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In Reply to: RE: T-IVa ribbons? ... posted by Davey on March 23, 2015 at 20:12:18
Surely, Davey - I agree it was placed there deliberately. :-))
However, you went on to say "There are valid reasons to position an attenuating resistor in an alternate location in a filter network. It will achieve a slightly different action in the network depending upon placement and Magnepan may have understood that and taken it into account for their objective."
In fact it was not an "attenuating" resistor - they provided connectors to insert one of these (instead of the default link) - it was a "compensating resistor", to cope with the change in the ribbon resistance, compared to the previous value of 3ohms.
Now, you and I can model the difference in filter response which the resistor's position causes, using lspCAD ... but I suggest Magnepan didn't have such useful tools, back in the 70s & 80s. The way I see it, it would've been slightly more difficult to place the 1ohm resistor right next to the ribbon - it was easier to put it right behind the fuse. So that's what they did! They should've changed the HP XO values to compensate for having a 2ohm ribbon ... but, again, that would've been extra work - so they didn't. Then they went back to a 3ohm ribbon with the 3.3 ... and the issue went away.
Regards,
Andy
Follow Ups:
I believe it was intentional as they wanted the lower sensitivity 2 ohm ribbon to be high passed higher up
1. Because the ribbon was being high passed too low originally and was not robust enough even in the new thicker 2ohm version
2. since the contribution of the ribbon lower down made the speaker somewhat bright
Yes, that could well be. However, when I rebuilt my passive XOs - and subsequently, with my active XO - I always put the 1ohm resistor right next to the ribbon and used the original XO point ... and have not had any blow-ups. :-))
Andy
We are talking early 80s so the likelihood of blown tweeters from Adcom 555 565's or ampzillas clipping was very high.
80's , i know of maggie owners losing tweeters in 2015 ... :)
[We are talking early 80s so the likelihood of blown tweeters from Adcom 555 565's or ampzillas clipping was very high.]- Satie
Well, now we're getting into semantics. :) My definition: A series resistor is an "attenuating" resistor, and parallel/shunt resistor is a "compensating" resistor. I think you'll find that my definition is correct if you peruse some engineering texts.
Back then they didn't have lspCAD but, amazingly enough, they actually still had electronics engineers and could easily analyze/test this circuit for relative differences of the resistor position.
I've already modeled the Tympani IIIA schematic in an old SPICE program a number of years ago. I can translate into lspCAD if you like.......if I can still find it on my old computer.
Cheers,
Dave.
Thanks, Davey but when I said "III" & "IIIa" I was meaning the original 3-series - the MG-III & the MG-IIIa. I know what ribbons these models had - and I now have a T-IVa ribbon in my 'Frankenpans'.
As dar as I am aware, the Tympani III did not have a true-ribbon.
Regards,
Andy
Well, my goodness, we're going to have be more specific on the models we're talking about here. Roger was talking about the Tympani IIIA's, with his resistor comment, was he not?
Yes, I'm aware they don't have a ribbon tweeter and the "normal" IIIA's do. :)For reference, above is the difference between the two possible positions of the 1 ohm resistor (with the two ohm driver) in the Tympani IIIA tweeter network.
Dave.
Edits: 03/24/15 03/24/15
No, I was talking about the Tympani IVa.
(I have the Tympani IIIA too but I did not mention them here.)
It sure was nice of the OP to come back and elaborate on his query. For all we know he's working on some other non-Maggie ribbon tweeter.
In fact, that looks to be exactly the case. :)
http://www.audioasylum.com/cgi/m.pl?forum=hug&n=172687
Dave.
I guess I was responsible for this thread about Magnepan ribbons, just an "ass"umption on my part. I was tempted to ask which model he was asking about - thinking about the MG 2.5 or 2.6 with a 1.45 ohm impedance - but never did. It didn't even occur to me that he was asking about a non-Magnepan tweeter.
The Raal ribbon uses a transformer to get the load impedance to ~8 ohms (above 1 Khz) which is why the OP could not measure the impedance of the ribbon, only the DCR of the primary. From the data I found the impedance below 100 Hz is ~1 ohm which is probably the actual DCR of the ribbon element and unlike the Maggie ribbons it is not a pure resistive load.
I would be willing to wager a guess that the actual ribbon diaphragm -ZMin is .3 ohm and not 1 ohm ...
Regards
Not your fault. A little Googling by the OP and he could have easily answered his own question. As you did, I easily found the impedance plots of the RAAL tweeters on their website.
My goodness. :)
Dave.
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