DSP Active Crossover Before the Power Amps

DSP Active Crossover Before the Power Amps

DSP Active Crossover Before the Power Amps

Most amplifiers send a full-range signal to the speaker and let a passive crossover inside the cabinet split bass and treble. That works, but it is not always the best solution.

Passive crossovers need large coils, capacitors and resistors. Good inductors are expensive, bulky and not perfectly transparent. The crossover also works after the amplifier, at speaker power level, so part of the amplifier output is lost in the crossover network instead of going directly to the driver.

This project uses another approach: the crossover is done digitally, before the power amps.

The input signal goes into our ADAU1701 DSP board. The same board handles the front panel controls and the active crossover. After that, the already-filtered signals go to the power amplifier channels.

The important part is simple: the amplifier does not waste power in a passive crossover. Each amp channel drives the speaker section it is meant to drive.

Why Use a DSP Crossover?

The main reason is control.

With a passive crossover, every change means changing parts. If the crossover point is wrong, you replace coils or capacitors. If the slope is wrong, you redesign the network. If the tweeter is too loud, you add resistors and lose more power.

With DSP, the crossover is part of the SigmaStudio program.

  • Change the crossover frequency
  • Change the filter slope
  • Adjust bass and treble balance
  • Correct driver polarity
  • Add tone shaping
  • Add protection filtering
  • Switch between full-range and bi-amp routing

No expensive coils. No trial-and-error soldering. No wasting amplifier power in large passive components.

For this prototype, the starting point is a 2-way Linkwitz-Riley crossover at 900 Hz, 24 dB/oct. The exact value can be changed later depending on the speaker and measurements.

Implemented with Our ADAU1701 Board

The amplifier uses the same DSP board concept from our ADAU1701 guitar and audio projects. The board already includes the analog audio input and output stages, plus the physical controls. SigmaStudio defines what those controls actually do.

For this amplifier, the front panel remains simple:

  • Input
  • Volume
  • Balance
  • Tone
  • Power Level
  • Amp Mode
  • Power Indicator

There is no screen and no menu system. From the outside, it still feels like a normal amplifier.

Inside the ADAU1701 program, those controls are mapped to the actual audio behavior. Volume controls the system level. Balance works before the crossover. Tone is handled digitally instead of with a traditional analog tone network. Power Level can reduce the output level, for example full power or -12 dB. Amp Mode switches between regular full-range operation and bi-amp operation.

The front panel stays simple, but the signal routing is flexible.

Signal Flow

The ADAU1701 DSP board is the control and crossover section of the amplifier. The front panel controls and the DSP active crossover are not separate hardware blocks. They are part of the same DSP board and SigmaStudio program.

The simplified signal flow is:

  • Audio input enters the ADAU1701 DSP board
  • The ADAU1701 handles front panel control logic and DSP processing
  • The DSP program creates the active crossover
  • Filtered outputs go to the power amplifiers
  • The amplifiers drive the speaker drivers directly

Regular Mode and Bi-Amp Mode

The amplifier has two useful modes.

In regular mode, it works like a normal stereo amplifier. The main left and right outputs carry full-range audio and can drive normal passive speakers.

In bi-amp mode, the DSP crossover is enabled. The bass outputs receive the low-pass signal. The treble outputs receive the high-pass signal. The speaker must be wired for separate bass and treble input.

This gives a practical amplifier that can be used normally, but can also drive a speaker system without relying on a passive crossover.

SigmaStudio Program

The SigmaStudio program is the real crossover board.

A simplified version of the program is:

  • ADC input
  • Front panel control mapping
  • Regular / Bi-Amp mode switch
  • Full-range output in regular mode
  • Linkwitz-Riley crossover in bi-amp mode
  • Low-pass output to bass amplifier
  • High-pass output to treble amplifier

The current crossover is a starting point, not a fixed rule. That is the point of doing it in DSP. If the speaker needs 700 Hz, 1.2 kHz, a different slope, or a level correction, it can be changed in software.

This is especially useful when working with small drivers or custom speaker boxes, where a standard off-the-shelf passive crossover is usually a compromise.

What Problem This Solves

This project is about avoiding the usual passive crossover problems.

  • Expensive inductors
  • Power loss in crossover components
  • Fixed crossover frequency
  • Difficult tuning
  • Impedance-dependent behavior
  • Poor flexibility with custom speakers
  • Amplifier power being sent through parts instead of directly to the driver

With an active DSP crossover, the split happens before amplification. The amp channels receive only the frequency range they need to amplify. The speaker drivers are controlled more directly. The crossover can be changed without rebuilding hardware.

That is the real reason for the project.

Not because four amplifier channels are impressive. Because a digital crossover before the power amps gives better control over a custom speaker system.

Related Resources

This build uses our ADAU1701 DSP board as the control and crossover section. More details about the board are here: A Universal Modeling Guitar Pedal Built on ADAU1701.

The power amplifier section is based on Rod Elliott’s Project 3A amplifier: 60-100W Hi-Fi Power Amplifier.

For custom speaker, amplifier or DSP builds, we can also design dedicated boards around the same idea: physical controls, SigmaStudio DSP processing and the exact crossover/routing needed for the project.

Conclusion

This amplifier is built around a simple idea: make the crossover digital and put it before the power amps.

The ADAU1701 DSP board handles the front panel controls, tone, mode switching and active crossover. The power amps then drive the speaker sections directly, without a passive crossover wasting power or locking the design into fixed components.

For custom speaker work, this is much more practical than designing passive crossovers with large coils and guesswork. The crossover can be tuned in SigmaStudio, adjusted and reused for different speakers.

Finished amp front and back panel:

 

 

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