Modular Audio Amplifier System

Project Roadmap

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 a flexible platform to experiment with different circuits. From an economic point of view, the housing, power supply and output stage of each amplifier are the most expensive parts. Versatility of the housing and power supply ensure that a broad spectrum of amplifiers that can be built. 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?

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

Example Assembly

Example Assembly Top View

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 power 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 in the range of +30dB
  • Driver and output bias stage / Error correction stage. Gain ≈ 1
  • Single emitter follower or single source follower for current amplification. Gain ≈ 1
  • Loudspeaker protection / Power on delay
  • Amplifier Power supply (Rectification, reservoir capacitors)
  • Auxiliary Power supply (Rectification, reservoir capacitors) to boost front end rails
  • Amplifier control circuitry (Transformer startup / protection)

Currently I have several modules under development:

  • OPamp based pre-amplifier
  • VFA front end
  • CFA front end
  • Error correction driver stage (Hawksford / Cordell)
  • Maybe also 4 or 6 pairs of lateral MOSFET source follower output stage

Here are the finished modules:

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

Block schematic

Below block schematic illustrates how an example amplifier could be connected together. The wiring scheme is a bit unusual as there are three star points and also four floating auxiliary supplies to boost the front end power supply rails. The transformer audio input modules guarantee that no ground loop can form when interconnecting with other audio equipment. Each amplifier channel has its own star point for minimized circuit loop area. Due to having very large supply capacitors at the output stage, this makes sense. Also, the output stages feature low value resistors to both smooth the supply even further and also to separate the channels from each other. Following this approach, the number of auxiliary power supplies has to be doubled in order to maintain the channel separation. The third star point is right at the main power supply for safety reasons.

The actual scheme, especially the mains AC input, is more complex and has been simplified for clarity. I colored some nets in order to visually untangle the connections a bit.

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 or redesigned without affecting any of the other modules. The resulting amplifier can be configured an many ways and quickly reconfigured.