Modular Audio Amplifier System

Project Presentation

Abstract

The basic idea of the modular audio amplifier system is that any amplifier can be broken down in basic building blocks. The combination of those building blocks yields a variety of amplifier configurations. The underlying idea is to have a flexible platform for experiments with different circuits. From an economic point of view, the chassis, power supply and output stage of each amplifier are the most expensive parts. Versatility of the chassis and power supply ensure that a broad spectrum of amplifiers can be configured in this environment. The small signal stages cost is negligible, but has huge impact on amplifier behavior and performance. Both the mechanical and electrical interface needs to be standardized to some extend in order to ensure flexibility.

Idea and inspiration

When I was a child, one of my favorite toys was Lego. I mostly owned simple bricks, but found it fascinating how many different things I could build with just simple bricks. A tiny set of Lego Technik fascinated me even more. Electronics is quite similar to Lego: With just a few different parts, a lot of very different things can be built. With this in mind, why should I restrict the countless options to set up an amplifier to one specific build instead of having unlimited possibilities?
I explain the design philosophy behind this project in a separate article.

Chassis

The amplifiers chassis is the single most expensive part. Especially at low volumes, like a single piece, any customization becomes expensive. The heat sinks and bottom plate feature a regular grid of mounting holes in 20mm pitch. This restricts all modules mounted inside to adhere to this 20mm pitch and in turn offers flexibility to mount modules anywhere. I evaluated different mounting hole grid spacing and found 20mm to be the best. The front panel holds an illuminated power switch. On the rear, audio input and output interfaces are standard D-flange type suitable for XLR and SpeakON and power inlet is 3 pole IEC type. All rear interfaces are PCB mounted.

Grid System

Empty Chassis

Above illustration shows the custom modified empty chassis.

Example Assembly

All modules are mounted using M3 hexagon shaped stand-offs. Most common height of the stand-offs is 10mm.

The three amplifier modules are mounted to the radiator. From right to left this is the front end, driver module and power output stage.

In the rear of the chassis, three modules are stacked on top of each other on 10mm spacers with 40mm spacers in between the modules.

The lowermost module of the stack is the transformer startup control.

On top of the transformer control module resides the loudspeaker protection module. It serves as power-on delay and DC voltage protection.

The topmost module is for the audio signal input and could be a pre-amplifier. Shown here is my audio signal transformer module.

The power toroid transformer is located in the center of the chassis. It is fixed to a mounting plate using the typical toroid transformer mounting kit. The mounting plate is just a PCB with the 20mm hole grid.

A second mounting plate on top of the power transformer allows to install modules on the real estate that otherwise would be lost.

Here are two small auxiliary power supply modules and a transformer DC blocking module installed on top of the power transformer.

Right behind the front panel there is the main power supply unit located. The module rectifies the AC voltage and provides a lot of capacitance for smooth DC output.

On top of the rectifier and capacitor module, there could be another mounting plate to carry further modules.

In this example assembly I put some very basic 5x20mm fuse modules on top of the power supply.

With appropriate size of the filter capacitors, it would be possible to stack two power supply modules on top of each other for dual mono configuration of the amplifier. Four transformer secondaries would be required for a true dual mono setup. In one of my amplifiers I have two power toroid transformers with two secondary windings each so a dual mono setup would be feasible. However, I have a clever power distribution network that is almost as good as a dual mono setup and do not plan to build a true dual mono amplifier.

The chassis could accommodate even larger transformers. Below example shows a toroid transformer that is too large to fit on the mounting plate. I have such a transformer left over and plan to use it for one of my modular amplifier configurations.

The oversized toroid transformer conflicts with other modules. One solution could be to install the rectifier and smooting capacitor module upside down so that part of this module is above the toroid transformer.

With the second radiator equipped with the amplification modules and the missing parts of the chassis installed, this example configuration of the amplifier is complete.

Power supply

The power supply should be as flexible as possible. A transformer with two primary (2x115V) and four secondary windings would be ideal. Two primary windings ensure adaption to different regions of the world and four secondary windings allow different output voltages. Each secondary winding could have roughly 30V to 35V, allowing to parallel windings in order build low voltage class A amplifiers or connect the windings in series for double output voltage in order to build high power class AB amplifiers. For the prototype, I plan to reuse transformers from my old amplifier built around year 2000. Back then, I used two transformers with two 28V secondaries each with 160VA each as I could not find any single more suitable transformer.

Modules

The modular amplifier is broken down into following modules:

  • Signal conditioning / pre-amplifier
  • Front end: Voltage gain stage. Gain of +25dB
  • Driver and output bias stage / Error correction stage. Gain ≈ 1
  • Emitter follower or source follower output stage for current amplification. Gain ≈ 1
  • Loudspeaker protection / Power on delay
  • Amplifier high power supply
  • Auxiliary low power supply to boost front end rails
  • Amplifier control circuitry (Power transformer startup / protection)

Additional to the completed modules, currently I have following modules under development:

  • CFA front end
  • Error correction driver stage (Hawksford / Cordell)
  • Maybe I will also design a 4 or 6 pairs of lateral MOSFET source follower output stage (unlikely)

Here are the finished modules:

Most of the modules can be combined to yield a huge variety of possible amplifier configurations.

Configurations

Here are the amplifier configurations that I built already:

(For simplification, only the audio signal path is shown.)

Configuration #1

The power supply is configured for low voltage operation (25V DC) and the amplifier operates in full class A.

Configuration #2

The power supply is configured for low voltage operation (25V DC) for evaluation only and will be configured for high voltage operation (50V DC) once evaluation is successful. The amplifier operates in class AB.

Summary

The big benefit of the modular approach is low risk of investment and evaluation. In case any module fails to deliver the performance or sound expected, this single module can be replaced without affecting any of the other modules. This way, each of the amplifier modules can be perfected on its own. The whole amplifier can be configured an many ways and quickly be reconfigured. Different amplifier configurations may measure and sound differently and the amplifier can be configured to yield the best performance with little effort.