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| TL071 distortion - bad amplifier design? |
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| AngraMelo:
Hey Spec, I did not have the BC556 on hand, I just ordered some. As soon as they get here Ill make the changes |
| spec:
--- Quote from: AngraMelo on December 12, 2018, 09:01:45 am ---Hey Spec, I did not have the BC556 on hand, I just ordered some. As soon as they get here Ill make the changes --- End quote --- Well done. Practically any small signal PNJT will do for the top of the VAS. Practically any small signal PNJT with a VCE of 45V or over will do for the level-shifter. |
| Mark Hennessy:
As the PDF is still open on my computer... Earlier the importance of PCB layout was mentioned. I appreciate that this is for much later, but the attached screengrab from the book illustrates just one cause of distortion caused by PCB layout - so-called "Distortion number 7: NFB takeoff point distortion" by Self. Quoting the numbers given in the text, the distortion rose from 0.00097% to 0.0027% when making this mistake. We're only talking about a few mm of difference in terms of where the connection was made. Obviously this won't be audible, but as I said in an earlier post, it's satisfying to avoid traps like this when the fix is free and easy, and doesn't cause any other harm. Another example: using the ubiquitous LM3886, it's possible to make a pretty decent amp with 1kHz distortion in the 0.003% region. My lash-up - detailed here: https://www.markhennessy.co.uk/microamp/ - achieves this, despite the crude construction (I used an LM4780, which was a dual LM3886). A colleague designed a PCB to employ a pair of LM3886s, but the distortion was huge - from memory one channel was around 1% and the other was more like 0.1%. He had a hard job believing that the PCB layout could be the cause, despite the differences between channels, which was a pretty major clue, but agreed to humour me and revise the PCB. He incorporated most of my suggestions, but not all. Result: much improved distortion, but still not 0.003% and still different per channel. After some track cutting and bodge wires to implement the last of the changes I suggested we did get it very close to my version in the end. One day I'll write all this up on my website, along with photos and measurements. It can be really quite dramatic - and is solid engineering that is very definitely not in the realm of audiophoolery. Capacitor choice can fall into that category, however. Check out DIY Audio if you want to see folk who believe that properly implemented passive components sound radically different - and if you want to know more about capacitors specifically, seek out Cyril Batemans landmark work on this. All that said, I'll repeat my earlier recommendation - you won't find a better book on the subject than Self's. If you understand the basics of how transistor circuit work, then do yourself a favour and give it a go. Even the old PDFs that can be found on-line are great - later editions expand on the subject, but don't contradict what was originally written, and that's because it was very thoroughly researched right from the start. |
| David Hess:
--- Quote from: Mark Hennessy on December 12, 2018, 08:35:39 pm ---All that said, I'll repeat my earlier recommendation - you won't find a better book on the subject than Self's. --- End quote --- Bob Cordell's book might be better but they are both excellent. |
| Mark Hennessy:
--- Quote from: David Hess on December 13, 2018, 01:31:55 am --- --- Quote from: Mark Hennessy on December 12, 2018, 08:35:39 pm ---All that said, I'll repeat my earlier recommendation - you won't find a better book on the subject than Self's. --- End quote --- Bob Cordell's book might be better but they are both excellent. --- End quote --- I have Bob's book, and agree that it is pretty good. Personally, I think Self's is better; it goes deeper and is more rigorous. And yet manages to be more readable - to me, at least. Individual taste will obviously be a large factor. Of course, Self has had a lot of criticism over the years because of his strong anti-subjectivist stance. Not surprising, I guess. Rational audio engineers will always have an uphill battle when threads like this exist: https://www.diyaudio.com/forums/chip-amps/40676-try-potentiometers.html :palm: |
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