Whot to reverse engineer a LM3886? - Page 1
Forum section: electronics / beginners

author: MrZwing
author_rank: Regular Contributor
timestamp: January 19, 2016, 12:37:32 pm
content: HelloI know this is a weird question since it all exists in an IC package.but is there any way to find out how to build a full size LM3886 Circuit?with the transistor/mosfet values and so on?would just be a fun thing building it since it is a pretty good audio IC.

author: dom0
author_rank: Super Contributor
timestamp: January 19, 2016, 01:07:15 pm
content: If you take a look at the equiv. schematic you see that it is a pretty ordinary audio PA circuit. It's a good PA IC, but not great for an audio PA, anyway (for example, 0.03 % THD+N at PN= 30 W).What makes it a good IC is not so much the amplifier, but the extensive protection circuitry. Duplicating those is part difficult and part nearly impossible in a discrete design. E.g. the fast die temperature sensing, precise SOA protection, ...

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 19, 2016, 01:11:35 pm
content: They can design circuits in monolithic silicon that can't really be reproduced with conventional discreet components.The internal circuit is not published and wouldn't be all that helpful.The LM3886 is nice and convenient, as a single-package monolithic chip.And at 0.1% distortion, it is probably good enough for most users who listen with their ears (vs. what they see).But if you want to build an equivalent ~60W audio power amp, I would look at these...http://sound.westhost.com/projects-0.htm#pwrAnd particularly this one....http://sound.westhost.com/project3a.htm

author: macboy
author_rank: Super Contributor
timestamp: January 19, 2016, 03:45:28 pm
content: Quote from: Richard Crowley on January 19, 2016, 01:11:35 pmThe "equivalent schematic" is in the data sheet. As usual, one can only guess about the various parameters of the devices, and it is full of the usual shortcuts, like implicit current sources (rather than showing the explicit construction of them). There is enough there for someone experienced with audio power amps to build something that will resemble the LM3886, but it won't be a LM3886. And the protection circuitry is completely missing.+1 to sound.westhost.com audio projects. Highly recommended even if only to read rather than build.Quote from: dom0 on January 19, 2016, 01:07:15 pmYou missed a zero, the THD+N is ~ 0.003% (at 1 kHz, and rising above that which is not unusual). By the numbers, the performance is quite good... not extraordinary but better than monolithic audio amps that preceeded it. Of course at anything near this level of THD+N, one spec doesn't tell the whole story.

author: dom0
author_rank: Super Contributor
timestamp: January 19, 2016, 04:18:00 pm
content: I didn't, I referred to the (very first) figure in <unnumbered table on page 4>. 30 W / Rl = 8 ?, 60 W / Rl = 4 ?,full audio band, Av = 26 dB ... THD+N typ. 0.03 %.Figure 16 and others show the usual course of distortion vs. frequency for almost any amp ; rising distortion toward lower frequencies (mainly thermal and bias shifting effects), lowest distortion in the range of 100 to 1000 Hz, rising distortion beyond that due to the roll-off of loop gain (which is why it is most common to specify at 1 kHz, it's a) audio mid band and b) a region where typical amplifiers achieve optimal performance and of course because historically THD measurements were quite involved (and still are, beyond <-120 dB, that is), measuring just a few spot frequencies was much easier than creating extensive THD vs. f plots).

author: MrZwing
author_rank: Regular Contributor
timestamp: January 19, 2016, 06:01:30 pm
content: Quote from: Richard Crowley on January 19, 2016, 01:11:35 pmHmm I will certainly take a look at it could be fun to build something a bit more advanced then a simple LM3886 circuit.the thing is I don't really know what to look for to know if the amplifier is good or not. and the LM3886 was mentioned in so many places to be decent that I thought "eh why not".Still very new to the electronics hobby.but thank you all for the answers

author: Kleinstein
author_rank: Super Contributor
timestamp: January 19, 2016, 06:15:38 pm
content: The shown simplified schematics is likely less than half the circuit. The protection part is still missing.The other point is, that monolytic chips can rely on essentially same temperature and paramters for several transistors. With discrete parts allready a simple current mirror is much less accurate. So you may not be able to just copy the chip. Also some on chip parts are not available as discrete parts: e.g lateral PNPs (generally lousy transistors by high reverse base - emitter rating).A difficult part in a good amplifier is the thermal design to get the bias setting very stable - as to get the low distortion rating not only in cold or steady state conditons. This problem is much easier on chip.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 19, 2016, 06:49:51 pm
content: Quote from: macboy on January 19, 2016, 03:45:28 pmWow, that's interesting. There is apparently more than one data sheet. The one I have (from TI) does not have an equivalent schematic.QuoteThe TI data sheet I have bills the chip as: "THD+N 0.1% from 20Hz to 20KHz"

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: January 19, 2016, 06:53:23 pm
content: I have extensive experience with the LM3886, and would say that properly implemented, it makes an excellent amplifier. The specifications given on the datasheet are reasonably conservative in my experience. A distortion of 0.003% at 1kHz into 4 ohms at 60 watts is an excellent result indeed - especially for the cost. Bear in mind that 0.003% is 90dB of separation between fundamental and harmonics+noise. Just compare that to previous-generation ICs like the TDA2030. Or these horrid class D things that some folk seem to like!It's certainly true that with the help of Douglas Self's books, you can improve on the performance by using discrete components. And there is great intellectual satisfaction from doing so (got the t-shirt!). However, in the real world, there's no real point (beyond marketing perhaps, who might not like the idea of trying to sell an expensive amp that uses cheap ICs - not that Jeff Rowland had a big problem with that). Really, once you've got the distortion to 0.1% or less, then you need to look to other things - especially headroom and clipping behaviour. Admittedly, these ICs can be a bit uncivilised when clipping - it's worth considering aSIMor similar.They can be put in parallel to drive lower impedance loads - which helps with heat sinking as the thermal resistance (junction-case-sink) is reduced. They can be bridged as well. You can get the best part of 200 watts into 8 ohms from 4 or 6 of them in bridged-parallel mode.Having said all that, if the goal is to learn about transistor amplifiers, then using the LM3886 as-is will teach you relatively little. My advice would be to get hold of a copy of "Audio Power Amplifier Design Handbook" and study it - which is no chore as it's up there with "The Art of Electronics" for readability. Then get experimenting. You'll quickly see that as well as potentially being able to improve on the LM3886, there are plenty of ways to screw it upHappily, a lot of what you learn will be equally applicable to ICs, as PCB layout and wiring topologies make a surprising difference. Just a mis-routed earth track can increase distortion by orders of magnitude!

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: January 19, 2016, 06:54:34 pm
content: Quote from: Richard Crowley on January 19, 2016, 06:49:51 pmhttp://www.ti.com/lit/ds/symlink/lm3886.pdfA decent selection THD+N graphs on page 11(Equivalent schematic on page 7)

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 19, 2016, 07:30:51 pm
content: Bzzzzt! I quickly scrolled right past the equivalent schematic.However, the opening paragraph on the first page of the data sheet says:QuoteIf performance is better than 0.1%, then it is curious why they say that in their description.

author: dom0
author_rank: Super Contributor
timestamp: January 19, 2016, 07:39:22 pm
content: It's the power spec, not the THD spec - for monolithic audio PAs the output power is usually specified for 0.1 % THD these days. Older devices tend to specify power for 10 % (!) THD.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: January 19, 2016, 08:39:03 pm
content: Quote from: dom0 on January 19, 2016, 07:39:22 pmYes. Often, 1% is used. It just depends...It can make quite a big difference if you have an amplifier that has a slow rise in THD as you get close to maximum - e.g. a "soft clipping" valve amplifier. With solid-state stuff using normal levels of NFB, it's not quite so dramatic, but still potentially useful for marketing reasons - especially if you give "loose" specifications that don't include the THD level. Picking 10% rather than 0.1% takes the LM3886 goes from around 60 to just over 70 watts (figure 21). That's only 0.7dB approx, but that might be "useful".You do need a "stiff" PSU if you intend to extract the maximum from these chips into 8 ohms. The absolute maximum supply voltage is 84V, and unregulated supplies will obviously sag under load. 70V loaded gets you the figures just quoted. I generally aim for 45 watts into 8 ohms, which you can readily get using an off-the-shelf 25-0-25V transformer which gives roughly 35-0-35 unloaded. If you want to squeeze more out, a custom-wound 28-0-28 might do, but it's sailing a bit close to the wind if the mains voltage is 10% high. It's safer to pick a regulated supply, but these obviously come at a cost and a lot of complications. I prefer to either bridge or pick something like anLME49810, which just requires the output transistors of your choosing. These perform really well - not quite in Douglas Self territory, but certainly knocking on the door. For a commercial design that needs to be turned around quickly, I recommend these.Sorry for the OT ramblings.

author: dom0
author_rank: Super Contributor
timestamp: January 19, 2016, 08:51:53 pm
content: Beware that the LME49810 and it's brothers (-11, -13, -30) are being discontinued (life time buy). Some other LME chips are discontinued as well. LM4562 / LME49720 persists, but not in DIP.

author: Audioguru
author_rank: Super Contributor
timestamp: January 19, 2016, 08:58:36 pm
content: Quote from: Mark Hennessy on January 19, 2016, 08:39:03 pmThe excellent LME49810 and its sisters are not made anymore. They were designed by National Semi before Texas Instruments bought National Semi and they are linear but TI has class-D power amplifier ICs.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: January 19, 2016, 09:17:06 pm
content: Yes, it's a real shame. Texas took an axe to the National line, sadly - I remember when the "Overture" range was absolutely massive; I have a databook (a real one) from 2004 that lists them all. Still, LM3886s are so cheap that it doesn't make sense to stock the lower-powered-but-otherwise-virtually-identical versions as well. I'm not sure when the LME49810 went, but there's still some in the supply chain and I've got a load here still for my own projects and repairs. It's been a while since I last put one in a product, as most folk prefer fully discrete solutions for marketing reasons. If they are happy with chips, bridged LM3886s usually work out cheaper and easier.They still make several class B amplifiers, but when you go looking for them on the Texas site, they are in the class D categoryQuoteAt least they still make the rather wonderful PGA2310, which I've been using for more than a decade now. But then, that wasn't a National part

author: Audioguru
author_rank: Super Contributor
timestamp: January 19, 2016, 10:28:13 pm
content: National Semi made a class-D amplifier IC but it was sold only for a few years, maybe it did not work well, like the Tripath "class-T" amplifier ICs.

author: dom0
author_rank: Super Contributor
timestamp: January 19, 2016, 11:11:56 pm
content: Quote from: Mark Hennessy on January 19, 2016, 09:17:06 pmThe notice was fairly recent, September or Oktober (2015). I dunno about last order / last ship, probably one and two years later, respectively. I acquired a nice stock of the relevant parts for myself that'll last me.I think they said the reason is that these are made on some old production line that is decommissioned.

author: Mark Hennessy
author_rank: Frequent Contributor
timestamp: January 19, 2016, 11:19:47 pm
content: Quote from: dom0 on January 19, 2016, 11:11:56 pmThank you