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THE AUDIO IMPEDANCE METER USES & TIPS SUPPLIED BY SOUNDCOM WHOLESALE REF. J. COOPER; 17/10/2012
GENERAL As a field instrument, and apart from basics such as screwdrivers/cutters/pliers etc, the audio impedance meter is probably the most useful and important tool to have in an audio maintenance kit! It’s quite surprising how many audio field technicians do not have this type of meter at their disposal as, without it, many fault tracing hours can and will be wasted trying to find short circuits and disconnections in speaker runs as a DC resistance meter DOES NOT WORK! Only by transmitting a frequency, (usually 1KHz) can the speaker load, i.e. watts, and the line conditions be analysed. Some may say that this type of meter is expensive however, believe me, it will pay for itself many times over, not only in saved man hours but also in keeping down frustration levels and avoiding damage to system equipment. Although its’ primary purpose is to test speakers, speaker lines etc, it has many other uses for the field technician that can just make the job a bit easier. In order to discuss what it can be used for, let’s describe what it is first. What is an AUDIO IMPEDANCE METER Typically, an audio impedance meter generates a square wave output signal at nominally 1 KHz, which it then uses to show the impedance (‘resistance + reactance’ as a value in ohms @ 1KHz), either by moving coil needle deflection or digital display. The voltage of the 1KHz applied to the speaker line and load can be anywhere from ±0,5V and higher, depending on the brand and the method of test. To maintain reading accuracy, there will probably be range selection by button(s) or rotary switch etc, and these should be used to bring the reading somewhere around mid-scale of the most appropriate range selected for the test. It is unfortunate the reading will most probably be in ohms whilst the delivery amplifier load capability for comparison will be in watts, the reason being that the resultant wattage is dependent on the amplifier system delivery voltage that is selected, i.e. 100V – 70V – 50V – 25V etc. Conversion of realised impedance to wattage load is; WATTS = System Delivery Voltage² Impedance in Ω (ohms) As an example, 100 ohms @ 100V delivery = 100 Watts, but only 50 Watts @ 70V delivery
So, what is an audio impedance meter? a. It’s a device that can be simply a low voltage level 1KHz tone generator. b. Because of ‘a’, it can be used to quickly test any pre-amp input, balanced or unbalanced, from microphone to line level. See note under ‘OTHER USES and TIPS’ before applying. c. It’s a device that can read both DC resistance and AC impedance. d. It’s a device that, because it can tolerate a short circuit on its’ output leads without excessive battery current drain, makes an excellent continuity tester. e. It’s a device that, in conjunction with an inductive tracer, can assist in cable pair tracing.
OTHER USES and TIPS Note that for the following to be safe, it is essential to know the output level and operational characteristics of your impedance meter as, depending on the inputon-test impedance and sensitivity, meter input level could damage it, especially that of microphone inputs. As a minimum, apply the test for as short a time as possible to start with. For safer operation, consider a volume control on the output as these tests below are only to confirm input integrity. Also, there must not be any signal transmission through the device being tested. a. Check out a dynamic microphone. Select the best range for 600 ohms, apply the tone and Microphone OFF, the reading should be a short circuit (most microphones provide this to an input when OFF to prevent interference noise injection). Microphone ON, the tone will be heard via the microphone capsule, and the meter will show the coil impedance.
Do not use this method on an ECM or phantom power type microphone. b. Check out a differentially balanced input. This type of input typically has a 3 terminals XLR female socket, these being Signal Ground (1) / +ve input, HOT (2) / −ve input, COLD (3). To check that both inputs are functional, i. Connect the meter between Signal Ground and ‘+ve input’. Tone output should be detected. ii. Repeat ‘i’ between Signal Ground and ‘–ve input’. Tone output should be detected. c. Check out a balanced transformer isolated input. Connect the meter across the +ve and –ve input and confirm tone output OK. d. Check out input common-mode rejection. As this applies to a differentially balanced input, it is the ability of the balanced input to null out any cable pair noise interference. Connect one lead of meter to Signal Ground and the other lead simultaneously to both +ve and –ve inputs. No output should be detected.
e. Check out a differentially balanced output (line out etc). This type of output typically has a 3 terminals XLR male socket with Signal Ground (1) / +ve output, HOT (2) / −ve input, COLD (3). Both outputs should exhibit the same output impedance within 1% and be low, i.e. ≤ 500Ω. Connect alternately between Signal Ground (1) and +ve output, HOT (2) – and – Signal Ground (1) and −ve input, COLD (3) to confirm. f. Check out an unbalanced output (REC out etc). Usually a RCA or mono phono type socket, repeat as for ‘e’ above between ‘signal ground’ and ‘signal’ connections. g. Use as a Continuity Tester. One of the characteristics of an audio impedance meter is its’ ability to accept short circuit as one of its’ functions, similar to a resistance meter. A specific test when audio hum is affecting a system is to test between device CHASSIS ground (electrical) and audio SIGNAL ground (pin 1 above). Many audio devices provide this as standard which, personally, I disagree with as it can easily cause hum, buzz and instability on audio systems, and is also known as ‘earth looping’. If this is the case, and there is no ‘GROUND LIFT’ switch, locate where this link is in the device and remove.
Ensure that ‘CHASSIS GROUND’ remains effective for electrical safety purposes. In closing, the standard functions of an audio impedance meter can be used to troubleshoot many sound system problems quickly and effectively, and I’m sure there are more uses than I have highlighted in the foregoing.
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