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Old 3rd February 2011, 09:04 PM   #1
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Default Why do i need a preamp?

I'm balancing on the idea wether to build a preamp or not. However, i stumble upon some major questions. I have only one source, a DAC, so a multiple input preamp is out of the question. The DAC already has a line level unbalanced output.

So the main thing is: why do i need a buffer circuit -like a diamond buffer from Sjostrom Audio- before i put the signal through a stepped attenuator? The signal already is line-level. Right?
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Old 3rd February 2011, 09:20 PM   #2
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You don't. The signal is already line level. In fact some would argue that any unneeded processing degrades the signal.

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Old 3rd February 2011, 09:48 PM   #3
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Why is it that many DIY and commercial designs do put all of their signal through some kind of opamp/buffer circuit? (eg: one buffer with a multiselector to put the different inputs through it) It just seems wrong.
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Old 3rd February 2011, 09:58 PM   #4
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Most 'preamps' are for input selection and volume control. Not an issue if you only have one input but if you have several, you don't want to be swapping cable all the time.
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Old 3rd February 2011, 10:00 PM   #5
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Originally Posted by MJL21193 View Post
Most 'preamps' are for input selection and volume control. Not an issue if you only have one input but if you have several, you don't want to be swapping cable all the time.
That part, i understand. But what i don't understand is that the input switched signal always go through the gain stage in many preamp designs.
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Old 3rd February 2011, 10:03 PM   #6
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A buffer is a current amplifier. In other words it has no voltage gain but a low output impedance.
In some cases a buffer can work well when the output impedance of the source (DAC in this case) is "ḧighish" an the input impedance of the amplifier is "lowish" (some solid state amps have a rather low input impedance of 10k or so).
An impedance mismatch has a bad influence on the sound stage in my experience.
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Old 3rd February 2011, 10:09 PM   #7
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Originally Posted by MJL21193 View Post
Most 'preamps' are for input selection and volume control. Not an issue if you only have one input but if you have several, you don't want to be swapping cable all the time.
That part, i understand. But what i don't understand is that the input switched signal always go through the gain stage in many preamp designs.
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Old 3rd February 2011, 11:25 PM   #8
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the volume control in a preamp is almost always before the gain stage. Depending on the value of the volume control, 500k 200k or 10k the signal quality of the output of the volume control may be effected by the cables belween the preamp and the power amp. In my old home I had a 20 x 30 foot room (7x10 meter) the preamp, turntable, tuner, Reel to Reel tape recorder etc was in one corner and the speakers were on the other side of the 30 foot wall. The speakers were bi amped and the amplifiers sat on the speakers. The cables between the amplifier and the pre amp were 50 ft (17 meter)long at least. I had a passive preamp, and it just couldn't drive the cables. I had to put in a buffer stage.

If you have a particular situation where cables are short and you know all of the impedances, you may be able to get by without a gain or buffer stage. The same setup may not work elsewhere.

if your volume control is 50k and your load is 100k, no real problem. If your volume control is 100k and your load is 10 k, oh oh.

Some preamplifiers have 2 volume controls one on the input and one on the output. These were ganged together. This setup had 2 main purposes. when the volume is down it would mask the noise of the preamplifier stages, 2 when the volume is down it would guarantee that the input stage of the preamp was not overdriven.

you may notice that many power amplifiers have an input level control. It mainly serves to limit the level from the preamp so that the preamp volume is not at the 9 o'clock position and the amplifier is already clipping.

you could make your own preamp where some of the inputs are buffered and some not. It just involves some switch wiring.
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Old 3rd February 2011, 11:43 PM   #9
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Originally Posted by SimonV89 View Post
That part, i understand. But what i don't understand is that the input switched signal always go through the gain stage in many preamp designs.
Because it's easier to send everything through the gain stage than to add a switch to bypass it?
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Old 4th February 2011, 12:44 AM   #10
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When ordinary switches are between one contact and another, the amp following has an "open input" which can recieve rf noise from whatever else is in the area. This tends to be heard as "pop". Popping is hard on speakers and your ears. Rotary selector switches on old preamps like the PAS2 grounded everything and only let them go when they reached the proper input index. Rotary selector switches are expensive.
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