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Interfacing: Impedance and Signal Level

 

It is common to refer to various connections in a system as "low impedance" or "high impedance".  This often causes confusion.  When trying to determine if you can connect source component A to load component B, forget the terms "high Z" and "low Z".

(Also, to avoid confusion when discussing connections, keep signal flow in mind.  Folks often say "...I connect my amp's inputs into my mixer's L/R outputs...".  The signal does not go from the amp to the mixer, it goes from the mixer to the amp.  One needs to say "... I feed my mixer's L/R outputs into my amp's inputs...".)


When you want to connect two pieces of gear (a "source" and a "load"), you need to ask these questions:

1) What signal level (voltage) does the receiving (load) device's input want to see?  (With rare exception this would be mic level, guitar level, line level, or speaker level.)

2) Is the source device's output able to produce the required voltage?

3) What is the minimum load impedance into which the source component can produce the required voltage?

4) What is the input impedance of the load device, and is it within the range that can be driven by the source component?

Why do we not consider the receiving device's input impedance first?  Because it is usually irrelevant (unless we are talking about guitars).  Let's take a look at a few common loads that we might want to drive.

The "low impedance" microphone input on a mixer:  Most mics are specified as being something on the order of 150 Ohms.  So the mic input's impedance must be around that, right?  Nope! Commonly it is about 2000 Ohms (2K Ohms).  (150 Ohms is the output impedance of the mic, and is of no concern to us.)

That rack of five power amplifiers behind the speaker stack:  All the channels are daisy chained.  The resultant input impedance is: 2K Ohms - the same as the mic input.  (The output impedance of the mixer or crossover that's driving these amps is between 50 and 150 Ohms - in the same range as that of the mic, and again not of concern to us).

The difference between these inputs is the signal level (voltage) that the load device requires.  The typical signal level needed to drive a mic input that's set up for a vocal mic might be around -30dBu (about 0.025 Volts).  With a blasto rock band playing, that stack of power amps could require around +12dBu (about 3.0 Volts).  Plug a mic that's putting out -30dBu into the amps and you'll get nothing.  Plug a mixer output that's doing +12dBu into a mic input and you'll get a grossly distorted signal (if it does not blow the mic input).  (We can of course turn down the mixer's output level, but the result would have a lot of hiss.)

We need to see if the mixer's output is designed to deliver the required voltage into the load impedance we have.  A typical mixer will deliver + 12dBu, +20dBu, or even more into a 2K Ohm load, so this is no problem.  However some entry level mixers, EQs, or crossovers may be limited to driving a 5K or 10K load at this signal level.  (Once the load impedance decreases below a certain point, the voltage that the source can safely deliver into the load decreases.)  Our cheapo mixer can likely drive a 2K Ohm load fine at a reduced signal level, so if we use amps that only need say 0dBu, we might be ok.  (Of course if we can afford that many amps, we're not likely to be using a cheapo source component.)

A common confusion I see is when someone wants to change an unbalanced line to a balanced line, such as when going from an unbalanced mixer out down a snake to a balanced amp input.  Because the amp's input is an XLR, and a DI's output is an XLR, I often see folks recommend the use of a DI (Direct Box).  These folks are forgetting about the signal level.  A DI reduces an instrument level, line level, or speaker level signal down to mic level, and is not the appropriate device for the task.  It might be made to work, but the output level from the source will have to be turned up very high, and the DI will be overdriven.  A 1:1 line level isolation transformer is the appropriate device for this task.  This can be a high quality Radial, or an Ebtech Hum Eliminator.

An application where we can cheat on the normal voltage vs. load impedance requirements (in order to save some money) is in the driving of headphones from a mixer's aux outputs without using a headphone amp.  A console line out is typically designed to do 0dBu, +12dBu, 20dBu, or even more into a 2k Ohm load.  Lets say it's +12dBu. +12dBu into 2K Ohms is about 3 volts @ .002 amps (the power is 4.5 milliwatts).  What will happen when you try to drive a set of 60 Ohm or even 8 Ohm headphones?  Take a look at the specs of a typical headphone: 100dB SPL with 1 milliwatt of input.  For a 60 Ohm set this is a signal level of 0.25v, @ 0.004 amps.  For an 8 Ohm set, this is a signal level of 0.09v @ .012 amps.  Into the intended load impedance, our console is capable of 4.5 milliwatts.  Our headphones need about 1/4 of that.  When driving headphones the voltage/current relationship is way off of what the console was designed for - the phones need more current than the console is meant to supply, but at a great deal less voltage.  Will it work?  So far, I have yet to find any line level device (mixer, EQ, compressor, CD player, etc.) that cannot do it.  They have all sounded pretty good too.  A friend was running his rehearsal room this way, using one aux out to drive five sets of headphones.  Even through 30 feet of cable the phones were plenty loud and sounded just as good as they did when one was plugged into the console's headphone output.  (He recently did some rewiring and modified the console so that all the auxes are pre-fade, so he could run one set of phones off of each aux and one set from the L/R mix, for a total of five).


If there's something you'd like to see added, please email me.

 

 

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