Oceana A100 Audio Power Amplifier

Contents This page... Overview Schematics Circuit details Construction details

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Overview

This audio power amplifier has been designed and constructed by Glenn Baddeley, a member of the M.A.C. (Melbourne Audio Club Inc.).

Features:

• Operates in class A mode,
• Delivers 100 Watts RMS into an 8 Ohm resistive load,
• Solid state design with fully regulated power supplies and overload protection,
• Output stage consists of 8 high-power darlington power transistors mounted on two massive heatsinks,
• Signal passes through a capacitor at only only one point, on the amplifier input,
• Rest of the circuit is DC coupled, including the negative feedback path,
• Output DC offset is maintained at less than 1 millivolt by a servo circuit.

The physical appearance is two (mono-block) wooden cabinets (varnished pine) with large black heatsinks on the sides of each cabinet. The large heatsinks are required due to the continuous heat dissipation of over 200 Watts.

The amplifier has been heard on many occasions over the past few years, from large groups in auditoriums, to small gatherings in private homes, to extended individual auditions. It has received favourable comments such as "effortless", "transparent", "rock solid", "detailed". Apart from these subjective assessments the amplifier has met my design criteria of inaudible distortion products and sufficient power for driving average loudspeakers to high levels.

The Oceana A100 amplifier design and the schematics are copyright © 1997 Glenn Baddeley. All rights are reserved. No responsibility is taken for errors or omissions.

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Schematics

Click on the PDF icon below each sheet to open a high quality schematic for printing or full-screen viewing.

To read PDF files you will need the Adobe Reader software. Click on the icon to go to a web page which will allow you to download the free software. Adobe Reader is a registered trademark of Adobe Systems Incorporated.

Sheet 1 - Power Transformer and Rectifiers

(1 page, 30KB)

Sheet 2 - Series Pass stage of Regulator

(1 page, 32KB)

Sheet 3 - Regulator

(1 page, 56KB)

Sheet 4 - Power amplifier

(1 page, 45KB)

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Circuit details

Power Amplifier

Refer to schematic sheet 4.

The power amplifier is based on the classic three stage bipolar design (input differential pair, voltage amplifier stage, power output stage) with overall negative feedback applied to set the gain. The general theory of operation is well described elsewhere on Internet. The variations and specifics in this design are as follows:

The output stage is biased to operate in Class A into a resistive load up to the full power output rating. The output stage quiescent current is set by adjusting R69 to give 2.5 Amps shared through R72-75 (and also R76-79). 0.3125 V should be across across any of the resistors. See the design equations for details.

The feedback network (R56,R66) is fully DC coupled and does not contain a band-limiting stabilisation capacitor. The output quiescent voltage point is maintained at less than 1 mV by a very low frequency low pass filter servo (Q34) which biases the input stage (via R61,R52). The B-E junctions are protected from over-biasing by diode limiters Q32 and Q33.

(I intend to provide more information when I get the time. Contact me if you would like me to explain some aspect of the circuit or you have questions).

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Construction details

To be supplied...

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This page has been viewed times since 21 March 2007.
© 1999, 2007 Glenn Baddeley.
http://home.pacific.net.au/~gnb/audio/a100.html was created 13 December 1999 and last updated 21 March 2007.
Report problems and send comments to Glenn Baddeley.