The project which I will describe here is not, like Jim’s pre- and poweramp, a high-end design. Many so-called "serious audiophiles" will shrink from its schematic in horror. But those of you who take the trouble to build it may be in for a pleasant surprise.

[contact Aren van Waarde about his project]

Background

About one year ago I looked into my large junk box, and discovered some ancient power-MOSFETs (Hitachi 2SK135). They were bought in the early eighties for a project which never materialized and they were gathering dust in my attic. Can I do anything sensible with them? I wondered. What about building a small amplifier for my little son? (the wonderful thing about having a son is that it gives you an excuse for building all kinds of toys).

So I started looking through my collection of old electronics magazines, and I finally ran into a schematic which seemed simple and easy to reproduce (at the latter point my expectations were not quite correct - see below). It was published by Per Hojlev, a Danish engineer from Copenhagen, in the January 1985 issue of Wireless World (page 75). As you will see from the schematic (fig.1), it is a single-ended class A design using just one power MOSFET per channel. The FET is driven by a series of three CMOS inverters in series. Some of you will be horrified by the idea of using inverters in an amplifier. But they are cheap and apart from the fact that they are not very low-noise devices, there is little against them. As it turned out I also had 4069 i.c.’s in my junk box, so I could start building a prototype without having to visit a store...

The first prototype which I made didn’t work because I had confounded source and drain of the MOSFET (Don’t smile. There is a mistake in the Wireless World text...). The second prototype worked, but it became scaldingly hot since it had too little heat-sinking. Then I decided to design a printed-circuit board and to use a bigger heat sink, and I built a third prototype (stereo pair). When tested individually on my workbench, the left and right channels seemed OK. But as soon as I put them in a cabinet and hooked them up to the input and output connectors, I was in trouble. They became unstable and started to oscillate in an unpredictable manner at different volume levels. Since I was born and raised in the province of Zeeland, I am stubborn and tenacious. So I built a fourth prototype, hard-wired with very short connections between all components, not using any PC board. This prototype (stereo version) worked well and I am happy with it, although it still becomes rather hot after prolonged use.

The parts and the power supply

There are very few parts in the amplifier. I didn’t use expensive components, but took what was in my junk box. The input coupling capacitor is a 2.2 uF/250 V MKT type from East German origin. You may use a better capacitor which could give better results (polypropylene, paper-in-oil). I have not yet tried whether this makes a significant difference. This capacitor sets the low frequency cutoff point (12.5 Hz in this case).

The output capacitor is a 2200 uF/63 V electrolytic from decent West German stock (Siemens). You may bypass this with a 10 uF polypropylene if you like, I have not done this. All resistors are 0.6 W metal oxide, except for the 8.2 Ohm / 10 W which is a wire-wound type from Vitrohm. The volume potentiometers are 470k log mono from the East German brand RFT. You may use an Alps, Bourns, Noble or Penney & Giles if you are rich and don’t want to be called a heretic.

Each channel is fed from a + 15 V, 2 A stabilized power supply consisting of a 78S15 (see fig.2). These i.c.’s should also be mounted on an adequate heat sink. The transformer is a 2 x 18V, 80 VA type from Amplimo (known as ILP in the UK and Plitron in the US). Connection to the speakers is through 16 A four-way binding posts, the input connectors are standard RCA jacks (gold plated insulated RCA jacks are of course better). Input wires consist of Prefer low-noise OFC microphone cable, output and power supply wires of 4 mm insulated cable for measuring purposes.

The sound

Every home constructor thinks that he has made something special. It is difficult to look objectively at your own products. However, I must confess that several amplifiers which I made in the past have failed to impress me. It was different in this case. My wife, who has better ears than I and has no interest in electronics, also noted the difference. I will try to describe the strong and weak points of the sound as honestly as possible. As speakers I used bass-reflex cabinets containing a single 5" full-range driver (the design resembles L’Audiophile Le Petit, there is no cross-over, sensitivity 89 dB/W/m).

The sound is very natural, with an inherent ‘rightness’ and ‘musicality’ which I find pleasant. Even after prolonged listening there is no listening fatigue. After the amplifier has warmed up (about 10 min), you simply forget the installation and you are drawn into the musical performance. The ambience of the recording venue is reproduced quite well. Bad recordings sound bad, good recordings very good. I must confess that I prefer classical chamber music and small jazz combos. Orchestral recordings sound good, but in loud and complex passages you notice that the amplifier runs out of steam (output power is less than 2 Watts in 8 Ohms!). This is not an amp for Wagner fans, head bangers or rock-and-roll addicts. The attic in which I listen is quite small (4 x 3 meters) and I listen in the near field (about 1 meter between my ears and each speaker cabinet). With Lowther or Fostex speakers the amp may fill a bigger room with sound. Small details are reproduced surprisingly well. I took tape after tape from the storage room and I heard things which I had not noticed before. Small bells and triangles, harp sounds in the back of the orchestra, the turning of a page. Human voices sound very good - there is no trace of cross-over hardness or sibilance (this is pure class A, remember?). The proverbial "veil" between you and the sound seems to be very thin or absent.

I consider this amplifier a good introduction to single-ended designs. Hobbyists which don’t like to pay a lot of money for their first attempt at SE amp building may try it and get hooked. You can build a copy of this amp for as little as $ 60 and have lots of fun. If you like it you will probably proceed to a Zen amplifier (Nelson Pass), or a single-ended tube design. But that will set you back a great deal more. And you may join the Lowther club....

Construction details

Assembly is critical, as I told you, so proceed as follows. Mount the power MOSFET on a large heat sink with TO3 holes, using a mica insulator and a liberal amount of heat-conductive silicon grease. Mount the CD4069 i.c. on the other side of the heat sink, between the pins of the power MOSFET with its plastic back glued to the heat sink. Now hardwire all components directly to the pins of MOSFET and the 4069, with all connections as short as possible. The power resistor (8.2 Ohm / 10 W) can be flush-mounted against the heat sink. However, the input and output capacitors should be mounted 90 degrees square to the heat sink. The sink will get hot, and the capacitors should not touch it since this will reduce their lifespan. Especially electrolytic capacitors must be kept at low temperature. As you might have guessed from this description, I use one 4069 i.c. per channel though it would be theoretically possible to use one for stereo. Forget about this unless you are fond of HF oscillations...

Amplifier (one channel)
IC1 Philips 4069
Q1 Hitachi 2SK135
C1 Input coupling capacitor, 2.2 uF 250V MKT (RFT)
C2 Power supply decoupling capacitor, 0.1 uF 100 V Siemens
C3 Output coupling capacitor, 2200 uF 63 V axial Siemens
R1 5k6 0.6 W 1% metal oxide
R2 100 Ohm 0.6 W 1% metal oxide
R3 470k 0.6 W 1% metal oxide
R4 330k 0.6 W 1% metal oxide
R5 8.2 Ohm 10 W Vitrohm wire-wound
P1 470 k log potmeter (RFT)
J1 Standard RCA jack
J2,3 Four way-binding posts 16A (red and black)

Power supply (stereo)
IC1,2 78S15
D1,2,3,4 B80C25000
C1 4700 uF 63V axial JRC
C2,2' 0.47 uF 63V Wima MKS
C3,3' 0.1 uF 50V Siemens multi-layer
S1 Mains power DPDT switch
T1 Amplimo 220:2 x 18V, 80VA (both secondary windings in parallel)
F1 Fuse 1 A fast