L12/2 version 5. The Ugly Truth. FULL CIRCUIT DIAGRAM.

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@MichaelBeeny 19 сағат бұрын

I commented in the video that this was a true DC amplifier. As a few of you pointed out it's NOT. What I meant to say is that the frequency response goes down to almost DC. FLAT. It's quite hard to measure the last few Hz but as far as I can measure it simply does not roll off at all. Sorry If I miss led you. I will show this on the next video.

@cobar5342 Күн бұрын

Well done Michael. Whilst frustrating, attempting to solve a problem is a joy in itself. Even if the result is not what one wants. Good on you and Ron

@stickmenwithrayguns Күн бұрын

Here's a couple of suggestions to be implemeted in the mentioned order until the problem is "hopefully" solved.: 1. Replace the R1 ( 2.2K ) input resistor with a 0.47uF MKT capacitor ( This is not a DC coupled amplifier anyway. ) 2. Q9 & Q10 are serving as capacitor-multipliers stabilizing the voltage to the differential-amp & voltage driver stage. Adding 100nF MKT capacitors parallel with C5 & C8 + C6 & C9 will not hurt. 😉 3. The voltage across Q12 is very small ~1.2V This is mainly due to the Quasi config used by Q11 & Q13. This Vce voltage of Q12 could in fact be too small to be stable. 🧐Start removing C7 from the VBE multiplier. The current in R14, R15, R16 is far too low in my opinion. I suggest replacing these resistors with ones that are 1/10 the value => R14: 560 ohm, R15: 470 ohm, R16: 220 ohm trim. ( This will allow the current in the resistor network to be much higher than Ib @ Q12 thus enhancing stability. ) Adjust the trim-pot in order to obtain ~20mA thru the power stage without an input signal. 4. The circuit is using Quasi-coupled transistors in both differential amp and the power section. 🤨This structure can cause oscillations. ( try to replace Q1 & Q2+ Q3 & Q4 with 2 normal NPN Darlington transistors. ) 5. For some strange reason the high side current source to the differential stage and the "voltage driver stage" current source shares Q6, R12 & C2 as stabilizing elements with only 1.2V across Q6. This provides a constant current of 4mA to the diff-stage & around +30V on the high side of R11. This may work as intended, but the usage of "shared regulators" could potentially create an unwanted signal path.🤔 Start with a 100nF MKT parallel with C2. If this doesn't help proceed changing R11 to 220ohm and connecting it to a separate adjustable voltage of ~ 1.2 Volt lower than the emitter of Q9. ( F.x. a 3.3V zener parallel with a 100nF MKT and 5K trim + 22K to ground. ) Connect the center-pin of the trim to the modified R11= 220 ohm and adjust the current thru R13 to ~10… 15mA. IMPORTANT. It's crucial that you have completed "Step 3" so that current in your drivers Q11, Q13, Q14, Q15, Q16, Q17 is less than ~20mA ! Use a current limited power supply set to max 50mA ! ) 6. The gain is controlled by [ R17 / R18 ] with C3 limiting the DC gain = 1 and -3dB @ ( 2*pi*330 ohm*1000uF)^ -1 = 0.5Hz. Changing C3 to 47uF bringing the -3dB up to 10Hz, would be a more conservative choice. ( The gain should be less than 30 with the input being maximum 2 Vpp to avoid clipping. ) While tinkering you should replace R18 with a 560 ohm resistor. ( I guess you could make this into a real DC coupled amp by shorting C3, but you will have to add "offset adjust" to get 0V output at idle, which this circuit doesn't have. ) 7. Q18A + Q18B is a Widlar current mirror. ( no remarks here. ) This is a very simple amplifier yet with some strange approaches and also cutting corners. 😬 IMHO. Don't buy this design. If you want something that is more thought thru, get the open-source Beta22 either as a kit or semi-finished product that only needs a power supply. 👍

@ScottGrammer Күн бұрын

I would also suggest an output inductor before the R30/C10 stabilization circuit. A few microhenries in parallel with a 10 ohm resistor should suffice.

@Poult100 Күн бұрын

It will be interesting to scope around the circuit to see if the source of the instability can be found. Watching with interest!

@ronschauer839 Күн бұрын

In process... 😁

@barrymayson2492 Күн бұрын

I am a rf circuit guy so think differently but I would definitely get a sensitive probe on a SA you might see something or not but hf oscillating in tight circuits is definitely a thing I watch for. After getting bit a few times. The small handheld SA are brilliant at bringing higher testing facilities to us mortals. Have several bigger SA but the little one is always my go to. Good luck with finding the problem.

@trsconsulting791 Күн бұрын

Hi Michael, try installing a small capacitor around 10pF across R17 to limit out of band HF gain. You could also try a small capacitor around 68pF from Base to Collector of Q7. The suggestions below are excellent, the Quasi-coupled transistors in the differential input stage is unusual and could be problematic. The addition of a choke in the output as suggested is a good practus.

@Douglas_Blake Күн бұрын

Exactly ... Looking at the scope traces we can see very high frequency oscillations on the test signals. These fuzzies are high frequency oscillations under load. Bypassing R17 with a small value cap will increase feedback, reducing gain at higher frequencies which, fuzzies or no, should improve that amplifier's stability quite noticeably. As a suggestion, they could look into various values, with an eye to limiting the bandwidth to about 30khz. It might also help to bypass C3 with a ceramic cap to compensate for value reduction at higher frequencies. I also agree that a more or less standard Zobel filter at the output would probably help.

@MichaelBeeny 19 сағат бұрын

Chokes will be added in due course; however, it will not help in this case. At present I'm only feeding resistors as loads. This would only help when feeding real loudspeakers. PS we are almost there!! watch this space.

@stickmenwithrayguns 16 сағат бұрын

@@Douglas_Blake Indeed. 👍It's a good idea to limit the bandwidth both up and down. 🤗For audio purposes nobody needs anything outside audible range. In order to eliminate unwanted oscillations you will foremost need to place some "HF limiting elements" in the feed-forward loop like C4 on Q8. You could apply the same method ( just with with bigger values ) to Q11 and Q13. If HF-limiting elements is placed only in the feed-back loop, the amplifier could become incapable of handling "out of band" noise because this will be undetected. 😕

@stuartwadsley Күн бұрын

Hi Michael, I have now received two in kit form and will hold off building them up until I hear more, hopefully some good news. Thanks for your efforts.

@MichaelBeeny 19 сағат бұрын

We are getting there Stuart, I don't think you will have long to wait.

@ronschauer839 Күн бұрын

*Thanks very much Mike.* 👍👍 Reducing the gain from its "as-shipped" 31X down to around 11X by means of some simple resistor changes decreases the distortion levels at ANY output amplitude (even just short of clipping levels) by around 70%. More on that later. The instability of the QI circuit (and it is *very* unstable indeed) can be improved with a few changes. Unfortunately, although they appear simple on paper they are somewhat complex to perform on the actual PC board. This is because of the convoluted board layout. There is yet another relatively simple change that appears to get rid of MOST (but not all) of the general board instabilities. However this needs quite a bit more testing. Finally, even more reductions in distortion can be had by another handful of simple component value adjustments. More on this later as well. Please stay tuned... Oh, and *IF* you can find version 3 or version 4 boards, our best advice at this point would be to purchase them rather than this version. Be very careful though because many sellers (such as on eBay) are showing version 3 or version 4 boards in their listings. However, if you then order them you are very likely to receive version 5 boards instead. So be certain to ask the seller what version they will ACTUALLY be shipping before hitting the "buy" button. *Buyer beware..!*

@ivanlukic2360 Күн бұрын

Greetings from Serbia. I see that this v.5 doesn't have 100R base stoppers at the driver transistors. In the earlier versions, which were more like straight copies of Doug Self's Load Invariant amp, these resistors provided some kind of stability. In my opinion this is not ideal solution depending on combination of used semis and does not provide unconditional stability. These Sziklai (CFP) amps are very tricky concerning stability. As I explained on other forums, all you need to do is to put 220pF cap between B and C of PNP driver transistor (in this case 2SA1837). Everything will work just fine.

@Buzz_Purr Күн бұрын

Thank you for posting the new schematic! I noticed these differences with V4: - The base of Q5 is now decoupled by C2, fixing a mistake in V4. - The clamping diodes on the collectors of Q18A and Q18B have been removed. - R9 is now 150Ω (was 100Ω in V4) so the current in the differential input-stage is lower. - Q9 and Q10 in the supply rails are a valid way to filter out noise, in my opinion. - Not decoupling the main power rails is outrageous! - R21 and R26 are now 33Ω (was 100Ω in V4), so the current in Q11 and Q13 is higher (ok, I guess). - Q11 and Q13 used to have a base resistor of 100Ω, for high frequency stability! - The input transistors Q1 ~ Q4 are now in a very different configuration. In V4, there were two single input transistors with two cascode transistors. This forms a pure current output, feeding into a current mirror. In V5 however, Q1 and Q3 are kept at a constant current, increasing the input impedance. Q2 and Q4 are emitter followers, feeding into a current mirror! (Q18A and Q18B) That seems pretty bad to me, the emitter of Q2 looks straight into the miller capacitance of C4/Q8. It would make much more sense to replace Q1 ~ Q4 with a pair of PNP darlingtons. Then you would have pure current sources feeding into the current mirror again. ======= Lowering the current in the input-stage may make the clamping diodes obsolete. But where does the amplifier clip now? R5 and R6? That would limit the output swing and increase distortion prematurely. ======= You're right, version 5 seems to be a failed experiment.

@fredcailloux2526 Күн бұрын

Thank you for your work Ron. I didn't build my kit since I can't match with my signal tracer the transistor he came with. Now if Michael thinks the result is atrocious and the PCB layout is bad there is no magic solution for this. Spending money on a piece of "Turd" for fun? Maybe, but I'm not in a hurry...R.I.P LJM L12-2 Ver 5

@ClausDreisig 2 сағат бұрын

Hi Michael and Ron. I did some simulation on loop stability using the schematic you have shown in the video. I have used LTSpice and the Tian probe method. Usually close to reality. My initial investigation have shown that the amplifier without an output coil (I chose 800nH) can be or are close to unstable. Without the coil the amplifiers gain margin (8ohm) is 11dB and phase margin is 85 degree. With the coil gain margin is 20dB and phase margin is 74 degree. If you load the amplifier with 8ohm/1uF the amplifier reacts wildly. This amplifier badly need an output coil in order to garantee stability. There could of course be other problems like paracitic oscillations, but at least put an 800nH output coil//3.3ohm eg. on the speaker terminal. If I can help with info on the open loop issue, please say so. BR/Claus

@andymouse Күн бұрын

Well to my untrained eye there is nothing I see that is 'special' I would note the absence of a 'Zobel network' on the output, my next thought would be 'Miller' compensation but 100pF is reasonable value which sadly does start to point at layout. Anyway, again I look forward to what trained eyes can see !!

@ronschauer839 Күн бұрын

We thought the same thing. Mike and I have both tried increasing *and* decreasing the 100pF CDOM capacitor's value. Either way we went things got worse in regard to the output instabilities, although he saw a ray of hope at 47pF. However, even that did not work when I tried it, it merely moved the parasitic "fuzz" to a different spot in the waveforms. The instability is up into the multiple megahertz region, by the way. In the end, sadly, it apppears very likely that this will prove to be a horrendously bad board layout.

@andymouse Күн бұрын

@@ronschauer839 Thanks for the info ! and we shall see :)

@michaellundsrensen2292 15 сағат бұрын

I don't like the The Sziklai Darlington Transistors Q1, Q2 and Q3, Q4. Cascodes are often tricky to stabilize in my experience! Try changing Q1 and Q3 to BC546B (or equals). Get the legs corect. You don't need the Sziklai coupling. My 5 cents! Happy Xmas!!

@cyrilmarchand237 Күн бұрын

Thanks for video mister Beeny. You make me an unbearable suspens! I can't wait to see what happens next.

@MichaelBeeny 19 сағат бұрын

I can hardly wait myself Cyril lol

@cyrilmarchand237 18 сағат бұрын

@MichaelBeeny 🤣

@jim9930 Күн бұрын

Assuming you got the schematic correct (?), the emitters of the driver transistors should be connected to the 'tops' of the emitter resistors of the output compound pair (collectors). As shown, it is NOT a compound szlikai pair; but rather two common emitter gain stages ...absolutely disaster waiting to happen (you'll be lucky to get something like that stable even of you pick all the X-istors for Hfe, Vbe, Ft, Cob etc). There is NO local current feedback around the output pair - it is intended to stabilize the Hfe droop of the output power transistors under high current load (ie Near the rail !!). Driver package L with stray C makes a really nice 'negative resistance' oscillator as the Hfe changes! Cut the traces and connect them where they belong. Add .1uf ceramic bypass caps at the supply rails of the output transistors and after the cap multiplier - you or the designer are not smarter than conventional wisdom. And another thing, there is NO DC in any recording, recording mic or loudspeaker; stop believing the bullshit nonsense about a tiny bit of offset as a real concern - 100mv does damn little to alter the sound of any decent woofer. As a matter of production tolerances, there is some "dc" offset built into every driver when the surround or spider are glued; even the best high dollar drivers have OFFSET - quit worrying about silly stuff. If you actually knew/measured the klippel offset, you can add a small fixed current to correct it { actually makes a real difference with some underhung midrange drivers }. OK. amateur hour is over, the bill is in the mail... 😎

@ronschauer839 Күн бұрын

The schematic is correct. Make of it what you will. It appears to simply be a dreadful board design. Performing signficant surgery changes to it will be a bit questionable. Hey, neither of us designed it, it is what it is. Kindly do not shoot the messengers.

@MichaelBeeny Күн бұрын

What a dreadful and patronizing person you are! Firstly, we did not design this circuit. If you had watched ALL the video, you would know it was designed by LJM. All we have done is to reverse engineer it as received and are looking for ways to make the board stable. I respect everyone's right to have an opinion, but your comments are bordering on being rude. You might well be the first person to be banned from my channel. So, think on and place your bill where the sun doesn't shine.

@jim9930 Күн бұрын

@@ronschauer839 Glad to help. Board surgery should be a simple fix on this one. And another thing, the compound input transistor pairs are a waste of silicon - leave out the NPNs and short the BE pad(2.2k). It is only adding excess phase to the open loop response.

@Poult100 Күн бұрын

@@jim9930 I think you're tone needs a large filter. Make of that, what you will.

@terencekaye9948 Күн бұрын

Is there an instability throughout the amplifier ? Does it show up at or past a certain point? No output chokes?

@MichaelBeeny Күн бұрын

No, the amplifier, out of the box does not have chokes on the output, it should and will on my board. This is not the issue as the test results are on a resistive load, so not applicable in this case. Progress is being made, too early in the day to publish, but we do have a working sample that is stable. I will update as soon as we are 100% happy with the result. Just got the QI to get stable now!

@andymouse Күн бұрын

Merry Christmas ! in case this your last upload before the holidays. I have often wondered about those pre packaged current mirrors, I mean it should be a great idea as the two devices are matched (trimmed at manufacture) and of course as thermally bonded as can be, I just haven't heard people say much about them. I will try them in a front end one day ! I will look forward to the next part but now I will go watch the first again for a refresh ! Oh yeah, have you ever worked on really expensive kit and I mean £25,000.00p and up, if yes was it interesting please :) !

@MichaelBeeny Күн бұрын

Yes, they are a good idea but tend to be quite expensive. They were used all the time in the 70s. They are just NOT fets lol Regarding expensive amplifiers, no I don't offer a repair service at all. All my friends are poor, like me!

@ronschauer839 Күн бұрын

The "FET1" position on the PC board appears to have been laid out to accommodate a genuine dual FET, or at least one or two particular part numbers of them. Then it didn't work out quite as they had hoped (but I am purely guessing at this..!). Fortunately (for them) if you take a commonly available commercial dual NPN transistor and flip it upside down ("dead bug" mounting) the connections work out OK without changing the board layout. The connections to make the pair into a current mirror are done externally to the device package in the board layout itself. To be honest, it appears to us that the real reason it was done was simply to save board space. However no matter how you look at it, there are so many design flaws on this PC board that we haven't even begun to describe them all.

@andymouse Күн бұрын

@@MichaelBeeny and me :)

@ClausDreisig Күн бұрын

Of course the amplifier is unfit for commercial sale as is, but if you want to find the culprit, a loop antenna can be a good help. BR Claus

@jarlesagheim9710 Күн бұрын

Hi Mike. I have bought the “ old” l12 amplifier. Thought I give it an go. I have an 40 vac transformer with is good size. Is 40 volt ac to high voltage for the board.? -

@RandomUser6947 Күн бұрын

2x40vac will give you around 2x57vdc. That is a bit on the high side for the L12, I think they officially state that 2x55vdc is the absolute maximum. But as usual, they like to live on the edge of what is safe. I would recommend you stay around 30 to 33vac on the transformer. It will give you between 42 and 47vdc after rectification and smothing caps. That also leaves you with a little room for mains voltage variation.

@peterlarkin762 20 сағат бұрын

@RandomUser6947 Solid advice. Recommended voltage is around 45VDC.

@jarlesagheim9710 19 сағат бұрын

Thank you for the advice. It is appreciated 👍⭐️

@MichaelBeeny 19 сағат бұрын

@RandomUser6947 I tend to agree, Because I wanted max power, I used a DC of 50 volts. I would tend to regard this as a maximum. The main thing is to use large heatsinks, the cooler your electronics is the longer it will last. Don't use a higher voltage unless you really need it.

@jarlesagheim9710 17 сағат бұрын

Can anyone recommend a nice easy ,ish, good sounding amplifier board- preferably with an 55 v ish rail. The L9 accuphase E405-mod. Has anyone tried this board?

@ClausDreisig Күн бұрын

My comment with a link to TI seems to have disappeared? Well! I just wanted to say that I have had good use of a small loop antenna (TI has a video on youtube) in search of a culprit transistor that oscillates. There are a number of things in the design that could cause this, but it is easier to measure it. Solder a coil, eg. 6-8 mill diameter and 5-6 windings to a peace of coax with a BNC connector in the other end. Connect it to the scope and probe on the board. It should show you which part of the amplifier is bad. It could of course be other problems, but then you have ruled out local problems. BR/Claus

@vladnurk4710 Күн бұрын

Even with this issue can I get a board to play with?

@MichaelBeeny Күн бұрын

OF course, I just want people to understand the issues as of now.

@vladnurk4710 Күн бұрын

@@MichaelBeeny yes i could help used to design analog & digital circuitry

@ronschauer839 Күн бұрын

Sure, but buyer beware... 🙂

@vladnurk4710 Күн бұрын

@@ronschauer839 I am across the ditch so where to purchase

@MichaelBeeny Күн бұрын

@@vladnurk4710 Just type L12/2 ver 5 on Ebay or Ali, lots to choose from. Confirm with the seller EXACTLY which boards the have before you part with any money.