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Old 13th January 2011, 10:46 AM   #1
polop1 is offline polop1  United Kingdom
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Default preamp - at the most basic level

Hello there,
I've been interested in diy audio for a while now, but I have never quite understood the fundamentals of using a passive pre-amplifier, and for the first time I think I have the need for one.

the output from my laptop is incredibly noisy, and very noticeable when playing at low levels - which is what I do mainly. I have been recommended that I attenuate the output of my laptop and play the music at higher levels - thus reducing the signal to noise ratio.

I have no real need for the fancy stuff you guys use that either costs lots of money or requires allot of money, I just need say one that just divides the signal by 5 or 10 or some other appropriate amount.

impedance matching is apparently important, having the input resistance of the following system be at least 10* the output of the previous system I think is the guide.
does this mean that I should construct potential dividers (for each channel) in the kilo ohms range, as the output of the laptop will likely be in the ohms range, and the input to the amplifier in the 10kilohms range.

so to sum up potential divider to divide by 10 will be a 900ohm and a 100ohm resistors - input from a laptop headphone plug, output to a amp 6 ?

your feedback will be very helpful - thanks
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Old 13th January 2011, 11:48 AM   #2
Mooly is online now Mooly  United Kingdom
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You have to remember that attenuating any given signal will not improve the SNR.

However if the SNR from the laptop improves as you increase it's output then doing as you suggest may well help.

Values... as it's a H/phone output it will drive 1K easily but normally that would be considered a very low load for a "line level" source. So yes, try it From a technical view it's good that the amp input sees a low source impedance which makes it less prone to stray pickup and minimises HF losses due to stray capacitance. In this case you have no such worries.
If it were a "true" line source then those values would need to be a least 10 time greater.
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Last edited by Mooly; 13th January 2011 at 11:51 AM.
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Old 13th January 2011, 12:52 PM   #3
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The source of noise from your laptop is likely the switching power supply...try using it on battery & you will notice a big difference!
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Old 13th January 2011, 01:55 PM   #4
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It it is switching psu junk and you want to run plugged in, then you'll need to isolate the outputs.

One way is to use an isolation transformer for each channel such as a Jensen or Lundhal, fairly expensive, but works.

If you are feeding the signal to a receiver with an optical input and you have an optical output available on your laptop, use an optical cable to provide galvanic isolation. If your laptop doesn't have an optical out there are usb sound cards that will get you there for less than a single quality audio transformer.
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Old 13th January 2011, 10:01 PM   #5
polop1 is offline polop1  United Kingdom
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it is not the psu, no change when i take it out. surely the potential devider option (passive preamp) is a good solution as i can only get to roughly 12 out of 100 on my laptop before it is too loud for student halls. also this will allow more more increments in volume as only having 12 between off and a bit to loud generally, tuning it for background listening can be difficult.
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Old 14th January 2011, 06:35 AM   #6
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by polop1 View Post
it is not the psu, no change when i take it out. surely the potential devider option (passive preamp) is a good solution as i can only get to roughly 12 out of 100 on my laptop before it is too loud for student halls. also this will allow more more increments in volume as only having 12 between off and a bit to loud generally, tuning it for background listening can be difficult.
You'll just have to try it... I think it will be as good as anything.

The only practical issue is that you can sometimes be fooled into thinking you know how much you need to attenuate the signal and the reality being different.

Maybe wire a preset pot to one channel and decide from that what ratio is needed before making it for real.
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