Meadowlark Audio Legacy Driver Specs

Pat McGinty

Pat McGinty

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In an effort to assist those seeking replacement drivers, I've just started a Google spreadsheet into which we will pour our best information. I intend to add the remaining models and probably the crossover circuits as time and memory allow.

Some drivers continue to be available. Unfortunately others do not. The Kestrel tweeter is perhaps the worst problem. To the best of my understanding - a few years ago, the great driver makers Peerless and Vifa were acquired by Tymphany which discontinued the Vifa brand, moved a few Vifa drivers into the Peerless line and dropped much of the Peerless line. On the bright side, we sourced many of our drivers from Scan Speak that continue in production.

Here's the link:
https://docs.google.com/spreadsheets/d/1WPe6uCxpNITBWOZnt8q7pAQcB7WSvus5nABAX6cu59s/edit?usp=sharing

The sources cited happily vend single units to retail customers.

One tidbit of advice: if you're having difficulty extracting a bad woofer that seems stuck to its gasket - don't mess up your baffle - just cut the cone out and grab hold of the chassis. It's more fun that way.

Happy Days!
Pat McGinty
 
Welcome to the Haven, Pat. Great to have you onboard!

Thats an incredible service to provide for those of us who own and love your speakers.
 
Welcome to The Haven, Pat!

Always thought your original Kestrels sounded pretty good with medium powered tube amps. :)
 
Hi Pat.
Sad to see what has happened to Vifa, once the ubiquitous manufacturer of drivers. It's a shame. Their stuff never sounded bad.

Out of curiosity, did you ever try the Revelator in the Shearwater? Certainly not a twin electrically, but it would drop right in. :)
 
Hello All -

The considerations when swapping tweeters is just a tad more interesting than we'd really like. Two three dimensional transfer functions sum. Acoustic output is a frequency/amplitude/direction function and impedance is a frequency/amplitude/phase function. The filter takes the sum of those into account so it's not likely that any substitution will perform precisely as the original part did.

But if you enjoy fiddling around with passive filters - by all means, have at it. The Revelator is one hell of a tweeter. Besides great detail, naturalness and dynamics, it'll withstand a rough use. Filter work by trial and error can be a very amusing process. And in the end all that matters is the subjective result; if you like it better it IS better.

You may have to adjust the baffle rebate depth and the through hole.

Shearwater's tweeter is the D2905/9500, but notice that the D2905/9700 is built on the same chassis - so it should physically drop right in. It sports the dandy SD-2 motor, same as the Revelator series. So that may be the best place to start.

FWIW - Shearwater's high pass filter consists of a 5ohm shelving resistor, a 7mfd series cap and a resonance shunt consisting of an 65uF, 0.7mH and 8 ohms series, in parallel with the load. The shunt specifically addresses the fundamental resonance of the 9500. If you compare the resonances of the 9500 and 9700 you'll see that the 9700 has a higher Q at about the same freq - so you'd look first at reducing the resistor in the shunt to increase the circuit's effect. The primary purpose of that circuit is to stop the driver from over working at resonance. My first crack would be adding another 8 ohmer in parallel to the existing 8 ohmer to net 4 ohms. this will double the magnitude of the shunt - which, by eyeball, looks about right. If you're working completely by ear: if you notice a raspiness on peaks - that's the tweeter thrashing itself at resonance - begin reducing that R value until it subsides.

https://www.madisoundspeakerstore.c...ak-classic-d2905/9500-1-tweeter-textile-dome/

https://www.madisoundspeakerstore.c...ak-classic-d2905/9700-1-tweeter-textile-dome/

Best!
Pat
 
Pat McGinty said:
Hello All -

The considerations when swapping tweeters is just a tad more interesting than we'd really like. Two three dimensional transfer functions sum. Acoustic output is a frequency/amplitude/direction function and impedance is a frequency/amplitude/phase function. The filter takes the sum of those into account so it's not likely that any substitution will perform precisely as the original part did.

But if you enjoy fiddling around with passive filters - by all means, have at it. The Revelator is one hell of a tweeter. Besides great detail, naturalness and dynamics, it'll withstand a rough use. Filter work by trial and error can be a very amusing process. And in the end all that matters is the subjective result; if you like it better it IS better.

You may have to adjust the baffle rebate depth and the through hole.

Shearwater's tweeter is the D2905/9500, but notice that the D2905/9700 is built on the same chassis - so it should physically drop right in. It sports the dandy SD-2 motor, same as the Revelator series. So that may be the best place to start.

FWIW - Shearwater's high pass filter consists of a 5ohm shelving resistor, a 7mfd series cap and a resonance shunt consisting of an 65uF, 0.7mH and 8 ohms series, in parallel with the load. The shunt specifically addresses the fundamental resonance of the 9500. If you compare the resonances of the 9500 and 9700 you'll see that the 9700 has a higher Q at about the same freq - so you'd look first at reducing the resistor in the shunt to increase the circuit's effect. The primary purpose of that circuit is to stop the driver from over working at resonance. My first crack would be adding another 8 ohmer in parallel to the existing 8 ohmer to net 4 ohms. this will double the magnitude of the shunt - which, by eyeball, looks about right. If you're working completely by ear: if you notice a raspiness on peaks - that's the tweeter thrashing itself at resonance - begin reducing that R value until it subsides.

https://www.madisoundspeakerstore.c...ak-classic-d2905/9500-1-tweeter-textile-dome/

https://www.madisoundspeakerstore.c...ak-classic-d2905/9700-1-tweeter-textile-dome/

Best!
Pat
Click to expand...

Baffle rebate depth? ummmmm.... not a term I’ve heard before!

How about making the big jump to something like the d2904/7100-03? Pretty different electrically, I admit, but it would be a heck of a tweeter.

https://www.madisoundspeakerstore.c...d2904/7100-03-wide-surround-black-face-plate/

The 9700 sure sounds like an easy mod. Is the crossover in the bottom of the speaker again? It’s such a bloody fantastic speaker, it seems almost sacrilegious to make any changes to it.

What made you chose the 9500 over the 9700? Or over the Revelator for that matter. Was it purely a matter of a needed price point? What the market would bear? Always interested in the compromises needed to make a successful product.

Any regrets with the Shearwater design? Things you wish you could have done differently? Particular moments of brilliance?
 
Prime Minister said:
Baffle rebate depth? ummmmm.... not a term I’ve heard before!

How about making the big jump to something like the d2904/7100-03? Pretty different electrically, I admit, but it would be a heck of a tweeter.

https://www.madisoundspeakerstore.c...d2904/7100-03-wide-surround-black-face-plate/

The 9700 sure sounds like an easy mod. Is the crossover in the bottom of the speaker again? It’s such a bloody fantastic speaker, it seems almost sacrilegious to make any changes to it.

What made you chose the 9500 over the 9700? Or over the Revelator for that matter. Was it purely a matter of a needed price point? What the market would bear? Always interested in the compromises needed to make a successful product.

Any regrets with the Shearwater design? Things you wish you could have done differently? Particular moments of brilliance?
Click to expand...


The D2904 Revelator series is fabulous. We used the ring radiator variant in Blue Heron II and loved it for its dynamics, detail and naturalness. It's also a very rugged part that can withstand relatively low frequency. So it should be quite forgiving if you're up for doing the filter work that will be needed to paste it into Shearwater. You're paying a little bit for amplitude capabilities that you're not going to use, but who cares? The filter is in the bottom making the process a bit easier. ((FYI: most HOTRODs were wired with Cardas Litz for which you'll need a solder pot to make new terminations so you might want to retain the existing terminations.))

Shearwater was designed in 1994 before the 9700 variant or the Revelators were introduced. And, sad but true, one of our bigger problems was that we were already putting too much in the box which forced us to run too lean. So upgrading that tweeter then raising prices probably wouldn't have been a the best choice. We really needed to sell more big speakers to make up for the tepid margins on the small ones, so that's where the effort went.

I'll blow the dust off Shearwater's story to the best of my recollection. When we introduced our first product, Kestrel, we had unwittingly arrived at the 995 price point that dear, departed Jim Thiel had just vacated. Dumb luck really, the Kestrel was in immediate demand and we really had to strain to keep up. Also, at the time I had no idea that it was impossible to launch a speaker company with a single model; you had to have an entire line. I had no idea what I was doing. For years I took trash talk about that from other makers.

Anyway, right away our newfound retailers began demanding a higher price point so, hard pressed for time and resources, I just scaled up the Kestrel, thickened the cabinet walls and used better drivers. Scan Speak was an obvious choice, still is. For the bass system alignment we took a nonstandard approach. At the time the industry was fixated on F3 but I had been observing subjectively that the price for F3 was poor "speed". Bad word, misleading, better: "signal tracking". So I looked only at rise and settle behavior by using gated sine waves. Baseline, four cycles, baseline. In that way I could see how faithfully the system tracked transients sweeping through the frequencies of interest at full amplitude, optimizing the arrangement of the cabinet and its internals. I still use that method.

The filter came together faster and more easily than any before or since. Some take months. Some many months. Some die on the bench. Shearwater took one day that ended with "Holy Shit! That was easy!". So that little birdie just leaped out of the nest.

Considering what could be done by us at that time, no, I have no second thoughts about Shearwater. It worked so beautifully because the design was single mindedly "form follows function" and it was dead-nuts simple. I mean, 24 years later it still seems to hold its own, no?

Back then there was a sign on the lab wall: "Believe your heart, not your analyzer". I still use that method.
 
Pat McGinty said:
The D2904 Revelator series is fabulous. We used the ring radiator variant in Blue Heron II and loved it for its dynamics, detail and naturalness. It's also a very rugged part that can withstand relatively low frequency. So it should be quite forgiving if you're up for doing the filter work that will be needed to paste it into Shearwater. You're paying a little bit for amplitude capabilities that you're not going to use, but who cares? The filter is in the bottom making the process a bit easier. ((FYI: most HOTRODs were wired with Cardas Litz for which you'll need a solder pot to make new terminations so you might want to retain the existing terminations.))

Shearwater was designed in 1994 before the 9700 variant or the Revelators were introduced. And, sad but true, one of our bigger problems was that we were already putting too much in the box which forced us to run too lean. So upgrading that tweeter then raising prices probably wouldn't have been a the best choice. We really needed to sell more big speakers to make up for the tepid margins on the small ones, so that's where the effort went.

I'll blow the dust off Shearwater's story to the best of my recollection. When we introduced our first product, Kestrel, we had unwittingly arrived at the 995 price point that dear, departed Jim Thiel had just vacated. Dumb luck really, the Kestrel was in immediate demand and we really had to strain to keep up. Also, at the time I had no idea that it was impossible to launch a speaker company with a single model; you had to have an entire line. I had no idea what I was doing. For years I took trash talk about that from other makers.

Anyway, right away our newfound retailers began demanding a higher price point so, hard pressed for time and resources, I just scaled up the Kestrel, thickened the cabinet walls and used better drivers. Scan Speak was an obvious choice, still is. For the bass system alignment we took a nonstandard approach. At the time the industry was fixated on F3 but I had been observing subjectively that the price for F3 was poor "speed". Bad word, misleading, better: "signal tracking". So I looked only at rise and settle behavior by using gated sine waves. Baseline, four cycles, baseline. In that way I could see how faithfully the system tracked transients sweeping through the frequencies of interest at full amplitude, optimizing the arrangement of the cabinet and its internals. I still use that method.

The filter came together faster and more easily than any before or since. Some take months. Some many months. Some die on the bench. Shearwater took one day that ended with "Holy Shit! That was easy!". So that little birdie just leaped out of the nest.

Considering what could be done by us at that time, no, I have no second thoughts about Shearwater. It worked so beautifully because the design was single mindedly "form follows function" and it was dead-nuts simple. I mean, 24 years later it still seems to hold its own, no?

Back then there was a sign on the lab wall: "Believe your heart, not your analyzer". I still use that method.
Click to expand...

Do the crossovers unplug? Can they be worked on without lugging 60+ pounds of Shearwater to the bench every time? The idea of doing the upgrade is both tempting and daunting. I've never heard a Revelator that didn't sound great, but these speakers are absolute brilliant as they stand. Their is a real musical magic to what you created, and I'd hate to compromise the music to improve the sound. Do you think the upgrades woukd be musically worthwhile, or would I just be changing flavours? Decisions!

Good thing I haven't started even thinking about woofers! :)

Very interesting story on the Shearwater development and the Kestrels. You must have upset some of the competition greatly. That is easily the most musical speaker I'd ever heard at that price point. Some others might have sounded better, but no one made music better.

If we are discussing history, how did the Kestrels come about anyway? They were pretty unusual at that time. And in this time too, come to think of it.

Wonderful sign! More manufacturers would do well to keep that in mind. I was listening to a lineup recently of a major manufacturer, and while the sound was exciting and consistent up and down the line, musically they were dead and boring. That flaw was not from a lack of engineering resources!
 
Prime Minister said:
Baffle rebate depth? ummmmm.... not a term I’ve heard before!
Click to expand...
"Baffle rabbet depth" would be my guess. The little inset 'shelf' upon which the tweeter flange sits, so that the face is flush with the baffle. (Credit that damned AutoCorrect for that one.)

This thread is a great read! Thank you.
 
We call the housing into which the tweeter flange fits a 'rebate'. Wood shop talk.

Yes, the filters are in an isolation chamber at bottom and you can easily disconnect the lead-in wires and clip lead them out onto the floor. Do not underestimate just how interesting the problem will be; the tweeter does not simply paste onto the woofer; the two parts need to marry happily so both high pass and low pass will come into play.

And, yeah, please don't start thinking of the woofers. Woofers are very particular about their alignment. Half of the sound they make goes backward into the cabinet where it must be treated just right or the other half - the half you're listening to - won't be happy.

I do worry about improving just the trebles; there are hundreds of forgotten bookshelf speakers that had superior treble and so-so everything else. It's better that quality stays consistent across the band.

An example is Kestrel which is merely good from top to bottom but relentlessly pleasant. A great tweeter would have ruined it.

The short story about Kestrel's origin is that I built 100 speakers and the first 99 didn't cut it. Ask my wife! You can see a few of them here:

http://meadowlarksings.com/whoweare.htm

Like any aspiring designer, I started with the idea that I could pick the right drivers, build a nice box, use my analyzer and win. Wrong, wrong, wrong and wrong. It took that many attempts and some help from a brilliant scientist friend to learn about the rest of the engineering issues at hand. The funny techy details of that path would take some time to relate.

Anyway, to the start of the Kestrel, prototypes in hand and excited to show them off, I headed up to Los Angeles from my home in San Diego to see if anyone cared. At my third call Elliot Midwood of Acoustic Image in Studio City was kind enough to take a listen. He played Ella Fitzgerald's 'Cry Me a River' then asked to keep them. Wow! Great! While driving back home I realized that I had taken no notes at all. No drawings. No schematics. I started to fret. Once home I found a message on the answering machine from a retailer in Michigan who had heard it through the grapevine and wanted a pair. Back to Elliot's! Then it began, sold orders started coming in. Finally the homeowners association caught wise about what was happening in my garage which forced me to operate the business properly in a nearby industrial park. When we showed Kestrel at the 1994 CES on the morning of the first day a well known Chinese distributor came in, sat and handed me a CD. He listened to exactly the same track that I had been using to evaluate vocals - a particularly tricky one - Rebecca Pidgeon's 'Spanish Harlem' on Chesky. Crimony! He wrote a PO for 20 pair. We signed up a handful of domestic dealers, started advertising in Art Dudley's Listener Magazine and we were off to the races. We ended up making thousands of them.
 
Pat McGinty said:
We call the housing into which the tweeter flange fits a 'rebate'. Wood shop talk.

Yes, the filters are in an isolation chamber at bottom and you can easily disconnect the lead-in wires and clip lead them out onto the floor. Do not underestimate just how interesting the problem will be; the tweeter does not simply paste onto the woofer; the two parts need to marry happily so both high pass and low pass will come into play.

And, yeah, please don't start thinking of the woofers. Woofers are very particular about their alignment. Half of the sound they make goes backward into the cabinet where it must be treated just right or the other half - the half you're listening to - won't be happy.

I do worry about improving just the trebles; there are hundreds of forgotten bookshelf speakers that had superior treble and so-so everything else. It's better that quality stays consistent across the band.

An example is Kestrel which is merely good from top to bottom but relentlessly pleasant. A great tweeter would have ruined it.

The short story about Kestrel's origin is that I built 100 speakers and the first 99 didn't cut it. Ask my wife! You can see a few of them here:

http://meadowlarksings.com/whoweare.htm

Like any aspiring designer, I started with the idea that I could pick the right drivers, build a nice box, use my analyzer and win. Wrong, wrong, wrong and wrong. It took that many attempts and some help from a brilliant scientist friend to learn about the rest of the engineering issues at hand. The funny techy details of that path would take some time to relate.

Anyway, to the start of the Kestrel, prototypes in hand and excited to show them off, I headed up to Los Angeles from my home in San Diego to see if anyone cared. At my third call Elliot Midwood of Acoustic Image in Studio City was kind enough to take a listen. He played Ella Fitzgerald's 'Cry Me a River' then asked to keep them. Wow! Great! While driving back home I realized that I had taken no notes at all. No drawings. No schematics. I started to fret. Once home I found a message on the answering machine from a retailer in Michigan who had heard it through the grapevine and wanted a pair. Back to Elliot's! Then it began, sold orders started coming in. Finally the homeowners association caught wise about what was happening in my garage which forced me to operate the business properly in a nearby industrial park. When we showed Kestrel at the 1994 CES on the morning of the first day a well known Chinese distributor came in, sat and handed me a CD. He listened to exactly the same track that I had been using to evaluate vocals - a particularly tricky one - Rebecca Pidgeon's 'Spanish Harlem' on Chesky. Crimony! He wrote a PO for 20 pair. We signed up a handful of domestic dealers, started advertising in Art Dudley's Listener Magazine and we were off to the races. We ended up making thousands of them.
Click to expand...

Yes, while it is tempting to think one can improve an already great speaker by just "improving one thing", usually these things end up going sideways. I'm thinking restoration is the better concept.

The Kestrel really is a magical speaker. While I comment so much on the Shearwater, which admittedly is better, the Kestrel is just... special. I got a chance to visit my old Kestrels in the new owners home recently, and was blown away again by the shear musicality of them. It's not that they SOUND better then a lot of other, often much more expensive speakers, they just make music better then anything close to their price range. Or often many multiples of it.
 
BTW, Pat, what was the thinking around the foam surround on the Shearwater tweeter? Was that due to the angle of the front baffle?
 
Pat McGinty said:
We call the housing into which the tweeter flange fits a 'rebate'. Wood shop talk.

Yes, the filters are in an isolation chamber at bottom and you can easily disconnect the lead-in wires and clip lead them out onto the floor. Do not underestimate just how interesting the problem will be; the tweeter does not simply paste onto the woofer; the two parts need to marry happily so both high pass and low pass will come into play.

And, yeah, please don't start thinking of the woofers. Woofers are very particular about their alignment. Half of the sound they make goes backward into the cabinet where it must be treated just right or the other half - the half you're listening to - won't be happy.

I do worry about improving just the trebles; there are hundreds of forgotten bookshelf speakers that had superior treble and so-so everything else. It's better that quality stays consistent across the band.

An example is Kestrel which is merely good from top to bottom but relentlessly pleasant. A great tweeter would have ruined it.

The short story about Kestrel's origin is that I built 100 speakers and the first 99 didn't cut it. Ask my wife! You can see a few of them here:

http://meadowlarksings.com/whoweare.htm

Like any aspiring designer, I started with the idea that I could pick the right drivers, build a nice box, use my analyzer and win. Wrong, wrong, wrong and wrong. It took that many attempts and some help from a brilliant scientist friend to learn about the rest of the engineering issues at hand. The funny techy details of that path would take some time to relate.

Anyway, to the start of the Kestrel, prototypes in hand and excited to show them off, I headed up to Los Angeles from my home in San Diego to see if anyone cared. At my third call Elliot Midwood of Acoustic Image in Studio City was kind enough to take a listen. He played Ella Fitzgerald's 'Cry Me a River' then asked to keep them. Wow! Great! While driving back home I realized that I had taken no notes at all. No drawings. No schematics. I started to fret. Once home I found a message on the answering machine from a retailer in Michigan who had heard it through the grapevine and wanted a pair. Back to Elliot's! Then it began, sold orders started coming in. Finally the homeowners association caught wise about what was happening in my garage which forced me to operate the business properly in a nearby industrial park. When we showed Kestrel at the 1994 CES on the morning of the first day a well known Chinese distributor came in, sat and handed me a CD. He listened to exactly the same track that I had been using to evaluate vocals - a particularly tricky one - Rebecca Pidgeon's 'Spanish Harlem' on Chesky. Crimony! He wrote a PO for 20 pair. We signed up a handful of domestic dealers, started advertising in Art Dudley's Listener Magazine and we were off to the races. We ended up making thousands of them.
Click to expand...

Thank you for sharing how Meadowlark began. Great story.....! :)
 
Prime Minister said:
BTW, Pat, what was the thinking around the foam surround on the Shearwater tweeter? Was that due to the angle of the front baffle?
Click to expand...
The foam tweeter surround served two functions: it reduced dispersion just a bit making the speaker a bit easier to place, especially in relation to side walls, plus it rebated the driver just enough for the dome to clear the grille. At the time we did whatever we could to avoid conventional grille frames because of the reflections they caused - hence the elastic sock style and the rounded edge profile. (Today we achieve that objective by a tricky inset grille that requires tooling then not available to us.)

Angling the baffle also served two functions: effecting coincident vertical alignment of the two acoustic centers, plus it pointed the woofer's axis upward just a bit. With the shallow filter, some small unwanted HF will spit out; fortunately at freqs where the driver begins to beam. So the arrangement kind-of shoots that little bit over your head.
 
Prime Minister said:
Yes, while it is tempting to think one can improve an already great speaker by just "improving one thing", usually these things end up going sideways. I'm thinking restoration is the better concept.

The Kestrel really is a magical speaker. While I comment so much on the Shearwater, which admittedly is better, the Kestrel is just... special. I got a chance to visit my old Kestrels in the new owners home recently, and was blown away again by the shear musicality of them. It's not that they SOUND better then a lot of other, often much more expensive speakers, they just make music better then anything close to their price range. Or often many multiples of it.
Click to expand...

Thanks for the kind words. That musicality came from a couple of simple observations that lead to a simple idea.

Early on, I observed that woofer low pass filters tended to demand more and more parts to achieve a perfectly flat response. The seduction of complexity while looking at the F/A plot on one's analyzer is damn-near impossible to resist. So one finds oneself adding a Zobel, then a shunt cap, then the outband stuff looks a little rough so you steepen the filter. It's like putting icing on a wedding cake until it's perfect. But I also observed that, never mind the low pass inductor, the addition of the next additional part just sucked the life out of the thing. Then it gets worse from there.

Paul Klipsch made the same observation following WWII when parts were in short supply. To paraphrase him: "Every time I take a part out it sounds better."

I recall giving a demo at a San Diego Audiophile Society meeting during which I merely switched in-and-out a Zobel. (Zobels are an RC circuit in parallel to the driver intended to flatten the rising impedance caused by the voice coil's inductance. Zobels were then omnipresent in competitors' products, probably still are). The results were not subtle. But me, still wet-behind-the-ears, failed to realize that I had just diminished each guy's speakers. Frowns all around.

There are a nasty and complex set of adverse trade offs, but I resolved to keep the life in the music by restraining myself to a single inductor as the woofer filter. That was the cornerstone idea that resulted in the line sounding as it did.
 
Pat McGinty said:
The foam tweeter surround served two functions: it reduced dispersion just a bit making the speaker a bit easier to place, especially in relation to side walls, plus it rebated the driver just enough for the dome to clear the grille. At the time we did whatever we could to avoid conventional grille frames because of the reflections they caused - hence the elastic sock style and the rounded edge profile. (Today we achieve that objective by a tricky inset grille that requires tooling then not available to us.)

Angling the baffle also served two functions: effecting coincident vertical alignment of the two acoustic centers, plus it pointed the woofer's axis upward just a bit. With the shallow filter, some small unwanted HF will spit out; fortunately at freqs where the driver begins to beam. So the arrangement kind-of shoots that little bit over your head.
Click to expand...

I do have to order the material to replace my tweeter foam. As it sits, and while it was a valiant effort by the previous owner, I don't think this is how they are meant to be:

20180219_145052.jpg
 
At first we used the type of open cell poly foam used by HVAC guys:

https://www.amazon.com/24x300x1-Foa...4264&sr=8-4&keywords=air+filter+foam+1/4+inch

I got that idea from David Wilson who, at the time, was using it as grille material on his Watt/Puppy. Later we went with a denser foam (that we had to buy in large quantities) because it worked better.

Obviously the trick is cutting it cleanly. We outsourced that process to a packaging house adept at making and using custom cutting dies. Today you'll do your best with an Exacto knife, which could be pretty tricky. If you can cut two discs from some stiff material as jigs then squeeze foam between them you might have a better shot. In addition to using a sharp edge, I imagine you could cut it with a hot wire if you can work that out. Inconel wire and a variac is the usual way but a little imagination might work out a hack. 'hope you get there!
 
Pat McGinty said:
At first we used the type of open cell poly foam used by HVAC guys:

https://www.amazon.com/24x300x1-Foam-Air-Filter-Roll/dp/B00RJU7FLQ/ref=sr_1_4?ie=UTF8&qid=1521464264&sr=8-4&keywords=air+filter+foam+1/4+inch

I got that idea from David Wilson who, at the time, was using it as grille material on his Watt/Puppy. Later we went with a denser foam (that we had to buy in large quantities) because it worked better.

Obviously the trick is cutting it cleanly. We outsourced that process to a packaging house adept at making and using custom cutting dies. Today you'll do your best with an Exacto knife, which could be pretty tricky. If you can cut two discs from some stiff material as jigs then squeeze foam between them you might have a better shot. In addition to using a sharp edge, I imagine you could cut it with a hot wire if you can work that out. Inconel wire and a variac is the usual way but a little imagination might work out a hack. 'hope you get there!
Click to expand...
Thanks, Pat!
You don't by any chance have any dimensions on the foam left, do you?
 
Sorry, no. The CAD files are long gone as is my personal pair. I'm only guessing: 5.125" OS, 2.5" IS. Am certain the thickness was 0.25".
 
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