If I put my notes here, I might be able to find them again later!
Preamps are not line level.
Posted 1st April 2013 at 04:57 AM by rjm
If you don't think about it too hard, you'd imagine that the signals in the phono stage would be smaller than the signals in the line level preamplifier stage that follows it. Or that the signals in the DAC/CD player would be about the same level or slightly lower than the signal in the preamplifier.
As usual, the answer is "it depends". It depends on the sensitivity of your speakers, how loud you are listening, and the voltage gain of the amplifier. It also depends on whether we are talking about a MM phono stage or low output MC.
My point is simply this: the volume control is an attenuator, and at the typical "9 o'clock" position the input signal is reduced in magnitude by about 35 dB.
That cuts it back down to being comparable to the output of a moving magnet cart, and much, much smaller than anything found in a DAC stage.
It means you absolutely, definitely, positively must spend as much effort and care in the line preamp with respect to grounding, shielding, and general low noise techniques as you would in a phono stage.
You cannot let your guard down just because it is a "line stage".
As usual, the answer is "it depends". It depends on the sensitivity of your speakers, how loud you are listening, and the voltage gain of the amplifier. It also depends on whether we are talking about a MM phono stage or low output MC.
My point is simply this: the volume control is an attenuator, and at the typical "9 o'clock" position the input signal is reduced in magnitude by about 35 dB.
That cuts it back down to being comparable to the output of a moving magnet cart, and much, much smaller than anything found in a DAC stage.
It means you absolutely, definitely, positively must spend as much effort and care in the line preamp with respect to grounding, shielding, and general low noise techniques as you would in a phono stage.
You cannot let your guard down just because it is a "line stage".
Total Comments 4
Comments
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I agree completely with what you are saying, low noise techniques are always a benefit.
However, if you have a linestage with much less gain, the attenuation applied is less, and therefore the noise issue is lessened.
Still, this in no way detracts from using good techniques at all times, in all stages of the signal chain.Posted 4th April 2013 at 01:12 AM by 6L6
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Thanks for your comment.
"No more gain than you need" is a good maxim, but the volume control should still be maintained within a reasonable range (8-10 o'clock is ideal). Insufficient gain means pushing the volume position to much higher position, increasing the impedance seen by the inputs of the following stage and here again adding noise.
Some people tried to address that by making the volume control change the circuit gain rather than attenuate the input signal. Rather than a solution, that can be better understood as exchanging one set of problems for another one.Posted 4th April 2013 at 02:54 PM by rjm
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A very valid point!
How about keeping the total gain low-ish and placing the pot on the output? Your argument about output impedance still holds, but it probably wouldn't bother most properly-designed amps.Posted 6th April 2013 at 10:47 PM by 6L6
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That's another approach which has been done before. Solves the noise problem, but the issue of headroom now becomes a concern. Also the output impedance will develop into a very big problem if the preamp is used with a long interconnect, a largish volume pot (50k) into 5-10 m worth of interconnect capacitance is likely to audibly roll off the treble, and, what's worse, the roll off will depend on the volume position!Posted 9th April 2013 at 12:17 AM by rjm
