Pleiades power amplifier continued, Ο τελικός ενισχυτής Πλειαδες, συνεχεια

Your own risk.


Trying to tidy it up in a box instead of crocodile cable clips in all directions on the floor.


It sounds amazing connected to a Sony 2 way APM speaker. On these speakers, the tweeter (paper cone snall speaker) is first order connected ie with just a series capacitor. A modern Philips ribbon tweeter oak wood speaker has not been connected yet.


All in all just 7 components including speaker?


2N3055 power transistor with HEATSINK.
10KΩ biasing resistor
9V battery
Fuse
Speaker
4.7KΩ log potentiometer with
Switch


The speaker is OTL (output transformer less) connected to the collector as collector load. About 300mA pass through it and it seems to operate fine.


HEATSINK is important to minimize the chance of thermal runaway.


Rb=10KΩ is connected from collector to base.


The 4.7K pot is connected as variable resistor, rheostat, in series with the base. Only 2 terminal of the pot have been connected.


On the other side of it, it is connected to the CD player. The more the pot is turned clockwise the more the CD player line out is loaded and bass responce drops. This is presumably because there is a series output capacitor used for blocking DC. This bass drop may be useful for compensating the equal loudness ear brain frequency responce curves also called Fletcher Munson curves. Otherwise the frequency responce of the Pleiades amplifier tends to 0 Hz as there is no coupling capacitor from power transistor to speaker. Even the quiescent current passes through the speaker.


So everything is in class A (electrons flowing all the time) and the amplifier sounds amazing and much louder than it's very small power output suggests. The mid band is a joy to hear and the high frequency detail unbelievable. Bass is excellent.


Everything is to be put including speaker and input connectors in a small cigarette like box.


It is a mono block amplifier so both channels a fed to it. The singer and instruments come from a nearly point source giving the illusion that they exist in the room.


It may be nice to make this amp to an active speaker on a beautiful box like those designed by Jacob Jensen for Bang and Olufsen (B&O). Or a self contained music (un)boom box player.


Setup, signsl path:


Dionne Warwick definitive collection CD, Arista - Sony Poportabke D-EJ758CK CD player line out - Pleiades power amp - Sony APM-078 speaker - reverberant room - listener's ear-brain


Reference:



2 transistor miniature AF Amplifier - K. Jones - page 41 - Audio Amplifiers - J.R. Davies editor - Data Publications LTD LONDON