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In Reply to: RE: My MG-20 manual lists the midrange-ribbon crossover as 3 KHz. posted by Al Sekela on October 22, 2009 at 15:00:20
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.
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