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Old 16th September 2006, 07:28 AM   #1
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Default Need to change passive xover frequency

I have a high pass 2nd order @ 125hz, it has (series)100uf with 5mh to ground the impedance is unknown as it is a electrostatic.
I need to shift that 125hz frequency to 200hz, can anyone work out for me the simplest way?.
As the 100uf is made of multiple poly-props, could I just say take off about 40uf and make it 60uf instead of 100uf, would that shift the xover up from 125hz to around 200hz?

Cheers George
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Old 16th September 2006, 12:12 PM   #2
rabbitz is offline rabbitz  Australia
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You need to do the inductor as well as the cap and assuming the impedance is around the same, then 62.5uF for the cap and 3.125mH for the inductor whatever is available.
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Old 16th September 2006, 01:01 PM   #3
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Thanks rabbitz,I take it from that you worked out what impedance (ESL) the crossover is seeing? Can you let me know?
Sorry I should have posted the impedance graph before

Cheers George
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File Type: gif monolith impeadence graph.gif (89.3 KB, 153 views)
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Old 16th September 2006, 01:04 PM   #4
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And here's the cross over but I'm now active on the bass low pass 24db @ 200hz
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File Type: pdf monolith 3 passive crossover schematic.pdf (9.4 KB, 23 views)
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Old 16th September 2006, 02:43 PM   #5
rabbitz is offline rabbitz  Australia
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Hi George

In my original calcs I assumed impedance was equal which is close to true on a normal woofer.

Two formulae are used for 2nd order:
C=V/(Rf)
L=R/(Vf)
C cap value, V is a variable depending on type such as butterworth and LR but these cancel out in the calc so are ignored, R is impedance, f is frequency. Since V is the same, then for example for a cap, V=CRf then CRf for 200Hz = CRf for 125Hz which gives C(200Hz)=CRf(125Hz)/Rf(200Hz) which becomes:
C=(100x8x125)/(4x200) = 125uF

That impedance graph makes a huge difference and at 200Hz would be 125uF (yes goes up) and 1.56mH. As the impedance drops, cap value increases and inductor value reduces for a set frequency and 125Hz and 200Hz are not very far apart.

phew...
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Old 16th September 2006, 10:48 PM   #6
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Martin Logan Monolith III with IIIp (passive xover)

Thaks rabbitz, sorry for the headache, that's what scared me when it saw the impedence graph, I asume that the graph is of the whole speaker as a complete product with the standard passive xover and bass driver insitu, and not just of the panel itself, I have no way of finding out.
But also the fact that now the bass part of the passive xover is gone becuse it's now active, would that change the graph much at the 200hz region of the els part of the graph.

The reason I'm going this way is that i tried to active xover the esl with the 24db LR active xover and did not like what even very good opamps did to the sound (sterilized it) and the passive correction components were left in place, so the balance was the same, just the 100uf and 5mh were removed. the active sounds two dimentional and more (mechanical/sterile) than the passive.

I need to shift that xover up to 200hz because the the active bass can go up that high (since a driver change) and if esl have a weakness its in the lower to mid bass in that they need more slam in that region, this is why the shift from 125hz to 200hz.

I think I should somehow measure the impedence @ 200hz of my esl's with just the passive corection networks in place and not the 100uf and 5mh but I've never done this, need to learn.

Cheers George
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Old 17th September 2006, 04:03 PM   #7
rabbitz is offline rabbitz  Australia
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Might pay to post a question in the ESL...etc section as those guys might be able to help you.

I have zero knowledge on those suckers.
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Old 19th September 2006, 01:24 AM   #8
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Managed to plot the impedence from 50hz to 1100hz, the red trace is planel only with the standard ML high pass xover intact, the blue trace is the way I want to use it with the ML high pass xover minus the (C2A & L3)100uf and 5mh but with everything else in circuit even the LCR is in circuit.

I need to cross over at 200hz with different values of the 100uf and 5mh, the blue trace is the one I believe is the one to calculate the new values of C2A and L3,
To me the impedence at 200hz (blue trace) looks as though it maybe hard to have an effective 2nd order at the impedence of 14ohms with it climbing and falling so steep after the 200hz point, what do you think?


Cheers George
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Old 19th September 2006, 01:57 AM   #9
lndm is offline lndm  Australia
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An interesting challenge. The downturn after 400Hz appears offputting.

I wonder whether it wouldn't be worth also plotting the impedance phase and then putting this into a crossover simulator to do visually.

You'd want, then, to have an acoustic response plot too. You could sim the response from this both with and without the original L and C, thereby getting a visual on the slope of the original crossover, and hence something to mimic as you go up in frequency.

Even if you don't want this slope, it may assist you in visualising what it was you had been listening to in the past, and hence make a more informed decision.
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Old 19th September 2006, 02:08 AM   #10
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Thanks lndm, then again Martin Logan used a 2nd order at the 125hz (C2A & L3) and it's just as steep on both sides, so maybe it would work fine at 200hz also,
Would you or anyone else know the values for C2A & L3 for 200hz when it's seeing the 14ohms now?

Cheers George
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