Turbo Mode: Dual-LM1036 Psychoacoustic Enhancement Project

The prototype is built with two LM1036M, now on its third revision, this time using conductive-plastic element potentiometers rated for over 100,000 rotational cycles. These pots feel exceptionally smooth and introduce no electrical noise during adjustment.

The initial LM1036 prototypes dates back to 2014, using both the 20-pin DIP and SOIC packages. Although, now considered obsolete, originally released in the early 1990s by National Semiconductor, it remains widely available through online sellers. For detailed electrical characteristics, refer to the Texas Instruments LM1036 datasheet (SNAS525C).

The schematic diagram shown reflects the actual hardware used in the build, incorporating a dual-LM1036 configuration, an onboard 3-pin linear regulator for local power conditioning, and all associated control potentiometers, coupling capacitors, and protection components required for the completed signal-conditioning module.

The images show the fully populated LM1036M PCB assemblies. These fabricated versions are used in the current prototype. Two PCBs are required to implement the cascaded configuration for this project. The LM1036M SOIC 20 device is soldered on the bottom side of the PCB which keeps the footprint compact and helps reduce noise and interference because the top layer is primarily a ground plane. An onboard LED and a 78L12 regulator were added for operational convenience. The regulator requires an input voltage greater than 12 VDC, with a maximum allowable input of +35 VDC.

What the Dual-LM1036M (stacked) Turbo Mode is doing:

• The first LM1036M shapes the initial loudness contour from the source.
• The second LM1036 applies the user’s desired tone contour for the amplifier.
• Both LM1036M loudness circuit mimics this psychoacoustic behavior. Stacking two LM1036M compounds the contour, giving the ear a “fullness boost” without increasing actual gain.
• The listener perceives more weight, more sparkle, more presence — the Turbo Mode sensation.
• Their voltage-controlled amplifiers multiply, creating nonlinear shaping instead of simple addition.
• The noise floor stays low because LM1036 is inherently quiet.
• The audio power amp suddenly receives a shaped, energized, psychoacoustically boosted signal.
• Your ear hears density, presence, forwardness, and clarity that you did not hear before.

What the Fletcher–Munson Curve Does to the Stacked LM1036M Pair

The Fletcher–Munson curve shows how human hearing changes with volume. The stacked LM1036M pair uses this psychoacoustic behavior to its advantage. Fletcher–Munson curves show how loud each frequency must be for your ear to perceive it as equally loud. They prove that human hearing is not flat — our ears change sensitivity depending on volume.

At low listening levels, your ear becomes less sensitive to:

• Bass — it disappears first
• Treble — it thins out
• Midrange — stays most audible

At higher SPL, the curves flatten, meaning bass and treble “come back” and the sound feels balanced again. This is exactly what your ears experience when you turn music up or down.

Fletcher–Munson curves show how human hearing changes with volume — bass and treble need more energy at low levels, and flatten out at higher levels. The LM1036 loudness contour is designed to mimic this behavior.

Stacked LM1036M Prototype Wiring Diagram

QA400 Audio Analyzer Testing for THD, THD+N, SNR and Frequency Response.

The single LM1036M shows classic hi-fi behavior: THD at 0.11%, and the noise+distortion at –64 dBFS across the audio band.

Stacking two LM1036Ms drops harmonic peaks by 10–12 dB, cutting THD to 0.05% and pushing N+D down to –74 to –76 dBFS.

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