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Old 1st March 2004, 09:50 PM   #1
Sherman is offline Sherman  United States
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Default ML TQWT 2-way? (kinda long)

I'm planning on building a ML TQWT (straight, not folded) using Pioneer B20FU20-51FW drivers. I ran the parameters using Martin King's Mathcad worksheets and the frequency response SPL is virtually flat (looks like 93dB +-2) from 35hz to the limit of the sheets (10K). The problem is that the Pioneer driver "only" goes to 17K and looking at the specsheets it appears it will be down 6dB well before that.

So... I'm thinking of adding a tweeter using only a highpass cap rather than a crossover, maybe 1.5uF for about 12k-13k? The tube will be 60" internally with the driver at 50%, nice ear height from my couch.

Questions-
1- Would you mount the tweeter in the baffle next to the FR driver?

2- Would you enclose the rear of the tweeter using a cup of some sort or just let it open into the tube?

Thanks for any opinions.
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Old 1st March 2004, 10:14 PM   #2
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Speaker design is wrought with compromise. I went shorter than ideal for size (WAF) reasons. This limited my LF response but it also "tightened" up the bass. The B20 is not the most detailed driver and the tweeter I chose gave respectible performance. Therefore I chose a much lower XO point than you are proposing. This made it necessary to use an inductor. Taking the minimalist approach I came up with a 1st order series XO that rolled off the Pioneer before it's upper midrange breakup. Others don't seem to mind it and XO at 7-8Khz. I doubt you can get away with just a cap (but you might) due to the high efficiency of most tweeters. Likely you'll need to pad the tweeter with a resistor or two. I mounted the tweeter off-center which brings it as close to the Pioneer as possible. Tweeters are chambered so there's no need to add another one.
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Old 2nd March 2004, 12:35 AM   #3
Sherman is offline Sherman  United States
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Quote:
Originally posted by Timn8ter
Speaker design is wrought with compromise. I went shorter than ideal for size (WAF) reasons. ..snip... The B20 is not the most detailed driver and the tweeter I chose gave respectible performance. Therefore I chose a much lower XO point than you are proposing. This made it necessary to use an inductor. ... snip... I doubt you can get away with just a cap (but you might) due to the high efficiency of most tweeters. ...snip... I mounted the tweeter off-center which brings it as close to the Pioneer as possible. Tweeters are chambered so there's no need to add another one.
Thanks for the info. I'm new at this and the help is greatly appreciated.

I have a little concern about the WAF as well but I built a pair of fairly bad looking little towers using 4 of those little 4" Pioneer drivers per and a tweeter and she has allowed them to remain in the family room so there may be hope for the TQWTs if I do a decent job finishing them!

Which tweeter did you go with? I haven't chosen one yet. In my towers I just used a cap but I have 4 "woofers" to a single tweeter. I was afraid I might have to do a "real" crossover with this TQWT.

I was thinking of possibly using an adjustable Lpad on the tweeter to make tweaking easier. Not too much at Parts Express.

Being a beginner at DIY audio stuff I didn't know tweeters were all chambered. That certainly makes sense now that I think about it.
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Old 2nd March 2004, 01:19 AM   #4
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I used the GR-T6 tweeter from GR Research. They had just introduced them so I was able to get a matched pair for $30. I see they are now $60 per pair so I feel fortunate. Another reason for the choice was that having two Pioneers in parallel makes impedance matching with the tweeter easier. I only use one series resistor on the tweeter leg keeping the efficiency up. There are many acceptable tweeters that can be used. One recommendation would be to avoid tweeters that are flush with the faceplate. Having the diaphram recessed in a tapered "horn" works better. Have a look at the
GR Research tweeters for an example. You may want to look at my BV2 speaker . I haven't put many details in as I'm still tweaking the design although I feel I'm getting very close. I think the L pad is a great idea. It the heck out of changing resistors.
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Old 2nd March 2004, 02:34 AM   #5
Sherman is offline Sherman  United States
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Wow, nice cabinets! If I produce something like that I'm sure they will be allowed to live in the family room. (Of course then I'll have to move my tube amp there permanently... or build another one!)

I was thinking of a little less expensive tweeter, something like the Audax TM025F1 (276-152 ) or the Tang Band 25-669B (264-800). (OK, a lot less expensive.)

I still think I'd like to try the crossover higher up but leave room to move the frequency. I'v been thinking of building a box onto the back of the cabinets to house the XO. That would allow easy changes. Maybe just screw a RS project box to the back with grommets and mount everthing in there.

I am planning on driving these with an 8 watt pp tube amp so I hope I don't lose too much in the XO and Lpad.
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Old 2nd March 2004, 05:03 PM   #6
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I used the Audax before too. It has nice dispersion. Should work fine.
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