preamp vs. amp?

As a newcomer to electronics and teaching myself along the way, I 've
come to a obviously simple question that does not seem to be answered
in any of my books. What determines whether an amplifier setup I've
made should be called a pre amp or just amp,in other words ,what type
of voltage or current inputs would a preamp handle versus an
amplifier.? Does a preamp assume a certain signal to noise ratio that
an amplifier design would not? What's the dividing line? Thanks for
any help. Jk

 

There is no dividing line, just accepted practise. A 1kW amp might be
considered a pre-amp, if it was driving a 100KW transmitter.

Typically a pre-amp refers to low output current, and small input
signals. An output of a pre-amp, might be a small or a large signal,
i.e. from mv to several volts. Its all a bit arbitrary really.

Pre-amp, is also somewhat of a redundant term. Just sticking to amp on
its own is all that's really required.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

That would be called an exciter or buffer amp. I assume the OP was
referring to audio amplifiers, and not RF amps. But even RF preamps
are for very low level signals in a receiver's front end.

But industry standards are a line level of 0 dBm which is 1 mW into
600 ohms. One could assume that anything that amplifies line levels
(for instance a power amp) is an amplifier, and anything that
amplifies very low level signals, and equalizes them, is a preamp.

But HiFi nuts have a box they call the preamplifier, which is really a
combination of things. It's a preamp for a phono cartridge and
microphone, and a switch to choose what program source to listen to.
Another requirement for a preamp is to have low noise to amplify very
low level signals.

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Can I ask what led to that assumption. I would assume he was talking about
RF as I deal mostly in RF circuits, and the audio circuits on the radio's I
maintain are all built on one IC.
I would have an RF preamp in almost every radio to incerease the incoming
signal.

But even RF preamps

Yep. And easily overloaded if not careful during design.

 

I would usually refer that term to a completely different field.

Ho hummm . Its all arbitrary. Anyway, a buffer is *typically* a unity
gain urrent amplifier, not an voltage amplifier.

I assume the OP was

I didn't assume anything, other then the name.

This isn't relevant. The term pre-amp doesn't assume audio industry
standards at all.

And also that anything that does this as just an "amplifier" as well. It
doesn't require the "pre" bit at all.

Not at all. There is no assumption of low noise.

I personally feel the term pre-amp a bit redundant. It doesn't serve
mush purpose. Other than the convention in the above mention pre-amp
hi-fi unit, it is probably better to dispense with the term.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

On the radios (no apostrophe needed) that I maintained, called RADARs,
a half megawatt exciter drove a 5 megawatt output magnetron (actually
an amplitron).

[snip]

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Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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Buffer amps are used in RF equipment to prevent the modulated stage(s)
from loading the xtal osc, and may (and usually do) have both current
and voltage gain, because they're transformer coupled.

Maybe I should've said 'requirement for an audio preamp..' above.
That would've clarified it. But either way, it's quite obvious that
any preamp that amplifies low level signals is going to have low
noise; if it doesn't, it's going into the repair shop! ASAP!

Yeah, mush is what you get if your preamp is not working right!

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Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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There is no real dividing line. Basically, if a relatively low signal is
amplified in multiple stages, the first one is the preamp. Sometimes
however, especially when dealing with low-power amplifiers, the whole thing
is in one chip where it does not really make sense to call a part a preamp
unless you are talking of the internal design of the chip. In audio
equipment, microphone signals are usually preamplified before being fed to a
common amplifier with a switch and multiple inputs. So are signals from
cassette player heads and phono cartridges (Thx, Mr. Watson). In radio
technology, a preamp often amplifies low antenna signals before a tuner
picks the needed one (usually the tuner comes with an internal preamp, so no
external device is needed). If you want to listen to AM radio stations that
are very far off so the signals are too low for your receiver to handle, the
device used to 'bring them nearer' is very likely to be called an antenna
preamp. In general, a preamp is a device that amplifies a signal to a level
less than the one finally required, so one or more additional amplification
stages are used to complete the operation. There is however no need for the
word 'preamp', it is quite reasonable and much simpler to call every device
that makes a signal 'stronger' an amp.

Dimitrij

 

The word preamp has been traditionally used to indicate a low level,
low noise amplifier. There is a need for a word that differentiates a
low level, low noise amplifier from a regular amplifier, and the word
preamp is a suitable choice.

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that will give you a choice and save you money(up to half).
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Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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I simple don't agree with this low noise tradition assertion at all. For
starters, "low noise" itself doesn't really mean much. In fact, the term
"a low noise pre-amp" is often used, which implies that there are low
noise and noisy preamps.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

I haven't really thought about consistency, but one reason there are
"preamps" is because someone has put an amplifier in a separate box
and it's output is supposed to feed something else.

In other words, the name is applied because of it's standalone nature.

So you've got an old shortwave receiver that could use better sensitivity.
You build or buy a preamp, to put between the antenna and the receiver.

Or, you've got an audio power amplifier that doesn't have anything but
a volume control. You build or buy a separate box that has a phone
preamp, source selector switches, and tone controls. This would be a preamp.

That doesn't quite explain "phono preamps" in stereo receivers, or
for that matter why the low level stages in said stereo receiver might
be referred to as the "preamp" stages. I would suggest that perhaps
it's because of the external box definition. Maybe in those early days
of hi-fi, the low level stages were always a separate box (or weren't called
preamps in the early days) and when they were integrated into the main
amplifier or receiver, the name stuck to define the area.

Michael

 

I recall a preamp to be primarily a voltage amp while a power amp is a
current amp. This is however a huge generalization, use at your own risk.

 

IME, there are noisy preamps and noisier preamps ;-(

 

It comes from the fact that in the olden days, all the mics and
crystal/ceramic phono cartridges and tuners had a high-impedance,
high-level output (which standard has evolved into 1VRMS into
high-impedance); when some pundit came up with low-impedance
(with concomitant lower induced-noise pickup - look it up ;-) )
magnetic phono cartridges and dynamic mics, they needed to
amplify those pesky millivolt signals up to something easier
to work with. Designing an amp with a 3 dB noise figure (for
example) is a HECK of a lot easier when your signal is 1VRMS
than when it's in the sub-millivolt range. Ergo, you concentrate
your harder design efforts on the more sensitive parts.

And you can use the same tone control for all of them, and
sometimes switch in fixed precomp/decomp ("equalization")
curves.

But it turns out that elfa needed a crossover network, after
all; and hopefully I've been some assistance in that department.



Cheers!
Rich

 

Shit!!, I hope you had a fancy lead codpiece.

 

Not needed, unless you are incapable of doing proper work. Only an
idiot would work on high power microwave equipment when it is in an
unsafe condition. Only a total fool would fire it up with any loose,
missing, or damaged waveguide, and a dummy load or antenna. On the other
hand, if you do fire it up under those dangerous conditions, you deserve
what you get.

 

What? An apostrophe i's alway's needed. If there
werent any apostrophe's, wed never know when a word
wa's about to end with an 's.

-- Jim L.

 

LOLPIMP! ;-)