What's The Point Of A Pre-amp.

[streaml1ne]

Why do I see people using pre-amps for gear that already output a line level? I thought the point was to bring a source to a line level for further amplification. Am I missing something?

(edit: This applies to hi-fi stereo gear.)

Edited January 10, 2005 by streaml1ne

[dex Otaku]

Tape heads do not transduce at line level.

Phono cartridges do not transduce at line level.

They also do not reproduce all of their bandwidth ina linear fashion. Or, another way to look at it: they do reproduce in a linear fashion is compensation is added to the signal [pre-emphasis] before applying it to the media, and removed [de-emphasis] after transduced back to an electrical signal.

This is why tape has different EQ positions for different tape formulations [affecting the EQ and bias freq when recording and EQ during playback]. It's also why phono preamps have standards determining pre- and de-emphasis curves so response comes out "correctly" i.e. the desired bandwidth is flat [-tish] from bottom to top.

It is the same priniple that applies to a microphone. Analogue devices generally use a transducer to "read" the music; a microphone is a transducer, same as a tape head, same as a coil or crystal in a phono cartridge. The signals induced by any of them are miniscule, in millivolts.

A preamp is used to amplify that to a managable level to make longer cable runs more practical [feet instead of inches] and noise rejection better, amon' other things. Some devices also have pre- or de-emphasis in their preamps. Microphones generally don't, but many have selectable cut or rolloff filters, especially if they also contain other electronics of their own [such as internal power in the case of a condenser].

Most gear that use line-level don't actually go through preamps. In this case, it's a piece of gear that is almost a misnomer; they usually have buffering preamps in them [which add noise but are great as components in, say, an electronic source-switch that you can run from a cheap IR remote control]. Higher-end units like this are simply passive switches that do not signal processing except for attenuation in the main "volume control".

So. In the case of stereo equipment, a preamp generally is nothing more than a glorified switcher. Of course, it would probably also have a phono preamp in it, legitimising the name.

[streaml1ne]

Tape heads do not transduce at line level.

Phono cartridges do not transduce at line level.

They also do not reproduce all of their bandwidth ina linear fashion.  Or, another way to look at it: they do reproduce in a linear fashion is compensation is added to the signal [pre-emphasis] before applying it to the media, and removed [de-emphasis] after transduced back to an electrical signal.

This is why tape has different EQ positions for different tape formulations [affecting the EQ and bias freq when recording and EQ during playback].  It's also why phono preamps have standards determining pre- and de-emphasis curves so response comes out "correctly" i.e. the desired bandwidth is flat [-tish] from bottom to top. 

It is the same priniple that applies to a microphone.  Analogue devices generally use a transducer to "read" the music; a microphone is a transducer, same as a tape head, same as a coil or crystal in a phono cartridge.  The signals induced by any of them are miniscule, in millivolts.

A preamp is used to amplify that to a managable level to make longer cable runs more practical [feet instead of inches] and noise rejection better, amon' other things.  Some devices also have pre- or de-emphasis in their preamps.  Microphones generally don't, but many have selectable cut or rolloff filters, especially if they also contain other electronics of their own [such as internal power in the case of a condenser].

Most gear that use line-level don't actually go through preamps.  In this case, it's a piece of gear that is almost a misnomer; they usually have buffering preamps in them [which add noise but are great as components in, say, an electronic source-switch that you can run from a cheap IR remote control].  Higher-end units like this are simply passive switches that do not signal processing except for attenuation in the main "volume control". 

So.  In the case of stereo equipment, a preamp generally is nothing more than a glorified switcher.  Of course, it would probably also have a phono preamp in it, legitimising the name.

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Thanks, Dex. That cleared up alot.