To Recap or not AR91

[DavidR]

The old glue can (depending on type) be made to release easier by dribbling a small amount of isopropanol (rubbing alcohol) on it and allowing it to 'soak' for a bit.

[Aadams]

55 minutes ago, Markolone said:

Usually I leave the control flat and I use only the loudness control.

About the recording I'm using CD's

Sorry, I should have been clearer.  I meant multi band like 5 to 10 band equalizer?  Music is CD;  Is it Rock, Classical, etc?

[Markolone]

33 minutes ago, Aadams said:

Sorry, I should have been clearer.  I meant multi band like 5 to 10 band equalizer?  Music is CD;  Is it Rock, Classical, etc?

No, I don't use equalizer.   About the music, I like Blues, Rock, Classical, Jazz, Country, Pop.  For the test I heard some of my preferred pieces:

- Dire Straits:  "You and Your Friends"  (very well recorded, it would probably make many speakers sound good)

- Beethoven:  piano sonatas plus Symphony

- Queen: "Bohemian Rhapsody"

- Norah Jones

- AC/DC

[AR surround]

A few questions / observations:

1) You said that employing the attenuation switches did not seem to make much of a difference, even between the 0 dB and -6 dB positions.  That should not be the case.   There should be a very noticeable difference between those two positions.  Something may not be right with that circuit.

2) When you went into the speaker, did you check that the wiring was in accordance with the schematic?  When I got my pair of AR915's, I found that someone had previously gone in an "rearranged" the wiring.

3) Did you install the F&F bypass capacitors?

4) The caps that AR used were known to have a very high ESR.   You might find it necessary to add a small amount of resistance to tune the speakers to your taste.  That  is why my first question about the attenuation switches is so important.  When I recapped a pair of Boston A70's with NPEs, I regretted replacing the old gray Unicon caps.  The A70's had no attenuation switches, so I needed to add both a 0.5 ohm resistor and then a F&F bypass capacitor to achieve what I considered to be acceptable response.

 

 

[Stimpy]

1 hour ago, AR surround said:

4) The caps that AR used were known to have a very high ESR.   You might find it necessary to add a small amount of resistance to tune the speakers to your taste.  That  is why my first question about the attenuation switches is so important.  When I recapped a pair of Boston A70's with NPEs, I regretted replacing the old gray Unicon caps.  The A70's had no attenuation switches, so I needed to add both a 0.5 ohm resistor and then a F&F bypass capacitor to achieve what I considered to be acceptable response.

 

 

A shame the Unicon capacitors aren't still available.  Great sounding NPEs.  Very dynamic.

[Markolone]

On 5/1/2023 at 4:15 PM, DavidR said:

The old glue can (depending on type) be made to release easier by dribbling a small amount of isopropanol (rubbing alcohol) on it and allowing it to 'soak' for a bit.

Thanks for the suggestion.  I didn't know, so I forcibly pulled out the first two capacitors, then left the others glued so as not to risk damage 😑

[Markolone]

14 hours ago, AR surround said:

A few questions / observations:

1) You said that employing the attenuation switches did not seem to make much of a difference, even between the 0 dB and -6 dB positions.  That should not be the case.   There should be a very noticeable difference between those two positions.  Something may not be right with that circuit.

2) When you went into the speaker, did you check that the wiring was in accordance with the schematic?  When I got my pair of AR915's, I found that someone had previously gone in an "rearranged" the wiring.

3) Did you install the F&F bypass capacitors?

4) The caps that AR used were known to have a very high ESR.   You might find it necessary to add a small amount of resistance to tune the speakers to your taste.  That  is why my first question about the attenuation switches is so important.  When I recapped a pair of Boston A70's with NPEs, I regretted replacing the old gray Unicon caps.  The A70's had no attenuation switches, so I needed to add both a 0.5 ohm resistor and then a F&F bypass capacitor to achieve what I considered to be acceptable response.

 

 

Thanks for your feedback.  Here under my answers:

1) I agree with this observation in fact I was convinced to find a faulty switch, or some craft modification. From the conditions, I believe that no one has touched it since 1980. I measured the resistance in the different positions of the switch with a tester and it corresponds with those of the project, even with considerable precision! At this point the doubt arises that the old capacitors had a very high NPE such as not to be affected too much by the addition of resistors. By the way, the capacitors have no printed branding.

2) Yes, it matches with the official pattern. Before starting the work, in addition to the photo, I also mapped all the components and related connections on paper, which I checked several times.

3) Yes, I installed 1F&F for each cap (5 in total per speaker)

4) as soon as I have time, I try again the comparisons more calmly, using the switches 

 

Some additional considerations regarding the real value of the caps, measured during the change (see my previous picture).

On the tweeter:  the former cap was 4,7uF, the new is 3.9uF (close to the project)

On the midrange: old value:  9.3 uF vs 8.1uF

If I’m not wrong, the mix between the above situation should cause a moving of some load from tweeter to the midrange.  The new condition should be more close to the original project, anyway.

Next step:  I will reopen the box and I will check again. In the meantime I think if there are some special check that I can do.   Unfortunately I don’t have an instrument to test the NPE.

[AR surround]

2 hours ago, Markolone said:

At this point the doubt arises that the old capacitors had a very high NPE such as not to be affected too much by the addition of resistors. By the way, the capacitors have no printed branding.

 

2 hours ago, Markolone said:

Next step:  I will reopen the box and I will check again. In the meantime I think if there are some special check that I can do.   Unfortunately I don’t have an instrument to test the NPE.

Do you mean ESR, Equivalent Series Resistance?

[Markolone]

38 minutes ago, AR surround said:

 

Do you mean ESR, Equivalent Series Resistance?

Sure!  sorry

[DavidR]

3 hours ago, Markolone said:

On the midrange: old value:  9.3 uF vs 8.1uF

The 8uF cap is a shunt capacitor for the mid-range driver. Frequencies go to ground.

The series cap for the mid driver is the 24uF.

[fedeleluigi]

On 5/1/2023 at 2:49 PM, Markolone said:

There is a perceptible difference between the two speakers

Hello Marklone,

To be more scientifically sure of this difference you should swap all drivers from one speaker to another. Actually the difference could depend more on some differences in the drivers (they are around 40 years old) than on those of the crossovers.

On 5/1/2023 at 2:49 PM, Markolone said:

I tried to switch the position of the speakers thinking of a difference due to the environment, but it was not that.

You did well. In reality during the comparison the two speakers should "ideally" be in the same place in the room (as even the same speaker in two different places in the room will sound differently).

Therefore, when making the comparison, I suggest you place the two speakers at least next to each other and swap their positions from time to time. Obviously the musical signal must be the same for the two loudspeakers and not the right/left one of a stereo signal. If you don't have the mono option on your amplifier you can use  an RCA splitter on your source (CD player, etc) and use only the right (or only the left) signal for both speakers

 

On 5/1/2023 at 2:49 PM, Markolone said:

After several listens I'm tempted to say it sounded better before.

That wouldn't surprise me!

The ESR of the capacitors may affect not only the attenuation of the driver and the crossover point (when placed in series) but also the damping operated by the crossover filter (when placed in parallel). In other words, the use of capacitors with ESR different from the original ones can compromise the target response of the speaker.

Ken Kentor (kkentor) and David Smith (speaker dave), two leading speaker designers writing on this forum have pointed this out on different occasions in this and other forums.  Read for example what Ken Kentor says in the thread below about  replacing capacitors.

 

 

 

[Markolone]

16 minutes ago, fedeleluigi said:

Hello Marklone,

To be more scientifically sure of this difference you should swap all drivers from one speaker to another. Actually the difference could depend more on some differences in the drivers (they are around 40 years old) than on those of the crossovers.

You did well. In reality during the comparison the two speakers should "ideally" be in the same place in the room (as even the same speaker in two different places of the room will sound differently).

Therefore, when making the comparison, I suggest you place the two speakers at least next to each other and swap their positions from time to time.

 

That wouldn't surprise me!

The ESR of the capacitors affects not only the attenuation of the driver (when placed in series) but also the damping operated by the crossover filter (when placed in parallel). In other words, the use of capacitors with ESR different from the original ones can compromise the target response of the speaker.

Ken Kentor (kkentor) and David Smith (speaker dave), two leading speaker designers writing on this forum have pointed this out on different occasions in this and other forums.  Read for example what Ken Kentor says in the thread below about  replacing capacitors.

 

 

 

About the first suggestion, to exchange the drivers, I can do it but originally the two speakers had a similar sound.

Regarding the type of capacitors, I changed the type only for two NPE (one on the tweeter and one on the mid);  maintaining the other 3 NPEs (2x woofer, 1 x mid).  I tried also to use the switch in order to introduce resistors to attenuate the level on both tweeter and mid (usually I selected 0dB).

Today I reopened the speakers to check again the circuit, that is exactly the original.  I tested again the functionality of the switches:  the value of the resistance introduced by the switch is by design.

In the next days, I will continue to test using different input.  Currently I'm not able to understand how the bandwidth is moved. I think this information is the starting point to find an adjustment.

The last (funny) hypothesis should be the following: because some capacitor value is more close to the original project, it could be that 40 years ago they sounded like this.   But my ear had became accustomed to the old sound.

Last but not least:  ask to my wife / brother / friend to choose the best speaker, a blind test of course. But I'm afraid of the feedback 😱

 

[fedeleluigi]

 

11 hours ago, Markolone said:

About the first suggestion, to exchange the drivers, I can do it but originally the two speakers had a similar sound.

It may be that before the capacitor replacement you didn't pay much attention to the sonic differences between the two speakers.

 

11 hours ago, Markolone said:

I changed the type only for two NPE (one on the tweeter and one on the mid);

Using the 8.2uf polypropylene cap in parallel with the midrange may have altered the damping and therefore the frequency response around the crossover frequency (with the tweeter).

 

On 5/1/2023 at 2:49 PM, Markolone said:

After several listens I'm tempted to say it sounded better before

Without accurate measurements of both the capacitors you used and the recapped speaker it's hard to tell if the degradation you hear compared to the original speaker is real or not.

 

11 hours ago, Markolone said:

The last (funny) hypothesis should be the following: because some capacitor value is more close to the original project, it could be that 40 years ago they sounded like this.   But my ear had became accustomed to the old sound.

In my experience frankly it has never happened to me that replacing the old capacitors in AR speakers made the sound worse. There is generally an improvement especially when the old capacitors are out of specification.

Just follow the simple rules mentioned by Ken Kentor in the thread I linked.

Personally, for AR speakers, I only use bipolar audio electrolytic capacitors (usually Bennic or M.D.L.) from reliable sources (unfortunately there are many fakes). I always measure them to select them. I never use the small bypass capacitors.

[Markolone]

Thanks Luigi for your suggestions.

About the measurement of the old and new caps, here under the outcome:

Probably the two replacement in orange caused the main changes in the sound, for two reasons:  different values (even if the new values are closer to the original design) and the type (NPE to poly). Fortunately the two old capacitors are still glued even if shortened the terminals. 

I could try to restore them.

I have a question:  what can be the ESR value for a NPE cap in the worst case?   

 

 

 

[AR surround]

On 5/4/2023 at 5:48 AM, Markolone said:

I have a question:  what can be the ESR value for a NPE cap in the worst case? 

Other's who have measured that ESR might be able to answer that question.  However, as implied in other posts above, you should be able to effectively raise the ESR of those series capacitors by using the attenuation switches.  A 3dB reduction is large and audible.  A 6dB reduction is huge.

[RoyC]

On 5/5/2023 at 7:03 AM, AR surround said:

Other's who have measured that ESR might be able to answer that question.  However, as implied in other posts above, you should be able to effectively raise the ESR of those series capacitors by using the attenuation switches.  A 3dB reduction is large and audible.  A 6dB reduction is huge.

At 1Khz (the frequency at which the stated value of a capacitor is labeled), ESR of decent new npe caps is seldom higher than .5 ohm. It, however, can increase rather dramatically as frequency decreases. This is especially pronounced with lower cap values. The capacitance of higher value npe's will increase as frequency decreases, but ESR is less affected. Unfortunately even though this behavior is the same for all npe's it can vary with era and age, making them much less predictable than film caps.

Technically the behavior of new and old npe's can affect crossover points/slopes, and sensitivity. This is further complicated by differences in the behavior of new vs old npe's. While exhibiting essentially the same behavior, older npe caps can present higher series resistance at all frequencies. The effect can be a "warmer" or "smoother" sound for some people. Others may find it to be less "detailed" or "muted" and prefer the boost of new npe or film capacitors. It usually comes down to a matter of preference...especially considering the many additional external variables.

In other words, it is entirely possible for someone to prefer the "sound" of the original capacitors over any type of replacement.

It should be noted that the vast majority of capacitance (and ESR meters) used by hobbyists measure at extremely low fixed frequencies, and are unreliable when it comes to discerning the above characteristics of npe capacitors. High value npe caps (typically above 20uf) almost always measure on the high side with these meters. (Since film caps vary very little, the basic meters work just fine for those.) Playing with tiny-valued by-pass film caps can be fun and interesting for audiophiles, but are relatively insignificant compared to the obvious measurable variables associated with npe behavior.

In light of the above, film crossover capacitors (any type or brand) are extremely predictable, and measure nearly ruler flat across the audio spectrum with barely measurable amounts of ESR at any frequency. These typically result in a brighter/more aggressive effect in the upper mid/lower high frequencies, where our hearing is the most sensitive.

Below are Ken Kantor's comments from the link provided earlier in the thread.

"Electrolytic cap technology was starting to get good around the time the MGC-1's were built. I have never had to replace any in my pair, but I realize that that might be, to some extent, luck. At any rate, quality speakers made in the 80's should have an indefinite lifetime, provided they were not exposed to really excessive power or ambient heat. Speakers made in the 50's, 60's and 70's a totally different story, and almost always need new caps.

I suppose you need to use your intuition, your ears and maybe a good capacitance meter. If you do decide to replace, try to find reasonable matches to the originals, and avoid expensive "improvement" parts. We knew what we were doing when we voiced a crossover, and the parts characteristics are factored into the response. It you replace an electrolytic with a film, for example, you might wind up with a treble boost, or an underdamped resonance."

Roy

 

[Markolone]

Thank you for this very interesting and instructive analysis. There are many variables involved and a one-size-fits-all solution cannot be taken for granted. On the other hand this concept applies to the entire HiFi world and is also part of its charm.

Meanwhile I continued to listen for hours and hours; the strange thing is that in the end the two speakers now sound very similar. I have two hypotheses to explain this: the first is that in the first tests I had left the attenuator in the central position (-6dB) convinced that it was all the way to the left; very strange because I had tested all the positions, but coincidentally the sound perceived at the beginning is similar to what occurs with an attenuation in the medium frequencies.

The second hypothesis seems a bit science fiction to me and concerns the "running in" of the capacitors; I've read elsewhere that the Mundorf need to work many hours. Furthermore, the models I bought are of the "RAW" type which does not seem to be the best for this use. But the difference seems excessive to me.

Whatever the reason I'm glad I got the original sound back; probably the capacitors of these 1980 ARs were of very good quality.

For now I don't touch anything anymore, but in the future I could calmly try to replace the 24uF NPE in series at the medium with an equivalent polyethylene.

If it weren't necessary to disassemble the woofer every time, it would be much easier to replace one component at a time and have fun testing.

[AR surround]

5 hours ago, Markolone said:

For now I don't touch anything anymore, but in the future I could calmly try to replace the 24uF NPE in series at the medium with an equivalent polyethylene.

Glad to hear that things have ended up well.  If you like the way your recapped speakers sound, you might not want to challenge fate and drive yourself crazy in the process.

[RoyC]

16 hours ago, AR surround said:

Glad to hear that things have ended up well.  If you like the way your recapped speakers sound, you might not want to challenge fate and drive yourself crazy in the process.

Agreed!

Roy

[DavidR]

In the end it really only matters what it sounds like to you.

[Markolone]

14 hours ago, DavidR said:

In the end it really only matters what it sounds like to you.

I agree!   now I enjoy the music.

I bought these speakers a couple of months ago from a person who hadn't used them for years and had in the meantime sold all the other components: amplifier, etc...

Obviously I immediately compared them with my old (not too much old) AR35BX. The differences are evident, but all in all the 35BXs did their job considering they are of a lower class. The more time passes, the more I notice the differences, obviously in favor of the AR91s. I wonder if it's just training my ear or if the 91 needed a break-in. The woofer refoaming was done by the owner just before he sold them.

 

[DavidR]

Enjoy them !