Carlspeak
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Posts posted by Carlspeak
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>Hi again;
>
>I added to the topic, but lost everything last night.
>
>Here goes again.
>
>I have a sample of, I suspect, 2 AR-2, speaker terminal boards
>complete.
>
>There is an L-pad on both boards, rather than a pot, with only
>2 of the 3 terminals connected to anything.
>
>One L-pad has embossed on the rear, CM24067 10 ohm 140911,
>the shaft turns with a definite smoothness, needing very
>little effort to turn, as if brand new.
>
>Not anywhere near the screechy, hair raising screech of the
>typical 16 ohm Aetna-Pollock pot, nor the dragging feel of the
>windings, as the rotor passes over the wire.
>
>There is a copper wire coil, approximately 2" in
>diameter, no markings on it, wound on a short piece of wooden
>dowel.
>
>There is 2 cute little metal body caps, about 1" x
>1" x 1 1/2", with 3 terminals each, taped together
>with what appears to be 1/4" masking tape covering most
>of the manufacturers embossed markings.
>
>One cap has 200VDC - 50'C 30Z embossed on it's top.
>
>The other cap is covered with the 1/4" tape.
>
>The other board is identical, except the L-pad has 140918
>embossed instead and this L-pad has an original shaft knob.
>
>I would need to remove tape from one or the other to see the
>cap rating.
>
>The 3 terminal screws are in a triangular configuration.
Vern:
What I suspect you have is someone's Rube Goldberg attempt at upgrading the crossover of an AR2a. I'm not an EE and can't answer all the detailed questions in your first post. However, AR must have purposfully used 15 ohm rheostats instead of the typical 8 ohm L pads in their design. 15 ohm L pads are darn near impossible to find today. So, again, your predicessor chose to cobble up something.
Yes, the original rheostats are a bit scratchy when adjusted. However, that should be done infrequently once the speakers and room acoustics are settled in and you've adjusted them to your satisfction.
It's evident, at least to me, that speaker development in the 60's and 70's was a bit of a seat-of-the-pants process. There were no T/S parameters or computer models back then. Tweeters were in a rapid state of development and I'm sure their performance varied quite a bit out of the manufacturing plant and AR didn't want to spend a lot or resources testing all of them to find suitable matches.
IMO AR developed a way to compensate for these variances by adding adjusting mechanisms to help balance each driver's output to the listener's listening satisfaction. Just look at all the speakers Kloss was involved with in the early days. Almost all of them had some means of adjustment. Without this novel flexibility, I wonder how sucessful AR would have been?
Contrast that era with today's. How many loudspeakers do you see sold today have similar adjustments? I suspect VERY few. Driver to driver T/S parameters are relatively more consistent and thus, designers can tune a crossover design to specific components during the development process that maintain sonic output within a very narrow range.
Numerous modern speakers claim + - 2 dB response over a very broad frequency band. I doubt AR could have done that back then with a fixed crossover design that had no adjustments.
What are you plans for these terminal boards?
Remember, it's all about the music
Carl
Carl's Custom Loudspeakers
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>Just to fill in some blanks....I got my AIIIs from Peter
>personally...having helped him in a number of ways...and later
>I served the family as a Board member...so I am very familar
>with his speakers...
>The woofers...particularly in the AIIIs is not an easy part to
>find...I think the only folks who can touch them are Snell.
>The tweeters and midranges were modified from stock units...
>
>Internally the speakers were wired with Kimber cables...as
>were the connectors from the top to bottom unit...
>
>the most vulnerable unit historically was the mid range...the
>problem with the woofers is that over time the foam surrounds
>broke down with time....depending upon the environment they
>are in
>
>the drivers were matched unit to unit and within the
>system...the matching was done in the crossover...remember
>typical production tolerance of drives is likely to be =/-
>3db...and you couldn't use the ones that were on the minus
>side...they went into other...less critical production
>models....this is why its important to properly replace units
>with the correct one...and then pad it down as
>required...assuming you have the gear and reference standard
>to do this....
>
>John
I've worked on a few Snell xovers from the Classic model A to JII's. All have the same sliding, wire wound resistor set up in series with the tweeter. All crossovers were mounted on a board and fastened to the inside wall of the cabinet. This tells me that it was impossible to make the matching adjustment will all the drivers installed in the cabinet (model A excepted) without a great deal of tedious trial and error. This must have been done externally somehow. Perhaps a knowlegable reader could enlighten us all as to how and why this set up existed. I have found it unique among speakers I have worked on over the years. With this out-of-the-box adjustment process, I suspect it was used primarily for SPL matching to overcome unacceptable production variation in the efficiency of individual drivers. It is basically analogous to an l-pad, but in this case, once adjusted, a screw was tightened and thereafter, no changes were possible without removal of the crossover. I personally think Kloss's rheostats allowed more flexibility of adjustment by AR owners in their own environment. Albeit, the rehostats are well known for their propensity to fail due to corrosion.
As John correctly points out above, readers considering replacement of Snell drivers take heed to be sure they don't screw up the balance in SPL's. However, if the balance isn't right, a knowledgeable speaker 'builder' could measure the SPL differences and bypass Peter's original, sliding resistor with an appropriately sized, audio grade one. Or, tinker with Peter's clever little sliding resistor within its range of adjustability.
One final thought regarding SPL matching. Numerous makes and models of speakers from the 'classic' era (AR, Advent, Snell, etc.) all had some means of adjustment to balance the tweeter with the other units. Rheostats, switches, sliding resistors, L-pads were common on many speakers of that era. Remember, dome tweeters were in the early stages of their development and perhaps their SPL's couldn't be controlled very well during manufacture - necessitating the use of series variable resistance.
Remember, it's all about the music
Carl
Carl's Custom Loudspeakers
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>The Snell area is in the pipe - hopefully by next weekend
>I'll have it up.
>
>Mark
It's been over 2 months now since your above post regarding adding Snell speakers to the library area of the Classic Speaker Pages. Any light at the end of the tunnel?
Hope your recent hard drive issues have been resolved.
Remember, it's all about the music
Carl
Carl's Custom Loudspeakers
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>Thanks. I just emailed my mailing address.
>
>It you are worrying about the electrolytic cap, then replace
>it. It should be clearly marked for value in the schmatic and
>probably is easy to read on the cap itself.
>
>I do not recall the size (usually woofer circuits have a large
>cap like my Proacs have a 7 mf cap).
>
>In other forums on DIY speaker building people are rabid about
>getitng electrolytic caps out of rebuilt classics like these.
>The general rap on them is they leak, change value and so they
>change the well designed pass of low frequencies to some other
>area where the speakers perform less than peak. So, if this is
>correct, you may even hear something more from the bass
>units.
>
>You can get a great replacement from Madisound
>(http://www.madisound.com/) and they even take PayPal. For
>example, a 7mf audio cap can be as low as $3 each for a Solen
>cap (reasonable quality) to a Hovland musicap (about $35
>each).
>
>I'd go somewhere in the middle or with the Solen cap for the
>woofer circuit. Sonic caps are in the middle and my favorites.
>Not for sale at MAdisound though.
>
>Last thing worth mentioning and it is "Snell" like.
>Peter's crossovers (not always later Snell crossovers) that I
>have seen and held all have a cascade approach to caps. I
>haven't seen the woofer cap in the A's (I don't remember it
>anyway), but the other caps in the crossover are all
>cascaded.
>
>The general rule for doing this is easy to work with. The
>total cap value is separated to use at least two caps so the
>value adds to the value needed. Snell often used three.
>
>The rule is that the big cap is one value, the next cap is
>about 1/10th that value and if you use a third then the third
>is about 1/10 of the second. So the third is for speed really
>and acts like a by-pass cap.
I've worked on Snell models A, EII and JII and saw no evidence of cascading. I saw much evidence of bypassing a series or parallel wired bundle of electrolytic caps with a better quality PP film type cap. It's my suspicion Peter tended to stock a limited number of cap uF values like 4's and 8's and so forth and wired them every which way to get the net uF value he wanted. Below is a link to my web site. It includes photos of a Model A crossover as originally produced and then my upgraded version. In a few areas, Peter bundled up to (8) 4uF electrolytic caps and bypassed the bundle with a ceramic PP cap that was in the pF range. That little disk type cap was usually at the bottom of the bundle and provided a relatively flat surface to apply hot melt adhesive to.
http://www.classicloudspeakerservices.com/gallery.html
Remember, it's all about the music
Carl
Carl's Custom Loudspeakers
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>My Type a's do not sound any better bi-amped. I still have a
>set hooked up with an NAD bi-amp set-up, but only because I
>haven't gotten arround to changing the set-up. But I see no
>noticeable difference i biamping. I have also done this with
>upper level Adcoms.
>
>Bi-wiring is easy and I always do it whether I can hear the
>difference or not. Snell recommends it in the original
>literature.
>
>Here's the scoop on the crossover caps. Snell uses a technique
>that cascades smaller value caps to make the capacitance value
>they need. So, as you have likely seen, the caps are stacked
>on top of one another. Usually they use three to four. Usually
>the last is really small in actual value and I suspect it acts
>as a by-pass cap (for tweeter and mid circuits).
>
>I am taking up too much space with my coments. But I'd not
>change the midrange unless I had to because of damage. Snell
>sells a replacement. The tweeter I have changed in my A-II's
>with the Snell set and I like the sound a lot. I bought a
>replacement set for my other two Type A's, but never put them
>in. So, I guess the amount of work to replace it has kept me
>out of the other sets. But the tweeter is a nice upgrade, I
>think.
>
>I have opened a lot of older speakers like the fine AR3a's and
>some Bozaks, Klipsch and so on and all major speakers this age
>I have seen appear to have cheap looking or ugly acoustic
>damping in them -- if that is what you are commenting on. I
>think it;s just wall insulation. I am not replacing mine.
>
>The electrolytic caps are items people seem to run to and
>replace. They go bad, they change value, and they were never
>made "audio grade."
>
>You've made the right choices with Soniccaps. I use them.
>
>The only thing I would be careful of is modifying the speaker
>so it can not be bi-amped. But onlybecause ot may change value
>should you ever decide to sell.
>
>That's my opinion.
>
>Now, I'd love to have a copy of the Type A cdossover diagram.
>Will you be posting it or sharing it?
>
>
Check out the Snell in the library area of this web site. I sent a diagram and pictures to Mark some time ago in the hopes he would post them there. If they're not there ask Mark what happened.
Remember, it's all about the music
Carl
Carl's Custom Loudspeakers
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Be careful ordering 15 or 16 ohm pots from electronic suppliers. I've tried to get some from Parts Express and MCM Electronics and both had oversize bodies the would not allow two to be installed side by side at the current center distance which, if I remember is about 2.25 inches. As far as lame, I mean Layne Audio is concerned, the web site looks inviting but....customer service is terrible. There is no on-line ordering system and the phone is hardly never answered, nor are voice mails ever responded to.
I just finished refurbishing 2 sets of AR3a's and ended up cleaning up the old pots and re-installing them.
Carl, The speakerdoctor
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Hi Daniel:
Those were simple DC resistance measurements.
Carl
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>What are the impedances of each of the three drivers in the
>AR3a? Not that I would ever consider replacing them with
>anything different
... >
>Thanks --
>--Daniel
I worked on a pair a while back with serial no's 47405 and 43070. With my V.O.M. I measured 4.0 ohms on the 3700-1 Woofer, 5.2 ohms on the HR 4500-1 mid and 2.7 ohms on the HR4600-1 tweeter.
Carl,The SpeakerDoctor
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Can anyone help with information on repairs or who does repairs on these late 70's Advent Bi-Amped Powered speakers?
Carlspeak
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AR pots & L-pads AR-2?
in Acoustic Research
Posted
Exerpt from Soundminded's post:
"The problem with this scheme is that as you adjust it, it changes the resistance the crossover network sees and this not only adjusts the speaker volume, it changes the crossover frequency because its exact value is an element in the way this filter circuit works.
An L-pad is designed to change the volume of the speaker without changing the resistance the crossover network is connected to."
Your explanation of the differences between pots and L-pads didn't answer the vital question aluded to in Vern's original post:
WHAT TRULY WAS AR'S INTENT IN USING POTS?
1) To adjust both volume AND frequency?
2) To adjust volume only and in the process acepted the fact that frequency would also change.
The former would seem to me to be an undesirable design objective.
AR's design was rather novel in any case. I wonder if they had it patented at the time. What say you Tom T.?
p.s. my response to Vern was solely my opinion on why things happened the way they did. I never worked for AR. However, I've worked on a number of different classic speakers and my experience and observations led to these opinions.
Remember, it's all about the music
Carl
Carl's Custom Loudspeakers