R/E/P > Dan Lavry

The Signal Path...

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donnie7:
Welcome Dan! Would like to discuss the signal path, Both at microphone level and line level. Also could you cover proper gain staging and and its importance in optimizing audio quality. Thank you all!

danlavry:
“Welcome Dan! Would like to discuss the signal path, Both at microphone level and line level. Also could you cover proper gain staging and and its importance in optimizing audio quality. Thank you all!”

This is the first answer on this forum. It is long.  Let me know if it is too long, too basic, too complicated… So here we go:

Microphones put out little signal energy:

Dynamic mics (not powered) have the task of picking up air vibration over a small area (the membrane) and converting that to electrical power. With so little mechanical energy, the electric energy can not be any higher…

The condenser mic does receive some DC power (typically 48V phantom for pro mics). The DC is used for charging the “capacitive membrane” (which makes the condenser work), but it is also used for amplifying the signal output power.
But even the condenser mic is very limited. The energy supplied to the condenser mic is in the order of  magnitude of 100mW, and most of it is for DC biasing of some active circuitry. The signal output power peaks are rarely above few mW (milli Watts)

So far I am talking power, not voltage, because power is what counts. But we can now switch to the “normal” language – voltage. Say you get 1mW signal out of a condenser into a 250 Ohms AC load. It amounts to 0.5V signal. Of course getting a 1V signal out of a condenser takes loud sound and close mic placment. At feet away, soft sounds perhapse a dynamic mic, and the signal may be a few microvolt…

So we need obviously need to amplify the signal. Tiny analog signals are too delicate, and there are 2 reasons for that:

First, the circuits handling our tiny signals generate noise. Such self generating noise is “there all the time”. It is random noise. We want the noise to be very low, much lower than the signal. Lower noise is how we get closer to “noise free” signal.
Unfortunately, we can not amplify the signal only. The signal present at the input of a microphone preamplifier is made of both music and noise. The amplifier circuit does not know music from noise. It all gets amplified (treated the same way).
So the input stages of the micpreamp is a “specialty” circuit. It has to be designed to generate as low noise as possible.

Second, we need to deal with noise and undesirable electromagnetic pickup into cables, circuit board traces and all other physical components. A given noise interference has some power level, say 1mW. It can be “manifested” in many ways. A 1mW on a 1 KOhm yields 1V noise signal.  But 1mW on say 10 Ohms yields only 0.1V noise (20dB less voltage).

So we want low resistance for immunity from interference, high resistance for more voltage signal, low resistance for less self generating noise, proper resistance for maximum power transfer and so on… the “typical engineering set of tradeoffs”….

Another factor is: Balanced vs. unbalanced signal handling:

Unbalanced signal transmission is about sending some signal on some wire from a source driver to a load. The voltage level (signal) goes up and down with respect to some ground wire (or path). That is how most of electronics works. But long wires (an antenna in fact) and/or a high source and load resistance, pick up noise. You can help the situation some buy shielding the wire, and that is a “part way solution”. Picking say 10uV when the signal is 10V is OK (120dB signal to noise). But the same 10uV noise added to 100uV mic signal is only 20dB signal to noise… So what is good for line level may be terrible for mics.

Balance signal transmission is the method used for mic preamps. How does it work? You have 2 wires, one carries the signal, the other carries the inverted signal. If wire A (pin 2 of the XLR) goes up to +1mV, wire B (Pin 3 of the XLR) goes to -1mV and so on. The signal receiver subtracts the 2 signals so in our example we get +1mV – (-1mV) = 2mV signal. If we make sure the wires are parallel to each other (or even twisted around each other). Occupying the same space, they will act as “identical antennas” and pick up the same unwanted noise.
Say they both picked up 10mV noise at some instant of time. The wire A now has 1mV+10mV=11mV. Wire B will now have (-1mV) +10mV = 9mV.          
Again, the receiver circuit subtracts the signals A-B = 11mV – (-9mV) = 2mV.
So the “trick” worked. The noise disappeared. The outcome is the same. If we add the same unwanted signal to both wires, subtracting one from the other adds zero total. Yet the subtraction of a signal from its inverted mirror image yields doubling of the signal. Cool concept.

So back to your question: Mic signals vs. line level (a few volts), or we even AES/EBU analog signal levels (higher than line level): A balance transmission with a shielded cable is a must for tiny mic signals. The other particular issues for micpreamp signals handling (amplification) are extremely low self generating input circuitry, and a good differential amplifier (for the signal subtraction A-B). We call it CMRR (common mode rejection ratio). Common mode is the signal added to both inputs (A and B). Thus we call it named “common”.

Line level is easier to handle. With more signal, one can use noisier (self generating noise)    
circuits. Let me stop here for now. Give me some feedback (I can continue with more talk about line level issues, or get into more fine detail regarding what I touched on so far).

And lets start another thread on proper gain staging…  That is also a very good subject.    

BR
Dan Lavry

donnie7:
Dan, Bravo and APPLAUSE! Very comprehensive answer. Thoughtful, precise and by all means not too long. Glad to have you answer the first question on your board. We welcome you with open arms. Keep up the wonderful work you are doing! Wishing you all the best that life has to offer! With Regards and Respect, Donnie Dixon.

plughead:
Wow,

I gotta thank you as well, Dan: that explanation was so clear and informative, I'll be coming back to this forum often. Hopefully to better understand what the heck it is I'm trying to accomplish in this business...

Hat's off!

Roland Storch:
danlavry wrote on Sat, 02 October 2004 01:56


So back to your question: Mic signals vs. line level (a few volts), or we even AES/EBU analog signal levels (higher than line level): A balance transmission with a shielded cable is a must for tiny mic signals.Dan Lavry


I have a question to the shielding of balanced interconnestion.

Dr. Jahne from StageTec claims that with balanced cables and (important) a common mode rejection better than 90 dB unshielded cables are better than shielded ones. What do you think about that?

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