I sent Ken Stevens this link so he could weigh in. Since hes not a member here, he asked me to post his response...
"Thanks for sending this link to me. Let me attempt to illuminate this situation.
The noise which ANY line stage produces at minimum volume is its natural noise level, determined by the noise threshold of the input device, be it tube or SS. I'm sure DaveyF can attest that his SL1 noise level is very tiny at minimum volume (and its about 6dB lower in current SL1s).
When the volume control is raised, the line stage starts to amplify the noise from the input resistance. As the volume is raised, the input resistor becomes larger and the noise gets bigger, not because the amplifier's input device is getting noisier, but because a larger value input resistor simply produces more noise.
At a certain resistance level of the VC the resistance noise of the VC equals the noise of the line stage itself, and above that point the noise becomes strictly dependent on the volume control level. No line stage of any type can produce a lower noise level once the noise of the VC itself becomes dominant.
The SL1 line stage has a very low noise floor for ANY type of preamp, but once that VC starts going up there is nothing that ANY preamp can do to reduce the thermal noise of the VC (which is determined by the laws of physics).
Therefore the only way that one can make a line stage quieter at gain is to lower the overall resistance of the VC. A 10k VC will have 1/5 the noise resistance of a 50k pot at any given attenuation level.
Some preamps do indeed have low input impedance for this very reason, however these preamps are a tough load for most CD players, causing dynamic compression and introducing stress into the music. One of the biggest advantages of an active line stage is the fact that the higher input impedance vs a passive line stage allows the source to sound relaxed and dynamic. We aren't going to sacrifice one of the main advantages of an active line stage to reduce a noise which is basically inaudible in practice.
Regarding phono stage input noise, this is different because ALL active phono input stages have noticeable noise when running a low output MC. The SL1 has the lowest input noise of any other tube phono stage, but there are some SS MC stages which are up to 6dB quieter, depending on particular input device. We refuse to use a SS input device because many of the benefits of a tube phono stage, including vibrancy, dynamic life, emotional expression, and timbral purity are compromised.
In our current preamps we use an incredible MC transformer of our own design to improve the SN ratio by up to 17dB (in high gain mode) making it quieter than ANY SS or hybrid phono stage in the world. (Stereophile ran a test of the SL1 Renaissance which is online. Check out the incredible performance of the MC stage.)
I hope this is helpful."
Ken Stevens
Convergent Audio Technology
"Thanks for sending this link to me. Let me attempt to illuminate this situation.
The noise which ANY line stage produces at minimum volume is its natural noise level, determined by the noise threshold of the input device, be it tube or SS. I'm sure DaveyF can attest that his SL1 noise level is very tiny at minimum volume (and its about 6dB lower in current SL1s).
When the volume control is raised, the line stage starts to amplify the noise from the input resistance. As the volume is raised, the input resistor becomes larger and the noise gets bigger, not because the amplifier's input device is getting noisier, but because a larger value input resistor simply produces more noise.
At a certain resistance level of the VC the resistance noise of the VC equals the noise of the line stage itself, and above that point the noise becomes strictly dependent on the volume control level. No line stage of any type can produce a lower noise level once the noise of the VC itself becomes dominant.
The SL1 line stage has a very low noise floor for ANY type of preamp, but once that VC starts going up there is nothing that ANY preamp can do to reduce the thermal noise of the VC (which is determined by the laws of physics).
Therefore the only way that one can make a line stage quieter at gain is to lower the overall resistance of the VC. A 10k VC will have 1/5 the noise resistance of a 50k pot at any given attenuation level.
Some preamps do indeed have low input impedance for this very reason, however these preamps are a tough load for most CD players, causing dynamic compression and introducing stress into the music. One of the biggest advantages of an active line stage is the fact that the higher input impedance vs a passive line stage allows the source to sound relaxed and dynamic. We aren't going to sacrifice one of the main advantages of an active line stage to reduce a noise which is basically inaudible in practice.
Regarding phono stage input noise, this is different because ALL active phono input stages have noticeable noise when running a low output MC. The SL1 has the lowest input noise of any other tube phono stage, but there are some SS MC stages which are up to 6dB quieter, depending on particular input device. We refuse to use a SS input device because many of the benefits of a tube phono stage, including vibrancy, dynamic life, emotional expression, and timbral purity are compromised.
In our current preamps we use an incredible MC transformer of our own design to improve the SN ratio by up to 17dB (in high gain mode) making it quieter than ANY SS or hybrid phono stage in the world. (Stereophile ran a test of the SL1 Renaissance which is online. Check out the incredible performance of the MC stage.)
I hope this is helpful."
Ken Stevens
Convergent Audio Technology