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Old 5th December 2004, 09:07 AM   #161
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Hello,

I finished measuring and found that we speak of a - 0.7 dB level at 20 kHz, so I don’t really think that there is any reason to resort to a drastic solution like cutting with a dremel.

I have chosen (Like Tyler suggested) to mark the boards where the x100 setting is used, and adjusted these boards to have the right gain at 20 kHz using this multiplication setting. This way I will use these boards for midrange and treble, and the others can be used for bas and sub. I think this is the easiest way to get around the problem.

\Jens
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Old 5th December 2004, 10:18 AM   #162
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Quote:
Originally posted by moamps
Hi,

I have checked today if the reported problem "1dB dip on HF" is also present with the crossover I designed and published here on the forum. When I measured the dip on the HF filter, it was approx. 0.2dB. Then I went on to check if the problem would show up when I used a protoboard filter - dip was about 0.3dB, regardless of the opamap used (TL071, NE5534AN,LF35x,...). The dip is apparently caused by the capacitive divider, which is formed of a capacitor 1nF and a parasitic capacitor between the opamp's non-inverting input and the ground plane. Parasitic capacitance must be considerable in Jens's design, mainly due to (unnecessary) implementation of the ground plane on the PCB's top side.
On very easy way to reduce the influence of stray caps is simply to scale down all parts to lower impedance values.
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Old 5th December 2004, 12:14 PM   #163
moamps is offline moamps  Croatia
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Quote:
Originally posted by peranders
On very easy way to reduce the influence of stray caps is simply to scale down all parts to lower impedance values.
Quote:
posted by moamps a bit earlier
The negative effect can be avoided, as someone here has already mentioned, by using larger value capacitors and smaller resistors. Extreme caution is advised, though because that may cause the filter's input impedance to fall significantly in the passband region.
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Old 5th December 2004, 03:25 PM   #164
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Default resistor values

Sorry I didn't post this earlier. What is being found is consistent with the summary conclusions found in:

http://focus.ti.com/lit/an/sloa024b/sloa024b.pdf

The recommendation for Sallen-Key filter resistors is in the range of hundreds to several thousand ohms. Capacitance is the culprit.

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Old 6th December 2004, 09:56 PM   #165
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since i am finally ordering parts to populate my MOX boards, i had a question regarding values for the main board..

i will be using these exclusively with one pair of speakers, and they will be matched to those speakers. from the manual, i can see that the speakers use a 2-way, 6dB/octave xover at 1100hz.

instead of stuffing the mox board with all the values, i can just use what i need, right? and instead of jumpers, just hard-wire it to the speakers' requirements?

i assumed the above. the real question is what do i do with the q? do i just keep it at 1.0 for both HP and LP? thanks for any answers!
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Old 6th December 2004, 11:19 PM   #166
moamps is offline moamps  Croatia
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Quote:
Originally posted by cowanrg
....since i am finally ordering parts to populate my MOX boards, i had a question regarding values for the main board.....
Hi,
Theoretically, your conclusions are correct.

However, I would suggest putting such crossover elements that would enable you to choose slope 6 or 12dB/oct and crossover frequency from 800Hz to 3.15kHz. This would allow you to fine-tune the crossover because final crossover frequency in a 6dB crossover will be, in most cases, the result of combining the driver's natural roll-off and the crossover's slope. For this reason, you will occasionally notice that LPF may be set to 1.5kHz and HPF to 2 or 3 kHz, for example.

Besides, a bit of experimenting can't hurt.

Regards,
Milan
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Old 7th December 2004, 12:02 AM   #167
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im not sure why you are suggesting i experiment? i really dont want to mess around with different settings. do you really think i could improve upon the crossover frequency? these are planar speakers if that makes any difference... i have seen many people build either outboard passive xovers, or active xovers, and 99% of people use the factory xover point (the other 1% im just not sure about).

anyone else see a benefit in playing around with xover points? in the manual they even state if you are going to do an external active crossover to use 1,000 hz, not 1100 like the passive...
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Old 7th December 2004, 12:38 AM   #168
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Since it is only a few extra components, I'd definitely stuff enough to try a few things out. I mean, after all the effort it takes to build the speakers, amps, and MOX, don't you want to be sure the sound is the best possible? That is really the entire purpose of MOX, there is no need to say "I've resoldered this circuit 5 times, I give up!"
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Old 7th December 2004, 05:15 AM   #169
moamps is offline moamps  Croatia
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Quote:
Originally posted by cowanrg
...im not sure why you are suggesting i experiment? i really dont want to mess around with different settings...
Well, then do whatever you like. I don't see the point of me explaining to anyone the value of experiment... especially on a DIY forum.

Regards,
Milan
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Old 7th December 2004, 05:31 AM   #170
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sorry if i offended you in any way.

i am just ignorant with electronics, and VERY ignorant with loudspeaker design. as i understand it, a speaker has a pretty set crossover point. im just looking to replace the passive xover (with JUNK components) with this active one. thats all. i dont really want to reinvent the speakers. as many of you know, i have too many other half-finished projects

so, sorry if i came across rude, i didnt intend that. i am just unaware of any benefit with tamperint with xover points in a commercial loudspeaker. if there is one, im all for it, i just didnt realize there was.
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