Audio Electronics

I am often asked about various aspects of this somewhat confusing arena. I find I have to think back to all the short-cuts that I used in my occupation. Here are a few notes that I may choose to refine in the light of further interest and discussion.

HEADPHONE IMPEDANCE
Several articles quote typical 50 to 150ohms and up to 600 ohms. This is very different from my memory of so called Hi-Z types which were in Kohms. Further research needed. The impedance of my mp3 headphones is 18.8ohms each. That is at 1Khz - which is/was a common standard. Remember the XL increases with rises in frequency and you cannot get a reliable reading from a DC ohm multimeter!

So I've fixed my Inductance meter and the headphones for my mp3 player are 0.3mH and another similar set is 0.5mH. Is that sensible?

DECIBELS
This is a ratio unless you state an impedance. "Characteristic" impedance in the case of cable. We say this is, say, 22dBs up (or down). FROM WHAT? POWER & VOLTS are different when it comes to dBs. Read on. (1Watt is 1 joule / sec. This is energy as in calories).

There is confusion between various references. In particular the old Post Office Telephones standard for their nominally 600ohm network. Knowing that impedance one can calculate actual power for particular dBs (known as dBm). The standard is that 1mW into 600 ohms is set at 0dBm. The operation of the network is -13dBm. Very easy if you will settle for each 3db (of power - not volts) being double or half. So -3dB is half = 0.5mW. Do it again is 0.25mW then again is 0.125mW and one more time for -12dbm is 0.0625mW. That's the power for a telephone earpiece (well -1 more db actually). Why? Well to get 1mW we need an RMS voltage of 0.707 which is a peak ac wave of 1V. {0.707 is the root of 2} and 1/1.414 = 0.707. Now just almost forget it!

Forget it save for one more very useful thing. When people use dBs in audio it's useful to know that power (Watts) doubles for each addition of 3dB, as I just said. If one is measuring voltage however, it doubles for each rise of 6dB. So if you raise 3V by 6dB it becomes 6Volts. This Voltage calculation becomes useful because microphones for example are often quoted in dB outputs. Just as in music no-one ever says that E7 / C7 is actually a FLATTED seventh note in the scale, so it is that no-one ever says that the microphones are normalised to 1V (peak). {That's times 0.707 for RMS}.
So a -70db moving coil microphone will 70/6 = 12 X 6dB steps down approx. You divide 1Volt by 2 six times! (or by 2 to the power of 6)
1V is 1000mV so if you divide it by 128 you get 7.8mV peak as the output. Very slight error - but you can do it in your head! For RMS multiply by 0.707 = 5.5mV. Amplifiers are the same. The gain is quoted in dBs. It doesn't tell you much unless you assume the voltage going in. Now a passive guitar pick-up gives around 30mV. BUT NONE OF THIS SPEAKS OF ANY KIND OF IMPEDANCE. {An unloaded Voltage is reduced by input impedance}.

IMPEDANCE of CABLES
The capacitative reactance of screened cable has a greater effect when it shunted across a high impedance. High Frequency (HF) losses are therefore worse with long cables. {Typical value is 200pF/meter. One way around this is to use low impedance microphones with a step up matching transformer. Medium Z is 200-1000 ohms. Hi-Z is 50Kohm or more. Lo-Z is 30-50 ohms.

There is no absolutely pure impedance (Z). There's R+XC+XL in any practical component. They all contain: -
Pure RESISTANCE+Capacitive+Inductive = REACTANCE.
Z impedance formulas are for C in Farads (that's enormous and the largest unit we use is uF which is 1 millionth part or x 0.000001). For L this is in HENRYS which is also quite large. We use more mH and uH. They are 1 thousandth and 1 millionth part respectively. That's x 0.001 or x 0.000001.

SPEAKER IMPEDANCE
There's a standard. Well more than one! It started with - well no standard at all but then 16 ohms became common. With transistors came 8 & 4 ohms. The reason for the low impedance is to keep the coil assembly light & small. Then again thicker wire can be used for high Watt output powers.
For Public Address (PA - not Power Amplifier as in guitar), in buildings where there were speakers in many rooms & places, there came a "LINE" distribution and speakers were often around 100 ohms - or more - and all wired in series/parallel as needed to match to the nominal Impedance of the system. Transistorised outputs of any power are usually 8ohms, but tolerant to higher impedances without distortion.

Now then on to valve amplifiers and their speaker loads. In order to match the high output impedance of a valve (many Kohms), with a low impedance speaker, a transformer is needed. These are heavy & expensive but many musicians like the result they give. They are more commonly 16 ohms, or two 8ohm units in series. It is absolutely vital that you do not operate without the speaker load. There will be almost certain damage to the transformer from undamped back EMF. {TIP: - Good idea to use a shorting jack with a load resistor to replace the speaker if it is unplugged}. It's not so much the valve damage that you have to worry about. They are relatively rugged & cheap - even now. It's the damage to the transformer as it's interwinding insulation breaks down. They are expensive and much more difficult to fit.
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