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I am in the process of helping an inmate rebuild the passive XOs on his 3.3s, by providing my recommended circuit schematic (as the stock XO delivers a rather woeful overall (electrical!!) frequency response :-)) ).I started off using lspCAD to model the ribbon highpass filters of nearly all the true-ribbon Maggies - to whit, the IIIa/3.3/3.5/3.6/20/20.1 and T-IVa.
These schematics are all listed under "Tweaks" and looking at them, I am fascinated by the fact that every bloody model has a different -12dB HP roll-off ... yet they all have the same 2 ohm ribbon - except the 3.6 which has a thinner ribbon (which is 3 ohms). This is taken down much lower than all the other ribbons - I guess because the thinner ribbon is more robust and can take the increased current flowing at the lower frequencies?
I would've thought that Magnepan would've learn't during their product lifecycle that the ribbon was strong enough to take lower frequencies (without burning out) so, once they developed the 3.5 ribbon HP filter, they would've used this on the the 20. :-)) But no ... the -3dB point for the 3.5 is 3,900Hz while the -3dB point for the 20 is 7,500Hz!!
Anyway, for those of you who are interested, the -3dB figures are as follows:
* IIIa: 6,200Hz
* 3.3: 4,500Hz
* 3.5: 3,900Hz
* 3.6: 2,300Hz (thinner ribbon)
* 20: 7,500Hz
* 20.1: 7,400Hz
* T-IVa: 5,600Hz.IMO, the optimal setting for the ribbon in a IIIa/3.3/3.5 should be set around the 3.5 point of 3,900Hz. A bit lower is not a problem - I run mine at 3,400Hz - but certainly not as low as the (thinner, 3ohm ribbon of the )3.6 at 2,300Hz.
Regards,
Andy
Edits: 10/22/09 10/22/09
Andy,
You made me curious, so I took a quick look at the 3.3 crossovers in the tweaks section. (For the "Lars" 3.3/R values.)
The Bass Filter is a third order Butterworth
L2+L3 = 4.8mH
C2= 275 uF
L4 = 1.7 mH
For a nominal 4 ohm bass panel this provides a 200 Hz, third order roll off.
The combined mid & high bass filter is actually difficult to analyze without computer tools (Spice or Matlab). Fourth-order polynomial equations are not fun!
However, a simple approach is to model the ribbon filters as a series of two second-order LC-filters. With the Filter frequency equal to 1/[2 x pi x Sqrt(L X C)]
First Filter (external x-over, mid & tweeter)
C1=90 uF
L1=8.5 mH
freq = 182 Hz
Second Filter (tweeter)
C4=20 uF
L7=0.397 mH
freq = 1,786 Hz
The band-bass configuration is somewhat odd. I'd really have to derive that one.
At any rate, it would seem without an exact soulution that the ribbon filter on this model 3.3R is likely much lower than 4,500 Hz.
regards,
Jim
.
Andy,
I haven't seen the lspCad software before, looks like it is audio specific software.I derived the 3.3R fourth-order cross-over "Old School" last night while listening to some John Hiatt.
The input voltage is known, and there are two unknowns - the ribbon voltage (Vr) and the voltage between the two caps (V1).
KCL at Vr
Vr/R + Vr/L7*S + (Vr-V1)*C4*S = 0
KCL at V1
(V1-Vin)*C1*S + V1/L1*S + (Vr-V1)* C4*S=0
Solving for V1 = f(Vr) in the top equation, and substituting for V1 in the lower yields (after some manipulation)
Vr/Vin = (R*L1*L7*C1*C4*S^4) / [ R*L1*L7*C1*C4*S^4 + (L1*L2*C1+L1*L7*C4)*S^3 + (R*L7*C4 + R*L1*C1 + R*L1*C4)*S^2 + L7*S +R ]
There are four zero's at the origin, and four poles which are the roots of the denominator polynomial.
regards,
Jim
Edits: 10/23/09
So I let the computer do all the work!! :-))Yes lspCAD is for DIY loudspeaker nuts; Davey put me onto it many years ago. :-))
I only use it to model the XOs but, if you're making conventional cone-speaker monkey-coffins, you can model box behaviour as well. And it's got an extensive database of the Thiele-Small parameters for lots of cone drivers.
Regards,
Andy
Edits: 10/23/09 10/24/09
The schematic from Jian on the MUG tweaks page shows a 10 microfarad capacitor in series with the 3.1 ohm tweeter and a 0.7 millihenry inductor (tweeter and inductor in parallel).
Is this what you simulated?
My excel spreadsheet shows the -3db at ~5000 Hz (not 7500 as per Andy) and the crossover frequency at 2000 Hz not 3000 Hz. I am using the values listed in the tweaks page.Spreadsheet
I plan to procrastinate my demise for as long as possible. In the meantime, I practice by putting off all the little stuff.
Edits: 10/22/09
Hi Al,
Remember, the "crossover point" (between the mid lowpass slope and the ribbon highpass slope) is not the same as the -3dB frequency of the ribbon's HP filter. Typically, the crossover point will be around the -6dB frequencies on the two curves.
Yes, those values are what I used for the simulation - except I made one error (which I will correct/edit tonight, when I can run the simulation again). Because of the release date of the 20, I had assumed it must have the 3.5's 2ohm ribbon - not a 3ohm ribbon. (I didn't see the 3.1ohm value in Jian's schematic.)
That would certainly bring down the -3db point but can you please have a look at the back of your ribbon and tell me approximately how wide the corrugated aluminium strip is? My IIIa ribbons (and the 3.3 & 3.5) are about 1/4" wide ... whereas the 3.6 ribbon - which is 3ohms - is only about 1/8" wide.
Regards,
Andy
Hi Andy,
Sorry for taking so long to get the data for you... lots of chores today.
The MG-20 ribbons are 1/8" wide.
.
my 3.5r's have a 3 ohm ribbon. i've measured it many times. i think they started the 3 ohm ribbon on later 3a's. early 3a's had a 2 ohm ribbon and a 1 ohm resistor as i understand it.
The first IIIas had a 3-ohm ribbon. IIIas having ribbon serial nos. after SN 63744 have different ribbons ... Magnepan changed it to a 2ohm-resistance (presumably by widening it?) and so they introduced a 1-ohm resistor in series with the ribbon, to bring the total back to the 3-ohm value assumed by the IIIa crossover component values. (This is not the ribbon-attenuation resistor.)
My own IIIas have a 2-ohm ribbon (and the 1-ohm resistor) ... likewise the T-IVa ribbons you sent me are 2-ohms (but without the resistor).
Can you tell me how wide your 3.5 ribbons are? For instance, my 2-ohm IIIa ribbons are about 1/4" wide (as are your T-IVa ribbons) ... in contrast to the 3-ohm ribbon of the 3.6, which is only about 1/8" wide.
And when you measured the ribbon resistance, did the value of 3-ohms which your meter showed exclude the intrinsic resistance of the meter leads (which you can get by simply shorting the connectors)? Or was it the total of the ribbon resistance plus the meter leads?
Regards,
Andy
that's interesting. every once in a while i see an old set of 3a's on a-gon or ebay for a steal and am tempted to buy them just for the tweeter cores so i can have a spare set "just in case" . so do any of the 3a's have the same tweeter as the 3.5r's? (same cage and magnets)i'm not sure of the exact measurement, but the ribbon on the 3.5r looks narrower than the ribbon on the t4a to my eye.
Edits: 10/23/09 10/23/09
Hi Bart, I hope to be able to make a definive statement on that soon, when I've got some info from a couple of people. :-))
Aah, so the ribbon on the 3.5 looks narrower? Would you say it's more like 1/8" wide instead of 1/4" - if so the 3.5 definitely is different to the IIIa/T-IVa.
(Magnepan will not have more than 2 styles of ribbon material - so it's one or the other!)
And can you confirm that your reading of 3 ohms excludes the resistance of the meter leads?
Regards,
Andy
so you ended up with spares. the tympani tweeters are the same as 3a. i get .1-.2 ohms on a direct short of the meter leads and 3.1-3.2 on the ribbons. the ribbon element on my 3.5r is definitely wider than 1/8 inch. i would call it 3/16. that is using a tape measure not a micrometer.
You are already tri-amping so that the tweeter is independent of the other drivers, so how about going directly from the pre-amp to the treble amp only (without any filter) and using a pure tone generator and RS SPL meter to measure the acoustic response of the tweeter at 1 meter? Marchand has a free signal generator on his site.
BTW I am using a -3db of around 4k for my IIIa tweeter as I shifted the mid-treble xo point to ~2500k rather than the stock 3k.
I plan to procrastinate my demise for as long as possible. In the meantime, I practice by putting off all the little stuff.
Edits: 10/22/09
Yes - been there, done that!! :-))You would appear to have located it about the same as the 3.5 ... FWIW, I've been running mine lower for the last 5 years or so @ 3,400Hz (without any ill effects).
Regards,
Andy
Edits: 10/22/09
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