Hood 1969 Class A AmplifiersSimplistic Hood 1969 Class A Amplifier MJ2955 & 2N3055
John Laurence Linsley-Hood (1925 in Wandsworth, London – 11 March 2004 in Taunton, Somerset) was an English electronics engineer and designer of audio components. He was educated at Reading School, Acton Polytechnic, the Royal Technical College (Glasgow) and after world war two, at Reading University. In 1942 Linsley-Hood joined the G.E.C. Research Laboratries at Wembley, working on Magnetron development as junior member of a team. Joining the R.A.F. aircrew in 1943, he was transferred to labour on Radar, then subsequently worked with T.R.E. (Malvern) overseas. After returning to university Linsley-Hood joined the Windscale Research Laboratories of the Atomic Energy Authority. He was in charge of the electronics team in the Research Laboratories of British Cellophane Ltd. from 1954. John Linsley-Hood is best remembered by hi-fi enthusiasts for his "Simple Class A Amplifier", which he developed to provide a good-quality performance comparable to that of the classic Williamson amplifier. The design was published in Wireless World in 1969 (April 1969 issue, p. 148), and later updated in 1996.
The SES GOLDMUND BuildHood HD1969 PNP MJ2955 Class A 13W+13W Stereo Amplifier WLX
The plus-minus adjustment is made at the amplifier clipping point. This is done using an 8 ohm dummy-load on the output and a scope across the dummy-load.
I set the signal generator to 1000Hz. Increase the amps gain or volume until the output starts to clip. Adjust the Balance Adj until both + and – (top and bottom) of the waveform are equally clipping. The amplifier is an inefficient Class A design thereby the transistors get rather warm. This requires a sizable heatsink. The Quiescent Current Adjustment is used for biasing the output transister by balancing the voltage and current to increase efficiency. Though rated at 3.5 amps max, the bridge rectifier still gets hot at one amp current. So, I attached with thermal compound a 1x2 inch piece of aluminum to each rectifier to help keep it cooler.
The 200VA toroidal transformer I used had only two 20V secondary windings. I added a third 16.5V winding using 22 gauge insulated wire for the K155 preamp. I installed a bridge DC power supply filter module set to 13.5V to power the K155 and an LED front panel power indicator.
I also built an AC line filter. Everything is fused including primary as well as all three transformer secondary windings. Now I know the purests are having a fit over my addition of a tone balance control module. This simplistic module uses an MC4558 op amp that is widly used in upper-end analog studio mixing equipment. Much of the older recorded music you hear today went through one of these little op-amp IC.
Kit 155. Stereo Tone Control Unit which uses the MC4558 op amp provides +/-12 dB of bass and treble boost/cut, and channel balance control.
The resulting build is as clean as I've heard from an amplifier. The output is a tad more than 10 watts per channel before clipping. I could increase the current to get a few more watts, but I am powering a pair Fostex P1000-E with Fe103 Sol 8 50th anniversary drivers 8 ohms. With the tone and balance controls set to the top dead center, the frequency response is virtually flat from 20 to 20,000 Hz. With no signal connected and the volume control fully clockwise, there's no noise of any kind.
The case is a heavy all aluminum heatsink which was design for these amps. These were made for a company called Goldmund, a Swiss manufacturer that produced very highend audio equipment. The net weight of the complete amplifier is 11 lbs 10 oz (5.3kg). S January 2017
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