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In Reply to: question for Kurt Strain (power supply modulations of p-p vs SET) posted by WCT on July 29, 2002 at 12:38:40:
A single-ended amp has one connection to the power supply. It operates class A. I am comparing this class A amp to the class A push-pull, not the class AB PP, and I failed to make that distinction.A class A SE amp will draw current directly as a function of its instantaneous power output movements. A large swing on the output either draws more than the bias current "median" or less. It is not constant current over a note's cycle. If you place a scope probe on the B+ you will see movement of its voltage with time, somewhat resembling the music signal. The power supply with a filter cap can only store so much energy, and as the frequency of the signal goes down the magnitude of the drain on the power supply capacitor voltage goes up. This is because low frequencies have excursion away from the median bias point longer than higher frequencies, and so extra current draw lasts longer and less current draw lasts longer. This modulates the power supply B+ at low frequencies higher, which is the bass. If you have a scope, you can see that.
If the power supply moves so much on bass notes, it's diminishing the power swings available on the output. It becomes bass weak, and less solid, and more distorted. Adding capacitance can improve bass, and the price can come as a loss in midrange and high frequency quality by putting in lesser performing high capacitance caps that are slow to charge and discharge.
Parafeed helps out a lot by attempting to isolate the signal from the power supply. High impedance, high isolation CCS's go further with that, but there are many who like a little give with a choke there instead, a little warmth and ease is obtained that way, but not too much. All a matter of how you like it.
Then there's the class A PP. In the simplest example, there's the B+ that enters the center tap primary of the IT or OPT. Each tube is conducting all the time, and the sum of their currents at idle is the total bias current. When one tube conducts more than the other, if the tubes are linear and matched, the sum of the current is one tube conducting some more plus one tube conducting less by the same amount as the other went up. This in theory, if the balance and linearity is perfect, says that the instantaneous current draw is always constant. If it's always drawing the same current, then the B+ voltage does not move at all.
In practice, on average, a good matched set of tubes and transformer will move the current some, but it on average is 100 times less by this cancelling process. Another way to say it is that the PP topology reduces power supply modulation by 20 dB over SE. Also, the reverse is true. The amp is 100 times less susceptible to an imperfect power supply with ripple, noise, etc. Another way to state this is that the Power Supply Rejection Ratio, PSRR, is 20 dB improved by going PP.
The CCS's offer some very high PSRR for SE amps due to it forcing constant current draw on the power supply, and forcing constant current into the amplifier stage. The chokes act similarly, but not as well from a technical point of view. The bass improves dramatically with CCS in all stages. Cascoding and then bootstrapping helps this continue further and better into higher frequencies as well. The highs don't modulate a PS much if there's a good PS cap in there, but the help from these better CCS's work magic to take out the slop in those frequencies and allow more detail to go through.
But what's good for SET is good for PP. You can use CCS's in PP as well. There are many designs with PP differential stages before it gets to the OPT. And on the OPT instead of a B+ voltage you could design in a CCS there and tighten the stage more than parafeed SET, and this all still benefits the PP in terms of tightness.
Which all leads me to think from these design principles and observations of the idea that soundstage width is a result of great control of the amps to the speakers with no slop, as PP tends to help out, while depth is a result of a natural harmonic progression from a SET, where PP acts to cancel it out.
Kurt
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- Re: question for Kurt Strain (power supply modulations of p-p vs SET) - Kurt Strain 14:42:31 07/29/02 (0)
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