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?
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?
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)
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)
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.
http://www.dact.com/html/ac_calculator.html
I use a passive and prefer it.
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.
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.
lumanauw said:
David,
That's not true. If the preamp's input sensitivity is higher, your sound LEVEL will be higher, unless you turn down the volume. There is no reason why there would be any diference in sound quality from the difference in sensitivity. It just means that the power amp will reach its max output level with lower source level, so you need to turn down the volume more.
Jan Didden
Hi, Janneman,
Yes, you are right. I use wrong words (again). I don't mean sensitivity in term of gain, but "sensitive" in terms of higher input impedance. I'm thinking of signal voltage transfer. A better signal voltage transfer is possible if the (lower) source impedance is headed with a (higher) input impedance. This is possible if we insert a preamp. If the CD player has a certain output impedance (say 100ohm), the power amp has input impedance of 47kohm, then if we put, say Jfet preamp with input impedance of 1Mohm and output impedance of 47ohm, then (if the preamp itself is good), we can get better sound from the whole system. Am I right with this thinking of "sensitive"?
Yes, you are right. I use wrong words (again). I don't mean sensitivity in term of gain, but "sensitive" in terms of higher input impedance. I'm thinking of signal voltage transfer. A better signal voltage transfer is possible if the (lower) source impedance is headed with a (higher) input impedance. This is possible if we insert a preamp. If the CD player has a certain output impedance (say 100ohm), the power amp has input impedance of 47kohm, then if we put, say Jfet preamp with input impedance of 1Mohm and output impedance of 47ohm, then (if the preamp itself is good), we can get better sound from the whole system. Am I right with this thinking of "sensitive"?
lumanauw said:
Yes, I see what you mean, and I agree with that part. But I don't see how this leads to better sound. The difference between 47k load or 1M load (100 ohms source) is just a fraction of a dB loudness, undetectable unless you use precision test equipment. So, I don't see any need for a preamp here.
It is another case with a tube source with several k Zout, and a ss amp with low (5-10k) Zin, because the tube source may well increase its distortion if it has to drive the ss amp, as tube stuff is known to have often weak drive capability.
Jan Didden
Hi, Janneman,
. You are right, where the "better sound" is coming from, if we only look at the input-output impedance? If we thinking of "black box", there is no such thing as preamp (as black box) that will deliver 100% of what it's receiving. 99% signal delivery will still have 1% corrupted.
So, what is the need of preamp?
Now I'm confused myself
. You are right, where the "better sound" is coming from, if we only look at the input-output impedance? If we thinking of "black box", there is no such thing as preamp (as black box) that will deliver 100% of what it's receiving. 99% signal delivery will still have 1% corrupted. So, what is the need of preamp?
I'm currently passive Bi-amping. Each amp is an integrated with it's own 10K volume pot. I'm planning to build an external passive pre to control both and bypass the amps volume pots. So .......
Are there any extra considerations when using a passive pre to feed two power amps in parallel? The gain of both amps will be equal.
Many thanks,
Martin.
Are there any extra considerations when using a passive pre to feed two power amps in parallel? The gain of both amps will be equal.
Many thanks,
Martin.
Sonusthree said:
hi,
a few issues, but easily handled.
the effective load impedance seen by the pre-amp is Zin//Zin.
The effective cable capacitance seen by the pre-amp is Ccable+Ccable.
The input offset current of each power amp can interact if BOTH are devoid of DC blocking input caps.
Since one amp is handling low frequency, the input conditions can be optimised for that end of the frequency range. The amp handling the upper end of the frequency spectrum can have it's input conditions optimised for the higher frequencies.
Andrew, as always, thanks.
You have just given me another reason to reduce the input cap on the tweeter amp. I guess I will need to be conservative to minimise phase problems. I've ordered a stepped attenuator from Hong Kong so it'll be a while before I see the results.
How is your player setup? Does it have a volume control? Did it buzz while playing music?
It sounds a little like the volume was not lowered and the power amp was amplifying the noise from the player????
Tell us more.
You have just given me another reason to reduce the input cap on the tweeter amp. I guess I will need to be conservative to minimise phase problems. I've ordered a stepped attenuator from Hong Kong so it'll be a while before I see the results.
globalweb said:
How is your player setup? Does it have a volume control? Did it buzz while playing music?
It sounds a little like the volume was not lowered and the power amp was amplifying the noise from the player????
Tell us more.
Volume control. The ones available on source items (CD/SACD/DVD players are the only sources that have them, I think) just don't cut it. Then again, some pre-amps volume controls don't cut it either.
Switching is useful, as my partner likes listening to the radio and does not like crawling around behind power amplifiers (although I like the idea,
now that it comes up).
Oh, and loudness and tone controls - where would we be without them? Oh dear, it's time for the white coat with the long sleeves again - they're coming to take me away, ha ha!
Switching is useful, as my partner likes listening to the radio and does not like crawling around behind power amplifiers (although I like the idea,
now that it comes up).Oh, and loudness and tone controls - where would we be without them? Oh dear, it's time for the white coat with the long sleeves again - they're coming to take me away, ha ha!
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