Terminology Q: Preamp vs. Buffer

[mth5044]

Just a quick question about terminology. What is the difference between a preamp and a buffer? I was looking at JFET buffers, and found the AMZ and GGG buffers and compared it to the JFET preamp used in the Echoplex (which supposedly sounds good).

The two differences I'm seeing is that the 'buffer' has a supply connection to the gate of the JFET through a resistor, where the 'preamp' does not. The other (besides some part differences) is that the buffer takes the output from the source, where as the preamp takes the signal from the drain. It looks like you could take almost the same parts and move them around to get either a buffer or a preamp 

Can anybody shed some light onto what the difference is between these to? I have attached the two schematics.

Thanks

GGG JFET buffer: http://www.generalguitargadgets.com/pdf/ggg_buffer_jfet_sc.pdf
Echoplex Preamp: http://i958.photobucket.com/albums/ae64/clinchfx/EP-PRE/EP-PREbasicschem.jpg
or:

[R.G.]

The word "preamp", from pre-amplifier, indicates some circuit specially optimized to amplify tiny signals up to where other amplifiers can work on them. It carries the connotations of not damaging the tiny signal with distortion or adding noise while amplifying it up. It is within the context of the term "preamp" to do preliminary frequency shaping, as phono preamplifiers did.

The word "buffer" indicates that the circuit does not load down the input signals, which may or may not be tiny; further, it provides burly, manly output currents able to drive whatever is after the buffer, the idea being that it prevents the after-the-buffer from loading down the signal. Buffers often do, but may not necessarily have gain. Mostly "buffer" means voltage gain of one, but buffers with gain are also encountered often enough to not be rare or a contradiction.

Neither word has any implication of how the power supply is connected, how the buffer or preamp is biased, or where the output comes from. Do not confuse the abstraction of the word with the circuit that does the function, as you're doing now. Also, neither "buffer" nor "preamp" have any implication of "sounds good". That depends on the signal you put in and the loading/frequency shaping, if any that they do.

You're too far down in the mud. Come up a level.

[mth5044]

Thanks R.G.

[Mark Hammer]

A preamp can provide minimal buffering if it anticipates a low-impedance signal source (e.g., voice mic) but is fed a higher-impedance source (e.g., guitar).  But other preamps may be designed in anticipation of that high-impedance input source, so they accomplish buffering too.

"Buffering", however, implies an interface between a "from" and a "to".  So, a full buffering function would be one in which the input is designed to optimize reception of the anticipated input signal, and the output is designed to optimize transfer of that signal.  Some preamps do a nice job of buffering the input, but maybe not such a great job on the output.  Ideally, you would want both for something to be properly labelled a "buffer".  In most instances, you can safely assume that buffer stages you encounter here, that are described as such, will do both.

Some buffers are entirely passive, such as matching transformers.  The Les Paul Recording guitar uses low-impedance pickups, which need to be matched to the input impedance of a standard guitar amplifier input, so the guitar includes a buffering transformer onboard.

If a buffer is active, however, that circuit might provide some additional gain...or maybe not.  Some things called buffers provide only unity gain (output = input), while others provide some gain, and can be said to be both buffer and preamp stages.  The two functions, buffering and preamplification, are not mutually exclusive.  Sometimes you get one, and somtimes the other, and sometimes it's a 2-in-1 deal.

[Plexi]

Nice info here...

Why the Echoplex Preamp makes everything sounds so good?? 

[amz-fx]

Some buffers are entirely passive, such as matching transformers.  The Les Paul Recording guitar uses low-impedance pickups, which need to be matched to the input impedance of a standard guitar amplifier input, so the guitar includes a buffering transformer onboard.

Typically, low impedance pickups are low output, so I suspect that the transformer in the Les Paul is there to provide voltage gain to provide a stronger signal. The trade off is a higher impedance on the output of the transformer, but not a problem with the high Z amp input. With the transformer disengaged, the guitar can be plugged directly into the mixing console, which has plenty of gain.

Best regards, Jack

[mth5044]

Sorry Mark, never thanked you 6 years ago! So let me say now, thanks Mark!