Why do you need a preamplifier?

[globalweb]

Here’s a question for you knowledgeable people out there. Why do you need a preamplifier?
Searching the web I found these answers.
- If you have more then one source it’s easier then disconnecting the devices.
- If you have a turntable you need to amplify the signal before the power amplifier.
- If you don’t have a volume control on the CD/DVD player you need a volume control on the preamplifier.
- Anything else?

So if you use only a CD player. Can you connect it directly to the power amplifier? What could possibly go wrong?

[darkfenriz]

I don't need a pre for the same reasons.

[nikwal]

Volume control is nice.. everything else is just bloat ;-P

[Gordy]

If you use predominantly only one source, and that source has a good quality variable level output (i.e. a volume control that you believe to be of good quality), and there are no impedance mis-match problems between source and target amplifier, then you may not need a pre-amplifier.

However a pre-amplifier is a good solution for bringing together (any of) the following functionality:

Source selection
Record outputs (buffered ideally)
Volume control
Balance control
Home theatre by-pass (to use your 2 channel kit as part of a home-theatre system without swapping connections)
Tone controls (if it takes your fancy)
Filters
Mute
Unbalanced to balanced conversion
Balanced to unbalanced conversion
Impedance conversion (high in / low out)

[globalweb]

Hi Gordy
Thanks for answering. When you say “there are no impedance mis-match problems between source and target amplifier”. What do you mean?
I’m just a beginner so that’s why I’m asking.

[lumanauw]

Depends on the quality of the preamp.
If the preamp's input is more sensitive than power amp's input, then there will be better music (via preamp connection) compared if the source connected directly to the power amp.
"Impedance Bridging", Darkfenriz?

[mtlin12]

I need a good preamp or a buffer which can provide large current
to drive the signal cables' capacitive reactance.

Passive preamps seldom do this well.

[dshortt9]

If one is not careful about the source impedance, amp input impedance, and cable capacitance there can be a rolloff in the high frequencies. Here is a calculator:

http://www.dact.com/html/ac_calculator.html

I use a passive and prefer it.

[globalweb]

dshortt9

In practical terms. What does it mean? Will it brake any parts? Will it degrade the sound? Is it possible in the technical specs to see if there’s going to be any problem?
I’m sorry for being so inquisitive but I’m worried I going to brake something.

[Gordy]

1.
The output impedance of your preamp and the input impedance of your power amp form a 'potential divider'. The power amp 'sees' the signal at the junction of the potential divider.

To make sure that the maximum level of signal is available the output impedance of the preamp should be very small compared to the input impedance of the power amp. A good solution is input impedance of power amp = output impedance of preamp x 100.

This is not always possible. When the two impedances are closer in value the signal level 'seen' by the power amp will be lower. This is not ideal, however it may not be a big problem. It is very unlikely to break anything.

(At the very worst case it will cause high current to be drawn from the preamp, which may be excessive and cause the preamp protection circuits to work. However this is very rare).


2.
The output impedance of the preamp works with the capacitance of the interconnect cable to form a (low pass) filter which can cut some high frequency information. It depends on the values of both the output impedance and capacitance. Keeping the output impedance of the preamp low can help prevent this.


Most solid state equipment (CD players for example) have fixed output impedances in the range 50 - 1000 Ohms.

Passive preamps (such as potentiometers) have impedances of typically 10k to 20k Ohms. These vary according to volume setting. The simplicity of the potentiometer is then often negated by the poor interaction of the impedance and the capacitance of the cable, which is often audible as a loss of high frequency information. So if you use a passive preamp, keep your cables very short.