82.55.216.168
I have been encouraged to explore the possible improvements of performance of the recently posted OTL simulations by replacing the common cathode resistor of the differential amplifiers with a constant current source (CCS).
The key question is: what is the purpose of CCS in differential amplifiers?
The generally accepted answer is: to retain balanced operation with practical active devices (that in real world exhibit different electrical characteristics) and to enhance the common mode rejection.
In simulation world all modeled devices of a specific type (6AS7, 6SN7, 12AT7 etc.)are identical in nature and simulated audio signals do not usually develop a common mode component (unbalanced signal) unless the circuit is intentionally designed to do so.
As a matter of fact I have found that there is no major difference in LTSpice simulated spectra with or without using CCS.
There are of course minor differences but they are mainly due to numerical errors (differential circuit equations that are solved by numerical methods give always 'approximate' solutions).
And like all mathematical models there are limits of validity of the results due to these residual numerical errors.
In other words we might say the the 'numerical artifacts free' range of SPICE simulations is about -50dB/-60dB down the maximum signal power level (in terms of distortion -40dB=1%, -50 dB=0.1%, -60 dB=0.01% etc.), depending on the level of accuracy of the specific SPICE program.
This means that, within the limits of validity of the specific model, expecially when you use ideal HT voltage sources, the simulated SPICE spectra give an indication of the upper limit of performance of any given schematic, that is, the best performance that can be obtained with real world devices and by incorporating the best CCS ever known.
I fully subscribe the John Browskie point of view:
From http://www.tubecad.com/2008/03/blog0137.htm
"What could possibly be controversial about tube circuits? With the SRPP circuit it was my pointing out its intrinsic push-pull nature; with the patented Transcendent OTL, its patented zener trick not actually delivering true balanced drive to the output stage; and with the Circlotron amplifier, its plain simplicity and its inability to squeeze class-A blood from class-AB stones. With near-metaphysical anguish, many an SRPP devotee resented my differing perspective on the topology’s fundamental functioning. On the other hand, the most vociferous advocates of the patented Transcendent OTL and Circlotron amplifiers were those that had actually bought a patented Transcendent OTL or Circlotron OTL "Class-A" amplifier and who were greatly displeased with my uncouth disparaging of their expensive equipment; no reasoned arguments, no scope traces, no measurements, just the same How-dare-you... and I-just-spoke-to-the-guy-who sold-me-the-amp-and-he-says... retorts."
Best Regards
Luca Cellai
ecc230
Follow Ups:
Post a Followup:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2009, Audio Asylum, all rights reserved ]
[ Go Green ]
[ Take5 ]