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How to join the Q30 with a tandem of two 8 ohmKEF racetracks

 
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Wiesiek Lipowski
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PostPosted: Sun May 20, 2007 9:43 am    Post subject: How to join the Q30 with a tandem of two 8 ohmKEF racetracks Reply with quote

I would like to make DIY floorstanders out of old KEF Q30's and two 6x9 8 ohm racetracks each.

Should I leave the Q30 xovers alone and wire the bass drivers through a inductor only? What about the mid=bass drivers mismatch?

What value inductor it should be?

I recently obtained a pair of Q30's and have the 6x9 racetracks from my rejected old projects.

I'd be grateful for any helpful advice.

Wiesiek
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ColinR
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PostPosted: Sun May 20, 2007 10:19 am    Post subject: Reply with quote

I've no idea about the specifics, but a general proceedure would be something like this.


Wire 8 Ohm racetracks in parallel.

Take something like a Cantata bass section 7mH, 3.5mH and 100uF.

Halve the values of the inductors and double the value of the capacitor for 4 Ohm use.

Repeat the proceedure if you want to halve the crossover frequency, (Q95 bass unit's crossover ~120Hz).

Wire Q30 crossover in as if it were the mid/treble section of the Cantata.
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Wiesiek Lipowski
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PostPosted: Sun May 20, 2007 3:25 pm    Post subject: Reply with quote

Thanks a lot, Colin,

So, for 4 ohm use you suggest 1.75 mH, 400 uF to ground, and aprox. 0,9 mH as a low-pass filter for a tandem of 6x9's, (120 Hz) and the Q30 xover for the mi and treble units. Am I right?

Wiesiek
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ColinR
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PostPosted: Sun May 20, 2007 4:26 pm    Post subject: Reply with quote

Quote:
So, for 4 ohm use you suggest 1.75 mH, 400 uF to ground, and aprox. 0,9 mH as a low-pass filter for a tandem of 6x9's, (120 Hz) and the Q30 xover for the mi and treble units


Er yes abd no.

I got it wrong Embarassed .

You double the capacitors when going from 8 to 4 Ohms and again when you halve the frequency BUT you still need to double the value of the inductors when you halve the frequency.

So.

The correct ~125Hz crossover using effectivly a 4 Ohm unit will be.

7mH, 3.5mH and 400uF.

If the bass units need attenuating start playing with 24 Watt resistors in series with the +/- capacitor to dump some current - 1 Ohm would be a good value to start with.

Sorry for the error but it is Sunday Very Happy .
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Wiesiek Lipowski
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PostPosted: Sun May 20, 2007 6:21 pm    Post subject: Reply with quote

You mean the resistor "under" the new 400uF capacitor in the bass section to ground?

Wouldn't it rather increase the bass signals going through the inductors?
Wiesiek
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ColinR
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PostPosted: Sun May 20, 2007 6:59 pm    Post subject: Reply with quote

Quote:
Wouldn't it rather increase the bass signals going through the inductors?


No, providing it's value much less (~50%) than the dcr of the "bass unit", it just dumps some current reducing the power, hence a reduced spl.
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Wiesiek Lipowski
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PostPosted: Sun May 20, 2007 7:38 pm    Post subject: Reply with quote

Sorry to bother you again, but I still can't see how this resistor dumps the signal.

IMHO, if it were a pure shunt resistor to ground, it of course would do the trick, but since it is in series with the capacitor, it would rather hinder its job.

Another matter, though, in the Q30 xover, in its mid-woofer section, there is a 16uF cap with a 5 ohm resistor in series to ground. Perhaps I can give a 1-2 ohm resistor here to decrease the bass output from the B165. What's your opinion?
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ColinR
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PostPosted: Sun May 20, 2007 9:12 pm    Post subject: Reply with quote

Quote:
since it is in series with the capacitor, it would rather hinder its job


This is why they're called speaker designers Smile .


AC Joule heating P=IVcos° where ° is the power factor or phase angle.

You can play with Z=sum of Z's plus R's, but Z's are dependant on cos° if you want to play V=IZ

So you can either just heat up a voice coil and some inductors (the resistance of a capacitor is very small) or add a resistor effectively in parallel with the voice coil to reduce the power through it.

Quote:
there is a 16uF cap with a 5 ohm resistor in series to ground. Perhaps I can give a 1-2 ohm resistor here to decrease the bass output from the B165


If the dcr of the drive unit is >5 Ohm go for it, but I'd try 3.5 Ohms first.

If the dcr is <5 Ohm the circut is a C-R Zobel taking out a resonance.
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Wiesiek Lipowski
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PostPosted: Sun May 20, 2007 10:19 pm    Post subject: Reply with quote

Thanks Colin.

I've sent you an email.
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