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Old 3rd December 2004, 12:35 PM   #141
MBK is offline MBK  Singapore
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LOL!

Duh. I must have been measuring too much lately. No space left in brain for thinking of the obvious.

Thanks dude. That'll teach me.

MBK
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Old 3rd December 2004, 04:37 PM   #142
tiroth is offline tiroth  United States
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np. Now I am bugged about this again though; I had managed to convince myself that this was some freakish build error, since I could not explain it's cause and I had contrary evidence from another build of the circuit.
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Old 3rd December 2004, 05:10 PM   #143
MBK is offline MBK  Singapore
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FWIW, I calculated what capacitance to gnd would produce a -1 dB voltage divider. For 1 nF (1st order filter only) it is 120 pF, for 0.5 nF (2nd order with the two 1 nF caps in series) it is thusly about 60 pF. (I did the curves for the 1st order HP and lo and behold, the level only falls off 0.5 dB).

Current jfet input op amps have about 2-6 pF input capacitance, too low to count? But anyway, I saw the same effect with the bipolar NE5532 as well.

What *could* produce significant capacitance to ground is the double ground plane of the MOX PCB's. Now why does that not happen to Jens then Did he do his test with a prototype board which was single layered? Did he solder the caps at a couple mm higher from the board (that can't really be it though, since the actual connection is still at board level).

MBK
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Old 3rd December 2004, 05:24 PM   #144
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I made the measurements on the board pictured on my webpage:

Click the image to open in full size.

I'm not sure whats wrong to be honest.

The formulas dont really hide anything AFAIKT

\Jens
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Old 3rd December 2004, 05:27 PM   #145
tiroth is offline tiroth  United States
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Mystery...I do have the equipment to measure such small capacitances...

BTW Jens, I must say I was looking upon your Active Filter 1 with respect. I think I need to do something similar, if maybe a little smaller. I now have a total of 7 speakers to build based on MOX-designed prototypes, but I hate to have to use multiple 4"x6" boards. (3 ways) A little GEQ wouldn't hurt on some of the cheaper ones
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Old 3rd December 2004, 05:31 PM   #146
MBK is offline MBK  Singapore
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Jens,

OK, on thing I have differently is, I installed 47 pF C200 and C500 comp caps. Cutoff well into 200 kHz. Besides, this cap is isolated from the 1 nF caps by the resistors.

Tiroth,

that would be very interesting. I only have a multimeter. No chance below a couple nF (even if it does display random data).

MBK
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Old 3rd December 2004, 05:57 PM   #147
tiroth is offline tiroth  United States
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I found the comp. caps unnecessary with e.g. NE5532. In fact Jens and I had quite a discussion about this via email. I no longer remember whether or not I had any in-circuit in that old post.

Are you using any comp. caps currently Jens? They are certainly missing from that old board.
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Old 3rd December 2004, 06:49 PM   #148
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Nope, I found that they are not needed with the NE5532.

I will have to dig out my old protoboard and redo the measurement for the HP filter. I lost some of the data when my harddisk crashed.

I'll post results during the weekend.

(Regarding the filter one, Tyler mail me your adress and I'll send you a set of boards to try out)

\Jens
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Old 4th December 2004, 03:57 AM   #149
MBK is offline MBK  Singapore
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Re: C200 and C500 - these are just to set the gain for out of band HF to unity AFAIK. They are not strictly speaking necessary, just so that you don't accumulate HF junk over stages, especially when using high speed op amps such as OPA2134.

MBK
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Old 4th December 2004, 08:55 AM   #150
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Hello,

I just finished retesting the HP part of the module and found that I have the exact same problem now. (I must have missed the test on the 1nF cap earlier)

I did some simulations on the thing, and the problem seems to come from the frequency selection resistors dangling in free air when they are not used. The resistors will have some capacity to ground on the board, and when the multiplication cap is about the size of this parasitic capacitance the problem gets noticed.

I will have to do some more tests on it, to see if the problem can be resolved easily by means of removing the connection to ground underneath the resistors on the top layer. I’m still working on that though.

Regarding the C500 + C200 You should not go above 10pF as this will influence the performance of the filter at 20 kHz.

\Jens
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