flg,
I believe I experienced this firsthand, just yesterday.
A friend of mine recently acquired 5 TEAC AG-790 receivers for free, and he offered one to me. I wanted to know something about the amp so I downloaded the service manual. I had high hopes, and I was pleased to see a relatively simple design, but was then disappointed when I saw the cascaded differential pair for added gain.
Well I gave it a listen and it surely did disappoint; it was too shrill in the upper mids and highs for my liking. I did notice however, that they are using an electrolytic cap on the input and feedback cap that is not bypassed with a good film. Hard to say if that will cure the shrill or not. Not sure if I will bother trying to replace those caps though; it may just get passed on to the ex wife for her already shrill-sounding Sinclair Brighton speakers.
Attached is the schematic of the TEAC receiver amp. What do you think?
I believe I experienced this firsthand, just yesterday.
A friend of mine recently acquired 5 TEAC AG-790 receivers for free, and he offered one to me. I wanted to know something about the amp so I downloaded the service manual. I had high hopes, and I was pleased to see a relatively simple design, but was then disappointed when I saw the cascaded differential pair for added gain.
Well I gave it a listen and it surely did disappoint; it was too shrill in the upper mids and highs for my liking. I did notice however, that they are using an electrolytic cap on the input and feedback cap that is not bypassed with a good film. Hard to say if that will cure the shrill or not. Not sure if I will bother trying to replace those caps though; it may just get passed on to the ex wife for her already shrill-sounding Sinclair Brighton speakers.
Attached is the schematic of the TEAC receiver amp. What do you think?
Attachments
I concur, indeed it is obvious. But my question was not intended to imply the triode does and the pentode does not, but that isn't it obvious that they all do? In going into this question, despite that Zen informs me it is a well picked over discussion, we can try for an answer to a not so obvious question. Namely, what is the difference between a JFET and a SIT?
Thank you all, I'm really enjoying the great dialog on the many issues about negative feedback that just exploded in this thread. It's really helping me frame my thinking about Part III.
Thank you all, I'm really enjoying the great dialog on the many issues about negative feedback that just exploded in this thread. It's really helping me frame my thinking about Part III.
Indeed there is. That is certain.
It is obvious though, at least I thought?
The light is on
1. Black invented negative feedback as a tool to make a "perfect" power amplifier. By comparison, Schade developed [or invented] the Beam Tetrode as a "perfect" amplifying power tube. These 2 scientist pursued different objectives.
2. Black did not disclose in his patent one important consequence [or value] attendant to the use of negative feedback. Schade picked up on this one deficiency; but had to use Black's negative feedback in order to demonstrate it, and/or recognized it when it fell out inadvertently [invention, or pleasant discovery] from his studies.
3. Black did not declare in his patent that his negative feedback converted the characteristic performance of a tetrode [in a single tetrode power amplifier] to the characteristic performance of a single triode power amp.
4. Black did not show in his patent a figure like Fig. 35 in Schade's paper; which was an important consequence or fallout of his negative feedback tool.
5. Fig. 35 due to Schade in his paper was a consequence of his "discovery", and not Black's.
6. Schade's legacy will always be " Otto H. Schade used the tool of negative feedback invented by Harold S. Black to convert the typical or established characteristics of tetrode and pentode tubes {voltage variable current sources} to the characteristics of a triode which is a voltage variable resistor by comparison."
7. Schade stood atop Black's shoulders and used Black's tool of negative feedback to invent [no patent] another valuable tool we now call Schade Feedback.
1. Black invented negative feedback as a tool to make a "perfect" power amplifier. By comparison, Schade developed [or invented] the Beam Tetrode as a "perfect" amplifying power tube. These 2 scientist pursued different objectives.
2. Black did not disclose in his patent one important consequence [or value] attendant to the use of negative feedback. Schade picked up on this one deficiency; but had to use Black's negative feedback in order to demonstrate it, and/or recognized it when it fell out inadvertently [invention, or pleasant discovery] from his studies.
3. Black did not declare in his patent that his negative feedback converted the characteristic performance of a tetrode [in a single tetrode power amplifier] to the characteristic performance of a single triode power amp.
4. Black did not show in his patent a figure like Fig. 35 in Schade's paper; which was an important consequence or fallout of his negative feedback tool.
5. Fig. 35 due to Schade in his paper was a consequence of his "discovery", and not Black's.
6. Schade's legacy will always be " Otto H. Schade used the tool of negative feedback invented by Harold S. Black to convert the typical or established characteristics of tetrode and pentode tubes {voltage variable current sources} to the characteristics of a triode which is a voltage variable resistor by comparison."
7. Schade stood atop Black's shoulders and used Black's tool of negative feedback to invent [no patent] another valuable tool we now call Schade Feedback.
For tubes, yes to varying degrees. Remember it was the diode, then triode, then tetrode, then pentode (then beam tetrode?). The tetrode and pentode were developed essentially to isolate the electric field of the plate from the grid, and while there can never be 100% shielding, for all intents and purposes, there is no internal Schade feedback in tetrodes and pentodes (unless they are triode or UL connected). For triodes, the feedback is inherent by its design. I suspect SITs and V-FETS also have some feature of the design that permits the drain to influence the gate to a certain degree.
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I concur, indeed it is obvious. But my question was not intended to imply the triode does and the pentode does not, but that isn't it obvious that they all do? In going into this question, despite that Zen informs me it is a well picked over discussion, we can try for an answer to a not so obvious question. Namely, what is the difference between a JFET and a SIT?
Thank you all, I'm really enjoying the great dialog on the many issues about negative feedback that just exploded in this thread. It's really helping me frame my thinking about Part III.
Please include; if applicable the difference between a SIT and a JFET operating in Schade Feedback mode.
Well, the question appears to be teed up well with your nice summary, thank you.
I'm looking forward to working on this some more to see what I might be able to add in Part III from the field of electrostatics. That branch of electro-physics was clearly understood and used by Schade in his work, but essentially did not concern Black in his. And, in the end, electrostatics is what the designer of a power gain device (tube or transistor) deals with to achieve what we see at the terminals or in the datasheet. Most of what can be known about the influence of the plate or the drain or the collector on the gated portion of the device will be determined by the electrostatic structures internal to the device.
Thank you Dr. Mazzola for your comments. I am glad to be one of your students, and also a student of Mr. Pass, and Zen Mod , and many more great educators/contributors in/to the diyaudio forums.
Ultra-linear refers to the use of feedback from an output transformer tap direct to the screen grid of the beam tetrode or pentode output driver. Wikipedia says it is a special case of "distributed loading" taught in much earlier patents, but when a specific amount of this feedback results in a "sweet spot" like effect it is called ultra-linear. An example can be seen in the ubiquitous Dynaco ST 70 tube amp.
And I'm havin trouble with these way old patents. This covers it also
Patent Specification No. 496,083
Patent Specification No. 496,083
[QUOTEg=flg;4035670]may try US patent 2218902 Alan Blumlein[/QUOTE]
Thanks flg. It works using PAT2PDF - Free PDF copies of patents: Download and print!
Thanks flg. It works using PAT2PDF - Free PDF copies of patents: Download and print!
Thank you all for your inputs and information. More homework.
In an earlier conversation between Dr. Mazzola and poynt99, an intriguing point came about regarding Schade Feedback emanating from within a triode during its operation. I understood it as such, and did not want it lost. There is an inherent internal resistance or impedance between the plate and the grid; which is a straightforward pathway for an inverse voltage feedback [plate to grid] with a performance value like that from the external one taught by Schade for tetrode.
Is this inherent Intra Schade Feedback which gives a triode its characteristic of a voltage variable resistor?
In an earlier conversation between Dr. Mazzola and poynt99, an intriguing point came about regarding Schade Feedback emanating from within a triode during its operation. I understood it as such, and did not want it lost. There is an inherent internal resistance or impedance between the plate and the grid; which is a straightforward pathway for an inverse voltage feedback [plate to grid] with a performance value like that from the external one taught by Schade for tetrode.
Is this inherent Intra Schade Feedback which gives a triode its characteristic of a voltage variable resistor?
The short answer is, yes!
We often draw lines around the device and act as though what goes on inside is different from what goes on outside. Sort of "out of sight, out of mind." But the potential across the terminals of the device that is driving currents in the external parts of the circuit exist within the device as well. Device designers now a days use sophisticated two and three dimensional computer-based solvers of the so-called Poisson equation to understand the electrostatic distribution of the voltages within the device. The current flows (electrical charge transport) are modeled most often with something called the continuity equation.
Basically, all internal parts of the device typically have some influence, big or small, on all other internal parts of the device, which is managed by the device designer to produce a set of terminal characteristics that meet some specification (and that can be sold into applications). By device I mean a transistor or a vacuum tube.
We often draw lines around the device and act as though what goes on inside is different from what goes on outside. Sort of "out of sight, out of mind." But the potential across the terminals of the device that is driving currents in the external parts of the circuit exist within the device as well. Device designers now a days use sophisticated two and three dimensional computer-based solvers of the so-called Poisson equation to understand the electrostatic distribution of the voltages within the device. The current flows (electrical charge transport) are modeled most often with something called the continuity equation.
Basically, all internal parts of the device typically have some influence, big or small, on all other internal parts of the device, which is managed by the device designer to produce a set of terminal characteristics that meet some specification (and that can be sold into applications). By device I mean a transistor or a vacuum tube.
It would be trivial to turn the second diff pair into a cascode for the first
diff pair. I bet that would improve things.
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