Project Overview & Engineering Log: Mirage OM-7 Bipolar, FAST / WAW Reference Towers, Polk CS400 Center Channel

 1

Project Overview & Engineering Log: Resurrecting Audio Classics

1. The Mirage OM-7 Bipolar Restoration

• Project Status: Completed (May 2023)
• Design Philosophy: Bipolar / Omnipolar Room Integration (Ian Paisley / Andrew Welker legacy geometry)
• Cabinet Profile: 100 lb, dual-layer, acoustically dead flagship enclosures. Features a completely isolated, airtight sealed vault for the upper midrange array and a heavily braced, bottom-ported woofer section.
• The Mission: Total rebuild following a catastrophic amplifier DC-offset blowout that melted the factory driver voice coils and charred the internal crossover networks.

Custom Driver & Crossover Blueprint (Acoustic Logic)

Because factory parts were expencive, budget-friendly Parts Express/Dayton components were engineered to track perfectly with the cabinet’s native 360-degree acoustic signature via XSim and rigorous listening tests.

• Bass Low-Pass Filter:
  • Driver: GRS 8SW-4 (4-Ohm)
  • Network Topology: 2nd-Order Low-Pass (3.0 mH series inductor + 100 µF shunt capacitor)
  • Acoustic Function: Creates a smooth electrical cutoff near 290 Hz, forcing the budget woofer to stay strictly in its comfort zone while acting as an electrical sponge to keep chaotic midrange frequencies out of the bass cone.
• Midrange Band-Pass Filter:
  • Drivers: 2x Dayton Audio 4FR-8 (4-inch paper full-range) wired in a 16-Ohm Series String (In-Phase Bipolar configuration).
  • Network Topology: Modified 3rd-Order Band-Pass (6.2 µF series cap → 1.0 mH series coil + 10 µF shunt cap) with an integrated contour pad (2.0 mH coil || 12.5 Ω resistor).
  • Acoustic Function: Series wiring naturally matches driver sensitivity to the woofers. The 2 mH / 12.5 Ω contour network acts as a robust Baffle Step Correction (BSC) to tame the native "paper cone shout" and rising upper-mid glare. The 3rd-order section cleanly isolates the vocal band and prevents lower-bass over-excursion.
• High-Frequency Super-Tweeter Array:
  • Drivers: 2x Dayton Audio ND25FA-4 (1-inch neodymium domes) wired in a 8-Ohm Series String (Bipolar layout, mounted flush under factory grills).
  • Network Topology: High-Pass Shield (2.0 µF series cap → 1.4 Ω pad resistor + 0.15 mH shunt coil to ground).
  • Acoustic Function: Implemented strictly as a super-tweeter array cutting in near 9.5 kHz. Because the 4FR-8 full-range drivers extend incredibly high on their own, the tweeters are heavily protected from lower-mid voice coil heat, stepping in only to add transient "air," sparkle, and three-dimensional realism to live venue tracks and crowd applause.

---

2. The FAST / WAW Reference Towers

• Project Status: Completed (May 2023)
• Design Philosophy: Single-Point Source Imaging / Woofer-Assisted Wideband (FAST/WAW concept inspired by xrk971 geometries).
• Target Application: High-precision, pinpoint soundstage localization for secondary 5.0 media integration.
• The Mission: Taming high-performance but notoriously difficult rigid aluminum cones through aggressive, multi-stage passive filter design.

Driver Integration & Crossover Architecture

Unlike forgiving paper cones, aluminum drivers possess massive, razor-sharp high-Q mechanical breakup modes at their upper limits that require deliberate electrical suppression.

• Woofer Low-Pass Section:
  • Driver: Dayton Audio RS225-8 (8-inch Reference Aluminum Woofer)
  • Network Topology: Heavy-Dampened 2nd-Order Low-Pass (6.0 mH series inductor + 100 µF shunt capacitor)
  • Acoustic Function: Flat-lines the woofer's rising native voice coil inductance and forces a steep, early cutoff starting around 200 Hz. This ensures that the RS225's brutal 2 kHz to 3 kHz aluminum cone breakup peak is buried 30 dB down into the noise floor before it can corrupt the midrange.
• Wideband High-Pass Section:
  • Driver: Dayton Audio DMA90-8 (3.5-inch Aluminum Cone Full-Range)
  • Network Topology: Asymmetric 3rd-Order High-Pass Loop (33 µF series cap → 3.6 mH heavy shunt inductor → 4.0 Ω attenuation pad + 4.7 µF shunt cap across the terminals).
  • Acoustic Function: The massive 3.6 mH shunt coil acts as a literal bass shield, throwing low-frequency energy directly to ground to protect the tiny 3.5-inch cone from bottoming out. The post-pad 4.7 µF shunt capacitor acts as an electrical sponge, smoothing out the DMA90's aggressive high-frequency ringing and metal-cone artifacts, resulting in a seamless, fluid, crossfading point-source image that covers the entire vocal register without a crossover splitting the human voice.

---

3. Bench Notes & Manufacturing Methodology

All systems were constructed and validated utilizing custom rapid-prototyping infrastructure, ensuring lab-grade reliability:

• Crossover Layouts: 100% rugged point-to-point wiring, completely avoiding thin, fragile commercial circuit traces that lift under extreme thermal or current loads.

---

Introduction: Bipolar Ambient Realism vs. Point-Source Pinpoint Precision

When designing or restoring loudspeakers, the choice of physical topology dictates how the speaker interacts with the boundaries of your listening room. Two formats that represent fundamentally opposing ways of rendering an acoustic soundstage are Bipolar (Omnipolar) Geometry and Single Point-Source (FAST / MTM) Alignment. Understanding these differences highlights why specific speakers excel at certain tasks.

The Bipolar/Omnipolar Approach (The Room Envelopment Bubble)

Speakers like the restored Mirage OM-7 use identical driver arrays on both the front and rear of a heavy cabinet, wired tightly in-phase.

• The Reflection Mechanic: By radiating acoustic energy in a 360-degree pattern, the speaker intentionally sprays waves into the front wall of your room. These waves bounce back to your ears with a tiny, natural time delay.
• The Presentation: This physical crossfade expands the soundstage into a massive, enveloping 3D bubble. It completely strips away the physical "box sound," which makes it the ultimate weapon for chaotic, un-correlated phase signals—like a roaring live concert crowd track. The applause expands deep and wide because it mirrors the chaotic reflection patterns of a real theater.
• The Trade-off: While magnificent for ambient scale, this constant bouncing of rear waves blurs the absolute boundaries of the soundstage.

The Point-Source Approach (The Critical Listening Window)

For absolute critical listening, a single point source—like a highly refined FAST (Woofer-Assisted Wideband) array or a vertically symmetrical MTM tower—takes the crown.

• The Linear Wavefront: Instead of using the walls to diffuse the sound, a point-source system aims for absolute time-domain and phase alignment. Sound across the entire vital frequency register leaves the drivers on a singular, unified plane aimed straight at your ears.
• The Presentation: This alignment gives your brain pristine, razor-sharp instrument localization. A vocalist doesn't feel like a generalized presence floating somewhere in the room; they are locked down to a single, pinpoint square inch in space. You can easily map the exact physical width, depth, and spatial layers of the recording studio.

Both approaches are highly usable tools. The bipolar towers act like an open lightbulb that turns live venue energy into an immersive room experience. The point-source towers act like a high-resolution lens, stripping away the room's boundaries to lay bare the absolute, raw reality of the original master track.

---

4. Polk CS400 Center Channel Modification Log

• Project Status: Completed (Tuned by Logic, Ear, and Bench Experience)
• Design Philosophy: Acoustic Timbre Matching & High-Output Resonance Mitigation
• The Mission: Reworking a classic, high-output commercial center channel to smooth out a problematic, non-standard factory tweeter dome and pull its lower-mid presentation into perfect alignment with a custom FAST tower front soundstage.

The Problematic Factory Architecture

The Polk CS400 features a massive, robust cabinet with dual 6.5-inch mid-bass drivers flanking a proprietary, odd-sized trilaminate dome tweeter. Because of its odd physical dimensions, dropping a standard round aftermarket tweeter into the faceplate cutout is impossible without creating an un-sealed air leak. The factory crossover network left the tweeter sitting on a standard 2nd-order slope, causing it to distort down low and introduce a harsh, metallic shouting glare that ruined vocal pans across the front stage.

The Veteran-Builder "Ear & Logic" Modifications

Without relying on simulation software, decades of workshop experience guided a series of non-destructive, highly effective electrical and acoustic changes:

• The 3rd-Order Tweeter Shield:
  • Modification: Added a calculated inline capacitor right before the hot side of the tweeter terminal.
  • Acoustic Function: Shifted the factory network from a standard 2nd-order slope to a steep 3rd-Order Network (18 dB per octave), mimicking Danny Richie's X-Static layout rules. This aggressively blocked lower-midrange energy from hitting the delicate neodymium voice coil, instantly dropping distortion, smoothing out the top-end grain, and forcing the non-replaceable factory dome to sound smooth and organic.
• The External Binding-Post BSC Network:
  • Modification: Constructed a dedicated, parallel inductor-and-resistor Baffle Step Correction (BSC) network and mounted it externally right across the input binding posts.
  • Acoustic Function: When large center channels are placed near a wall or inside an entertainment cabinet, boundary loading creates an artificial, boomy "chestiness" in male vocals. By fine-tuning this external network with a specific resistor value, the lower-mid bass bloat was gently shelved down.

The Final Result

The modified CS400 completely lost its native mid-band glare and boomy box coloration. It now integrates into the 5.0 surround matrix, matching the fast, clean, aluminum point-source presentation of the main FAST towers. It serves as a prime example of using simple circuit logic to save a non-replaceable factory element.

Page settings Options Page moved to Trash