Originally published by Paul Stenning in ETI, December 1994

• Diagrams and Schematics

Introduction
The peace and quiet in our household has recently been shattered by the arrival of a Sega MegaDrive. For anyone who is not actually using the machine, the sound from Sonic the Hedgehog or whatever can soon become very monotonous and annoying. The obvious solution was a pair of headphones, but Sega did not feel it necessary to provide a suitable socket - nor did the manufacturers of out TV set!

There is only one audio/video output connector on the MegaDrive, and this is used for the RF modulator so that the unit can be connected to the TV. Fortunately this is a standard 9 pin mini-DIN connector, and the stereo audio output is on pins 8 and 9.

So all we need is a low power stereo amplifier capable of driving a pair of headphones, and some method of allowing this to be connected to the 9 pin socket on the MegaDrive, along with the RF modulator. Since cable mounting mini-DIN sockets are not readily available, I decided to have a flying lead from the amplifier, fitted with a 9 pin plug to connect to the mega-drive. A 9 pin socket is then fitted to the back of the amplifier, for the modulator.

The amplifier itself is mains powered, and the prototype was left powered all the time because the power consumption is minimal. I initially considered powering the amplifier from the DC output on the MegaDrive 9 pin connector, but this is only 5V which is not really adequate, bearing in mind that it would need significant decoupling to get rid of the noise from the digital circuits.

PLEASE NOTE THAT ANY DAMAGE CAUSED BY CONNECTING UNAPPROVED DEVICES TO THE SEGA MEGADRIVE WOULD NOT BE COVERED BY THE GUARANTEE.

This amplifier may also be suitable for other computer games machines, although the audio/video connectors will be different. You will have to work out the necessary connections yourself!

Circuit Operation
The complete circuit diagram is shown in figure 1. Only the left amplifier channel is shown, the right channel is identical with component references starting from 101.

Some of you may be wondering why I didn't use a dual power amplifier IC. The simple answer is that the unit was built using bits from my "junk box" - which contained plenty of op-amps and transistors, but no power amp chips! Two single op-amps were used in preference to one dual device because this simplifies the board layout.

R1, C1 and VR1 form a simple top-cut tone control. I had not originally planned to use any form of tone correction, but the sound through a pair of good quality headphones was found to be slightly fuzzy due to the audio signals being derived by digital methods. The tone control allows this effect to be minimized, although there is an inevitable reduction in the higher frequencies. C2 together with the track of VR2 causes the lower frequencies to be rolled off gently below about 30Hz.

The remainder of the amplifier is a standard non-inverting op-amp circuit with the addition of a class B push-pull output stage. The transistors TR1 and TR2 are held close to the point of conduction by R4 to R7. Cross-over distortion is removed by the use of a considerable amount of negative feedback (R10 and R3). This arrangement works well with a low gain low power amplifier such as this.

C3 and C4 gives additional negative feedback at high frequencies (above the audio range) to ensure stability. R8 and R9 limit the output current to a reasonable level if the output is short circuited.

The overall gain of the unit with the volume control at maximum is approximately unity. Obviously this is insufficient to drive the circuit into clipping, but gives more than adequate volume from the headphones. If you like having your ears blasted you could increase the values of R10 and R110 to 33K or even 47K - but I wouldn't recommend this unless you want to risk damaging your hearing!

The power supply is a conventional full wave split rail arrangement. This is not regulated due to the low current consumption of the amplifier. A 100mA transformer is adequate.