Maplin 50W amplifier module

Picture of 50W amp

These, along with most of Maplin's other audio kits, has now unfortunately been discontinued as part of Maplin's apparent move away from hobby electronics and into gadgetry.

This circuit has, in one form or other, followed me though my DIY audio career. My first ever audio project back in 1990 or so was to build a pair of these in a 2U rack-mount steel case. This was put away, however, once I discovered that my Croft valve power amp sounded far more satisfying, despite only half the power specification.

I then used this circuit (painstakingly hard-wiring four amplifiers on matrix board) as a basis for the bass amplifiers for my three-way speaker project. These had an upgraded power supply and various tweaks, including low-noise input transistors, upgraded capacitors and plenty of extra supply decoupling. Although they sounded rather good in comparison with the first incarnation, these were in turn consigned to the parts bin for my Stochino amplifier project

Their final glory is a reincarnation of the original project in the rack-mount enclosure and on the original PCBs, but with a large power supply using a pair of surplus 500VA toroidal transformers and ten 1000uF capacitors in parallel for each power rail, as well as several component upgrades. This remains as a spare power amp, standing in for each of my Stochino amps as these in turn are stripped for replacement of their mains transformers and uprating of output MOSFETs and heatsinks.

Its specifications are pretty average, with unremarkable figures for THD and slew rate, but it seems a sensible and reliable design for an active system, for example.

Specifications (from Maplin catalogue)

Power Output at 1kHz 4 ohm load                         8ohm load
One Channel: 72W RMS                          50W RMS
Two Channel: 49W RMS                          36W RMS
Input Sensitivity  
For Rated Output: 380mV                                     450mV
Frequency Response: Flat from 20Hz to 28kHz
Full Power Bandwidth  
(pulse tested at hf): 3dB down at 95kHz
Noise: <-100dBu
THD: <0.05% at 1 kHz
Damping Factor: 80
Input Impedence: 15 kohm
Slew Rate: 14V/us (at 10kHz)

Here is a page on building the Maplin amplifier.

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