Raul, I would refer you to the David Berning website, in the White Papers section, where amplifier transfer characteristics are discussed.
You may know that the "load line" of a given amplification device in an amplifier defines how it responds to the loads presented to it by the speaker(load) and how well it will control it directly and without feedback.
In my Berning amplifier, the load line is about 1.8 ohms, and it operates with no feedback, and is globally DC stabilized(something you seemed to think could only be in SS amps, and not be achieved with tube circuits, but is a feature of the David Berning ZOTL circuit). However, by using the much larger "effective turns ratio" that is provided by the unique ZOTL impedance conversion circuit, much more effective control of the speaker load by the triode is possible, than would ever be possible using any output transformer or traditional OTL ciruits. In fact, with the load line that is in my amplifier, according to the David Berning "White Paper",
"In an audio application, the tube behaves as if it is driving a 4k ohm speaker (the impedance conversion ratio goes as the square of the turns ratio), and the speaker acts and sounds as if it is being driven by a 0.6 ohm triode."
And this, with the use of ZERO FEEDBACK. The natural load line of the triode is sufficient to produce this, when using the ZOTL impedance converter design. So that all the linear transfer characteristics of the triode are preserved, and full control of the load is realized, while exceeding the capability(and eliminating any of the drawbacks) of any output transformers.
So, as you can see, the use of this design allows both a low output impedance AND zero feedback, and at the same time eliminates all the saturation problems and parasitic capacitances of typical output transformers in audio applications, and still allows the use of a single triode(no parallel tubes like traditional OTL) to control the speaker accurately and in conditions of shifting speaker impedances(within reason), based on its inherent transfer characteristics(load line)in conjunction with the Berning ZOTL impedance converter.
While David does give credence to your concerns about shifting impedances causing frequency response problems in certain amplifiers, it is shown that this amplifier circuit is not adversely affected by the things that you mention, and the load lines in the ZOTL circuit are very close to what would be expected from some MOSFET transistor amplifiers(without feedback). However, the sonic attributes of the triodes are preserved, and gives what might be called "the best of both worlds", as the control and speed are very much like solid state, and the transparency, tone, and harmonic qualities are all tube.
So, here we have a tube amp with a single output triode(per channel), no audio output transformers, no feedback, an effective output impedance of about 0.6 ohms, DC coupled and global DC stabilization, bandwith from 2Hz - 500kHz, high-speed switching power supply, RF-frequency tube heating, noise less than -100db, choke loading, DC powered, auto-biasing, brownout protection, and can even be turned on without any speaker attached without damage(just like a SS amp can), and weighs 5 pounds.
And best of all, it sounds like real music when you use it with a good speaker. It doesn't sound like most tube amps, and it doesn't sound like most SS amps. It sounds like the best of both.
Now, by anybody's numbers, a 1:13 ratio(damping factor 13) of output impedance to speaker impedance is going to be well within 1db(and probably within 0.5db) in frequency response variation over the audio range as long as a nominal 8 ohm speaker is used. Now, if you want to drive Apogee Scintilla's <1 ohm load with it, I'll agree that I cannot do it with this amp effectively. For the speakers that are intended for this amp, it will control them admirably and exhibit a quite flat response, and be musical.
Whew! I'm tired, and had about enough of this.
Regards,
Twl
You may know that the "load line" of a given amplification device in an amplifier defines how it responds to the loads presented to it by the speaker(load) and how well it will control it directly and without feedback.
In my Berning amplifier, the load line is about 1.8 ohms, and it operates with no feedback, and is globally DC stabilized(something you seemed to think could only be in SS amps, and not be achieved with tube circuits, but is a feature of the David Berning ZOTL circuit). However, by using the much larger "effective turns ratio" that is provided by the unique ZOTL impedance conversion circuit, much more effective control of the speaker load by the triode is possible, than would ever be possible using any output transformer or traditional OTL ciruits. In fact, with the load line that is in my amplifier, according to the David Berning "White Paper",
"In an audio application, the tube behaves as if it is driving a 4k ohm speaker (the impedance conversion ratio goes as the square of the turns ratio), and the speaker acts and sounds as if it is being driven by a 0.6 ohm triode."
And this, with the use of ZERO FEEDBACK. The natural load line of the triode is sufficient to produce this, when using the ZOTL impedance converter design. So that all the linear transfer characteristics of the triode are preserved, and full control of the load is realized, while exceeding the capability(and eliminating any of the drawbacks) of any output transformers.
So, as you can see, the use of this design allows both a low output impedance AND zero feedback, and at the same time eliminates all the saturation problems and parasitic capacitances of typical output transformers in audio applications, and still allows the use of a single triode(no parallel tubes like traditional OTL) to control the speaker accurately and in conditions of shifting speaker impedances(within reason), based on its inherent transfer characteristics(load line)in conjunction with the Berning ZOTL impedance converter.
While David does give credence to your concerns about shifting impedances causing frequency response problems in certain amplifiers, it is shown that this amplifier circuit is not adversely affected by the things that you mention, and the load lines in the ZOTL circuit are very close to what would be expected from some MOSFET transistor amplifiers(without feedback). However, the sonic attributes of the triodes are preserved, and gives what might be called "the best of both worlds", as the control and speed are very much like solid state, and the transparency, tone, and harmonic qualities are all tube.
So, here we have a tube amp with a single output triode(per channel), no audio output transformers, no feedback, an effective output impedance of about 0.6 ohms, DC coupled and global DC stabilization, bandwith from 2Hz - 500kHz, high-speed switching power supply, RF-frequency tube heating, noise less than -100db, choke loading, DC powered, auto-biasing, brownout protection, and can even be turned on without any speaker attached without damage(just like a SS amp can), and weighs 5 pounds.
And best of all, it sounds like real music when you use it with a good speaker. It doesn't sound like most tube amps, and it doesn't sound like most SS amps. It sounds like the best of both.
Now, by anybody's numbers, a 1:13 ratio(damping factor 13) of output impedance to speaker impedance is going to be well within 1db(and probably within 0.5db) in frequency response variation over the audio range as long as a nominal 8 ohm speaker is used. Now, if you want to drive Apogee Scintilla's <1 ohm load with it, I'll agree that I cannot do it with this amp effectively. For the speakers that are intended for this amp, it will control them admirably and exhibit a quite flat response, and be musical.
Whew! I'm tired, and had about enough of this.
Regards,
Twl
