Line and Microphone inputs

What is the difference between Line and Microphone Inputs

This is a question often asked in the forums, not necessarily as simplified as this but there is often ambiguity in the replies. So here we will look at what not to do and hopefully this will prevent mishaps in the interwiring of consumer and professional audio goods.

Firstly, the tried and trusted Decibel (dB)

The decibel is the logarithmic ratio of two inputs. It is not intended to go into great depth here so as to confuse the reader, here we are merely looking at driving consumer and professional audio systems without creating distortion and or damaging the input or output of one of the components.

0dBV is based on 1V R.M.S.  reference.

If the unloaded reference signal is known as 0dBu, then we have 775mV across a 600 Ohm load which dissipates 1mW of power. Yep, Bell Labs had this one taped, it’s an old telephone system standard. Power in dBm (decibel-milliWatts).  We now use dBu (unweighted) because input or impedance may not be 600 Ohms.

• Professional audio out is rated at +4dBu or 1.28V R.M.S.
• Consumer audio out is rated at -10dBV or 316mV  R.M.S.

What is R.M.S? 

If we have a 2V peak to peak sine wave the top or bottom half will be +1V or -1V peak, the R.M.S. value is 0.707 times this value or +0.707V or -0.707V.  (the equivalent to a DC voltage applied to a circuit).  In other words if we had a 20V p-p 50Hz (cycles per second) supply connected to a lamp, to get the same illumination we would use a (20/2)*0.707 or 7V DC supply.

Hopefully this sort of makes sense, ja?

Your professional mixer

The mixer has two inputs, microphone and line. The microphone max input level is +8.5dBu and the line +22dBu.  These are pretty much standard.

Go to Sengpiel Audio and in the left hand column of the table input these ratings:

• +8.5dBu = 2V R.M.S. max
• +22dBu = 9V R.M.S. max

As can be seen, the preamplifiers are very tolerant of the HOT signal as mic outputs are often in the order of a few mV.

The output of the mixer is stated as being +16dBu, typically about 5V R.M.S. – strong enough to drive most audio amplifiers in the professional grade category.

Distortion

Looking at the back of your audio monitors you will see (hopefully) an attentuator, -10dBu and +4dBu.  This would typically allow the amplifier to run both a HOT signal from your mixer or that of a consumer grade tape deck (-10dBV). The RCF Ayra 8 has three inputs, 6.3mm, RCA and XLR with variable input sensitivity and can be driven to full power fairly easily from a consumer grade output.

Unfortunately your consumer audio amplifier can and will be overdriven from a mixer and this is where “padding” is a useful solution.

Matching

Low impedance output to High impedance input.  This is a general rule, you do not want to overload the output impedance of any stage.  Except for tube power amplifiers, they require exact matching to the speaker and will be damaged if the load is disconnected.

It is sometimes acceptable practice to variably change load impedances for instruments such as a guitar to change waveshape and colour. Some preamplifiers have this facility as a matter of course.

I.e. A 47kΩ output impedance to 100kΩ is acceptable. A 47kΩ to 600Ω is not. Amplifiers: Solid state into any load as stated by the manufacturer but no lower. Tubes, into any load stated by the manufacturer but no higher.

Padding

The opposite to the amplification of a signal would be to attentuate. In most circles, the muso and sound engineering fraternity especially, the term “padding” is used.  In most circuits “padding” is done through a stepped resistive network and is strictly passive, meaning no power is required to operate.

Example

A typical example of a passive attentuator would be the 10kΩ / 1kΩ divider network to reduce speaker output to a low voltage drive source to another more power amplifier.  Assuming we have a 10W per channel amplifier which you need to drive a 300W per channel amplifier which needs 1V R.M.S. or 0dBV to get full power.

A 10W audio amplifier will deliver nearly 10V across an 8 Ohm load. Using a 10:1 or 11:1 divider network from series resistors we can get approximately 1V out.

The 10kΩ and 1kΩ series network is fairly common, with the top series resistor (R1) value increasing as the power is increased.  Note that this circuit should never be used for tube amplification which always requires a matched load (speaker).

Below:  Table showing relationship between the V R.M.S. across a speaker load to output dissipation.

The table above will reflect ratios of approximate 5:1, 10: 1, 15:1, 20:1 etc for a divider chain with a fixed R2 e.g. 1KΩ. Resistor ratings can all be 1/2W.  By using industry standard values, 4.7K, 10K, 15K, 18K etc or devaluing these values the out signal will run hotter. (actually 6:1, 11:1 etc).

Balancing

Balancing is an act.  Usually requiring a connector with three or more poles, ground, +Hot and -Return or Cold.  Noise gets picked up in the +/- lines and are cancelled.  Usually only used in professional audio due to long cable runs.

Line and microphone outputs are often balanced, either through a transformer or electronically.  When using short runs of under 5m a balanced signal path is often not necessary.  I have personally run 10m cable lengths without a hitch. Consumer audio is not designed for this because of the belief (often mistaken)  as either transformer or that the devices are next to each other.

Turntables cartridges have very low output signals, often requiring to be amplified / buffered before the next stage – they can be very susceptible to noise and interference.

Keep to the standard, + to pin 2 and – to pin 3 ground to pin 1.

Direct Injection or DI

The indispensible tool in your toolbox should be the DI or direct injection box. It is a life-saver.  Coming in either transformer or electronically coupled, taking a non-balanced signal to balanced, easy-peasy padding and sometimes gain enhanced as well the DI is as important to the consumer as the artist.

And you can make your own.

• A passive DI using a microphone transformer
• Rod Elliott’s own design – Active/Phantom battery powered DI Box

From my own experience, when meddling with consumer/proaudio buffering and/or amplifying the consumer gear to match the pro amp is usually the bigger problem, attenuating a signal is not.  I have both an Otari and Akai open reel tape system and the Akai pre-amplifier out is only a few milli-volts (stated as 800mV but running far less even with new tubes).  Because one needs to keep the all tube configuration the pre-amp is fed into a Pro Art tube pre-amplifier.  This is a total waste.  Tube pre-amp for this purpose on the cards.

Some cautions and phantom power.

Connecting line to mic is never a good idea but can done through a DI box. In pro-audio circles the mic input to a preamplifier is usually XLR but not always.  The same applies to the line out/in where XLR and standard jack sockets are often both used.

Always use a DC blocker capacitor to pass a.c. signals.  Condenser microphones use phantom power of around +48V which you definitely do not want to connect to your expensive home theater amplifier, neither the output of a tape deck or turntable.

6.3mm jacks very seldom carry phantom power but check first.

If in doubt read the small print.

Conclusion

As can be concluded matching and driving consumer audio with proaudio equipment is often easier than the reverse.  Often a mixing desk has a facility to connect consumer electronics, usually stereo RCA.

Read up on small signal amplifiers, especially op-amps.

Don’t disconnect the speakers from tube amplifiers to drive other equipment.

Calculators and further reading:

http://www.daycounter.com/Calculators/Decibels-Calculator.phtml

http://www.sengpielaudio.com/calculator-db-volt.htm

Sound pressure levels – Sengpiel Audio

• Attenuation – Electronics Tutorials
• Audio Master Class – padding
• Gain Structure – audio input and output levels – Support Biamp
• Harmonic Cycle +4dBu and -10dBV

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