Unused features in the Telefunken RM 300


The RM 300 was the main amplifier of Telefunken's 1981 top-of-the-line Hifi system T 300. Though all other components in this system really represented what Telefunken was capable of at that time, the RM 300 lags a bit behind, in the sense that it lacks functions that were present in previous generations' amplifiers, or even in lower-class amplifiers of the same series.

The most obvious thing - of course - is the relatively low output power of 'only' 2x50 watts RMS. Though this might seem sufficient for most home uses, it may become a problem when using more 'advanced' speakers, which usually have a lower efficiency and need a large amp. For example, the RA 200 of the lower-class T 200 system delivers 2x70 watts, and its immediate precedecessor, the STM 1/CM 20, even 2x85 watts.

Other things are maybe less obvious, but still annoying. For example, the RM 300 has connectors/clamps for two pairs of speakers, but they cannot be operated simultaeously. All other Telefunken amps with two speakers connectors had this option, so why not the RM 300? It can't be a question of overloading the amp, since in an 'A+B' setup, the speakers are connected in series, so the amp's load is even lower than with a single speaker pair.

Furthermore, the RM 300 lacks the LED VU meters known from all 'better' Telefunken amps. Though some people regard them as superfluous gimmicks, they are usually a good selling argument and it is difficult to understand why they were left out.

A closer investigation of its innards reveals that the RM 300 is a derivative of the CA 10, a smaller integrated amplifier of the 1980 series of Hifi systems. Is it possible that some options on the PCBs were simply left unpopulated for the RM 300 and wait for being discovered? Let's see...

The Speaker Switchover Section

The speaker sockets are located on an extra PCB, together with the relays that turn the pairs on and off. Each relay is operated by an extra switch at the amp's front, and the way these switches are wired up assures that when both switches are on, both relays are off. However, it is clearly visible that there are provisions for a third relay:

RM 300's Speaker Board

A closer investigation of the RM300's schematics reveals that this relay gets engaged when both switches are on. The CA 10 even has this third relay! Below is a simplified schematic about how the this relay operates in conjunction with the other two:

Speaker Switch Logic of RM 300 As one can see, speaker pair B's relay intercepts the speaker's 'hot' wire, while speaker pair A's relay intercepts the speaker's ground wire. If both switches are pressed, the 'A' and 'B' relays are both off, and the 'A+B' relay connects both speakers in series. The huge advantage of this approach is that it requires only one relay contact per channel, so the whole circuitry gets along with three relays that have simple '2xON' contacts. On the minus side, it is crucial that not all three relays get operated at the same time; while operating any two relays at the same time is uncritical (and very well possible as a transitional state), turning all three relays on will short the amplifier's output!

The switch logic theoretically prohibits such a situation, but I was not entirely sure about this and inserted additional 1 Ohm / 5 W resistors in series with the 'A+B' relay. Everyone has to decide for himself whether these resistors are necessary; the CA 10 does not have such resistors.

For one channel, inserting the resistor was quite simple because it just meant soldering in the resistor in place of a wire bridge; for the other channel, I had to cut a trace on the PCB and drill an additional hole. Below is a photo how the 'enhanced' speaker PCB:

RM 300's Enhanced Speaker Board

It is not necessary to find the exact same Rapa or Siemens relays that Telefunken used; this type of relay size and pinout seems to be a standard type that is manufactured by a couple of companies; I bought relays made by Schrack in an electronics store that had '2xUM' contacts, but it was simply a matter of clipping off the unneeded pins. Just be sure that you get relais wit 24V-coils!

Connection for LED VU Meters

LED chains on amplifiers seem to be a somewhat controversial piece of equipment: some people like them, others regard them as superfluous stuff. Nevertheless, it seems Telefunken had plans to add one to the RM 300 respectively CA 10 (neither have). A closer investigation of the main PCB show that there is an unnamed, unpopulated connector that can provide all necessary signals:
Pin Number Signal
1406 right channel via R 430
406 left channel via R 1430
508 positive supply (+33V) via DB 430
507 ground
509 negative supply (-33V) via DB 428
So after populating the connector (directly soldering cables to the PCB would seem rather unelegant to me...) plus two resistors and two wire bridges we have the signals and the supply we need for the meters. What I did was to more or less copy the amplifier/rectifier circuit from the STM 1's schematic. Since the OpAmp used in this circuit cannot run with more than ±18V, two transistors and two zener diodes serve as simple regulators. One could also use standard three-legged 7818/7819 regulators, but I didn't have them at hand, and a simple transistor regulator doesn't require much more effort.

The positive regulator transistor also supplies the LED driver chips, so it need a small heatsink, while the negative supply only has to deliver a few milliamps to the OpAmp. There is plenty of space in the RM 300, so the board has a generous size:

Rectifier/Regulator Circuit for RM 300's LED VU Meters

About the choice of the LED drivers: Telefunken originally used its own U 237 / U 247 combo in the STM 1, which is however not available these days any more. The 'good old' UAA 180 however can still be had, and it can drive up to 12 LEDs.

The question still remains why Telefunken included provisions for a feature and never used it. In case of the CA 10, the answer might be simple: the front is full, it would have been difficult to fit in the LED VU meters somewhere. That is not true for the RM 300, it has plenty of space. So the only idea I have is that either the designers had to hit a certain price tag, and the VU meters just like the relay simply didn't fit any more on the bill.

Of course, the reason might simply be an aesthetic one: The whole T 300 system is marked by its timeless and elegant design. A flickering LED display without a necessary function simply disturbs this approach. I probably wouldn't have done this either, hadn't the previous owner already made the necessary cutouts in the front...

Mysterious Pre-Amplifier

There is another unused area on the main PCB - and it's a rather huge one, and it is unused both on the CA 10 and RM 300. It took me a bit of time to deduce the circuitry from the traces, and it seem as though it is a small two-stage transistor amplifier per channel that drives the actual input. So it seems that Telefunken originally thought about making the input sensitivity higher, but later decided that a sensitivity of about 1 volt is enough...

RM 300's Unpopulated Preamp Section

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©2002 Alfred Arnold, alfred@ccac.rwth-aachen.de