Carlspeak
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Posts posted by Carlspeak
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Nice work Carl.
I suppose we're getting into the "mods" area now but have you looked at the 12" Mavin EM-TL3006Y-8 Woofer? They claim it's a drop-in replacement for the Large Advents. All the T/S parameters are shown on their website but it's mostly Greek to me
I know the OLA Advent woofer was 10" in a 12" frame but wonder if this (or any) 12" would give the bass extension you say the Madisound/SS replacement lacked.
The sales pitch on their site says
"Proven and tested, it sounds just as good, if not better than the original Advent drivers. Over all still a very good sounding speaker, much like we remember, with a bit more low end punch".
-Kent
Thanks Kent. Looking at the specs, some of which don't make sense, I feel it will not work as well as the MS/SS one. For starters, the Fo (should be Fs) is higher than the MS woofer and Qts puts the woofer in a boom box category.
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Nice work Carl, I've been thinking of doing something very similar with that Dayton tweeter, given
it's LF capability and price. Though I'd expect to use a different crossover.
Are your measurements with the actual OLA box or the customers?
Were they grill on or off?
I have both. Also with and w/out the grille. Most or all of what I have shown is w/out the grille.
BTW, the customer revised his grille a bit from original by putting the 6 velcro blocks on the grille instead of the baffle board. So, just the velcro strips are attached to the BB so it looks much cleaner with the grille off. I can provide the other tests privately if you request them. However, with or w/out the grille didn't make a significant difference to the FR.
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Lastly, stuffing changes for bass extension. Near field woofer FR tests were done with 1.5 lbs OCFg and minimalist stuffing (see plots below). Note the increase in the height of the response hump in the 80-120 hz range with minimalist stuffing (right plot). With 1.5 lbs OCFG, but Q was about 0.9. With minimalist stuffing (back wall only), Q was raised to 1.2.
After working with the customer on stuffing tweaking, he advised he was most satisfied with the sound with the installation of a 20X20X2 inch thick open cell foam block folded at the sides and placed against the back of the cabinet. This foam can be obtained from Joanne's Fabrics and craft store. Also 2 inch thick blocks of foam were added at the top and bottom of the cabinet.
One can work with either minimalist FG or foam stuffing wich is needed to mitigate internal cabinet reflections and target for a "Q" of >>1.0
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On to the tweeter. I identified an excellent high powered, low distortion tweeter from Dayton. The RS28F-4 model. Crossed at about 1 kHz, 2nd order the tweeter has better 3rd harmonic distortion than the original. The tweeter has a 20 uF series Erse MET cap and a parallel 0.5 mH 20 ga air coil. The dip in the 1-2 kHz range of the OLA was solved by running the new woofer full range w/o a series coil. Tests were run to see if some BSC could be added with a 1.5 mh coil, but it wasn't significant enough to warrant sacrificing the solution to the 1-2 kHz response dip. After some final voicing tests with the customer's participation it was decided to add a 3 ohm resistor in series with the tweeter. The new crossover does not have a switch like the original. I felt it wasn't necessary. Below are plots of near field response tests done on the Dayton tweeter, 1st with a very high uF cap just to see it's general reaponse, then with the final 20 uF cap chosen and 3rd with the 0.5 mH shunt coil added.
The masonite adapter plate is quite simple. Copy the size of the OLA tweeter aluminum plates size and hole pattern and put a 3 inch diameter hole in the middle with notches at the 3 and 9 o'clock position to clear the speaker wire tabs on the Dayton tweeter.
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If that metal frame woofer in the pics in the OP looks familiar it's because it a commercially available drop in woofer sold by Madisound, Simply Speakers, vintage-AR and maybe a few others. I tried it first because it represented the simplest replacement as far as fit and finish goes. The new drop in has it's advantages and dis-advantages. Above 200 hz, the new woofer is identical to today's original, vintage metal frame woofers. Follow the upper blue lines in the two plots below. These tests were run with each woofer in the same OLA box with original foam blocks stuffing. One plot can pretty much be overlaid on top of the other. So, the upper bass and lower midrange music reproduction will be essentially identical. Below 200 hz, the new woofer doesn't have the bass extension of originals. It seems the spider is somewhat stiffer. However, some of that loss in bass extension can be made up by modding the cabinet stuffing to deliberately raise the "Q" of the speaker well above 1.
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This thread is intended to help those who have either rescued curbside castaways, or bought a pair for a few cents on the dollar that have blown drivers. You can turn them into great sounding Advents with the purchase of new woofers, tweeters and a simple crossover for a little over $300.
All this has come about as the result of a recently completed project with a customer who came to me with an objective of creating a whole new OLA with all new parts and even new cabinets he wanted to build for himself but, at the same time, retain the original Advent sound. The new cabinet is not essential to this discussion because he duplicated the basic size, internal volume and baffle board locations of the woofer and tweeter. I was charged with identifying the drivers and crossover design. He built his own cabinets. I had two OLA's in my possession to use as prototypes to mod at my whim. They were of a vintage that had metal frame woofers and foam blocks for stuffing. Those same speakers are now FS in the classified section of this forum. After he built the cabinets, he sent one to me for final assembly and testing to verify the results I had gotten with the modded OLA prototype.
At the far left below is the end result of the project. Next to that is a pic of what is possible for a salvage job. It shows the new woofer and tweeter with an easy to fabricate masonite adapter plate installed in an OLA cabinet. I call it the Ni-OLA or New, improved OLA. It's improved because the new crossover and tweeter have resulted in a flatter frequency response above 1 kHz and increased HF extension flat almost to 20 kHz; something the original 'fried egg' tweeter doesn't come close to. Also, distortion is lower and efficiency is a bit higher at over 92 dB w/2.83 V @ 1 M.Third from left is an on axis FR test of the Ni-OLA. To it's right, FR tests of the OLA with original fried egg tweeter and crossover. Follow the upper blue line which is the on axis response. Note the dip at 2 kHz and spiking and roll off at 15 kHz.
Note to the techies, most of the FR plots I've posted were done with 1/12th octave smoothing.
Project details continued in next post.
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Ah, you are a patient man, Carl.
I will be very interested in your findings.
Roy
It's the Sigma Xi in me that drives me to experiment with stuff!
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If I ever get an extra Snell 8 inch Vifa bass/mid to play with, I'll try a refoam, test FR and then add the fillet by hand (
) and retest to see what the effect was on FR above 1 kHz. -
It appears my theory regarding the purpose of the bead of polymer has been confirmed. I asked the question of the folks at AudioNote in the UK about the presence of the bead on original Snell E and J foams and the response was its purpose was to "help terminate the standing wave in the cone". To me, the last 5 words are another way of describing cone breakup modes. They create frequency response peaks above 1 kHz that are sometimes difficult to surpress with typical woofer LP crossover topologies.
AudioNote has basically adopted the continuance of the Model E design but in a very high end version. They also sell replacement foams for the original E and J Vifa drivers, but they are quite expensive.
In one case I am aware of, the Spica TC-50 and 50i, Jon Bau designed a special circuit that effectively tuned out those response peaks in the original 5 inch Polydax bass/mids but led to unique driver matching and service codes which described the character of the peak.
I was told by AudioNote I could use a PVA wood glue type of adhesive for application to create a bead on new, refoams. However, application by hand will be difficult to produce a neat job. The originals were clearly done by machine.
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Interesting rara. The 'plot' thickens! Perhaps TT or Roy C could contribute. I wasn't aware some of the AR woofers had this. I was aware that some AR woofers had an application of something that extended down the slope of the angle attach portion of the surround and also onto the cone itself, but always thought it was just a generous application of a sealant at the inner edge of the surround. When doing refoams of these woofers I always found it convenient to peel the inner portion of the surround off with the aid of that 'treatment'.
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Many of the popular model E and J models had the 8 inch Vifa M21WN-07 woofer. The original foam is very supple and had a large roll.
What I discovered recently was a small bead of a non-hardening polymer was applied to the crease where the inner side of roll meets the angle attach area. I can only speculate what this was for, but think it might have been a way to dampen cone break up modes at high frequencies in the 1-5 kHz range.
This filling in with the polymer in the crease reminds me of the filleted foam Boston Acoustics (and others) foam are known for. Perhaps the same reasoning could be applied there.
Anyone have any thoughts on this. Check your Snells foams (if original) for this clear bead of polymer.
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Baski, you asked about changing the binding posts. A word of caution to you and others who may be contemplating such a change; you must ensure that whatever binding posts are used, they must remain insulated (as are the originals) from the aluminum nomenclature plate used as the back panel.
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If the baffle is 3/4" mdf, how can the woofers be recessed 3/4 inch? Flush mounting the woofer (and tweeter for that matter) couldn't hurt, but I would be very surprised if the change would be audible. A coil on the woofer would probably be the most cost effective upgrade, and would definitely be audible.
There's a second board glued to the back of the 3/4 inch BB that the woofer is screwed to. The woofer is designed to run full range. Putting a coil on it will cut it off too soon and probably create a dip in the response somewhere in the 1-3 kHz range.. One of the advantages of these Winslow Burhoe designs is the woofer. It's very difficult these days to find a woofer that has a relatively smooth roll off whilst running full range; thus negating the need for a 1st or 2nd order LP filter to suppress any woofer peaking above 1 kHz.
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When it comes to diffraction, every little improvement helps. The ultimate diffraction free speaker is the sphere or egg shape design. This is just one version of the egg shaped type... http://www.seelectronics.com/se-tv/andy-munro-se-munro-egg-monitor-a-revolution-in-monitor-design
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The AR3a 'Improved' model did away with the large frame and deep inset of the driver for, I believe, the exact reason: reducing diffraction. Something the AR engineers learned about during the course of the speaker's life. That was only one of many tweaks and improvements made to the 3a during it's life. I'm sure the historians who post here could fill in much more detail on those improvements.
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ra ra, the reason tweeters are set off center is to lessen tweeter-induced cabinet edge diffraction issues.
Here is a WWW article that does a good job of explaining what I've just written.
http://www.rjbaudio.com/Audiofiles/tweeter%20diffraction.html
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During a recent refoaming job I noticed the 003 woofers are set way down in a recessed cavity that measaures 3/4 inch deep. If these were my speakers, I'd try installing a 5/8 inch thick shim to bring the woofer out flush with the baffle board's face. In my view, this would accomplish two improvements.
1) Mitigate diffraction effects from the woofer edge wavefront interacting with the sharp edge of the baffle board recess hole.
2) Better time align the woofer's acoustic center with the tweeter's. It won't be ideally aligned but improved never-the-less.
Since these were not my speakers to experiment with, I'll await another opportunity when the owner allows me to 'tinker' a bit with them.
I can only wonder what Winslow Burhoe's thinking was when he purpousfully recessed that woofer down so low in the first place.
I also replaced the NPE caps with 10 uF PET film caps. Both originals measured about 9.9 uF at 10 hz and 8.7 uF at 20,000 hz. The replacement cap measured a consistant 9.8 uF throughout the 10-20,000 hz range.
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Here's a system impedance and phase of an AR11. Note the cabinet resonance is on par with AR3a at about 42 hz. Q is a bit low at 0.6. This AR11 was fitted with a new tweeter from vintage-AR which is a modded ABtech tweeter. I'll be writing more about the tweeter soon in a new thread.
Most of the impedance above 100 hz runs in an amp-comfortable 4-8 ohm range with a dip to 2 ohm at around 9 kHz. I don't recall how the level switch was set for this test.
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Another attack of the spambots the past couple of days. Sorry for the delay in getting their stuff erased. I've been away from my computer all day.
Carl
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bennybee, see my response over at AK.org
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I believe there is a break in when new caps are installed. Whether this is electrical, mechanical, emotional, psycho- acoustical, or, a combination of those, I can't say for sure.
I was told by engineers at Mundorf that they 'cook' caps on request (for a few $$ more!). So, they seem to believe in it, as well as some of their well pocketed customers.
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The hobby of audio is definitely one place where the law of diminishing returns prevails. I did an informal assay of speakers years ago and plotted price vs performance and found that at about the $2000 level and beyond, what you got for rapidly escalating $ spent was minor tweaks that were less and less apparent.
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Yes, I am familiar with Tony Gee's cap ratings. I wrote a paper a few years back about his ratings. At the time I found a high correlation between his ratings and the voltage rating of caps he was reviewing. I surprised him with this finding and he subsequently agreed to conduct a special rating of 3 caps at my request. All that's history now. I haven't followed his ratings recently. However, this paper has over 1300 downloads since it's addition here at CSP. There was high suspicion that Tony's ratings could have been influenced by his prior knowledge of the cap's size and cost. Read the paper, check out his current ratings and judge for yourself......
http://www.classicspeakerpages.net/IP.Board/index.php?showtopic=5828
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Is this the link? if so thank you interesting read. http://www.cartage.org.lb/en/themes/sciences/physics/electromagnetism/electrostatics/Capacitors/Applications/BasicConsiderations/BasicConsiderations.htm
No, that's not the one. However, I fixed my link.
) and retest to see what the effect was on FR above 1 kHz.
You can now salvage un-salvageable OLA's
in Advent
Posted
Sorry, didn't look at those. Obviously, there are so many to choose from and I don't have the resources to evaluate a large number. For this project I favored not sacrificing midrange = to that of the original metal frame over bass extension.,