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My attempts at a design of a 3 stage amplifier
My attempts at a design of a 3 stage amplifier
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Old 24th June 2019, 07:03 PM   #561
Mooly is offline Mooly  United Kingdom
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My attempts at a design of a 3 stage amplifier
Was it this one (post #404)

My attempts at a design of a 3 stage amplifier
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Old 24th June 2019, 09:36 PM   #562
edbarx is offline edbarx  Malta
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Simulations with the servo posted by Mooly worked. Again, the very good distortion figures did not suffer as a result of this addition. The short circuit current is composed of triangular pulses with a base of 23uS and peak of 33A. The frequency of these pulses is 1000Hz. With higher frequencies the triangular base is shorter but the frequency is higher. It is indicative at high frequencies short circuiting the output has a higher probability of causing damage. For this reason, caution can never be ignored.

I am attaching the latest version of the schematic. The good news about this is the actual physical circuit only needs additions with no deletions. I made a big effort to achieve that goal. The actual physical circuit works and it would be a real pity to dismantle it.
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File Type: asc 202019_06_24.2_final_amplifier_circuit.asc (18.4 KB, 17 views)
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Old 26th June 2019, 03:31 AM   #563
mjona is offline mjona  New Zealand
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If a Thiele stability network is a must have then C10 at 1uF is ten times what it ought to be. 1kHz is a much easier test frequency for THD than 20kHz the loading on your amplifier will be increased by 20 times due to the reduced impedance of C10 at 1uF.

I have suggested getting rid of the 1uF for that reason on more than one occasion. You should test for THD at 20kHz and run a 10kHz square wave test as well before going ahead with your build.
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Old 26th June 2019, 05:05 PM   #564
edbarx is offline edbarx  Malta
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The 1uF accounts for the worst case scenario of powering a speaker with very long speaker cables. In my very first posts of this thread, I was warned that with the kind of voltages used in this amplifier, when using very long speaker cables, amplifier can self oscillate. So, the 1uF is not part of the circuit, but it represents the speaker cable's parasitic capacitance.

The THD is often quoted for 1kHz but I am also simulating for 16kHz to calculate the THD at the topmost end of the human acoustic spectrum. These figures are higher than what is obtained at 1kHz but they are still below 0.2%.

The square wave test was also conducted. Since there is a parallel inductor resistor network in series with the load, there is some ringing but not at RF frequencies. I remember, earlier in this long thread, similar charts have been judged as good results.

I would like to ask some electronics guru who deeply understands the behaviour of real life components, whether using an opamp to correct the DC offset voltage, would also correct the stray voltages that caused the crackling and the mains hum/buzz? I remind any readers that the latter imperfections were quite weak.

My reasoning goes something like this. The hum, buzz and crackling are not part of the signal. So, they are not accounted for in the input stage and should be seen as extraneous voltages without an input drive. Because of this, they should also be cancelled by the opamp.

I remind readers I am fallible, and my insight might not be as deep and refined as reality.
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Old 28th June 2019, 10:35 AM   #565
Holmer is offline Holmer
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A DC servo does not correct output deviations from the input signal, but from Zero Volts. Its reaction time it slowed down to seconds on purpose, or it would ‘correct’ the music as well, leaving nothing.

So no, it cannot reduce crackling and buzzing, it cannot distinguish them from (wanted) music. It just keeps the _average_ output voltage at zero.

What you mean is error correction, or just negative feedback.
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Old 28th June 2019, 11:23 PM   #566
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by edbarx View Post
The 1uF accounts for the worst case scenario of powering a speaker with very long speaker cables. <snip>
The servo will not resolve the issues you have raised already pointed out. It is good to see you have abolished the zener diode reference for the cascode in the LTP in the simulation and replaced that with a resistor with a parallel capacitor. If you have not implemented that in the actual hardware you should test to see what difference that makes. The way you drew up the circuit suggested the zener current flowed through the earth reference point for the nfb decoupling arm to earth which is not in keeping with best practice.

The worst imaginable scenario is that of having the transformer centre tap to earth or the supply current to earth currents flowing through the input earths for noninverting and inverting LTP as these will result in the sorts of results you describe.

The correct way to wire up an amplifier has been the subject of recent discussion here -How to wire up an Amplifier
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Old 29th June 2019, 09:14 AM   #567
edbarx is offline edbarx  Malta
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Quote:
Originally Posted by mjona View Post
It is good to see you have abolished the zener diode reference for the cascode in the LTP in the simulation and replaced that with a resistor with a parallel capacitor. If you have not implemented that in the actual hardware you should test to see what difference that makes. The way you drew up the circuit suggested the zener current flowed through the earth reference point for the nfb decoupling arm to earth which is not in keeping with best practice.
The mentioned changes has been implemented in the hardware. According to my hearing, it seems, the issues have been attentuated, but I am cautious to be objective. Confirmation bias is something I want to guard against.

Quote:
The worst imaginable scenario is that of having the transformer centre tap to earth or the supply current to earth currents flowing through the input earths for noninverting and inverting LTP as these will result in the sorts of results you describe.
The zero volt terminal from the power supply is the terminal that conducts most current. This means, hum and buzzes are superposed on the theoretical 0V. For this reason, all decent electronic textbooks introduce novice learners to the practice of using a star ground with no loops. The 'centre' of the star is the point where a power supply feeds its current; the many star arms are used for the various subcircuits including signal subcircuits. To avoid coupling of hum and buzzes, the centre point is also where an electrolytic capacitor is connected. The same technique is used for the supply rails. Relatively long tracks are further supply decoupled using small electrolytic capacitors, which may also have pF ceramic capacitors in parallel.

Added Later; an anecdote:
Throughout this journey, I was always curious to understand, why the input stage's current source experiences modulation of its output current. Yesterday, a bell rang in my not so alert mind, that the differential pair's emitters follow the input signal voltage. This means, if a signal is 2V peak to peak, the differential pair's emitters also experience a 2V peak to peak voltage swing. This swing effectively changes the voltage seen by the current source's collector resulting in a collector current modulated by emitters_voltage_swing/Ro, where Ro is the current source's output impedance.

The most vexing analytical complexity of transistors, is the fact, most parameters are non-linear, and our human brain, is not very well equipped to predict the value of non-linear relationships. The transistor itself is an exponential device with currents starting to flow from well below the greatly simplified 0.7V of Vbe.

Something I would like to ask is whether bipolar transistors operate using the curious Quantum Mechanical effect of tunnelling. A bipolar transistor is a silicon die with three distinct 'impurity' diffusions with the base region having the least of diffusions. The signs of diffusions are such that two depletion layers are formed, one at the boundary of the emitter and base, and the other at the boundary between base and collector. The polarities are such that the equivalent of two diodes back to back are formed. So, under normal conditions, no current should reach the third layer from the first. However, and this however is very curious and interesting, when a small forward voltage is applied to the emitter base region, electrons will be accelerated into the base gaining kinetic energy, that is enough for them, to shoot straight into the collector region, notwithstanding the presence of an inhibiting electric field. According to what I can recall from Physics, this is Quantum Tunnelling. Am I correct?

Last edited by edbarx; 29th June 2019 at 09:26 AM.
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Old 30th June 2019, 07:40 AM   #568
mjona is offline mjona  New Zealand
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I resolved all the earthing issues I had with the methods described in the link provided. With a magnetic cartridge and full volume there is no audible hum at all.

Modern transistors are planar. As I understand it a PN junction would be made by diffusing a P impurity material into a wafer of N type material so where these doped areas of material come into contact you will have a greater amount of P type silicon on one side of a boundary and a greater amount of N type silicon on the other.

Thus being in intimate contact electrons will diffuse across the boundary so the Fermi levels on both sides are equal causing a relative shift in electrical potential between the two sides.

For the rest on what happens as a consequence of this there are plenty of resources you can study on the web that you can access for further information.
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Old 30th June 2019, 04:05 PM   #569
edbarx is offline edbarx  Malta
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Sadly, the web has become an unreliable resource for objective scientific research and learning. Filtering search results, where permitted, can help, but if a learner needs advanced texts, one is faced with getting only opinions, and very basic material. Please, do not suggest wikipedia, or similar open resources that can be edited by anyone. These are very very unreliable. It seems to me, notwithstanding of the advent of the internet, one is back to square one, when the internet did not exist: only good quality books from reputable publishing houses can guarantee high quality scientific and technical texts.
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Old 1st July 2019, 02:15 AM   #570
mjona is offline mjona  New Zealand
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Try this link to get started on your journey and check out "the read more prompts" if you don't get what you want use the information you see to search further into the topic from another source Transistor Structure | Transistor Fabrication | Electronics Notes

There are papers from researchers at universities and technical institutes.
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