Postao/la dadod » 28 tra 2013, 22:45
Nisam siguran da je bas jasno sto se krije iza ovog jednostavnog pretpojacala. Evo kako Charles Hansen opisuje u hvalospjevima svoje ruku djelo - Ayre KX-R.
Naravno to pretpojacalo je usminkano s dodatcima koji ne cine nista s zvukom ali djeluju impresivno kad ih se opisuje:
"But what really caused the buzz was the declaration by Ayre founder and chief designer Charles Hansen that the KX-R, with its use of a technology he calls Variable Gain Transconductance (VGT) to control the volume, would set new standards for signal/noise ratio. ------
"In the KX-R, the idea is very simple," Hansen said. "We simply use a variable resistance instead of a fixed resistance. Changing the value of the resistance changes the gain of the circuit. With a variable-gain circuit there is no input attenuator, as is normally found in a preamplifier. This brings about several advantages: The input impedance is not restricted by the value of the volume control (input attenuator). I mentioned that with conventional attenuators, the frequency response can vary with volume. In contrast, the KX-R has an input impedance of 1M ohm per phase, and the frequency response goes out past 250kHz regardless of volume setting. Another benefit is that the signal path is simplified, and there are fewer switches in the way. -------
"With VGT, the signal/noise ratio is constant regardless of the volume setting. A typical listening level might be 20dB below full gain, which translates into an effective increase in S/N ratio of 20dB. This is on top of an already quiet circuit, and improves the resolution of low-level detail audibly."
If VGT were simple, Hansen observed, everybody would be doing it. "I'm not saying we invented this. PS Audio's VCA technology sounds like they might be doing something along these lines, and I suspect CEC might be doing something in the same ballpark. I don't know, because nobody's talking about the real intricacies of their designs—and neither am I."
Key to VGT was Ayre's development of what they call EquiLock circuitry for the MX-R monoblock. "In a conventional circuit, the gain transistor has a load, usually either a resistor or a current source," Hansen said. "When the current through the gain transistor changes, then the voltage across the load also changes, which, in turn, means that the voltage across the gain transistor is changing. In fact, all of the parameters (transconductance, capacitance, etc.) vary when the voltage across the transistor varies.
"The EquiLock circuit adds another transistor between the gain transistor and the load. (In our case, the load is actually a current mirror.) This extra transistor holds the voltage of the gain transistor at a fixed level while still transmitting the changes in current to the load (the current mirror). By stabilizing the voltage across the gain transistor, all of the parameters of the gain transistor are also stabilized. The circuit is very similar to a cascode circuit, which has been used by other manufacturers, but EquiLock is an improvement over a conventional cascode circuit."
Kako to sve lijepo zvuci, a pretpostavljam da sve sto se krije iza toga je current conveyor, nesto slicno kao ovo moje pretpojacalo. Slicano je i GainWire samo sto sam postavio regulaciju pojacanja na ulazu a ovdje je tocno tako kako Hansen to radi u KX-R pretojacalu.
Nisam siguran da li cu napraviti i ovo pretpojacalo, imam toliko zapocetih projekata a i GainWire jos nije posve zavrsen. Ako je netko zainteresiran za ovo pretpojacalo mogu dizajnirati stampane plocice, napajanje bi bilo tipa shunt, slicno kao kod GainWire samo za manji napon i manje snage.
Dado
