low cost simple John Linsley Hood 1969 class A amp kit build

A simple build of one of the basic JLH 1969 Class A kits into a dumpster dived old amp chassis.

JLH 1969 found object chassis

Used a 120W SMPS after reading in the DIYaudio forums lots of people getting positive results combining this older “classic” DIY design with modern quality switched mode power supplies.

I decided to build this to dabble further with Class A amplifiers after being quite impressed with a very well made DIY S.E.T. Class A valve amp I got to loan. One of the few gear changes my wife has made unsolicited positive comments about the sound.

The kit a low cost Chinese one from Aliexpress. Nothing flashy but it had all the parts needed. No instructions but it appears a quite straighforward reference implementation of the 1969 circuit, but with modern transistors, so nothing difficult there.

I originally ordered the kit from EBAY but it never showed up.. but the vendor refunded my money and I ordered it with a tracking number from Aliexpress.
I used most of the parts from the kit as they looked OK quality – I did replace the input coupling caps with 2.2uF ERO MKT1822.. I have a big bag of them, and evidently they are the Best Capacitor in the World

Apart from the capacitor change, all the other cables and connectors are non-exotic, functional components from Altronics and Jaycar.

Used a hefty Australian made heatsink from Altronics – which appears to be an MF30-75 heatsink from Conrad Heatsinks of Melbourne. The SMPS was also sourced from Altronics, chosen as it had the lowest noise and ripple measurements I could find.

The heatsink is almost the same dimensions as the black aluminium faceplate on the old amp, so I’ve attached it so the heatsink sits on the front covering most of the holes from the previous amps input selectors etc. The final result turned out quite well for a “found object” chassis.
Got to learn a useful new tool/technique, with some advice from Col, Dgtlmoon & Wade – using a “tap” to make threads on the heatsink so I could attach the transistors properly, and attach the heatsink cleanly to the aluminium faceplate.

I dropped the voltage on the SMPS to from 24 to 22VDC, which dropped the power use at the wall from 85 to 70W.. taking that as proportional to the output power, I guess at 22V it’s closer to 8W output – which still should be more than enough for the HF (greate than 600hz) of the active speakers I’m driving with them.

Heatsink gets to about 57 degrees with a room temperature of 28 degrees.

There are piles of forum discussions and sites about this amplifier and it’s variants on the net, but the main resource is at The Class-A Amplifier Site hosted on the always informative Elliot Sound Products web site.

Listening Impressions:
Nothing scientific, no double blind testing or measurements yet, these are just impressions, so take with a grain of salt as I could be suffering new toy syndrome:
Hard to say right now as I swapped over straight from the $6 TDA7297 amp – but it’s definately very smooth and pleasant. Big and luminous. I think it helps with the perception of stereo.

A little bit of hiss listening right at the waveguide, but this amp runs very quiet. More than quiet enough for use in a high sensitivity active system.

3 thoughts on “low cost simple John Linsley Hood 1969 class A amp kit build

  1. Minor update:

    Well the SMPS failed with a POP earlier today – it had been on and playing music for a few hours, and has been otherwise running like a champ since the start of the year.

    I’m glad I got it from a local reputable vendor as it was no problem to return it for another one over the counter a few hours later. Thanks Altronics.

    The replacement SMPS is identical except the voltage tune trimpot has a few volts lower at it’s lowest setting, I can run this @ 20VDC with 57W consumed at the wall, compared to the 70W it was using before – I guess that gives me 7W output. Still fine for the HF amp in an active system (greater than 600hz) I think.

    While testing the amp to make sure it was just the PSU I found it runs fine, though quietly, on 12VDC from a battery.

  2. Hi – I see you have used a bridge rectifier to lift the earth while still giving a low enough impedance to blow a fuse if you get a fault current to earth. Did you have an earth-loop noise problem before or was it just a preventative measure ?
    also
    Unless you run a pair of JLH stages in a bridge for each channel, you will still be drawing variable current from the supply line as the current through the top o/p transistor varies between zero and 2x idle. You will thus tend to “hear” the performance of the psu. Ideally you might want a generous electrolytic across the supply feed – although this can be a problem on start-up for some smps.

    Good luck,
    Steve

    • Yes, the bridge rectifier earth loop protection was a preventative measure, I’ve added it in to all/most of my builds to reduce the chance of eart loops, as per this great article.

      I have a few hefty electrolytic capacitors available which I was going to try to add in, but so far the amp has been sounding good enough, though maybe I could see if it makes it any better?