Ar11 mkII impedance measurements after recap

[giovanni56]

The measurements were taken with the midrange and tweeter switches at - 0db both and - 3db on the midrange and 0db on the tweeter, respectively. In my opinion the Ar11s are for certain amplifiers difficult to drive especially with the midrange and tweeter level controls at 0db, however, the frequencies of a music program between 1.8kHz and 20kHz, where the impedance decreases dramatically, have an average low energy content

[fedeleluigi]

Hello Giovanni,

Your 200003 woofers show a very high FS, about 50% more than the typical (16 Hz) measure of the made in US woofers: Did you use a very stiff surround to refoam them? 

The values of 24.2 Hz and 23.6 Hz that your last two plots show are more typical for Tonegen woofers.

I'll appreciate any clarification.

Luigi

[giovanni56]

16 hours ago, fedeleluigi said:

Hello Giovanni,

Your 200003 woofers show a very high FS, about 50% more than the typical (16 Hz) measure of the made in US woofers: Did you use a very stiff surround to refoam them? 

The values of 24.2 Hz and 23.6 Hz that your last two plots show are more typical for Tonegen woofers.

I'll appreciate any clarification.

Luigi

Ciao Luigi, the two woofers are Ar200003 that had the spider sagging, most likely the high Fs is not due to the foams I use, but to the treatment I did to regenerate them, the measurements were taken without any "break-in" after the refoam and the "respider," in similar cases after a good "break-in" the Fs averaged 19Hz.

[fedeleluigi]

@giovanni56 Hello Giovanni,

Thank you for the clarifications.

If you are interested in a comparison, in the link below I posted the impedance plots of AR-11 MKI, AR-10 Pi MKI and AR-10 MKII measured by some Italian magazines in the 70's. 

Unfortunately, they do not say how the switches were set during the impedance measurement of AR-11.

https://community.classicspeakerpages.net/topic/1408-ar-11-woofer-200003-measurement-refoam-repair/?do=findComment&comment=121942

 

[giovanni56]

1 hour ago, fedeleluigi said:

@giovanni56 Hello Giovanni,

Thank you for the clarifications.

If you are interested in a comparison, in the link below I posted the impedance plots of AR-11 MKI, AR-10 Pi MKI and AR-10 MKII measured by some Italian magazines in the 70's. 

Unfortunately, they do not say how the switches were set during the impedance measurement of AR-11.

https://community.classicspeakerpages.net/topic/1408-ar-11-woofer-200003-measurement-refoam-repair/?do=findComment&comment=121942

 

You're welcome Luigi. 
Looking at the impedance curves, I think the Ar11 MKI was with the switches set to 0db, as well as on the Ar10p but with the woofer control on 4p¬.

[fedeleluigi]

On 4/29/2024 at 11:40 PM, giovanni56 said:

Looking at the impedance curves, I think the Ar11 MKI was with the switches set to 0db,

You may note the minimum impedance value on the plot of the Italian magazine is about 2.8 Ohm between 6000 and 7000 Hz while your plot shows about 1.8 Ohm in the same frequency range.

[fedeleluigi]

On 4/29/2024 at 11:40 PM, giovanni56 said:

on the Ar10p but with the woofer control on 4p¬.

 As I wrote in that thread, for the AR 10 Pi the Italian magazine said the switches were set for the maximum output (4Pi, 0dB, 0dB for woofer, midrange and tweeter respectively).

[giovanni56]

On 4/30/2024 at 8:01 PM, fedeleluigi said:

You may note the minimum impedance value on the plot of the Italian magazine is about 2.8 Ohm between 6000 and 7000 Hz while your plot shows about 1.8 Ohm in the same frequency range.

Yes, I had checked the graph in the magazine which has an impedance trend in that frequency range higher than 1ohm, as I noticed the minimum of 4.8ohm between 60 and 100Hz which I never measured in the three pairs I had available, I always measured minimums between 3 and 3.6ohm, in the graphs I posted the value is 3.2ohm. The first time years ago that I noticed that difference taken by doubt, I checked the speaker components, and everything was okay. Such a difference cannot be justified, unless, someone has let's say, "softened" that graph or made an unintentional mistake; you may have noticed that the impedance values on the vertical axis of that graph are reported manually, in my opinion whoever did it, probably entered the impedance values on the vertical axis by shifting a few positions. I can assure you that the measurements I reported are scrupulously correct.

[fedeleluigi]

Hello Giovanni,

I agree with you. In the old thread I linked some posts before I wrote: "Anyway, the minimum of 4.8 Ohm at 80 Hz  seems a little high in comparision with that of the AR 3a improved that had the same 200003 woofer generation used for the AR 11 MKI  and a rather similar woofer low-pass crossover network." 

Unfortunately Renato Giussani, the engineer who probably carried out the measurement for the Italian magazine, died a few years ago and therefore no one will be able to clarify the apparent inconsistency.

[giovanni56]

20 hours ago, fedeleluigi said:

Hello Giovanni,

I agree with you. In the old thread I linked some posts before I wrote: "Anyway, the minimum of 4.8 Ohm at 80 Hz  seems a little high in comparision with that of the AR 3a improved that had the same 200003 woofer generation used for the AR 11 MKI  and a rather similar woofer low-pass crossover network." 

Unfortunately Renato Giussani, the engineer who probably carried out the measurement for the Italian magazine, died a few years ago and therefore no one will be able to clarify the apparent inconsistency.

I knew Renato personally, and during a chat in which we talked among other things about the hectic times of Hi-Fi in those years, he told me that it was not uncommon that some tests might contain errors due to the hurry to compose the pieces in time for the specialized magazines, then later corrections of the errors were published, in this specific case I do not remember corrections published in later issues, however, I agree with you that in that frequency range the impedance curve must be similar to that of the Ar3a Improved, because the differences on the crossover are minimal, although the latter in the low pass of the woofer has a lower inductance value than that of the Ar11, but a larger capacitor although it is damped by an R of 1.5ohm.

[Pete B]

I measured an AR11 back around 2006 and noted the dip in impedance around 6-7 KHz and also found that

the high pass crossover filter had amplitude peaking in that region around/above the crossover point.  The reason

for this very low impedance is first due to the very low impedance of the tweeter, and second the XO is passive

but when there is peaking due to the highish Q chosen for the XO filter the amplitude peaking "gain" has to come

from somewhere and this is by lowering the input impedance.  This is referred to as a transformer "like" circuit 

in electrical engineering.  From memory the tweeter filter is a 10uF cap and a .1 mH inductor, the cap is too large

and the inductor too small, also the crossover point is rather high.  A better .75" tweeter could be crossed much lower.

Also, if we had .zma impedance files for all of the drivers (I think that I do somewhere) we could produce an Xsim

simulation to plot the crossover filter transfer functions.

The DC resistance of the inductor and the ESR of the original NPE cap should be duplicated in any component 

replacements and for simulations.  If the NPE cap is replaces with a film type .47 to 1 ohm of resistance should be

places in series with the cap to emulate the ESR of the cap.  This will raise the system impedance in that region.

That low impedance is dangerous for many SS amps by triggering the protection circuits for any amp not designed

to drive 2 ohm or lower loads.  Many protection circuits are not well designed and may oscillate resulting in burned

out tweeters and potential damage to the amp.  It is a mess.  The Adcom GFA555 and 555II are good in that the only

protection is fuses that don't have such problems.