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Anyone Have AR-3a Measurements, System, Drivers


Pete B

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  • 2 weeks later...

Pete B:

re: your comments below...

>I'm seeing some interesting results from this simulation. The

>ESR of the 150 uF cap strongly influences the input impedance

>dip around 450 Hz due to slight peaking in the filter's

>amplitude response with very low ESR. This is common, I've

>seen it before. The capacitor ESR strongly influences the Q

>of the HF rolloff of the woofer XO. A very low ESR cap causes

>more peaking in the amplitude response and a lower impedance

>dip in the input impedance. Here the ESR of the cap lowers

>the Q of the filter and flattens the amplitude response.

>Using the cap's ESR is not the best way to control Q since it

>then becomes dependent on capacitor type, but it is the most

>economical way, use a low cost cap and eliminate the need for

>a series resistor.

Please consider the fact that the early AR-3a with its Alnico Woofer, 1.9 mH inductor, and 1 lb, 14 oz of Fiberglass did *not* use electrolyic capacitors. The dielectric would have been wax-impregnated paper capacitors that when new would have had a very low ESR (~0.02%). Although it is useful to model NPR capacitors, they were not used until the early seventies.

>The response with the 1.9 mH inductor looks much closer

>to the published response curves. The 2.85 mH coil might

>compensate for the newer higher Bl woofers, by reducing the

>upper bass (200 to 500 Hz) output and in a relative sense

>increasing the below 100 Hz output. It may have also been

>changed to give the speaker a better balanced sound in room.

>I'm not exactly sure just offering some possibilities.

The changes of stuffing (decrease to 1 lb, 4 oz) and coil (increase to 2.85 mH) likely came with the advent of the ceramic magnet woofer with its higher strengh as you note. It would be interesting to model how the inductance increase and stuffing density decrease affected the overall system Q. The NPR caps were introduced a bit later and their ESR values would not enter this analysis.

>The input impedance dip to below 3 ohms at 900 Hz in the above

>figure 20 is very interesting in that the controls are all the

>way down offering a simple 16 ohm resistive load to the

>crossover network. This is caused by the 50 uF cap resonating

>with the .88 mH inductor with a higher Q when the pot is all

>the way down. There is very little damping in the circuit.

>The .5 ohm series resistor was an attempt by the designers to

>reduce this impedance dip effect. "Upgrades" to a lower

>resistance inductor will also make this dip lower and

>therefore should not be used.

I agree! This is precisely why one should not *upgrade* inductors in the mid- and hi-range crossover section. Capacitor upgrades in these sections have shown remarkable improvements in clarity; however, the resistance provided by the inductor is needed. I assumed the original wax capacitors and high quality polypropylene capacitors both have very low ESR and improvements in the performance were due to other issues. It is possible that high-quality ppe film capactors performed much like early oil or wax capacitors and performance took a dip in the electrolytic era.

>pass filter is high and there is peaking in the amplitude

>response but it is also strongly effected by the ESR of the 50

>uF XO cap. Low ESR caps will raise the Q even higher and also

>raise the associated amplitude peaking. The .5 ohm resistor

>and inductor resistance provide at least some loss/damping.

>I'll try to estimate the ESR of the 150 and 50 uF caps based

>on this input impedance data.

Since the original 3a design did not use NPRs, one would have to restrict these last two remarks to late AR-3a editions.

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Hi John,

Thanks for your comments. Just wondering what you mean by NPR?

Did you mean NPE, non-polar electrolytic, or non-polar royalitic?

>Please consider the fact that the early AR-3a with its Alnico

>Woofer, 1.9 mH inductor, and 1 lb, 14 oz of Fiberglass did

>*not* use electrolyic capacitors. The dielectric would have

>been wax-impregnated paper capacitors that when new would have

>had a very low ESR (~0.02%). Although it is useful to model

>NPR capacitors, they were not used until the early seventies.

Yes I thought about all these factors, even the capacitor type first thinking it might be a paper type but then considering the value of 150 uF concluded that it must be an electrolytic type. I'm not an AR collector so I've not seen one to veryify this but I don't believe there's anyway to make a 150 uF cap in paper with any sort of reasonable physical size. It might be an electrolytic that looks like a paper type. Electrolytics were certainly available back in the 1950s and I believe this type is the only way to get that much capacitance in a reasonable size. It might be an oil type but I doubt it, perhaps someone else could offer some input.

>The changes of stuffing (decrease to 1 lb, 4 oz) and coil

>(increase to 2.85 mH) likely came with the advent of the

>ceramic magnet woofer with its higher strengh as you note. It

>would be interesting to model how the inductance increase and

>stuffing density decrease affected the overall system Q. The

>NPR caps were introduced a bit later and their ESR values

>would not enter this analysis.

I don't usually think of the inductor as altering the system Q since it's effect is usually in the mid bass, however it can start to look that way when the value becomes very large. I do agree that part of the reason for the increase in value was probably to offset the lower system Q to some extent. We do have some published data in this matter, the AES article for the original woofer shows a system Q of about 1, but the damping factor is given as 1, indicating that the source impedance was probably about 2.5 - 3.5 ohms. I would not be surprised if the system Q was about .8 or less with a normal low impedance amplifier. The modern systems (10pi, 11) list a system Q of about .7 which agrees with this observation. I believe however that there were two differnt magnet strengths for the ceramic woofer and this is supported by the published literature, as I noted in another thread. I also believe the later replacement woofers may have yet different specs.

>I agree! This is precisely why one should not *upgrade*

>inductors in the mid- and hi-range crossover section.

>Capacitor upgrades in these sections have shown remarkable

>improvements in clarity; however, the resistance provided by

>the inductor is needed. I assumed the original wax capacitors

>and high quality polypropylene capacitors both have very low

>ESR and improvements in the performance were due to other

>issues. It is possible that high-quality ppe film capactors

>performed much like early oil or wax capacitors and

>performance took a dip in the electrolytic era.

I like to avoid electrolytics when possible but I don't buy into the boutique capacitor claims. I basically agree with what your saying here.

>>Since the original 3a design did not use NPRs, one would have

>to restrict these last two remarks to late AR-3a editions.

As I said earlier I'm not sure that this is true, there are a lot of informed people here who do not offer their sources. Were you an AR employee or how did you come to this conclusion? I don't mind restricting my conclusions to the later AR-3a editions, really they should correspond to the edition used in the response graphs cited and can be extended to systems built with components of similar performance.

Pete B

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>Thanks for your comments. Just wondering what you mean by

>NPR?

sorry, finger slipped to next key; should have been NPE.

>

>Yes I thought about all these factors, even the capacitor type

>first thinking it might be a paper type but then considering

>the value of 150 uF concluded that it must be an electrolytic

>type. I'm not an AR collector so I've not seen one to veryify

>this but I don't believe there's anyway to make a 150 uF cap

>in paper with any sort of reasonable physical size. It might

>be an electrolytic that looks like a paper type.

>As I said earlier I'm not sure that this is true, there are a

>lot of informed people here who do not offer their sources.

>Were you an AR employee or how did you come to this

>conclusion?

By opening cabinets and examining their contents, destructively, if necessary -- that's the way experimental physicists are trained.

See attachment below.

The dual wax capacitor (150 & 50 uF) weighs 5 lb., 3 oz. It is approx 7.5 in long, 5 in wide and 2 in high. Its scale can also be seen from the nearby 2.5-in-diameter woofer and midrange inductor bobbins. The 6 uF wax tweeter capacitor is to the right.

At some point in the early 1970's, when NPE became reliable and inexpensive, AR switched to electrolytics. The original AR-3a used Alnico woofers until s.n. 38,500-to-39,000, corrresponding to about June 1970, based on recorded s.n. versus date stamp data.

post-100900-1125377552.jpg

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Well that is one big capacitor, do you know if there is multipoint attachment to the plates? I assume they're rolled plates. Have you measured the ESR? I have seen the 6/4 uF cap before. Experimentation is fine also, never saw any mention of how big that cap was, thanks for the info.

Pete B.

>>Thanks for your comments. Just wondering what you mean by

>>NPR?

>

>sorry, finger slipped to next key; should have been NPE.

>

>>

>>Yes I thought about all these factors, even the capacitor

>type

>>first thinking it might be a paper type but then considering

>>the value of 150 uF concluded that it must be an

>electrolytic

>>type. I'm not an AR collector so I've not seen one to

>veryify

>>this but I don't believe there's anyway to make a 150 uF cap

>>in paper with any sort of reasonable physical size. It

>might

>>be an electrolytic that looks like a paper type.

>

>>As I said earlier I'm not sure that this is true, there are

>a

>>lot of informed people here who do not offer their sources.

>>Were you an AR employee or how did you come to this

>>conclusion?

>

>By opening cabinets and examining their contents,

>destructively, if necessary -- that's the way experimental

>physicists are trained.

>

>See attachment below.

>

>The dual wax capacitor (150 & 50 uF) weighs 5 lb., 3 oz. It is

>approx 7.5 in long, 5 in wide and 2 in high. Its scale can

>also be seen from the nearby 2.5-in-diameter woofer and

>midrange inductor bobbins. The 6 uF wax tweeter capacitor is

>to the right.

>

>At some point in the early 1970's, when NPE became reliable

>and inexpensive, AR switched to electrolytics. The original

>AR-3a used Alnico woofers until s.n. 38,500-to-39,000,

>corrresponding to about June 1970, based on recorded s.n.

>versus date stamp data.

>

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>

>Almost forgot to comment on this plot below, this is in

>between, less peaking at 450 Hz than Tom's plot, but flatter

>than the one above. It says that the woofer was measured out

>doors and all drivers were into 360 degrees? Can anyone

>confirm how the woofer was measured, half space or full

>space?

>

Pete,

I forgot to comment on your observations above regarding the measurement solid-angle. AR initially measured all woofers in a free-field, 2-Pi (half-space) environment. This was done by literally burying the speaker in the ground, face-up, flush with the surface, facing into a 180-degree solid angle. The ideal echo-free environment is outdoors, of course, but it is very inconvenient, and therefore indoor anechoic chambers became a necessity for most measurements. The problem with nearly all commercial anechoic chambers, of course, is that they are not large enough to be “anechoic” at the lowest audio frequencies. Chamber large enough to measure into the deep bass frequencies are huge and enormously expensive, and exist only at such places as Harvard’s Acoustic Laboratory and those belonging to aerospace and automotive companies.

AR’s large anechoic chamber (one of AR anechoic chambers was transported in its entirely out to NHT’s Benicia, California site back in the 1990s) was echo-free above approximately 200 Hz, so AR developed a calibration curve that compared the outdoor 2-Pi woofer curve to the woofer’s curve in the chamber, and thus a meaningful anechoic response down to 30 Hz was possible. The tweeters were measured in the anechoic chamber facing into a 4-Pi, 360-degree “spherical” environment. Before the calibration curves was used, AR took the outdoor, 2-Pi woofer curve and spliced it to anechoic curves made above 200 Hz in the chamber.

A 4-Pi low-bass woofer measurement is impractical, of course, since the absolute output of a speaker system below the frequency of ultimate radiation resistance is influenced by the solid angle of the space into which it radiates. 4-Pi woofer measurement at low frequencies will be down by 6 dB (halving the solid angle doubles the energy radiated at low frequencies) compared to 2-Pi measurements. This is the reason for the bass-shy characteristics of most speakers if they are mounted out in the middle of the room up on a stand, or listened to out doors. Another way of looking at it is that if a speaker is designed to be “flat” down into the low frequencies when measured into 360 degrees, it will therefore sound intolerably bass-heavy when mounted in the usual 2-Pi fashion in a typical listening room. This was the very reason for the AR-10Pi Environmental control -- to enable flexibility in mounting the speaker in various positions to offset the affect of the solid angle on bass frequencies.

http://www.classicspeakerpages.net/dc/user_files/583.jpg

4-Pi anechoic-chamber curve of AR-3 woofer system

--Tom Tyson

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  • 4 months later...

Thanks for the plot Tom,

I had heard of the outdoor burried in the ground measurement many years ago, it is novel and interesting. The measurement methods used that you mention make sense and you confirm the move from half space to full space environments.

The graph that you show seems to have bypassed the crossover as the woofer response is well beyond 600 Hz, and it makes perfect sense to see the droop in LF response. My point has been that a 1.88 mH inductor will provide "flat" response into half space, and a 2.85 into full space. The inductors are appropriate (ball park) for the different test environments.

Pete B.

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