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| TL071 distortion - bad amplifier design? |
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| Zero999:
--- Quote from: Kleinstein on December 17, 2018, 10:57:46 am --- --- Quote from: Hero999 on December 17, 2018, 10:44:41 am ---........ --- Quote ---The more radical way would be to have one side a fixed current source and only use the OP to drive one side. Power supply suppression would still be a problem. A transistor in base configuration could help to transfer the signal as a current from ground references (at the OP) to supply referenced for Q5 or Q6. --- End quote --- Similar to what speck proposed earlier on in the thread? https://www.eevblog.com/forum/beginners/tl071-distortion-bad-amplifier-design/msg2030107/#msg2030107 --- End quote --- Yes exactly that kind of circuit I was thinking of. Somehow forgot it was already in this thread. --- End quote --- It should work well with local feedback. I haven't simulated the original version, but my gut feeling is it will oscillate and be difficult to stabilise, as it has a huge gain, on the order of 20k. Local feedback sorts that out. |
| Zero999:
--- Quote from: AngraMelo on December 17, 2018, 07:04:33 am --- --- Quote from: spec on December 17, 2018, 06:47:47 am ---AngraMelo What are your plans now? I suggest at this stage that the way forward is: [1] Correct/develop the TL071 amp and get it performing as well as possible to the latest mod standard that I posted. [2] Build a Self Simple Blameless Amplifier (SSBA). In regard to the actual build of the SSBA you have a few options: [1] Knife and fork [2] Your own PCB layout [3] Stock PCB [4] Stock SSBA kit [5] Ready built SSBA [1] By hand wiring using copper/brass strip/wire for the relevant signal paths, you can make a superb SSBA, but it would be a load of work and development [2] Like [1] above you could produce a superb board, but once again it would involve much work and development. And then you have to have the PCB manufactured. [3] Last time I looked there were a load of SSBA (or similar) boards on the market. [4] As [3] [5] As [3] I would recommend [5] from a practical point of view, but as you have said that you are in this for the experience, rather than to just have an amplifier, so perhaps [3] or [4] would suit. By the way you have already got eminently suitable output transistors, or would you like to buy another pair of output transistors for the SSBA? --- End quote --- I would like to go for option one!! As I friends says: aim on Mars to hit the Moon! hahahaha I know it will be a pain in the butt but hey, Im here for the ride! If it proves a challenge to far away from my paygrade lets try option 3/4 Ill send an email to the Signal Transfer Company and see about the shipping fees, they appear to be a Licensed dealer of the boards, if they are not please let me know so I can look for someone who can actually sell them. They are a bit pricy when doind the currency exchange (around double what I pay for my electrical bill per board not including shipping), so if I could etch them myself it would be awesome! Also, lets to the TL071!! --- End quote --- Sorry for taking your thread on a tangent and allowing myself to become distracted. The SSBA is a good design and is ideal for the beginner as it allows one to see how it works. Performance wise, I'm not convinced it'll be any better or worse than an op-amp+discreet power stage (I'm talking about a better design than the TL071 posted here of course) but that doesn't matter. The main advantage of a discrete op-amp, which is what the SSBA is, it's transparent i.e. the builder, who can probe and measure every single point in the circuit. An IC op-amp is just a black box, with no way to see what's going on inside. Actually I'd recommend building a smaller, lower power discrete op-amp, before attempting the SSBA, simply because it'll be easier. I'm thinking of driving an 8Ω load with <1W or even a basic op-amp circuit which has no practical use, other than to demonstrate the concept. Did you ever get the TL071 circuit working properly? If not, how about implementing the minimum number of modifications required to get it to work properly? |
| Kleinstein:
--- Quote from: Hero999 on December 17, 2018, 12:44:27 pm ---........ It should work well with local feedback. I haven't simulated the original version, but my gut feeling is it will oscillate and be difficult to stabilise, as it has a huge gain, on the order of 20k. Local feedback sorts that out. --- End quote --- The local feedback would likely still need some compensation (e.g. a miller-capacitor at the VAS). The high DC gain is usually not a problem. The problem would be more with to much bandwidth or too much gain in the MHz frequency range. For such an amplifier circuit it is a good idea to start with a lower power (e.g. lower supply voltage) version, to limit the release of magic smoke. However it would not simplify the circuits shown so far. The SSBA circuit can also work with a lower voltage with only minimal changes. It is more like one might consider additional transistors (e.g. cascode for the input stage) if the supply is above 40 V, so that one does not need high voltage rated transistors there. |
| David Hess:
--- Quote from: Hero999 on December 18, 2018, 12:11:45 pm ---The SSBA is a good design and is ideal for the beginner as it allows one to see how it works. Performance wise, I'm not convinced it'll be any better or worse than an op-amp+discreet power stage (I'm talking about a better design than the TL071 posted here of course) but that doesn't matter. The main advantage of a discrete op-amp, which is what the SSBA is, it's transparent i.e. the builder, who can probe and measure every single point in the circuit. An IC op-amp is just a black box, with no way to see what's going on inside. --- End quote --- The SSBA topology duplicates the most common operational amplifier implementation so understanding it goes a long way to understanding most operational amplifiers. The TL071 could be used in a much better design if applied correctly. I posted above what I would do although it violates unloading the operational amplifier's output. |
| spec:
--- Quote from: Kleinstein on December 17, 2018, 08:24:42 am ---after the corrected annotations. --- End quote --- The annotations have not been corrected. I posted the circuit for the SSBA in reply #100: that is the reference and has not changed. But what I am going to do is remove the annotations from the original schematic in reply #100 to avoid any further controversy. |
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