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In Reply to: RE: Active crossover DX3 in Azura 204 Altec 414 posted by Bryon on August 05, 2007 at 12:08:54
There is nothing good to be had by trying to force the paraglow and lowthers to handle much below 100 hertz (really anything lower than the horn's cutoff IMHO). So you really need two crossover points, a high pass to the paraglow/lowthers, and a low pass to the sand amp/414's.
Marchand sells a very low cost solution if you don't mind doing a little soldering and building your own "box". Look at the XM1. Their power supply is adequate but you could build something a lot better from scratch. If looks matter you can consider the XM9. Marchand is a lot better than the options you mention. If you really hate the idea of op amps on the lowthers consider the XM46, a passive line level crossover. But it is a very tough load for a tube based preamp. I'd ask Paul Joppa's advice (a step down transformer after the foreplay is one way for example).
Russ
Follow Ups:
Russ makes an excellent point. The Lowther drivers should not be run full range as they try to reproduce the bass, it will muddy the midrange through the doppler distortion, and force the amp to work harder than it needs.
However, instead of a separate crossover device which will reduce the transparency of the midrange, simply size the capacitor(s) in your amps to serve as high pass filters.
Retsel
F= frequency in hz
R= resistance in ohms
C= capacitance in farads
I did some online research and found this formula to determine the size of capacitor to use as a high pass filter in the Paraglows.
I was surprised at the values I ended up and as I am not overly competent in these matters, thought I'd appeal to those who are.
The DX3's are 16 ohm drivers but actual resistance is more like 10 ohms. Which value should I use for R? Can I ignore the supposedly small level of resistance that the output RCA jacks and speaker cable add?
If I want to aim at a cutoff 200 hz and I use 16 ohms, the capacitor value works out to .202uf so I can parallel a couple of .10 uf caps to get there.
If I use the actual resistance of 10 ohms, the capacitor should be in the neighbourhood of a .33 uf cap.
Is this the right formula to use for this purpose? Is my math even close to being correct [it's been nearly 40 years]. Am I missing anything obvious [or not so obvious?]
thanks again for your input
Bryon
I would determine the value experimentally since what the capacitor sees is the impedance at 200 hz not the nominal impedance of the driver. Also this impedance is seen "through" the transformer.
Before sizing and investing in all that money for the V-caps, I would make one more change (if you have not already). I would wire your output transformer as an autoformer, instead of the conventional parafeed setup that you have now. To do this, search this and the Bottlehead forum for autoformer and parafeed and you will read the instructions for how to make the conversion. This change provides a noticable improvement in transparency.
Once you make the change to the autoformer setup, try a capacitor in the 0.2 range (if you are like me, you have these sitting around) and see where the Lowther starts to roll off (measuring their output using a test CD). You can then ratio the 0.2 capacitor value appropriately to derive the crossover point at 200 hz.
Do you also have a coupling capacitor in this amp (the capacitor between the driver tube and output tube)? If so, you can size this smaller as well to have a second order crossover at 200 hz. This will also save you money with the V-caps if you plan on replacing these as well.
Retsel
Sounds like a good idea - I know you can increase the bass by increasing the value of the capacitor and so I guess the inverse would hold true as well.
The question is what value to use. The Paraglows came stock with 2.0 uf caps and I actually increased them to 3.3 to get improved bass. How much lower do I have to go? 1 uf? .47 uf? I was thinking about getting teflon v-caps for my Paraglows and this could be an unexpected savings as the price of the v-caps increases proportionally and significantly as the cap value increases.
thanks for the input
Bryon
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