Scan-Speak Event Transmission Line – Modernized Crossover Concept

Revelator 18W/8531G00 with three high-end tweeter options

1. Project context

This renovation is based on the classic Event Transmission Line enclosure:

• Height: ~100 cm

• Width: 22.5 cm

• Depth: 33 cm

• Material cabinet: 22 mm MDF front baffle, drivers flush-mounted

• Material baffle: 25 mm MDF front baffle, drivers flush-mounted

• True TL with internal diagonal panel, rear/top exit

The goal is not to modernize the concept, but to elevate its performance using current high-end drivers and a carefully optimized passive crossover.

2. Reference midwoofer

Scan-Speak Revelator 18W/8531G00

Key properties:

• 8 Ω nominal

• Sensitivity ≈ 87 dB / 2.83 V

• Low Fs, 28-30 Hz, moderate Qts → excellent TL compatibility

• Extremely clean midrange

• Requires controlled upper roll-off and usually benefits from a notch filter

This woofer defines the character of the system: natural, unforced, low-fatigue.

3. Three tweeter options considered

Option A – Scan-Speak Revelator D2905/990000

28 mm soft dome, large rear chamber
Character: maximum calm, tonal density, and long-term listenability

Option B – Scan-Speak Illuminator D3004/660000

26 mm soft dome, very low distortion
Character: highest resolution and air, most analytical

Option C – Morel ET338

28 mm soft dome, robust motor
Character: slightly warm, very musical, forgiving

4. Tweeter comparison (with the 18W/8531 in a TL system)

Summary

• D2905/990000 → the most mature and relaxed match

• D3004/660000 → the most revealing and modern

• ET338 → the most emotional and forgiving

5. Optimized crossover concept (common philosophy)

Target:

• Acoustic crossover ≈ 1.9–2.0 kHz

• Steep acoustic slopes (LR4-like)

• Flat baffle, flush mounting

• Stable impedance behavior

Woofer section

• 4th-order electrical low-pass (series / shunt topology)

• Slight series resistance (coil DCR or small resistor) to assist baffle-step balance

• Zobel network across the woofer

• Optional notch filter if measurements show residual breakup energy

Tweeter section

• L-pad for level matching

• 3rd–4th order high-pass

• High-quality series capacitors (this is where parts quality matters most)

6. Explanation of the auxiliary networks

6.1 L-pad (tweeter level matching)

What it is
Two resistors:

• One in series (Rs)

• One in parallel (Rp)

Why it is needed

• Tweeters are typically 3–5 dB more sensitive than the Revelator woofer

• Simply padding with one resistor destabilizes impedance and crossover behavior

What the L-pad does

• Lowers tweeter output cleanly

• Keeps impedance predictable

• Makes crossover tuning easier and more stable

Audible effect

• Proper tonal balance

• No “shout” or glare from the tweeter

• Better integration through the crossover region

6.2 Zobel network (woofer impedance correction)

What it is
A resistor and capacitor in series, connected in parallel with the woofer

Why it exists

• A woofer’s impedance rises with frequency due to voice-coil inductance

• Without correction, the low-pass filter stops behaving as calculated

What the Zobel does

• Flattens the woofer’s rising impedance

• Allows the crossover to work as intended

• Improves phase alignment and crossover symmetry

What you hear

• Cleaner midrange

• More stable imaging

• Less “blur” around the crossover

Many builders report: “I don’t hear the Zobel — until I remove it.”

6.3 Notch filter (woofer resonance control)

What it is
An LCR circuit tuned to a specific narrow frequency band

Why it may be needed

• The 18W/8531 can show residual energy around 4–6 kHz

• Even when crossed low, this can subtly affect clarity

When to use it

• Only if measurements or careful listening confirm a remaining peak

• It is optional, not mandatory

Effect

• Cleaner upper midrange

• More relaxed treble integration

• Improved perceived smoothness

7. Component quality philosophy (Jantzen Audio approach)

Without listing part numbers excessively, the guiding principles are:

• Series capacitors in the tweeter path
  → highest quality film capacitors (this is audible)

• Parallel capacitors and Zobel caps
  → good quality standard film capacitors are sufficient

• Woofer coils
  → air-core coils, DCR chosen deliberately to help tonal balance

• Resistors (L-pad, Zobel, notch)
  → non-inductive, high-power types

This approach keeps the system transparent but not analytical, perfectly suited to a TL loudspeaker.

8. Final perspective

With a flat 25 mm baffle, a true TL enclosure, and the Revelator 18W/8531, the Event design becomes:

• timeless rather than spectacular

• neutral rather than impressive

• musically convincing over long listening sessions

The choice of tweeter does not change quality — it changes character.

If you want, the next logical steps would be:

• translating this concept into measurement-based fine-tuning

• publishing a step-by-step tuning workflow

• or a direct comparison with Accuton/AMT systems like the Inn II

Just tell me how far you want to take it.