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. ❗ Caution All information is provided “AS IS”, without warranty of any kind, express or implied. ...
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. ❗ Caution All information is provided “AS IS”, without warranty of any kind, express or implied. Electro-Mechanical Relays The most common type of relays in this application are electro-mechanical relays. Their switch contacts can have a hard time switching off fault conditions like those found in power amplifiers connected to inductive loads like speakers. Reasonably good case: the relay opens quick enough to protect the speaker, but the contacts get pitted by the arc drawn while opening the fault current; this degrades the contacts so that the contact resistance rises - potentially to an extend where the relay has to be replaced. Worse case: the relay only opens too slow to protect the speaker’s voice coil, but gets destroyed in the process. ...
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. This is a very short blog post, because I’ve already shared most relevant information in the previous posts. ...
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 All information is provided “AS IS”, without warranty of any kind, express or implied. ❗ Caution The elevated voltages inside this amplifier can cause damage, severe injury, and death. If you are not authorized or qualified to work on such equipment, do not do it. If you follow along, you do so at your own risk. ...
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. ❗ Caution All information is provided “AS IS”, without warranty of any kind, express or implied. ❗ Caution ...
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. ...
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. ❗ The elevated voltages inside this amplifier can cause damage, severe injury, and death. If you are not authorized or qualified to work on such equipment, do not do it. If you follow along, you do so at your own risk. ...
The Technics SU-VX 700 is an integrated audio amplifier from the early nineties. It delivers 2x 90W into 8 Ohms and is specified with a THD of 0.007% (20Hz .. 20kHz, rated power). In my opinion it is one of the better Technics amplifiers. Whereas the sound of an amp is always subjective to a large degree there are some design aspects that are a bit more objective. What I particularly like about the SU-VX 700 is mostly related to its simplicity and repairability: ...