ok, but...

The existing crossover is perfect as-is. I can't see throwing a Zobel on it after the fact without it having some negative effect. And remember, I'd like to turn the rear tweet on/off as needed. I don't mind building a dedicated circuit for it. I have gobs of crossover parts. And I'm thinking 2nd order or higher might be better. I'm sensitive to excessive output at 8k.

Thanks!

Neil
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Old 8th August 2005, 09:19 PM   #6
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I asked a similar question awhile back.

The general concencus from folks with experience and testing of this suggested it was counter productive to imaging and coherency as a whole.

Perhaps with extreme fine tuning like the Von Schweikert VR range it could be good but I believe they actually implement an ambience retrieval circuit which shifts phase and only works from a very specific frequency.

Cross it too low and it will sound plain bad was the opinion but considered effort to refine the XO point and careful use of phase alignment could reveal more 'air' or ambience depending on the room.

IMO its complex and time consuming to perfect and the subject needs more understanding than just how to build a XO's. I'd say you need good measuring equipment, an intimate understanding of room relationships as well as time/phase. This is the type of territory that Linkwitz would excel at, most of us mere mortals don't have the understanding to do it correctly and do it well.
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Old 9th August 2005, 01:27 AM   #7
tktran is offline tktran  Australia
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Hi Neil,

Could you check your mail? I have a question that is urgent.

thanks,
Thanh
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Old 9th August 2005, 07:55 AM   #8
jcake5 is offline jcake5  United States
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DSP_Geek,

How is the rear tweeter wired in? I am confused. I understand the Zobel on the front tweeter and how it is wired. You lost me on the rear tweeter using the front tweeter resistor. Could you please explain using the the Plus and Negitive tweeter terminals as the wiring points. Is the rear tweeter in parallel to the front tweeter?

Please explain,
Eric
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Old 9th August 2005, 01:34 PM   #9
jomor is offline jomor  Greece
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Adding a Zobel in an existing crossover, will decrease the tweeter's spl a bit, and completely mess the PHASE matching of the drivers around the (critical) crossover frequency. interesting idea though (if it is designed from scratch)..
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Old 9th August 2005, 02:35 PM   #10
DaveM is offline DaveM  United States
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Hey Neil,

I would suggest simply trying things. I added a set of old peerless tweeters to my line arrays and they really opened things up (depth and image). They were crossed over at 3500 first order (in other words there was nothing but a cap on them.) This may or may not work for you, but try slapping together a textbook second order crossover and give them a listen. Throw in an L-pad to make the level adjustable. Play with the level as needed and see if that gets you what you want.

If there is no absolute right answer, try the easiest solution, you might be suprised.

DaveM

P.S. how is the finger doing?
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