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Old October 16th, 2005, 12:34 PM   #16
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thanx much

A.J. and John, thanx much for all the info. Now I have to figure out how to use it in my situation. In short, I have a PD 170, and I am considering either a Azden SGM-2X (impedance = 680 ohms) or an Audio Techinca AT897 (200 ohms). I will be doing documentary filming in India, and some of the footgtage will include village music. That is primarily why I want another mic and wonder if the AT is necessary for that, considering budget constraints. I called B&H to see if I could get a tech to help out, but they were too busy. Also, cannot get the discount on the Audio Techinica without an AES #. Any suggestions, help? Thanx again for all the help.

Ramdas
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Old October 16th, 2005, 12:55 PM   #17
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According to its spec sheet the input impedance for the XLR inputs is 10,000 ohms. The signal drop from the 200 ohm mike would be 0.2 dB and from the 680 ohm .6 dB i.e. less than a dB in either case and you'd never notice the difference so I don't really think impedance needs to be a factor in your choice.
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Old October 16th, 2005, 01:12 PM   #18
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Impedance
1. Voltage is the "electrical pressure" that makes current want to flow, very analogous to fluids driven by pressure. This analogy is often used in electronics courses.
2. Resistance is the simple opposition to current flow due to the chemical and physical nature of the conductor, some of which conduct better than others. It does not depend upon frequency.
3. Impedance is also the opposition to current flow, but it takes into account the phenomena of inductive reactance and capacitive reactance that occur when a voltage and the resulting current alternate in polarity (AC signals). They do depend upon frequency. Lower frequencies, like audio frequencies give higher capacitive reactance, and lower inductive reactance.
4. Large inductances give high inductive reactance, but small capacitances give high capacitive reactance. Inductance and capacitance values both tend to be very small in conductors, so capacitive reactance is more of a factor at audio frequencies in conductors than inductive reactance.
5. Inductive and capactive reactance are arithmetic opposites that cancel each other out by subtraction, so the bigger of the two dominates in any circuit.
6. Bottom line: Impedance is the square root of the sum of the squares of resistance and reactance.

Balanced circuits
1. Signals require two conductors.
2. Balanced inputs, outputs and cables are ones where the impedance of each conductor with respect to some reference, usually ground, are equal.
3. When used for a mono signal, 3-conductor XLR cable the sending and receiving ends have both of the internal conductors at the same impedance with respect to ground. THAT is what makes it a cable being used in balanced mode.
4. The sending device using balanced mono XLR cable normally applies the voltage between the two conductors, so they are 180 out of phase with each other if you look at their voltages with respect to ground at any instant. But that is NOT the definition of balanced. It is an elegant way of cancelling any induced noise without the need for active electronic circuits to do that at the receiving end.

Impedance matching
1. It is true that to transfer the most power at connections that the impedances of both sides must be equal.
2. However, in audio circuits, particulary in connecting microphones to inputs, often the goal IS NOT TO TRANSFER MAXIMUM POWER but to apply maximum VOLTAGE to the input. To make this happen, the impedance of the input should be MUCH HIGHER than that of the microphone. This is because the input amplifiers are driven by voltage, not power, and voltage divsion in a circuit is proportional to the impedances of the devices in the circuit. The amplifiers get any additional power they need pumped in from their own power supply.
3. The thumb rule is that it is desireable that the microphone's output impedance be 1/10 or less of the input impedance.
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Last edited by David Ennis; October 16th, 2005 at 02:20 PM.
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Old October 16th, 2005, 02:18 PM   #19
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Quote:
Originally Posted by A. J. deLange
According to its spec sheet the input impedance for the XLR inputs is 10,000 ohms. The signal drop from the 200 ohm mike would be 0.2 dB and from the 680 ohm .6 dB i.e. less than a dB in either case and you'd never notice the difference so I don't really think impedance needs to be a factor in your choice.
A.J. That is exactly what I needed to know. Much appreciation.
Ramdas
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Old October 16th, 2005, 03:11 PM   #20
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I don't want to turn this into EE 101 but a couple of points since you seem interested:

1. Voltage is a measure of potential difference i.e. the amount of work that is done on a unit charge in moving it from one point in a circuit to another.
2. Resistance is the reciprocal of conductivity which is the current that flows per unit of potential difference. Resistance does depend on frequency. At higher frequencies currents flow selectively near the surfaces of conductors. This is not an appreciable effect at audio frequencies. The amount of current which flows in a circuit in phase with the applied voltage is given by the ratio of the potential difference (voltage) to the resistance.
3. Capacitance is the amount of electrical charge stored per unit of potential difference between two isolated conductors. If an AC current is applied across a capacitance current will flow but that current will lead the applied voltage by 90 degrees. The amount of current that flows is the ratio of the applied voltage to 1/j*2*pi*frequency(in Hz)*capacitance(in farads). Thus -j/2*pi*f*C is called the capacitive reactance which decreases as capacitance or frequency increases. Electrically -j represents the leading phase and the fact that no energy is dissipated in a capacitor (except in real ones whose leads have resistance). Mathematically it means that the current is imaginary. Inductance is a measure of the magnetic charge stored per unit of current flow in a conductor. If a voltage is applied across a conductor with very small resistance the current which flows is proportional to the applied voltage divided by j*2*pi*frequency*inductance (Henrys). This current lags the applied voltage by 90 degrees. Thus j*2*pi*f*L is the inductive reactance which increases with inductance and frequency. j idicates lag. No energy is dissipated in an inductor (except real ones whose conductors have resistance). Total current is V/(R - j/2*pi*f*C + j*2*pi*f*L).

While capacitive reactances may be large at low audio frequencies they usually shunt circuits. Thus the bigger they are the less effect they have. It is when they capacitance becomes large enough that the capacitive reactance approaches the impedance of the source that capacitance loads a circuit appreciably. Consider a twisted pair with 10 picofarads per foot. No problem for a few feet but a mile long twisted pair will exhibit inter conductor capacitance of a fraction of .05 microfarad.

Whether inductive or capacitive reactance dominates depends on whether they are in series or parallel. In a parallel circuit the smaller sets the current flow. In a series circuit it is the larger.

What is most important here is that we are talking about transmission lines (cables) which exhibit a characteristic impedance whose value is (in an ideal line) the square root of the ratio of the inductance per unit length to the capacitance to unit length. What this means is that the input end of a long cable looks like a pure resistor with value given by sqrt(L/C) which is not a function of frequency (as long as the look is quicker than the time it takes a singnal to propagate out to the end and get back) unless the end is terminated in the characteristic impedance.

Now what is true is that a short mismatched transmission line looks like a shunt capacitance and that may be what you were trying to get at. Match it properly and it looks like a pure resistance. This is another argument for low source impedance if the line is mismatched.

Balanced Circuits

Here are a couple of definitions of a balanced circuit:

"A circuit having its conductors electrically symmetrical with respect to a reference potential plane (e.g., ground). The potentials between the two sides and ground are equal and of opposite sign. For example, a horizontal two wire line may be a balanced line." ( US Patent Office)

"a circuit in which two branches are electrically alike and symmetrical with respect to a common reference point, usually ground." (IEEE definition)

I suppose you could argue that equal impedances are required for the branches to be "electrically alike" but consider an op-amp with 100 ohm resistors connected to both (+) and (-) inputs, a 1000 ohm feedback resistor and a 1900 ohm resistor from (+) to ground. The input impedance on both legs is then 100 ohms but the gain on the inverting input is 10 while the gain on the non inverting leg is 19. The common mode (noise) rejection would be dismal. I'd say the circuit was not balanced.
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Old October 16th, 2005, 03:21 PM   #21
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Good job Fred!
Here is a link that explains it balanced and unbalanced.
http://www.jensen-transformers.com/an/an003.pdf
You are correct about the impedance also. In audio we are looking for maximum voltage transfer so the input impedance of a device should be at least ten times the output impdance of the device feeding it.

Sam
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Old October 16th, 2005, 03:29 PM   #22
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A.J., I have an MSEE from the University of Connecticut so, I'm biased toward the IEEE definition. I once wrote a FORTRAN program to emulate the Smith Chart for transmission lines just for fun. I just try to keep it at level here useful to most curious readers.
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Old October 16th, 2005, 04:14 PM   #23
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FORTRAN? Smith Chart? You've been at this as long as I have!
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Old October 16th, 2005, 05:06 PM   #24
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I was afraid I might be dating myself :>)
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