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How t measure the impedance of Ribbon tweeters?

173.169.122.75
Posted on March 22, 2015 at 19:12:21
chopper87@aol.com
Audiophile

Posts: 942
Joined: September 2, 2001
 I have quickly learned that I cannot measure the impedance of my Ribbon tweeters using the Ohm setting of my Meter. How can I determine the impedance of My Ribbon Tweeters? Thanks a bunch.

LA

 

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RE: How t measure the impedance of Ribbon tweeters?, posted on March 22, 2015 at 20:31:33
neolith
Audiophile

Posts: 4842
Location: Virginia
Joined: February 21, 2002
Contributor
  Since: December 2, 2004
 The 3 series (III-3.6) have a 2nd order HP filter, which means there is a shunt inductor across the tweeter. You will need to unsolder one of the connections to measure the impedance. OTOH the 3.3 and above all use the same tweeter (which is the same as the 20.x) and the impedance is 3.0 ohms. The IIIa has a different tweeter and the impedance is 2.2 ohms but this has a 1 ohm series resistor in the network.



"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

RE: How t measure the impedance of Ribbon tweeters?, posted on March 22, 2015 at 23:51:25
Roger Gustavsson
Audiophile

Posts: 2058
Location: Huskvarna
Joined: February 12, 2010
 The first ribbon tweeters were also 3 ohm, like in my Tympani IVa. It was replaced by the thicker 2 ohm ribbon + series resistor. Maybe to increase the reliability? Unfortunately, the position of the additional resistor caused a change of the frequency response. Magnepan should have placed just before the ribbon, not before the crossover. The later narrower (and thicker) 3 ohm ribbon still in production, also have thicker magnets. In theory it should have a higher sensitivity.

 

RE: How t measure the impedance of Ribbon tweeters?, posted on March 23, 2015 at 10:03:17
Davey
 Following Neo/Rogers comments......if you do attempt to measure the impedance of your ribbon drivers be very careful. These transducers are much less rugged than the mid/woof and won't enjoy low-frequencies and/or DC being applied.

Cheers,

Dave.

 

RE: How t measure the impedance of Ribbon tweeters?, posted on March 23, 2015 at 16:35:23
Norman M
 "The first ribbon tweeters were also 3 ohm, like in my Tympani IVa. It was replaced by the thicker 2 ohm ribbon + series resistor. Maybe to increase the reliability? Unfortunately, the position of the additional resistor caused a change of the frequency response."

The way that reads it could be interpreted as meaning there are IV-As having 3 ohm ribbons (the early ones) as well later IV-As having the "2 ohm ribbon + series resistor". I don't believe you actually meant to imply that, or did you?

 

T-IVa ribbons? ..., posted on March 23, 2015 at 19:09:44
andyr
Manufacturer

Posts: 12551
Location: Melbourne
Joined: September 2, 2000
 Are you saying your T-IVa ribbons are 3ohms by themselves, Roger - without the added 1ohm resistor?

You are quite right in saying that the original Magnepan ribbons - which came out on the MG-III - were 3ohms ... then (possibly when they changed to the IIIa?) it became a 2ohm ribbon plus a 1ohm res. Which, as you say, was put in the wrong place in the IIIa XO circuit! :-))

My own T-IVa ribbons measure the same 2ohm that my IIIa ribbons did - so I've been running them with a 1ohm Duelund in series, as my current ribbon amps can't handle a 2ohm load. However, at some stage I will be replacing my ribbon amps with the latest evolution of the circuit - which can take a 2ohm load - so I can then remove the Duelunds. :-))


Regards,

Andy

 

RE: T-IVa ribbons? ..., posted on March 23, 2015 at 20:12:18
Davey
 That resistor might have been placed there deliberately. 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.
On more complicated crossovers used in conventional speakers, you see this all the time.

Dave.

 

"That resistor might have been placed there deliberately." ..., posted on March 23, 2015 at 20:35:14
andyr
Manufacturer

Posts: 12551
Location: Melbourne
Joined: September 2, 2000
 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

 

RE: "That resistor might have been placed there deliberately." ..., posted on March 23, 2015 at 20:49:50
Davey
 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.

 

"Tympani IIIA schematic" ..., posted on March 23, 2015 at 21:46:37
andyr
Manufacturer

Posts: 12551
Location: Melbourne
Joined: September 2, 2000
 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

 

RE: T-IVa ribbons? ..., posted on March 23, 2015 at 23:41:30
Roger Gustavsson
Audiophile

Posts: 2058
Location: Huskvarna
Joined: February 12, 2010
 Yes, it is correct. My T-IVa ribbons measure 3 ohm. I have new spares and they are 2 ohm. With just 2 ohm the sensitity will rise, if the power amplifiers cope, the 1 ohm resistors takes it back to a similar level as the original 3 ohm ribbons. Another way would be to use longer strips of aluminium as cables for the ribbon tweeter. The spare set comes with three ribbons, it is posible to use one of them splitted in two as 1 ohm resistors.... I Think Apogee had some strange looking series resistors of aluminium foil in the Scintillas, also raising the impedance and lowering the sensitity.

 

RE: "Tympani IIIA schematic" ..., posted on March 24, 2015 at 06:22:51
Davey



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.

 

RE: "Tympani IIIA schematic" ..., posted on March 24, 2015 at 11:31:46
Roger Gustavsson
Audiophile

Posts: 2058
Location: Huskvarna
Joined: February 12, 2010
 No, I was talking about the Tympani IVa.

(I have the Tympani IIIA too but I did not mention them here.)

 

RE: "That resistor might have been placed there deliberately." ..., posted on March 24, 2015 at 12:15:19
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 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

 

RE: "to be high passed higher up." ..., posted on March 24, 2015 at 12:27:04
andyr
Manufacturer

Posts: 12551
Location: Melbourne
Joined: September 2, 2000
 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

 

RE: "Tympani IIIA schematic" ..., posted on March 24, 2015 at 12:49:27
Davey
 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.

 

RE: "Tympani IIIA schematic" ..., posted on March 24, 2015 at 17:39:01
neolith
Audiophile

Posts: 4842
Location: Virginia
Joined: February 21, 2002
Contributor
  Since: December 2, 2004
 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.



"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

RE: "Tympani IIIA schematic" ..., posted on March 24, 2015 at 18:49:49
Davey
 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.

 

RE: How t measure the impedance of Ribbon tweeters?, posted on March 25, 2015 at 02:54:16
jazzbeat
Audiophile

Posts: 293
Location: Kansas city,MO
Joined: February 18, 2004
 When measuring this low it is rather difficult to get real measurements. One thing you mention is the addition of a series resistor in line with the new ribbon. Do the math. A two ohm ribbon is going to have a lower voltage drop than a 3 ohm. The resistor is absorbing roughly one third of the signal that the previous 3 ohm ribbon received. You might try playing around with this resistor in value first. Lowering it should raise your tweeter output.


Cheers

 

RE: "to be high passed higher up." ..., posted on March 25, 2015 at 22:06:49
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 We are talking early 80s so the likelihood of blown tweeters from Adcom 555 565's or ampzillas clipping was very high.

 

RE: Series resistor before xover , posted on March 29, 2015 at 17:24:27
A.Wayne
Audiophile

Posts: 2527
Location: Front row center
Joined: November 30, 2011
 Means they can use the same xover , quick fix requiring no need to develope a new xover....


Regards

 

RE: "Tympani IIIA schematic" ...raal ribbon , posted on March 29, 2015 at 17:31:11
A.Wayne
Audiophile

Posts: 2527
Location: Front row center
Joined: November 30, 2011
 I would be willing to wager a guess that the actual ribbon diaphragm -ZMin is .3 ohm and not 1 ohm ...


Regards

 

RE: "to be high passed higher up." ..., posted on March 29, 2015 at 17:34:14
A.Wayne
Audiophile

Posts: 2527
Location: Front row center
Joined: November 30, 2011
 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

 

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