Technics SE-9600: New Driver Circuit Boards

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.

The amplifier circuit

The input or transconductance stage consists of a diffential input pair (left: TR101, TR102; right: TR201, TR202) with a common emitter resistor and collector resistors as a load; no fancy constant current source or current mirror can be found anywhere ;) DC balance is adjusted by varying (RV101; RV201) the common emitter resistance that is supplied by +24V power supply from the Power Source Circuit Board.

The voltage amplification or transimpedance stage is a common emitter amplifer (TR103, TR104; TR203, TR204) with a current source load (TR105; TR205) and a “bias spreader” circuit, consisting of a couple of resistors/trim pots, a thermistor and the thermal compensation transistors (TR13; TR14) on the Thermo Compensation Circuit Board (SUPA990) boards mounted close to the heatsink.

The driver/current amplification stage uses complementary emitter followers (TR108, TR109; TR208, TR209). Additional transistors (TR106, TR107; TR206, TR207) provide some form of a SOA protection for the output stage factoring in the output stage’s emitter resistor voltage drops. Those emitter resistors are conveniently placed on the driver circuit board.

To paint the full picture: The output transistors are paired up, resulting in a total of 4 emitter resistors and 4 output transistors per channel.

Ah, and Technics implemented a “feature” to increase(!) the effective output impedance (why, really?). This was done by changing the feedback and having an 0.33 Ohm in series with the speakers - four options, switchable.

The redesign

As mentioned in the previous post I wanted to keep the circuit close to what Technics intended. Nevertheless, slight changes and adjustments in component values are required for proper operation with the more modern replacement transistors I selected.

Components

ℹ️ Note

The component selection is meant for the redesigned board. The values might or might not work as replacement for a defective component on the old board.

ℹ️ Note

The schematic in the service manual has a couple of typos or Technics changed something along the way. Always compare the components/values with what is actually installed in your unit.

Diodes

Bipolar Junction Transistors (BJT)

Resistors, Capacitors

See blog post for the Power Source Circuit Board for my general approach.

Bucherot cell/output inductor

I didn’t change the bucherot cell or output inductor design. However, I formed new coils so I wouldn’t have to take the coil from the original board.

Required changes for performance/stability

This was a tricky one and I expected nothing less with those 3pF and 7pF ceramic capacitors in the feedback loop. Simulations are semi-useful; you really have to try it and solder the components directly to the board, since even a couple of centimeters of additional lead length will contribute a couple of pF of extra capacitance (and of course this will vary with lead position). This means taking the board out, soldering the components and inserting the board into the slot(s) and measure. Try and error. A lot of error, really.

I had to adjust some resistors (R102; R202) and capacitors (C106, C107; C206, C207), add another compensation network (R in series with C) and partially disable some of the feedback circuitry for the higher output impedance switch positions; all to achieve stability for all four output impedance switch positions and good performance. The latter change likely reduces the effect of this utterly pointless “feature” of an increased output impedance (meaning the values printed on the front panel are no longer correct). Truth be told, I couldn’t care much less.

Thermistor

A bit of testing went into finding a proper substitute for the thermistor. I finally opted for the EPCOS/TDK B57164K221J: Although it’s only 220 Ohm instead of the original 250 Ohms, its other thermal characteristics seemed to match the original thermistors reasonably well.

LED debugging indicators

For this board I added two debugging LEDs that light up when the rail voltage is above about 42V. This proved to be helpful once again; especially during my initial testing with the dim-bulb tester.

The PCB

The design uses cost-effective 1.6mm FR-4 TG135 two-layer boards (35µ) (HASL). Ideally one would opt for a reasonable thick gold plating (for the edge connectors). Since these boards are one-offs and I didn’t know whether I made mistakes in the layout (like board dimensions/outline, routing etc.) I couldn’t justify the hefty pricetag of ENIG/gold plating. That is also the main reasons I didn’t go with 70µ copper. Instead I use wide tracks on both top and bottom layers for the high current paths.

While the component placement comes very close to the original boards, there are slight and intentional deviations. For example, the footprints of the differential input pair were oriented in a way that allows for thermal coupling of the two devices.

I also tried to adapt the look & feel of the original boards to some extent by placing fiducials in a similar fashion like Technics did - those have no other purpose since everything in hand-built.

For whatever reason Technics decided to mount not only power resistors, but also diodes, that do not get really hot, off the board. This made the extensive use of plastic tubing and ceramic spacers plausible, albeit not always strictly required. Either way, I tried to replicate this interesting design with heat shrink and similar ceramic spacers. And let me tell you, this required quite some additional work - and makes this comparatively simple board look quite a bit more special, in my opinion.

By the way: I also tested a couple different thermal compensation transistors as a replacement for the original 2SC828 bjts on the SUPA 990 boards, but the amplifiers’ distortion performance degraded significantly, despite proper idle bias and a decent thermal behaviour. I haven’t debugged this further (maybe instability or something?), and for now kept the original 2SC828 on the original tiny SUPA 990 boards that I didn’t plan to replace anyway. Would be worth a second more thorough look in the future.

The new heatsinks for the TO-220F transistors are substantially larger, hopefully allowing for proper cooling of the fully plastic package (i.e. without a metal tab for electrical isolation). I ordered the heatsinks without noticing that this particular version uses a quick mount mechanism…

Tests

As stated previously, I did quite a lot experimenting and testing to hopefully make sure that the amplifier is stable with typical loads. There surely is room for improvement and more testing. For now though, I’m happy with the results.

I actually like having the debug LEDs glowing in the amplifier. But of course, those could be omitted to have a more original look.