John Curl's Blowtorch preamplifier part II

[syn08]

Originally Posted by simon7000

If we offset the power supply voltages then the hand off between output devices does not occur as close to the zero crossings, so it actually happens less often and is more masked by the desired signal.

I expect you can now begin to understand why tight tracking of the power supplies is not always a good idea. Also if you carry the thought a bit more you can see some of the advantages of a current output.

Simon,

Before anything else, I am not sure why you think de-balancing the power supply would translate (e.g.) the crossover distortions from zero. To me, it's clipping which would be de-balanced rather than the crossover distortions.

Secondly, strictly about zero crossing distortions, I am having some trouble following your thinking on this. So, are you claiming that e.g. crossover distortions are less "damaging" (to what?) if they do not occur around zero?

Looking in the frequency domain there seem to be a contradiction here. The sum of harmonics of a signal with a dead zone crossover distortion (like in the extreme case of a P/N complementary stage with zero bias) has a minimum precisely when the dead zone is around zero. That's because the time domain signal is symmetrical and an odd function of time, therefore the signal would have mostly odd harmonics. If the dead zone is not around zero, then the function is no longer odd (or even) and the harmonics are both even and odd. It is easy to show that the sum of harmonics is now larger.

I'm attaching two pictures. The first is the spectrum of a sine voltage having a dead zone of about +/0.6V around zero. The second is the spectrum of exactly the same signal, shifted by +0.25V. Which signal has overall larger harmonics?

Now, strictly from a Fourier transform perspective, the only impact of adding/shifting a signal by a constant is a Dirac delta function at zero frequency, while the signal spectrum itself is the same. So even in the perfect world of mathematics, perfect shifting from a perfect zero crossing would not have any positive impact on the signal spectrum itself.

So I still wonder why you think de-balancing/shifting the power supplies is a good idea. Is this some empirical result, based on perception rather than signal analysis?

[syn08]

Originally Posted by Bob Cordell

Separately, the offset you mention is not an unreasonable thing to do, and people have been doing it with op amps in audio applications for years. Doug Self's Crossover Displacement (XD) output stage is not much more than applying this principle to an output stage.ob

Bob,

My understanding of XD is that it actually avoids crossover distortions, by sinking/sourcing a current into the output, rather than displacing the crossover from zero to some +/- value.

This is also what people are trying to do (with dubious results) by connecting a resistor from an opamp output to one of the power supply lines, in an attempt to "linearize" the open loop response of the opamp output stage.

[Wavebourn]

Originally Posted by syn08

This is also what people are trying to do (with dubious results) by connecting a resistor from an opamp output to one of the power supply lines, in an attempt to "linearize" the open loop response of the opamp output stage.

Huh? "Dubious results"? Can you explain that, please?

[syn08]

Originally Posted by Wavebourn

Huh? "Dubious results"? Can you explain that, please?

It doesn't necessary do any good. Sinking (or sourcing) only is not a consistent or guaranteed solution. Not that there's anything else that you can do, other than using the opamp as designed.

[Bob Cordell]

Originally Posted by syn08

Bob,

My understanding of XD is that it actually avoids crossover distortions, by sinking/sourcing a current into the output, rather than displacing the crossover from zero to some +/- value.

This is also what people are trying to do (with dubious results) by connecting a resistor from an opamp output to one of the power supply lines, in an attempt to "linearize" the open loop response of the opamp output stage.

Crossover distortion is a function of output current, not output voltage. The XD circuit does exactly what you describe, but it does not avoid crossover distortion; it merely shifts its occurrence to a point where the net output current of the amplifier is non-zero (in the simple case by the amount of the pull-down current).

For example, if the pull-down current is a constant 1 amp, the crossover will still occur, but only if the (sinking) output current gets to 1A. Most of the time, with low-level program, the output signal current may not get this far, and if it does, the resulting crossover distortion occurs in the midst of higher-level program material. Notice also that the approach introduces an asymmetry into the circuit in that there is no crossover distortion for positive signal current swings.

The patent mainly covers circuit elaborations that make the process more efficient than just using a current source for pull-down.

Cheers,
Bob

[stinius]

Originally Posted by Bob Cordell

Crossover distortion is a function of output current, not output voltage. The XD circuit does exactly what you describe, but it does not avoid crossover distortion; it merely shifts its occurrence to a point where the net output current of the amplifier is non-zero (in the simple case by the amount of the pull-down current).

For example, if the pull-down current is a constant 1 amp, the crossover will still occur, but only if the (sinking) output current gets to 1A. Most of the time, with low-level program, the output signal current may not get this far, and if it does, the resulting crossover distortion occurs in the midst of higher-level program material. Notice also that the approach introduces an asymmetry into the circuit in that there is no crossover distortion for positive signal current swings.

The patent mainly covers circuit elaborations that make the process more efficient than just using a current source for pull-down.

Cheers,
Bob

Hi Bob
That is exactly how I understand the XD principle as well.

If somebody is interested I can scan the EW article from November 2006 where Douglas describes the principle.

Cheers

[syn08]

Originally Posted by Bob Cordell

Crossover distortion is a function of output current, not output voltage. The XD circuit does exactly what you describe, but it does not avoid crossover distortion; it merely shifts its occurrence to a point where the net output current of the amplifier is non-zero (in the simple case by the amount of the pull-down current).

For example, if the pull-down current is a constant 1 amp, the crossover will still occur, but only if the (sinking) output current gets to 1A. Most of the time, with low-level program, the output signal current may not get this far, and if it does, the resulting crossover distortion occurs in the midst of higher-level program material. Notice also that the approach introduces an asymmetry into the circuit in that there is no crossover distortion for positive signal current swings.

The patent mainly covers circuit elaborations that make the process more efficient than just using a current source for pull-down.

Cheers,
Bob

Exactly, but the power supply has no bearing with this mechanism. The power supply is a voltage source, delivering current on demand, not enforcing a current. My understanding from Simon's post was that he's claiming a voltage zero translation as being helpful.

[stinius]

Originally Posted by syn08

Exactly, but the power supply has no bearing with this mechanism. The power supply is a voltage source, delivering current on demand, not enforcing a current. My understanding from Simon's post was that he's claiming a voltage zero translation as being helpful.

Absolutely right syn08 and in my opinion it was a lot of other things that he stated in his post that doesn’t correspond to (at least) my view. In fact it was a rather confusing post.

[jacco vermeulen]

Did someone say power supply ?

[john curl]

That is only one of our AC inputs. The other one is for the optional phono stage.