OHM Acoustics "Walsh F" Speaker remakes - Page 4 - diyAudio
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Old 26th November 2006, 08:19 AM   #31
mamboni is offline mamboni  United States
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Photo #1 Schematic of felt damping to woofer cone
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Old 26th November 2006, 08:19 AM   #32
mamboni is offline mamboni  United States
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Photo #4: Walsh loudspeaker
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Old 26th November 2006, 08:20 AM   #33
mamboni is offline mamboni  United States
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Default Build your own Walsh loudspeaker

Build your own Walsh loudspeaker for about $250 in parts and materials:

Woofer: PIONEER W25GR31-51F 10" BUTYL SURR WOOFER (http://www.partsexpress.com/pe/showd...number=290-088)

Tweeter: TANG BAND 28-847SA SHIELDED NEODYMIUM DOME TWEETER
http://www.partsexpress.com/pe/showd...number=264-822

6.8 uf high pass capacitor (metalized polypropylene) to tweeter (http://www.partsexpress.com/pe/pshow...Number=027-427)

4 ohm power resistor in tweeter circuit.

Sonotube: nominal dia. = 12.5 inch height = 37 inch
Stuffing: acoustastuff ~50% of internal volume

SPEAKER REPAIR GLUE 1 oz. BOTTLE http://www.partsexpress.com/pe/showd...number=340-076
This speaker glue has consistency of Elmer’s glue when wet, dries fairly quickly (water base) to form a clear and very flexible soft clear rubber with excellent damping characteristics.


Walsh Loudspeaker: two-way X-over first order (6dB/oct) high pass filter to tweeter at 8 kHz
Woofer – full range (no crossover)
Alignment: Acoustic suspension (Vb=2.5 cu. Ft; Qtc = .816; Fc = 49 Hz (-3dB))
Frequency Response: 49 - 20,000 Hertz (-3 db, +0db)
SPL: 89 dB/watt Max Power: ~100W

Illustration: Cone with felt damping applied with speaker repair glue.
Photos #1(schematic) and #2: woofer with felt dampers and felt variovent in dustcap. Variovent consists of ¾” dia. Hole cut into center of dustcap and hole covered with three laters of felt to provide resistive vent.
Photos 3-5: final loudspeaker.
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Old 27th November 2006, 03:28 PM   #34
doggy is offline doggy  Canada
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Default Ohm model D

Years ago-circa 1970, I had these bookshelf Ohm model "d"; I believe they were 2 way paper tweeter with 8". The magnet on the 8" was square alnico with the open ends capped.

These made a distinctive bass when played loud, have not heard that again untill this day. Wish I had a nos pair. I blew the surrounds out and they ended up at the local landfill. The power amp was a Lafette lr1500ta receiver.

cheers
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Old 27th November 2006, 05:35 PM   #35
Ian J is offline Ian J  United Kingdom
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Default vwey interesting!

mamboni,
how do you think this would work with, say, a 6.5in driver - would the drop in radiating area mean a big fall in output? Would the smoother FR at mid frequencies need less/no felt damping?
Ian
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Old 27th November 2006, 08:03 PM   #36
mamboni is offline mamboni  United States
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I am certain that a 6.5 inch woofer would work very well using analogous cone modifications and architecture. Of course, a 6.5 in woofer will not go as low, be as efficient and go as loud as a 10 inch woofer, all other things being equal - there's the rub.

You would want to use a reflex (ported) design to extend bass output (unless you have a subwoofer). The ideal 6.5 inch woofer would have:

Lowest possible free air resonance
Low Qts, say 0.35 or less
Very low voice coil inductance
Extended high frequency response

As to the latter, I like paper cones over all other as they represent both excellent value and in fact are only slightly improved upon by expensive exotics like kevlar and impregnates. The paper cones provide excellent combination of low mass and reasonable internal damping - but cone resonance remains a major problem unless either the crossover point is set below the cone resonance (which of course obviates all the advantages of a Walsh operation, the intent being to the utilize the 'bending' cone's sound production but manage the resonance), or the cone mods I've outlined upon are applied.

I've listed the component drives and sundry supplies and parts. Instructions on construction are not needed - the design is the model of simplicity and the photos should make their building self-explanatory. Excepting for possibly the finishing work, these loudspeakers can easily be constructed in a day. You will be rewarded with an exceptionally clean and coherent-sounding loudspeaker which images and soundstages like no other, all for a few hours of work and a couple of hundred dollars.
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Old 9th March 2007, 12:42 AM   #37
R. Jamm is offline R. Jamm  United States
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Default Ohm Walsh by mamboni

mamboni

Do you have a detail plan on making the walsh type speaker-where to place the binding post, soldering the crossover, stuffing the tube, and where to buy the tube? Any detail information would be appreciated


Thanks.
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Old 9th March 2007, 02:16 AM   #38
mamboni is offline mamboni  United States
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I don't have detailed instructions. Let me describe the construct as it is the model of simplicity.

This is a closed box (acoustic suspension) alignment as dictated by the specific 10" woofer being utilized, the Pioneer driver part listed in a prior posting. The exact cabinet volume is not critical - the target is about 2.4 cu ft, which should yield a system Q of 0.7 - 0.8.

The cabinet is made of a 36" long 12" internal diameter sonotube which can be purchased at Home Depot or Lowes. For each sonotube, three circular pieces are cut out of a 4' x 2' sheet of 3/4" thick particleboard. Two are circles approx 12" in outer diameter cut to fit snugly inside the tube - each of these has a center hole cut into it of 9 1/8" diameter. One is glued into the sonotube interior at it's midpoint to provide rigidity. The second is glued to the end of the tube flush with the cut end - the woofer will ultimately be mounted onto this top-piece centered on the 9.125" opening. For the bottom of the sonotube, I chose a circular diameter, obviously solid, particle board cutout of 16" diameter.

The completed cabinets (drivers not installed yet) need only to be wired and stuffed. Any good quality 18g or 16g OFC wire will do. I installed the binding posts in the bottom 16" piece facing up on the flange. Holes are drilled so the wires can be run through then bottom piece and under it and back into the interior of the cabinet then up the the drivers. Two holes are drilled through the top piece behind where the woofer driver will sit.

Acoustastuff is added to 50% fill. The cabinets can be finished to taste: wood venier or textured paint as I did. Once the finish work is done, bottom spikes can be installed in the 16" bottom piece close to the outer edges.

Before installing the woofers, I cut gaskets out of 1/8
" thick cork sheet (available at Lowes) having 9.125" inner dia. and 10.5" outer diameter and glued to the top piece around the center opening (this damps the woofer metal basket vibrations). All wire holes are hot glued to make them air tight.

The woofers have at this point have had the felt damping triangles affixed to the paper cone as per my drawings and photo above using the glue specified [important as it dries flexible with excellent viscoelastic damping properties]( see earlier post). The dust cap mod consists of carefully cutting a 1/2" diameter hole in the center of the dust cap and them covering it with three circular sections of felt cloth as shown above - this acts as a variovent, largely eliminates duct cap resonance and greatly improves the upper midrange.

The woofers are screwed into place flush on the cork gasket and perfectly centered over the 9 1/4" cabinet opening. The wires are soldiered to the woofer, red to positive (important to observe polarity).

The tweeter is a 1 inch soft dome from Tagband, crossed over using a 5.5 microfarad high quality polypropylene cap in series with a 4 ohm power resistor (5-10W). As the tweeter is nominal 4 ohms, this should provide for a first order high pass slope with -3dB point at about 8 khz. That's all there is in terms of crossover - the woofer is wired directly and is run full range. Important: the tweeter polarity is wired positive tweeter terminal to positive woofer terminal (so in fact, as the woofer is firing down into the cabinet, the woofer is actually 180 degrees out of phase with the tweeter. Based on listening, this wiring sounds superior. I believe that the signal experiences approx 180 phase rotation across the bandwidth of the woofer so that at the crossover point of 8 khz the woofer and tweeter are ostensibly phase aligned).

The tweeter is attached to the top of the woofer pole piece so as not to obstruct the woofer's pole vent. The tweeter voice coil should be at the acoustic center of the woofer voice coil [in the vertical axis] so the drivers are time aligned. I used a 3" section of 1/2 " aluminum angle, epoxyed to the pole piece; then I glued the flange of the tweeter to the front surface of the vertical element of the aluminum angle. The tweeter thus is oriented with the flange in the vertical and it's voice coil centered above the woofer pole vent opening. The flange of the tweeter and the adjacent front top of the woofer magnet surface is then covered with felt to provide cosmetic uniformity and eliminate baffle diffraction.

The speakers are positioned so that the tweeters are toed in at 45 degrees and pointing into the center of the room. The tweeter axes cross well in from the the listener. The resulting sweetspot is extremely broad, so that many listeners will enjoy an equally excellent spacious soundstage with holographic imaging.

The speakers produce an exceptionally coherent and clean sound (vocal reproduction is outstanding) that is substantially omnidirectional until crossover when the sound radiation becomes progressively directional above 8 khz (approx. wavelength 1.25 inch over a 3 inch diameter tweeter baffle). This results in outstanding image localization.

The speakers can play extremely loud (over 100 dB) cleanly and without strain amd require no more than 100W of clean power. The speakers a very easy resistive load to the amplifier. The bass is tight and tuneful and in room one should achieve usable output easily to 40 hz. (anechoic about 49Hz - but closed box enjoys gradual rolloff below resonance and benefits greatly from room reinforcement).

If necessary, I can post some photos if you are unsure about a particular part of the assembly.
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Old 9th March 2007, 02:58 AM   #39
R. Jamm is offline R. Jamm  United States
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Default Ohm Walsh by mamboni

Yes, pictures of how the tweeter is attached to the woofer would be helpful and also pictures of where you place the crossover components. Basically any pictures showing the process would be helpful but what I have listed above would be a start.

Thanks.
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Old 9th March 2007, 03:42 AM   #40
mamboni is offline mamboni  United States
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I'll try to post some photos tommorrow evening (Friday); failing that Saturday morning. Trust me: this is a very simple and easy project.
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