Many complain about the original LCD of the good old HP 3478A bench DMM and other HP equipment of this era, because of its rather poor readability. So did I. Quite many projects have solved this issue using different approaches: From adding backlighting to the original LCD, using multiple OLED displays - one for each digit - to replacing the display with Nixie tubes. Despite some of the projects achieving great results I didn’t want to replicate any of those, often because of the complexity/quantity of parts to be soldered, aligned and so on.
I love 19" rack gear, and the little Sansui AU-317 is no exception. With 2 × 50 W into 8 Ω, it is a medium-power integrated amplifier, of course built in the late ’70s like many of the greatest “vintage” amplifiers. Its specs are not bad at all, but there are limitations too. Let’s check it out.
The Agilent E3632A is a programmable linear 120W DC power supply. I got my unit a couple of years back. Since then, I replaced the RIFA caps and the noisy fan. The other day it developed a little issue: The power switch plunger detached from the switch. And now the plunger is inside the unit. Let’s fix that and take one or two pictures along the way…
Many amplifiers use relays in series with the speakers to disconnect the speakers from the amplifiers’ output during power-on and in case of undesireable amplifier states. This includes protection against excessive DC offset on the speaker terminals that may be caused - for example - by a shorted output transistor.
A couple of month past since I restored my Technics SE-9600, but I haven’t shown any measurements yet. This is partly because I didn’t have an audio analyzer when I started the restoration. Now I do, but at the time I took the measurements, my test setup wasn’t good at all, resulting in really high THD+N figures. So you’ll get the THD (without noise) numbers as calculated by the QA403 audio analyzer. Actual results could be better than what is shown.
The main reason for the redesign of the PCBs was the strong smell of the cheap original boards - and I have a feeling that the Meter Circuit Board was particularly smelly, just because of the power resistors that generate a lot of heat. So I had to design a new Meter Circuit Board too.
The next thing to be redesigned are the two so-called Driver Circuit Boards (SUPA2590, SUPA2600). But first let’s have a look at the circuit design that this amplifier is built around. Caution
Most audio power amplifiers are supplied by an unregulated power supply: A transformer with a center tap, a bridge rectifier and then two large filter capacitors. All the ripple/noise rejection happens inside the amplifier circuit itself. The Technics SE-9600 is a fairly rare exception. Let’s have a look.
The Technics SE-9600 is a high end hifi stereo power amplifier build in the mid to late 70’s - a design about 50 years old. This couldn’t be more obvious: Everything is metal, everything is massive - the chassis, the transformer, the heatsinks, all packed nicely into a 19" inch form factor. And of course it has VU meters with three ranges. Worth mentioning is the input on/off switch - I like that - and the utterly pointless output impedance switch that is capable of making your favorite music sound flat and mushy by reducing the effective damping factor.
Homelabbing is fun, but can get expensive very quickly. Using somewhat older hardware may significantly reduce cost, and more often than not, platforms from just a few years ago handle all the services you throw at them with ease—unless, of course, you’re planning to deploy local AI workloads and the like. That’s why I came up with a simple plan to repurpose my old Ryzen desktop. Sort of… There’s one catch, though: like all 5000X series chips my Ryzen 5 5600X doesn’t come with an iGPU, and keeping an RTX 3070 in there wastes a ton of power even at idle-while it has way too little VRAM to be actually useful for local AI tasks. Never did a lot of gaming either. So I decided to run an experiment: pick up a relatively cheap Ryzen 7 5700G and later sell off both the Ryzen 5 5600X and the RTX 3070. The motherboard on hand is an ASUS ROG STRIX B550-F Gaming (Wi-Fi), powered by a Seasonic PRIME TX 750W. The power supply now is absolutely overkill, but that is what is in there. In its silent mode it should easily power two of the servers without spinning the fan once. This is made possible by its fairly good efficiency, so that’s a plus…
I’ve recently been restoring vintage Hi-Fi equipment, focusing on Technics preamps, integrated amplifiers, and power amps. My first project in this series is a Technics SU-8080 integrated amplifier, which arrived in decent cosmetic condition (and with printed schematics!). However, it was covered in dust like I’d never seen before and suffered from frequent channel dropouts, rendering it unusable.