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Tweeter Impedence

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Posted on July 23, 2020 at 11:26:58
Ozzie
Audiophile

Posts: 3817
Joined: August 1, 2002
 I am building a crossover between my soft dome tweeter and a ribbon / planer driver. The freq of the crossover is 10 KHz. The planer is listed as 8 ohms. The soft dome is listed as an 8 ohm impedence, whereas the DC resistance is listed as 5 ohms. Yes, I confirmed that. About 5.4.

With both at 8 ohms logged in a crossover tool they both would require 1 uf capacitors, and .25 mh inductors for an LR 2nd order at 10 KHz.

Question, I intend on padding down the soft dome a tad to make a more seamless crossover point. Would 2 additional ohms of resistance in series with the soft dome accomplish what I need? Asking as the DC resistance would then be just under 8 ohms. In the original design there was a 4 ohm resistor, with two more 4 ohm resistors, terminated, for tweaking if necessary.

 

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RE: Tweeter Impedence, posted on July 23, 2020 at 13:54:06
b.l.zeebub
Audiophile

Posts: 9361
Location: 52deg 28'N,1deg56'W
Joined: April 17, 2006
 The ribbon/planar is likely 8Ω because there is a transformer in there somewhere (try and verify this) but the nominally 8Ω dome is highly unlikely to be that at 10k.

At the very least find it's data sheet which should include an impedance curve and use that unless you can measure the impedance at the desired xover frequency.

There are L pad calculators online which should help finding the desired attenuation without changing the impedance.

 

short answer to the question: no, posted on July 23, 2020 at 14:21:37
mhardy6647
Audiophile

Posts: 16018
Location: New England
Joined: October 12, 1999
Contributor
  Since: October 23, 2016
 As the other reply suggested, if you want to add a fixed attenuation, use an L-pad calculator (the one I usually use is shown below) -- or add a variable L-pad (which is what I usually do, since I am lazy and I don't like to be tied down to one value). :)

The (nominal) impedance of the drivers is what you're going to care about for an L-pad.

IMG_0341


all the best,
mrh

 

RE: Tweeter Impedence, posted on July 23, 2020 at 17:45:17
Ozzie
Audiophile

Posts: 3817
Joined: August 1, 2002
 Thanx guys. The soft dome data sheet, hopefully attached properly, shows impedence of 8 ohms, with DC resistance of 5.3 ohms. On the curve it shows about 6 ohms @ 10 KHz, hence my thinking, maybe wrong, of going with 2 ohms in series, and living with that. I think I'll stick with the values from the L pad generator, and go with it, for - 2 Dbs. I'd almost prefer the planer to put out a tad more than the soft dome, and let it compensate for listener aging. ;-P

I was not able to find an exact data sheet on the planer driver, but it likely has a transformer in there, as it is heavy. I have found 8 ohms in the net.

 

One more thing, posted on July 25, 2020 at 07:43:10
Ozzie
Audiophile

Posts: 3817
Joined: August 1, 2002
 Plugging in the values to the L Pad tool 8 ohms, 2 Dbs of attenuation provides 1.63 and 31 ohm resistors for the series and parallel resistors. Phil Marchand surprisingly has already sourced these two exact values.

There is one thing I need to confirm. At 10 KHz the soft dome actually has an impedence of about 5.7, or so. Is it ok for me to simply add a 2.2 ohm resistor in series with the input of the tweeter (after the L Pad) to bring tweeters effective impedence value up to just about 8? I am ok if there is a slight bit more of attenuation.

 

A couple of observations on that Audax tweeter, posted on July 26, 2020 at 13:07:25
Brian H P
Audiophile

Posts: 1291
Location: Oregon
Joined: December 18, 2012
 First, I'm assuming you mean to lowpass the Audax and highpass the planar at 10kHz.

Why so high? And what model planar are you using? Is it a true "ribbon" with a transformer (as b.l.zeebub suggested), or a planar magnetic?

At 10kHz crossover frequency the planar will be a "supertweeter," only adding a bit of sparkle and shimmer in top octave. If it's capable of crossing lower, say around 5kHz, you would get much more of the benefit of its delicacy and micro-detail resolution. You would also be rolling off the Audax below the point where its off-axis response diverges significantly from the on-axis, which will help avoid a big dip in the off-axis response at crossover.

You will note that the on-axis acoustic response of the Audax starts rising above 7kHz to a 6dB peak at approximately 16kHz. This will effectively subtract one order of attenuation from whatever lowpass slope you choose. So an electrical second-order slope will yield a first-order acoustic response above your cutoff frequency. Which could be fine, and could blend nicely, with minimal phase error, with a first-order filter on the planar. At a 5kHz or higher crossover frequency, only the most fragile true ribbons would have trouble with that shallow a slope.

You will also note that the impedance of the Audax rises from 5 Ohms at 3kHz to 7 Ohms at 20kHz. That's not much, but could ever-so-slightly shallow the slope of the lowpass filter. The parallel element of an L-pad would largely ameliorate that. The Audax has enough sensitivity (about 92dB average) that you could put a 2 Ohm resistor in series with it, call it 8 Ohms, and use an 8 Ohm rotary L-pad behind that to precisely dial in the desired attenuation.

Most planar drivers, lacking voice coils, have wonderfully flat impedance traces, and can essentially be regarded as resistors. So if your planar is described as 8 Ohms, it almost certainly is, from DC to light. This removes a major headache from filter design -- textbook formulas will work just fine. If this were my project, I would put a second 8 Ohm rotary L-pad on the planar driver, and be able to precisely fine-tune the output level by fraction-of-a-decibel levels. Use good quality L-pads -- the higher the wattage rating, the more robust the build quality -- and install them with mounting "cups" set into the back panel of the speaker, so if they start to sound scratchy in ten years, you can easily access them to squirt in some DeOxit.

Just some food for thought there. Sounds like an interesting project -- have fun!

 

RE: A couple of observations on that Audax tweeter, posted on July 26, 2020 at 13:28:18
Ozzie
Audiophile

Posts: 3817
Joined: August 1, 2002
 Brian, thank you for the response. You answered my main question, that adding a 2 ohm resistor (2.2 chosen, and on the way) will give me the 8 ohms at the crossover frequency (where the speakers were in stock form). You are correct in the intended use of the ribbon; for sparkle. It is actually a Panasonic planer that supposedly was the basic design that led to the Infinity versions.

I am running a 3 way active setup, see my profile. Ill go with the L Pad values from the tool. After I nail it down I can further tweak via the active crossover itself, if necessary. As high up in freq as this is, will I really hear slight differences anyhow?

 

RE: A couple of observations on that Audax tweeter, posted on July 27, 2020 at 05:07:41
JURB
Audiophile

Posts: 2056
Location: North Ohio
Joined: May 29, 2016
 You probably don't need the resistor. Speaker manufacturers don't worry much about that.

An impedance higher than the DCR is expected in a tweeter if it has a coil. Simple law of physics. Actual impedance is the vector sum of DCR and Xl.

As far as hearing the difference, well if all you listen to is oboe music you won't hear it. Now a song like Hot N Nasty, which is very hard to record on tape, Dolby HX is almost mandatory, now then you might hear something.

That top octave is really nice to have, but is not used much on most material.

 

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