Miniature Class A Amplifier
Only 3 cheap bipolar transistors per channel
Designed by Flavio Dellepiane (RED Free Circuits site)
Used in our study
Associated equipment: Last updated: November 26, 2005
Sony Discman Compact CD Player D-171
Elektor Feedback FM Receiver (a design from 1974)
La Petite Audiophile speakers
The philosophy behind this minimalist design has been explained by Flavio Dellepiane on his highly interesting site 1. I have myself written an article about this fleapower amp from Italy in the American magazine AudioXpress 2. All relevant details are published there. A single channel of the amplifier can be built for only a few Euros, especially if you have a well-stocked junkbox.
Schematic (one amplifier channel):
Parts list (one amplifier channel):
P1: 47k log potmeter (separate for each channel)
R5: 1k5 (optional)
R7,R9: 100 Ohm
R8: 560 Ohm
R10: 1 Ohm 3W
(all resistors ½ W unless otherwise indicated)
(R5 and C5 are an optional bass-boost facility)
(these parts can be omitted, I did not use them)
C1,C2: 10µF 63V
(I omitted C1 and I used a MKT cap for C2)
C3: 47µF 25V
C4: 100µF 35V
C5: 150nF 63V MKT or MKP (optional)
C6,C7: 220µF 25V
C8: 1000µF 25V
Q1: BC560C (I used a 2SA970)
Sizable heatsink capable of dissipating 17 Watts continuously
Schematic (power supply feeding both channels):
I used a regulated power supply to feed the two amplifier channels, Q6 is mounted on a heatsink. The power transformer comes from a damaged TEAC amplifier. It has a secondary voltage of 60V with a center tap. D1 and D2 are fast recovery diodes, e.g. MUR860. It is a good idea to bypass C7,C10 and C11 with smaller MKT or MKP capacitors.
The sound of this amplifier is very good for such a simple design, lively and threedimensional. No cross-over hardness or other transistor nasties. However, don’t push it too hard for then you will begin to hear harmonic distortion. Singers will sound as if they keep both their hands, like a cup, in front of their mouth. If this happens, reduce the volume (or use speakers with a greater efficiency).
1 Flavio Dellepiane, 3-5 Watt Class-A Audio Amplifier (RED Circuit no.80), http://www.redcircuits.com/Page80.htm
2 Aren van Waarde, Study System, Part 3: Class A Transistor Amp, AudioXpress 36, no.4, pp.32-35, 2005.
Finally, a picture of the finished amp. Built in a wooden chest. The cover is the bottom of a damaged NAD amplifier. You will notice my poor construction skills. And… as Steve Bench said, my sister has inherited all the artistic talents in our family.