IRS2092S BTL Project 

This project demonstrates the modern BTL approach using dual IRS2092S PCAs, matched, thermally stable, and tuned for differential operation. The BTL implementation keeps the output power benefits—2× the voltage swing and 4× the theoretical power from the same rails, and at the same time  eliminating the return-energy imbalance that caused bus-pumping in early Class-D modules.

 

Project Plan and Status of the IRS2092S BTL Prototype

The current workshop build is a full-scale Class-D development platform based on the IRS2092S. This project demonstrates stable bridged-tied-load operation using two SOIC-16 IRS2092S amplifier modules, each individually validated for switching behavior, square-wave integrity, and music auditioning before being paired into the BTL configuration. The design evolves from earlier 2015 DIP-16 based prototypes (Proj55 and Proj56), with a fabricated 2-layer PCB, continuous top-layer ground plane, and reduced noise/parasitics for improved EMI performance.

 

Schematic diagram of the IRS2092S BTL Project 

 

Power delivery is handled by a high-current linear supply using a large transformer, rectifier stage, and bulk filtering. A dedicated relay/varistor protection board provides soft-start, AC surge handling, and uPC1237 speaker protection. The entire assembly integration is confined into a 12×16×1/8 in aluminum plate, with precision-machined standoffs, heatsinks, and structural brackets. The prototype is intentionally planned with open-frame, allowing full visibility of the signal path, thermal behavior, wiring discipline, and mechanical layout. It serves as a teaching platform, a mechanical showcase, and a test-bench artifact rather than a consumer enclosure.

Test results match amplifier responses published in IRAUDAMP7S reference manual and application notes, confirming proper switching behavior and stable loop operation. Listening tests show clean, controlled output, highlighting the familiar divergence between objective measurements and subjective listening, where both perspectives remain valid and complementary.

 

The project is almost complete and works as a monoblock, with one BTL channel fully tested and mechanically integrated. A second BTL channel will be added in a future phase to complete the dual-channel system, including matched wiring, thermal symmetry, and final enclosure refinement.

 

How does BTL work?

 

How to get more output power without raising the supply voltage — and solve one of Class-D’s oldest headaches (bus-pumping) at the same time.

A BTL stage drives the speaker differentially, meaning one amplifier pushes while the other pulls. One channel is non-inverted, the other inverted, and the load floats between them. This simple idea hits two targets with one stone:

• 2× the voltage swing across the speaker
• 4× the theoretical power from the same supply rails

 

The “two amps driving a floating load” concept evolved directly into the full-bridge switching stage used in today’s Class-D power amps — including this IRS2092S project.

 

In a full-bridge Class-D:

• Each IRS2092S drives a half-bridge
• Two half-bridges form a full-bridge
• The speaker sits between the two switching nodes
• The differential switching cancels return currents

 

Simplified Block Diagram of the IRS2092S BTL Project 

BTL AMPLIFIER – CORE IDEA

 

Goal:

Use two amplifier outputs, equal in amplitude but 180° out of phase, to double the voltage across the load and get 4× the power from the same supply rails.

 

Signals:

– Vin: small 1 kHz sine (reference)

– OutA: +Vsine (non-inverting path)

– OutB: -Vsine (inverting path)

 

Key relationship:

Vload = OutA – OutB

 

If:

OutA = +Vsine

OutB = -Vsine

 

Then:

Vload = (+Vsine) – (-Vsine) = 2 * Vsine

 

Let’s use an example of the actual Tek hardcopy numbers obtained from the IRS2092 BTL prototype. 

 

Given:                                           

  Ch1 ≈ 33.17 Vrms

  Ch2 ≈ 33.89 Vrms

 

We’ll use an average:

  V_rms ≈ 33.5 V

 

General power formula:

  P = V_rms² / R_load

 

————————————————–

1) Single-Ended (SE) Power

————————————————–

 

Example A: R_load = 8 Ω

 

  P_SE_8 = V_rms² / 8

         = (33.5)² / 8

         = 1122.25 / 8

         ≈ 140.3 W

 

Example B: R_load = 4 Ω

 

  P_SE_4 = V_rms² / 4

         = (33.5)² / 4

         = 1122.25 / 4

         ≈ 280.6 W

 

——————————————–——

2) Bridge-Tied Load (BTL) Power

————————————————–

Assume:

  – Two equal channels

  – Perfectly out of phase (180°)

  – About the same magnitude V_rms each side

 

Effective BTL voltage across the load:

  V_BTL_rms = V_rms(ch1) + V_rms(ch2)

            ≈ 33.5 V + 33.5 V

            ≈ 67.0 V

 

Power formula still:

  P_BTL = V_BTL_rms² / R_load

 

Example A: R_load = 8 Ω

 

  P_BTL_8 = (67.0)² / 8

          = 4489 / 8

          ≈ 561.1 W

 

Example B: R_load = 4 Ω

 

  P_BTL_4 = (67.0)² / 4

          = 4489 / 4

          ≈ 1122.3 W

 

————————————————–

3) Relationship SE vs BTL

————————————————–

 

For the same per-channel V_rms and same load R:

 

  – BTL doubles the voltage across the load:

      V_BTL_rms = 2 · V_rms

 

  – Power scales with V², so:

      P_BTL = (2 · V_rms)² / R

            = 4 · V_rms² / R

            = 4 · P_SE

 

So:

  P_BTL ≈ 4 × P_SE  (for same load R)

====

Recap (ideal math):

  V_rms per channel ≈ 33.5 V

  V_BTL_rms ≈ 67.0 V

 

  P_BTL_8 ≈ 561 W

  P_BTL_4 ≈ 1122 W

 

Reality for music:

  – Music has high crest factor:

      Peak power >> Average power

  – Average thermal load is typically:

      ~1/5 to 1/10 of “full-sine” rated power

  – So even if peaks touch those 500–1100 W numbers,

    the long-term average is much lower.

 

Verdict:

  – 8 Ω BTL: Very comfortable for music, even at loud levels.

  – 4 Ω BTL:Electrically and thermally OK for music,

    as long as:

• Heatsinking is solid
• Rails aren’t extreme
• Protection (OC, OT) is in place

 

An image of two SOIC-16 IRS2092S amplifier modules for BTL configuration—as one channel in a stereo system