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Active circuit to compensate for a dip ?
Active circuit to compensate for a dip ?
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Old 8th April 2007, 11:43 AM   #1
youyoung21147 is offline youyoung21147  France
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Question Active circuit to compensate for a dip ?

Hi !

I've built sealed boxes for Fostex F120A drivers, but I got a bad surprise when measuring the driver : it exhibits a -10dB dip at 5kHz !! (not a problem with the enclosure)

The speakers will be active filtered, so I can build an active circuit to compensate for that.

The problem is that I only found notch filters which do the opposite of what I want.

Is there a circuit that could help me shape this weird response ?


Thanks in advance !
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Old 8th April 2007, 06:16 PM   #2
myhrrhleine is offline myhrrhleine  Belize
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Have you checked to be sure it's not caused by the room?
move the speaker/mic and retest?
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Old 8th April 2007, 06:43 PM   #3
youyoung21147 is offline youyoung21147  France
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No, it is not an issue with the room or the enclosure or the driver.

Tested in a different room, on an open baffle, the result is the same.

And it is no problem with the mic either, other speaker measure normal...


Fostex really cheats on us in their datasheet...
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Old 8th April 2007, 06:55 PM   #4
myhrrhleine is offline myhrrhleine  Belize
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It's more common to bring down the peaks everywhere else than to raise a dip so much.
Anyway, most op-amp chip makers publish equaliser circuits which can boost that much.
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Old 8th April 2007, 07:17 PM   #5
youyoung21147 is offline youyoung21147  France
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Hey, that's not stupid what you say ^^

Have the nominal gain of the amplifier only at 5kHz, and reduce it under and above. That could work !

By the way, are there phase or group delay issues doing this kind of equalization ?
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Old 8th April 2007, 07:29 PM   #6
myhrrhleine is offline myhrrhleine  Belize
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There's always phase issues when equalising.
Equalise a bit at a time.
See if the results sound good.
Trust your ears.
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Old 8th April 2007, 07:35 PM   #7
joe carrow is offline joe carrow  United States
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I would say that the bigger question is- why is the dip there in the first place? Dips are generally somewhat less offensive than peaks. What can you do, acoustically, to get rid of this dip?
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Old 8th April 2007, 07:45 PM   #8
youyoung21147 is offline youyoung21147  France
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The dip is here because the drivers are like this, and I don't know why... Bad design for sure.

The solution would be to replace the F120A with some FX120, but now I've bought the F120A I have to deal with this sucking dip...

To get rid of it, I can use the equalizer of the audio player, the problem being that these speakers won't be used with a computer...

I could also build a horn with a very narrow response, but I have already built the enclosures, and I'm not sure about the result anyway.


As I will active filter these speakers, the only solution I see is an active circuit.

However, if you have another solution, I'd really welcome it !!!
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Old 8th April 2007, 08:09 PM   #9
consort_ee_um is offline consort_ee_um  United Kingdom
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You could use a circuit based on a graphic equaliser
See this thread and scroll down to post no 10:
http://www.diyaudio.com/forums/showt...threadid=88123
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Old 8th April 2007, 08:56 PM   #10
scottw is offline scottw  United States
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I don't really know anything but I'd like to make a guess.

The dip is around 5k and could be a number of things. One thing that comes to mind is at that frequency it could be from a reflection about an inch or so behind the cone. These speakers have a big magnet that covers(reflects) a lot of the backwave. You could try putting a little wool felt on the inside of the basket legs and over the outter edge of the magnet (without touching the spider) and make another measurement.

I'd try a mechanical solutions before adding more circuitry, but that's just me.

Scott
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