Update on the Pleiades derivative of simplified no negative feedback Musical Fidelity A1

 At your risk.

Refering to the original Musical Fidelity A1 schematic.

The idea is to try just a BD135 driving a 2N3055 driving an output transformer and no negative feedback.

A BD139 was used instead, a Mitsubishi 2N3055, a bifilar 1:1 output transformer istead of perhaps a more suitable 2:1 output trasformer. Air gap is made by sticking a layer of 3M scotch invisible tpae on the stacked Es.

Vb is 12V.

BD139 is mounted on a small heatsink. 2N3055 is mounted on a larger audio cassette case size heatsink, black aluminium. It run bare touchable hot at 24V.

4R7, 100 ohms, 300ohms resistors are used as on the Musical Fidelity output stage schematic.

Bias is adjusted by a 220Kohm in series with 1Mohm variable resistor from Vb to the base of BD139.

2N3055 Ic was set to about 400mA.

Signal path:

various CDs, Dire Straits, Barbara Streisand, Year of the Cat - Al Stewart - Sony CD Wlkman line out, Pleiades simplified striped down Musical Fidelity A1 - Marantz Imperial 5-G

It worked, it sounded loud, even louder at Vb=24V, Ic=800mA.

Changing 4R7 to 6R8 made for a somewhat more open sound.

Quick changes are posble thanks to using Wago snap connectors.

But there was lack of treble, as if one or two high octaves were missing.

Going back to such a single 2N3055, the sound so softer, less power output but better sound quality, a bit more treble.

Going back to the Pleiades Darlington BD650. It sounded much more open with extended high frequency bandwidth and deep bass too, as if 2 more high octaves were added. Loudness was between the 2 above extremes.

Why does it sound much better? Is it because there are no resistors around the transistors? Should a BD135 driving a 2N3055 be tried in this way with least possible resistors?

The Darlington BD650 at Vb=12V  sounds great at just Ic=100mA, saving charged battery time.

On a small heatsink it run luke warm.

Moving closer to the spekaer at abot 1m from 3 m the sound is even more impressive.

WHat makes this so simple Darlington amplifier stand out is how alive drums sound, hi-hat sounds for example as if there is a hi-hat inside the room, for example of Sultans of Swing - Dire Straits, or On the Border - Al Stewart.