Analogue signal levels in typical unbalanced hi-fi audio equipment are fairly well standardised so interconnecting different manufacturers' equipment is not usually a problem nowadays. Interfacing modern gear with vintage DIN connected hifi equipment can cause problems.
Typical Modern (post 1980s) Audio Signal Interface Levels
Inputs and outputs are usually presented on RCA phono connectors, with the impedance to the input being much higher than the output impedance. This means that the voltage at the signal output is not loaded much, more like a bridging connection rather than a matched condition. Typical levels are
- Line Output level (non CD players) 0.775Vrms
- CD player Output Level 2V rms
- Output impedance < 600 ohms
Line/Aux/CD Inputs
- Input impedance > 10kohms (often 47kohms)
1960s and 1970s DIN Hifi Audio Interface Spec
The DIN standard was very different. The specification called for a high impedance line output, more a current source, specified that it delivers an output voltage of 1mV rms for every 1kohm of load impedance. This originally did have the advantage that a single tape recorder input could work with dynamic microphones as well as the output of another recorder. By making the input impedance about 20k it could match a high-impedance dynamic mic (typically achieved with a step-up transformer in the mic body) delivering between 1 to 5mV at typical music sound levels.
The same input socket could handle a line level output, since it would have to cope with 20mV rms which is only four times the level for the microphone, because the low impedance reduces the line level output.
There was a price to pay for this flexibility, as line level signals were considerably attenuated and had to be boosted by the microphone amplifier. This was not such an issue for the mid-Seventies, when tape hiss could be significant, at about 35-40dB down on peak signal level, masking the amplifier hiss.
Alternatively, a dedicated line level input was sometimes provided, with an impedance of about 100k. Now a different problem reared its ugly head, for audio cable can easily have a shunt capacitance of 100pF per metre. A standard 1.5m length cable would have a shunt capacitance that would cause more than a 6dB drop at 15kHz relative to low frequencies, which is not a good thing for a hi-fi interconnecting system.
As a result of these deficiencies, and as hi-fi enthusiasts no longer required microphone inputs on their tape recorders anyway, the DIN interface spec fell out of favour as the noise and high-frequency performance of hifi systems improved over the 1970s.
Interfacing DIN Hifi Equipment to RCA/phono based HiFi Equipment
Coupling the output of a DIN standard piece of gear to a modern line input of 47 kilohms impedance should give a signal level of about 50mV, about a tenth of the normal signal level. This can normally be dealt with by simply turning the volume up.
Going the other way, however, usually results in distortion and overload - presenting a line level DIN input of 100k impedance with a 0.775Vrms signal is much more than it expects, and presenting it with a CD player input is 20 times more. The answer is to use a 10x (20dB) inline audio attenuator, or wire a 22k series resistor to the source and 2k2 shunt resistor at the DIN end of the cable. This keeps impedances low enough to avoid the HF losses and signals low enough not to overload the vintage gear.
An alternative purist approach to the DIN spec would be to simply insert a series resistance of 680k for a regular line output and a 1.8M resistor in series with the CD output, but cable capacitance should be kept below 80pF if a dedicated 100k DIN line input is used.
References
Audio & HiFi Handbook, Ian Sinclair, Newnes, 2000 ISBN 0750649755
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Richard Mudhar - Originally from London, now enjoying a less hectic pace of life in Suffolk, Richard Mudhar is a keen wildlife sound recordist and ...
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