Discrete audio amplifier project PT16 Negative feedback and stability

[@martinda7446]

That was a fine introduction to the subject.

[@MrAlFuture]

This was a great introduction too the topic. Thanks, John!

[@nowt1002]

Very good introduction to the subject. Nice and easy to follow.

[@bigbread9000000]

I bought the Bob Cordell book, it is at a much higher level, with that being said, I still am still learning a lot between reading his book and watching your video's!! Maybe at some point you can make a video about the Zobel network, it's purpose in the output stage of the amp!

[@ats89117]

Nice video, but the conditions you give for stability are necessary but not sufficient. You also need the slope of the phase line when hitting the zero gain line to be less than 40 dB per decade and preferably closer to 20 dB per decade. This is what drives the need for compensation circuits to have a zero at a frequency above the added pole frequency. This gives faster roll-off for a while from the second pole but then slows it back down to somewhere between the tow pole and single pole response before hitting the zero gain line.

[@JohnAudioTech]

Good info. Thanks for sharing.

[@joeyscott4299]

Excellent information ! Thank you John . I've been working on understanding negative feedback and this made things a bit more clear . Take care !

[@demoncloud6147]

Great video, even hater couldn't dislike ! I wonder why my university professor couldn't explain this way.

[@38911bytefree]

Brilliant ..... really food for thought

[@Helectronics]

Very informative and not hard to understand, thank you! Keep up this good work.

[@diabolicalartificer]

Thanks, great video, it answered a lot of questions I was puzzled about, IE poles, CL phase etc. BTW how would one draw a Bode plot for a real would amp? Measure gain and phase on a scope at various frequencies? Keep up the good work, DA.

[@johnyang799]

Simply most people don't measure them. To acquire bode plot, for chip amps you can directly get from datasheet. For discrete amps, it's generally used in simulation. Like ltspice Ti-NA (pspice based simulators). Tweak on the simulation and build to test stability using square wave and capacitive load(john has video on this). If the amplifier is stable under most conditions it's not needed to measure open loop gain etc. (and it's actually really hard to measure)

[@wagsman9999]

Great explanation - thank you!

[@lex_parsimoniae]

I like your cat.

[@aurthorthing7403]

I think you should add an opamp preamp with some EQ filtering.

[@markweikle5645]

Great job, thanks!

[@cunningham.s_law]

do you have videos on building eqs or reverb machines?

[@DAVIDGREGORYKERR]

has anyone tried using a long tailed pair based on the BC182L driving two BD131 transistors if you don't know what I am driving at google 5W Class A power amplifier, the long tailed pair replaces the BC212L PNP transistor.

[@johnneybot9542]

I heard Snickers ? Where is he? You forgot to mention tolerances in components like those resistors will make a difference to the overall gain

[@peteb2]

When i heard... "OK, it's favourite time".... why'd i expect The Snickers to come into shot?

[@sdgelectronics]

Meow!

[@johnyang799]

Helloooooo.

[@ahmedalshalchi]

Oh my GOD ... Steve became a cat ?!!! ... Oh no ! ... I thought him a lion , a predator , a butcher in electronics not a cat !...

[@steveswan5714]

John don't know if you can help but I have fault on my old MOSFET guitar amp it runs from + - 45v power but the speaker output has +40v but after a few seconds it falls away I can't find a schematic for it on line, tested all semiconductors and can't find the problem its a Dean Markley amp thanks for the teaching bids 👍

[@martinda7446]

Sounds like the amp is DC coupled. There seems to be an issue with power supply balance, one rail is stabilising faster than the other. This will cause a large offset at the output until both stabilise. If the amplifier works after it falls away then its most likely a power supply capacitor on one rail that is tired and leaky. If it doesn't work at all then....tell us the symptoms...

[@steveswan5714]

@@martinda7446 hi Martin thank you for that 👍 I first suspected the power supply as when switching off the amp the its seams to work OK for a few seconds until the caps drain ? another thing I found was if I put my finger on the + pre output transistor I can pull the voltage at the output almost to zero ! I replaced the bridge rectifier just to be sure it was not at fault

[@martinda7446]

@@steveswan5714 The problem with DC coupled amps is that any issue anywhere shifts voltages all through the amplifier. I'm gonna have a look for Dean Markley amps...I've never heard of them?

[@steveswan5714]

Hi Martin thanks again for spending your time on this 😊 I found that schematic to and was using it to try some diagnostics then I noticed it is for a tr version I have MOSFET one although I think they are similar to some extent both have TL02 op amp, I have fitted a socket and swapped out the op amp no change will I'm not sure if there is zeners in this but can't tell as no marking due to age ? Will have another look regards Steve

[@JohnAudioTech]

Hard to offer reasonable help without a schematic. Amplifiers like the one I'm building or most dual supply chip amps will keep the output offset zero even with badly offset supply voltage but that may not be the case for your amp. So if the supply voltages check okay at power up, I'd trace the offset back through the circuit. EDIT: I found a couple replies in the spam folder after I posted this (now released). YT likes to stuff posts in there for any reason.

[@johnyang799]

Good talk. Could be more concise tho.

[@diabolicalartificer]

John Yang I think the purpose of the video was a brief overview intentionally simplified and less concise in order not to confuse neophytes like myself. Have you done a video on this subject yourself BTW?

[@johnyang799]

@@diabolicalartificer No i haven't. This is pretty good. I love John's videos. It's just seem to be a little long and not really much in there. I get that it's for beginners and is supposed to be as simple as and as intuitive as possible. I look forward the next videos.

[@johnyang799]

@@diabolicalartificer For example I would love to see the bode plot a bit earlier. And a few equations would be help as well.

[@johnyang799]

@@diabolicalartificerThis video can be helpful kzbin.info/www/bejne/kIirhJRvrNKceqs this one as well kzbin.info/www/bejne/h3msn5ykdsxnf9U

[@diabolicalartificer]

@@johnyang799 Yep, know what you mean, good in-depth video's of some of the more advanced aspects of audio amp design are thin on the ground. I've found several Indian lectures on the subject but not having been to college or Uni they are too advanced. Also not found many good boks on the subject, at least ones within my budget.