Meaning of an output's "load impedance"?

[pedrito]

Meaning of an output's "load impedance"?

Hello,

What does it mean when you have "Load impedance" mentioned for an audio output in specifications?

Example:

From http://www.orban.com/products/radio/fm/5300/specifications/

---
Analog Audio Output
Configuration: Stereo. Flat or pre-emphasized (at 50µs or 75µs), software-selectable.
Source Impedance: 50 , electronically balanced and floating.
Load Impedance: 600 or greater, balanced or unbalanced. Termination not required or recommended.
Output Level (100% peak modulation): Adjustable from -6 dBu to +24 dBu peak, into 600 or greater load, software-adjustable.
---

The "source impedance" (output impedance) makes sense to me, but I can't wrap my head around what "Load impedance" (input impedance) means in this situation. How can load impedance even be a property of a driven output? Shouldn't this be a property of whatever load you connect to the output? What do they mean here?

Any help would be appreciated 🙂

 

[Brett]

It simply means that the input impedance to the next device eg mixing desk, poweramp etc should be 600R or greater, as per spec you quoted.

Few output stages on a line level device will like to drive less than 600R, and most will work best into more.

 

[pedrito]

Thanks, Brett. That makes more sense now.

Is it safe to assume that the "or greater" is to be assumed even if it isn't explicitly stated? e.g., if the specification just says "Load impedance: 150 ohms" for the output, would there be any reason you'd have to connect it to a device with a load impedance of 150 ohms exactly? I ask because I have another specification that just says:

Source impedance: < 75 ohms
Load impedance: 150 ohms

I'm assuming this limitation of the output stage is largely dependent on the amount of current it can supply to the load you connect? Or is there some other factor?

Thanks again

 

[Brett]

There's a couple of things here. Typicall source/output impedance for audio applications should be 1/10 the input Z (Z=impedance) of the following stage, eg Orban Zout = 10R, following stage Zin = 600R is fine.
Higher is usually fine, but there are limitations in some cases.

Most line stages will have a range of loads where they work best, and as they are loaded with lower Zin for the following stage, distortion will increase. In extreme cases a very low Zin of the following stage will reduce the voltage provided to it. The Zout of the source and the Zin of the following stage will form a voltage divider.
Voltage Divider Calculator
In the link R1=Zout and R2 = Zin of the next stage. If R1=R2, then the voltage at the input of the next stage is 1/2 what it would be if R2 were say 100x Zout.

600R Zins are not uncommon in pro audio.

All of the above is for audio frequencies. At higher frequencies such as radio/TV, or even coax leads for digital signal transfer, it is important that Zout = Zin.