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Old 4th December 2004, 09:05 AM   #151
Cobra2 is offline Cobra2  Norway
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Default Modification?

How about using a 10nF as the smallest cap, and make a new calculation (list) for the resistor-grid...?

Arne K
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Old 4th December 2004, 09:10 AM   #152
Bricolo is offline Bricolo  France
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Jens, how did you choose the 1nF value (and the others) for its frequency range?
Is there a rule of thumb or something?
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Old 4th December 2004, 09:43 AM   #153
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Well,

Regarding changing the smallest capacitor to 10nF is definitely a good idea, but the 0.1 multiplication cap needs to be 10uF. This is not practical (I think) but if you can live without that option it should work ok.

I’m not sure about the current draw when you use smaller resistors; I have to do some math on it first to be sure.

I can post a set of resistors, or you can just divide the know values with 10 and use these values with the bigger caps.

\Jens
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Old 4th December 2004, 09:44 AM   #154
MBK is offline MBK  Singapore
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Jens,

why 10 pF for c200/00? Parallel to 22k (R200/500) I calculate Fc of 154 kHz with C200/500 at 47 pF.

Re: the shelving LP filter formed by the resistors (because that's what it is basically, a capacitance parallel to a resistor in the feedback path), at least we know now why...

MBK
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Old 4th December 2004, 09:56 AM   #155
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10 pF because you will "see" the Cap more at a high Q than a lower Q.

The change is not big, so if you are happy with 47pF by all means use that

Both resistors that set the Q have to be taken into account

Have you tried simulating it just for fun?

\Jens
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Old 4th December 2004, 10:17 AM   #156
MBK is offline MBK  Singapore
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Jens,

I just use plain old paper and pen, never got around to learn simulators. So I go by formulas.

I considered only the R200 // C200 combo in conjunction with the R201-220 that set the Q. This in itself should be a shelving LP topology, with F1 (lower corner frequency) of F1=1/2*pi*R200*C200. But haven't considered the influence of the frequency setting network on the other side of the op amp output. That one's a bit heavy w/o a simulator

Ten again the problem we now have would not have shown up in a simulator...

So if you say 10 pF I believe it ...
I'll keep the 47 pF in though , I likely stick to the lower Q's.

MBK
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Old 4th December 2004, 10:32 AM   #157
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The problem did not shown up on paper either Nor in the simulator.

Had I known the stray cap was there, I would have taken it into the paper work I did for the project and I would have shown up.

\Jens
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Old 4th December 2004, 09:22 PM   #158
moamps is offline moamps  Croatia
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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. If the ground plane were removed, the value of the capacitor in question would probably drop so the capacitor's influence would be diminished. However, I do not believe the existing dip can adversely affect the filter's operation (in the case of LPF, parasitic capacitance may cause a small shift in the crossover frequency). My recommendation is to keep all this in mind when designing the next version Ground plane is not always useful when it comes to low frequency.

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. Since the application of the crossover we are discussing here is universal, it can be expected to perform inferiorly in some respects to any crossover that has been custom-made for an individual case.

Simulators are not "almighty", as they mostly simulate ideal case scenarios. A more sophisticated board design software (pCAD, etc.) should handle the problem, given enough input data.

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Milan
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Old 4th December 2004, 09:59 PM   #159
Cobra2 is offline Cobra2  Norway
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Default Fix-2! ?

Would little cut all around the resistors be enough? ( on the top layer? ).
Can be done in ~3,5 min. with a dremel....

Arne K
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Old 4th December 2004, 10:16 PM   #160
moamps is offline moamps  Croatia
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Default Re: Fix-2! ?

Quote:
Originally posted by Cobra2
Would little cut all around the resistors be enough? ( on the top layer? ).
Can be done in ~3,5 min. with a dremel....

Arne K

It's worth a try. Might help. Don't dremel your eyes out.

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Milan
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