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)

 

R1:                  100k

R2:                  12k 

R3:                  47k

R4:                  8k2

R5:                  1k5 (optional)

R6:                  2k7

R7,R9:            100 Ohm

R8:                  560 Ohm

R10:               1 Ohm 3W

(all resistors 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:           10F 63V

(I omitted C1 and I used a MKT cap for C2)

C3:                 47F 25V

C4:                 100F 35V

C5:                 150nF 63V MKT or MKP (optional)

C6,C7:           220F 25V

C8:                 1000F 25V

 

Q1:                 BC560C (I used a 2SA970)

Q2,Q3:           BD439

 

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, dont 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).

 

References:

 

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.

 

 

 

 

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