Buffer vs Line Driver?

[simon111]

Can anyone explain to me in laymans terms what's the difference between a buffer and a line driver?

I have seen buffers called line drivers and vice versa.

I always thought a line driver was for retaining the tone over very long cable lengths and generally used a transformer for this. While a buffer is ideal for shorter cables runs to put before your pedals to again retain the tone. (HF)

Any simplified explanations would be great.

Simon

[DuncanM]

I always thought a line driver was for retaining the tone over very long cable lengths and generally used a transformer for this.

A transformer will require a terminating with a specific impedance to get a flat frequency response, so they're generally used with things like mic amps.
I would suggest that :

Line drivers would generally have a low OUTPUT impedance so they can drive larger currents to overcome the capacitance of the cable.

Buffers would generally have a high INPUT impedance to prevent them from loading a previous stage.

Some buffers will act as line drivers and vice versa...

[Satch12879]

No layman's terms 'round here; everyone's gonna be smart and that's it!



It's true, there is a difference.  In short, a buffer amplifier is a bridging device; it presents a high impedance load to any source so to prevent signal losses.  They also invariably output at a low impedance which is suitable to interface with other bits of circuitry.  Buffers most often deal with unbalanced signals, can be stand alone or a component of a larger circuit.

A line driver is something slightly different; it's an amplifier that provides not only low output impedance but a high current output.

Read this:  

Practical Line Driving Current Requirements

The transformer can help getting that output impedance down and provide isolation, but does nothing for the drive current requirement; you need a well designed discrete or tube output stage, or an opamp to do the job.  Line drivers can be balanced - more common - or unbalanced.

[Transmogrifox]

I like Rane's notes & rants.  They offer a lot of useful information.  I got a good session of information about real-time analyzers and noise generators & such from their notes once.

It was good to read that.  I had never in my mind defined a difference between a buffer and a line driver.  I had always kinda figured, what's the point in using a buffer that doesn't have the current sourcing capability to drive your cable run? 

For  20 and 30 foot guitar cable runs, you can quickly deduce from that article that there is no difference between a buffer and a line driver for a guitar player (practically speaking).  He's talking about 500' runs of cable, as compared to our typical maximum on the order of 50'.  Divide all that by 10, and you probably really only need about 3-5 mA to drive your cables...and this is really unrealistically conservative for guitar, since the signal levels used as examples in the article were on the order of 20V peak.  We're talking about 2 Volts maximum.  Most of our DIY buffers are able to source and sink 5mA and have an output impedance of less than 200 ohms.  My opinion is it's good not to exceed 200 ohms output impedance if your buffer is stable in this condition. 

[DuncanM]

I HAVE seen transistor buffer sections that have a high(ish) output impedance... Not all buffers can drive lines.

[Stephen]

So the MXR noise gate line driver ........Is not a buffer YET a LINE DRIVER???   Is that what it is?

[R.G.]

Let's not get all tangled up in the terms.

"Buffer" is the more generic term.

All line drivers are buffers in that they present an easily drivable load to what drives them, and can produce enough output to easily drive what the line that they drive. What's unique about line drivers is what kind of line they drive.

In audio, the word "line" can mean a lot of things. It can be a balanced or unbalanced line, it can be a high impedance line, a low impedance line - or a MODERATE impedance line - and it can be high level, low level, speaker level or ... "line" level.

If you're talking pro audio and studio stuff, "line", "line level", and "line driver" all tend to mean signals of 0.775Vrms to 1Vrms as a 0db reference carried as differential signals on balanced-to-ground cables of 600 ohm nominal impedance. The 600 ohm impedance comes from the telephone company work adopting 600 ohms as a nominal line impedance before there were any pro audio setups.

Pro audio work needs to carry signals that are a number of db bigger than 0db across these cables, so it needs lots of volts to ensure headroom to not clip. The common practice is to use drivers that can deliver +/-20V or more to a 600 ohm line. The NE5532 can do this at low distortion, and is a longstanding favorite of pro audio equipment. The LM833 can too, I believe. Many common opamps can either not give enough volts out or enough current to do this.

But that's a highly specialized context for "line". Line can also mean just the wires connecting hifi components together, and so the RCA phono cables connecting CD players to power amps are also nominally "line level" cables. And in more general electronics, "line" has the connotations of a long-ish distance connection through a wire between whatever near-ish things you're connecting. So a phone line is truly a long line; home stereo has RCA lines; a guitar needs a suitable buffer to drive the high impedance line between it and the amplifier input to prevent treble loss. Logic circuits need 120ohm lines across the circuit board to prevent high frequency ringing; TV, FM, cable TV and other RF stuff is carried on 75 ohm coaxial lines, and need 75 ohm coaxial line drivers to drive them.

Buffers is what's easy to drive and can drive heavy things for you. Line drivers are buffers that are specialized for the kind of line that they drive. All line drives are buffers, but not all buffers are line drivers because they may drive other things than lines. And not all line drivers can drive all lines - it has to be the kind of line they were designed for, so saying "line driver" without saying what kind of line conveys no information other than it's a specialized buffer.