Composite amplifier: LM3886 + LME49720 - Page 1
Forum section: electronics / projects

author: JeanLeMotan
author_rank: Contributor
timestamp: February 28, 2019, 09:17:36 am
content: In the past weeks I've been working on an audio amplifier using the LM3886 and LME49720 in a composite, bridged topology. I started off with TINA-TI simulations of the basic circuit until I got it stable with a decent phase and gain margins, step response and all and then I moved on to the actual schematic and PCB.The whole thing is finished (so I think) and I want a second opinion before I move on to actually prototype this.There's the info.1. Goal:- 100W into 4 and 8 ohms. Not a hard requirement, I'm happy with anything in the 60-100+W range but since I didn't choose the speakers yet and I don't know their sensitivity I want some headroom.- THD as low as possible (for a hobby builder with zero XP).- Fun build that will teach me something. This, plus the previous point is why I choose the composite topology. Building just a LM3886 feels too much like lego: someone already did the work and I just connect the dots. The composite topology is a different beast on the other hand.- I want to design it to spec, not end up with smth good (or bad) by accidentThe attachments are in this order:a) Inverting part schematic used for simulationsb) Non-inverting part schematic used for simulationsc) Schematic used for phase-gain margin simulation/calculation (done according to this:https://training.ti.com/ti-precision-labs-op-amps-stability-3)d) Phase/Gain plot obtained from the schematic abovee) Gain rise calculated according to this (https://training.ti.com/ti-precision-labs-op-amps-stability-4). This confirms the values from (d)f) Step response calculated according to this (https://training.ti.com/ti-precision-labs-op-amps-stability-4)g) Clipping transient responseI attached the schematic used for simulation (non-inverting and inverting parts separately), the step response, the gain rise over the frequency range, the phase/gain margin plot.The full schematic with BOM and PCB is here:https://easyeda.com/jeanleflambeur/amp-LM3886-bpa200The initial discussion is here (https://www.diyaudio.com/forums/chip-amps/334273-composite-amplifier-lm3886-lme49710-3.html)Extra info and questions:In the PCB, I tried to keep the track lengths for the feedback loops (both the inner one and the outer one) as short as possible. This forced me to place every second LM3886 on the bottom layer to keep the design symmetrical. The PCB in that area is also crammed. Does it make sense? Am I going too far here (or not far enough)?I have 2 grounds in the schematic:- The signal ground (QGND) used for the input reference and the signal opamps- The high-power ground for the LM3886On the PCB I feed these in 3 points: High-power GND using a spade connector in the bottom-center of the PCB, input-gnd together with input using a 0.1" connector and low-power-gnd together with opamp-vcc and opamp-vee using another 0.1" connector. The idea is to merge these grounds on the PSU board.Does this grounding make sense?There are 4 power rails in the schematic: V+ (28V), V- (-28V), OV+ (15V) and OV- (-15V).V+ and V- are fed separately for each half of the amplifier: on the PCB I have 2 V+ and 2 V- connections. Is this ok or should I try to connect the power of the 2 halves on the PCB and feed it in only one point?I have a signal-ground plane over the central part of the 2 halves - both top and bottom in an attempt to reduce the QGND impedance (and to use the copper I'm paying for). Is this ok?The resistors I chose are metal-film, 0.1% and they are somewhat expensive and hard to find. The packages are all over the place (some 0603, some 0805, lots of MELFs) which ruins the aesthetic. Does it make sense to go for metal-film in this application? I'm not an audiophile, I like to measure and I'd like to choose components based on data and realistic expectations rather than feelings.Any other feedback you might have, it will be much appreciated.Thanks a lot!

author: nick_d
author_rank: Regular Contributor
timestamp: February 28, 2019, 09:53:51 am
content: That looks awesome. I am not really an expert on this, I just wanted to jump in and congratulate you on the detailed simulations and design work done so far. I will read the linked reference about how to simulate later on. From what you describe I think your grounding scheme sounds sensible. I will try to find time to examine the schematic later. In the meantime I'm keen to hear of progress. I also think the composite amp should be excellent when properly designed. There was another thread last month in which many engineers panned the idea, but despite several requests I never received a clear answer as to why, except the feeling seemed to be that it would be unstable and whatnot, my answer to that was the feedback loop might be harder to design but once done it should be OK.cheers, Nick

author: David Hess
author_rank: Super Contributor
timestamp: February 28, 2019, 02:36:42 pm
content: Quote from: JeanLeMotan on February 28, 2019, 09:17:36 amMetal film resistors do not suffer from excess noise so they are a good choice for audio applications but such tight tolerances are not required except in circuits like instrumentation amplifiers where common mode rejection is important.

author: JeanLeMotan
author_rank: Contributor
timestamp: February 28, 2019, 03:26:59 pm
content: Quote from: David Hess on February 28, 2019, 02:36:42 pmRegarding the tolerance, The amp consists of 2 bridged halves of 2 paralleled LM3886 each. The paralleled amps have to have the same gain so that they don't fight each other and the 2 bridged halves should have the same gain to reduce distortion. Using 0.1% resistors was my attempt at this balancing.There are DC servos for each amp to make sure the voltage offsets don't cause problems.

author: JeanLeMotan
author_rank: Contributor
timestamp: February 28, 2019, 05:40:50 pm
content: Quote from: nick_d on February 28, 2019, 09:53:51 amThanks a lot!It was quite difficult to get it stable, I spend more than one week tweaking it.The inner loop was fine as it's stable at >10x gain but the outer loop tended to oscillate at ~5 MHz due to the LME49710 being so much faster than the 3886. Anyway, the 2 compensation caps fixed this and I think I have now a very stable amp... simulation.The next step is to build it. The PCB layout is as tight as I could make it - the only unknown is the grounding scheme. I don't know what PCB parasitic capacitance and inductance will to do stability. Also not sure how reliable are the TINA-TI models for the chips.The composite topology is definitely proven and this exact choice of components and configuration is already present in a line of products, see here:https://www.neurochrome.com/modulus-86/So I know it's possible to stabilize.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2019, 07:07:39 pm
content: Quote from: nick_d on February 28, 2019, 09:53:51 amAgree, this looks like good data. Will take a closer look later!Tim

author: JeanLeMotan
author_rank: Contributor
timestamp: March 01, 2019, 06:25:16 pm
content: Quote from: T3sl4co1l on February 28, 2019, 07:07:39 pmDon't mean to rush, but did you get a chance to look at the schematic & PCB?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 01, 2019, 09:43:07 pm
content: Thanks for the reminder.Schematic and simulation -- can't find a fault. Those are damn good opamps, and as long as it's stable, you're set. I was about to complain that DC coupling allows the bias servo to saturate (if left biased long enough), but I realized it is AC coupled, the cap is just shorted over for simulation purposes.Layout -- flipping a THT component is a really awful hack though. I'd flip it back and clean up the routing, yes it'll be a bit fiddly but the result is easier to build, and more compact.Seems like it's missing an inner layer ground, or something? I see ratsnests on the layout, GND isn't fully connected.That's my other complaint: no ground pours or planes. Inner layer would clean that up nicely. 4-layer boards are cheap and plentiful, hardly worth arguing about, especially considering the amount of time invested in making a PCB layout in the first place -- you've more than paid for that luxury already.That also gives another routing layer, which may help relieve some congestion. In bipolar supplied designs, it's not really feasible to pour both supplies and ground; but you could pour the two important ones (VCC, VEE), and route ground on outer layers as just another signal. Which really, in a design like this, that's what it is!Mechanically speaking: don't be afraid to extend the board a bit further behind the power amp footprints. I don't know if you'll be using offset pinned components, but I've seen those before (the leads are formed so the pin rows extend in front of the component body, rather than being straight vertical overhead as the silkscreen suggests). Even if not, it's quite feasible to add a heat spreader to space the components off the enclosure wall / heatsink face. Just grease both sides, bolt it down and you're good.I'd go with bigger mounting holes, and more meat around 'em, myself -- ~0.125" fits an M3 or #4 well enough, but I default to #6 for this kind of thing, which is comfortable in a 0.15" hole. Leave at least 0.2" of board width ("meat") around the hole. This may be a personal preference thing as much as any; #4 screws aren't exactly weak or anything.The holes near the heatsink may not even be needed -- the component leads will take up that stress without much problem, and they'll take up the strain of mounting errors and thermal cycling as well. (That is kind of an iffy instruction to make: counting on component leads to take up flex is a risk factor for fatigued ("cold") solder joints. 2+ sided PTH boards help greatly with this, holding the pin through the entire board thickness. Compare to the old days of 1-sided punched phenolic board, solder joints only on the bottom surface: any lead stress at all causes failure. The motivation here is, if those nearby screws are used, they have much more leverage against the component leads, making them a risk factor for mounting errors or fatigue as well.) If you do use them, try to fit everything in place, then solder the power amp chips as the very final step, so there's as little lead strain as possible.If you can't fit a ground plane layer, and can't expand the board to allow ground pour plus stitching vias around most traces, then I worry that you'll have problems with high frequency crosstalk (especially in the MHz range -- this is NOT a mere audio frequency beast!), and possibly oscillation due to the accidental mutual inductances between supplies and signals, which are very difficult to predict when not using a ground-plane design. (If you extracted a model of the PCB, it could be put into SPICE, but that extraction is... nontrivial, at least with the tools handy right now.)Cheers!Tim

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: March 01, 2019, 11:40:24 pm
content: There is a fellow who has done this already and sells complete amplifiers to the DIY crowd. He has shared a lot of information here:https://www.neurochrome.com/taming-the-lm3886-chip-amplifier/Check it out.

author: JeanLeMotan
author_rank: Contributor
timestamp: March 03, 2019, 12:16:36 am
content: Quote from: RandallMcRee on March 01, 2019, 11:40:24 pmIt's his designs that inspired me - check out his Modulus-686 amp.Tom posted a great deal of info regarding the LM3886 and amp design in general that I really used for my amp. His website - and Bob Cordell's book - actually got me going in the right direction.

author: JeanLeMotan
author_rank: Contributor
timestamp: March 03, 2019, 12:37:08 am
content: Thanks a lot for the info Tim. You've given me quite a lot to process so it's a busy weekend for me. Now I'm trying out alternative PCB layouts based on your advice to see what I can come up with - but it's clear that I need to understand better PCB design for amps, especially in the presence of multiple separate grounds (signal, low and high power).Quote from: T3sl4co1l on March 01, 2019, 09:43:07 pmI was actually proud that I found a clever way to keep the feedback loop short by flipping the 3886... I was completely unaware that this is a bad practice.Not sure what will happen if I remove the flip - but the feedback loop will definitely be longer. Are there any reasons except aesthetics and volume?Quote from: T3sl4co1l on March 01, 2019, 09:43:07 pmYes, the low-power ground was missing a connection to the high power one. I intended to make this connection on the PSU-PCB but since I don't think there are big currents on the ground traces (as it's a bridged amp and the speaker doesn't return to ground) I think I will just connect them on the main amp PCB.Quote from: T3sl4co1l on March 01, 2019, 09:43:07 pmI played around with ground planes but I wasn't sure how to go around it. From what I've read, the best way seems to be to use a ground plane for the signal ground and route the power ground by hand.The new PCB does this (still WIP) on both the top and bottom layer. I still have to do some serious via stitching.Fixed another issue while was at it: the output zobel inductors were parallel and they could couple magnetically. Now they are 90 degrees apart.4-layer boards are super tempting and I did one in the past: way more fun than 2-layer ones. It does increase cost a bit and I think I will have to do at least 2-3 iterations of this board but compared to the final amp cost (PSU, components, etc) the PCBs will probably be peanuts. The thing is - I know it's doable in 2 layers so I take it more like a puzzle. If I fail completely and the whole thing oscillates and I don't have any clue why, then 4 layers it is. But I want to give 2 layers a try at least.Quote from: T3sl4co1l on March 01, 2019, 09:43:07 pmGood point about the holes. I will make them bigger and remove the ones on the amp side.The LM3886 footprint seems to match the datasheet spacing between the pins and the heatsink - so no extra space there. For now I don't need that extra routing space but good tip with the heat spreader. I'll think about routing power through there, see where that leads. Maybe it solves some of the ground place issue.Thanks a lot for all the advice & info!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 03, 2019, 03:51:25 am
content: I wouldn't worry about feedback path length, at these frequencies it could be meters before you'll notice. Assuming noise or other feedback doesn't couple in along the way, of course (hence why planes are so great), and assuming the path length isn't on the feedback node itself (which shouldn't be loaded by much capacitance, but this also can be compensated with more "speed-up" cap).Quote from: JeanLeMotan on March 03, 2019, 12:37:08 amThe heart of the argument I think is more about manufacturing and space. If those are unimportant (as a low quantity hand-built is), it's meaningless.I feel a kneejerk about it because it's very dissonant against what you see out there, a style issue -- this is however the weakest possible argument (and being honest enough to notice when this is the case, is a challenge unto oneself).But as for useful justification, it's a design-for-manufacture (DFM) issue.In production:- You'd have to hand-solder at least one pair of amps. No chance of running the board over a wave solder machine twice.- Heatsink screws on both sides is fiddly, and makes order of assembly that much more critical. (Probably can't mount the board before finishing those screws?)- Requiring access means whatever mounting for the board (standoffs/bosses) has to be very tall.On the other hand, all the same... if you're going for an industrial chic look, it might be a feature-not-a-bug. Say you tilt the whole board so the LM3886s look like they're in rows or something. Idunno.In the latter case, fiddliness of assembly is probably a feature-not-a-bug, too.QuoteYikes! Don't run a critical signal through a wiring harness!Incidentally, be very weary of advice you see about star grounding. There are some very beautiful (in the industrial-chic sense) implementations of this out there... and they stink to high EMI heaven! Fortunately, audio amplifiers are extremely lenient, so that most any beginner can put together a schematic (with pretty much any layout) and get acceptable results, at least assuming they aren't in an RF warzone.The better way is simply to route current paths, and signal loops, away from each other. With high frequency currents returning on the ground plane underneath the trace, loops are where you put them. In this case, it should be easy to locate the speaker terminals (if this were an unbalanced amp) near the power amp, and have the signal amp on the other side of that connection.Also, it's just clicking that this is intended for bridged operation... so you could have some opportunity to measure thedifferentialvoltage, and feedback with that. A differential sense amp, that's easy, but maintaining balance is actually a bit harder.A fully differential amp could deliver +/- output (feedback) signals, but, I doubt one exists that has nearly as good specs as the '49710.You could feedback to one phase (the "slave" or "follower"?), and leave the other (the "master") alone (similar to what they did back in the day: the paraphase phase splitter). That is probably a bit better than the split path method, but it feels inelegant.Point is, you're approximately doubling the errors in the system -- the errors from both outputs add (well, subtract, but assuming random variables y'know). This is not wrong to begin with -- you've doubled the output power too, after all!Using a single error amp, would reduce it to a single error, not a double error, so it's worth about 3dB SNR. (And, again, not that this is really needed, but given the spec of the '49710, I assume you're going for the best possible.)So, it's merely an incremental improvement, versus, there may not be any good way to implement it with off-the-shelf parts.I don't think I would do anything different. This is more to illustrate design analysis, the kind of contemplation and insight that any good design needs!QuoteAh, cool.FWIW, nearby (air core) inductors typically have k < 0.1, which isn't much, and I doubt you'd notice anything here.Actually, the ones on opposite phases of a given channel, are getting the same current anyway, so they act in series and the coupling is equivalent to a slight increase or decrease in total inductance. Should be no problem there. The total value would be slightly different from what's intended, but these are low-Q parts, errors over a factor of 2 would be cause for concern.Crosstalk between channels, via inductor, isn't going to be noticeable until near cutoff, either; keeping them isolated, really just feels better, and that's fine.QuoteActually, y'know, leave the holes in, in case they're useful -- see how it goes in the real build. Don't HAVE to put standoffs and screws in them. If nothing else, maybe they'll prove useful for wire ties? Who knows.That's another nice lesson -- prepare for options. Not many options to worry about on an amplifier, but it can be nice to have a number of footprints for testing different components and combinations when you expect you have a design that's going to need some fiddling.Or... conversely... if you're not averse to defacing a PCB, there are many things that can be changed on an already-fabbed board, for varying degrees of difficulty. Cutting traces and running wires is easy. Drilling holes -- if you have free space to do so without cutting or shorting anything out -- is pretty easy, too. Patching traces in a like-new manner, is more difficult, but can be done. Rebuilding board material entirely, well... So, even if you don't put holes in the design, they can be added (cough... subtracted?) later, if the space is still reserved for them.Tim

author: JeanLeMotan
author_rank: Contributor
timestamp: March 03, 2019, 11:24:59 am
content: Quote from: T3sl4co1l on March 03, 2019, 03:51:25 amI did some simulation of what effects the feedback path length has for phase margin.Attachment 1 shows no parasitic inductance: The Phase Margin is 64 degrees.Attachment 2 shows a 10nH parasitic inductance (which corresponds to 1 cm of trace length): PM = 56 degAttachment 3 shows a 30nH parasitic inductance (3 cm of trace length): PM = 43 degAttachment 3 shows a 100nH parasitic inductance (10 cm of trace length): PM = 29 degThe phase margin degrades somewhat although it's not clear for me what this degradation means: is it bad? is it acceptable? irrelevant?Anyway, according to this page, feedback track length is critical for performance:https://www.neurochrome.com/taming-the-lm3886-chip-amplifier/stability/Quote from: T3sl4co1l on March 03, 2019, 03:51:25 amThis is very interesting, I never thought about these issues. I guess it takes a lot of training and experience to design something for mass production.Do they actually teach this in school?Quote from: T3sl4co1l on March 03, 2019, 03:51:25 amThat's actually a great idea! I didn't think about that but it makes a lot of sense. The 49710 would correct a bridged-paralleled amp made with 3886 and their servos. Like this it will correct all issues coming from both the paralleling and bridging.This will also reduce the BOM and routing density/constraints.I wonder what this will do to the outer loop stability.Hmm, choices....[Later edit] Here's a new schematic where I use one LME49710 per paralleled group:https://easyeda.com/jeanleflambeur/amp-lm3886-composite2Simulations in progress.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 03, 2019, 08:41:12 pm
content: Quote from: JeanLeMotan on March 03, 2019, 11:24:59 amWith inductance added as shown in the article?No, that's output path length -- that fits under the rule of keeping current loops and paths tight and separate.In other words, you don't run load current through your feedback trace!Same net, but the branch carrying output voltage (not current) from '3886 output pin to the '49720. Modest L and C in that path should be very tolerable, of course you'll want to add some damping if it's incredibly long. I can't imagine you could have any problem even for a very circuitous trace on a board of these dimensions.QuoteA modest amount certainly looks to be acceptable, which tells you that a corresponding length of loop-sharing is acceptable. So, about an inch, give or take.QuoteThey teach very little practical engineering in school. School is about two things, only one of which they tell you about.At school, you learn theory, so that you have a toolbox, a framework to build hypotheses on.You also make connections with other students, faculty and professionals, which benefit your career track.Practical things, you learn on the job, and from observing what others have done in practice. More importantly however, understanding what reasons there might be driving those decisions. It is very rare indeed to find any kind of design documentation; even when picking up existing designs in full, you're largely left to figure it out yourself.I've almost never been asked to produce design documentation myself -- and I'm in the business of making designs for others!I think in some cases, they don't really need it (if they continue to engage with us for design updates, they won't need to know). Others, it would be beneficial, but maybe they forget to ask, or they figure the cost of producing that documentation would be more expensive than having their guys look it over.Well, interpretation is up to the reader. That process can vary in terms of accuracy. They may miss subtle points; or they may discover catches that the original designer never anticipated, or intended.Well, analysis is up to the writer, too, so that can vary in terms of accuracy.On a recent job, I got to read one of these rare documents, actually. I wrote several pages of notes and critiques on it in the process; it was, uh, sad to say, rather lacking in many points.Such is engineering. There is no simple process to determine capability and fitness. At best, you can get more eyes on it for review, but you'll get about as many opinions as eyes*. Good engineering is ultimately tested in the lab, and then the marketplace; but that's a very expensive test indeed, and you usually only get one or two tries of that before heads start to roll (slipped deadlines, exploded budgets, rolling revisions; fired managers, or teams..).*Heh, nice. This implies approximately two opinions for every person. Which is probably true enough.If it passes testing (EMC, shock and vibe, environmental..) first try, that's encouraging (I pride myself on having done this more than a few times), but you may ask if the product is more expensive or bulky than it could otherwise be.You could bring in an expert to review the design, but how do you verify someone's credentials? They're only going to tell you about their successes, of course.Well... in any case, there is good reason why product development is so costly...Tim

author: Wallyboy
author_rank: Newbie
timestamp: March 03, 2019, 11:53:45 pm
content: Quote from: T3sl4co1l=topic=172005.msg2236821#msg2236821 date=1551573428Actually the LME49724 is a very good candidate for the fully differential solution. Using this, both bridged halfs could use the same inverting arrangement thus lowering the large common mode signal that is present with the non-inverting arrangement (lower distortion).Walt

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 04, 2019, 05:41:51 am
content: Well hell, there you have it.Tim

author: bson
author_rank: Unknown
timestamp: March 05, 2019, 01:42:20 am
content: Great little project, can't wait to see how you like it.I'd give it an amplitude sweep also to look for compression as it gets too close to rail on the output. This can be used to set the peak output level, and from there determine the total amplifier gain. Then move as much off this to the LME49710 up front, off of the LM3886. The good news is the LM3886 specs 0.03% THD+N at +26dB, and presumably this improves correspondingly with a reduction in gain. The 49710 will have THD+N specs orders of magnitude better, so get all the gain you can up front.Do you really need a 100W amplifier? The higher the power, the higher its total gain needs to be, and the more you need to attenuate the input and feed it ever weaker input signals - unless you actually need the power. Maybe consider switchable gain on the LM3886 (with a dial or switches) so you can run it at lower power levels normally, but crank up the gain if you buy large speakers that really need that much power. From a numbers perspective, less gain is better.You may not need the DC servos; the offset will be Vos of the LME49710 times the total amplifier gain. The LM3886 offset will be removed by the feedback. If I recall the 49710 max Vos is on the order of 1mV, but realistically you get much less. Does any real world loudspeaker care about a 20-50mV offset?I'd also use the mute function to silence it at power-on, using something along the lines of the BJT below. (It seems to require ≥0.5mA sunk out of the mute pin to unmute.) With 100kΩ*120µF and Vdd=17V the base reaches 0.6V in about 0.4s, and then it will gradually open up until it sinks the full 0.5mA. If Vdd ramps at power-on it will take longer to unmute. Or something like it (just breadboard and see what works).Just a few thoughts. Cool project, and I may make one for myself to experiment with if you don't mind.I like the idea of using an LME49724 and differential inputs, especially if the input signal is going to be very weak due to high amplifier gain!Edit: oh, and another reason to mute it during power-on is the DC servos need to prime or you can get a nasty DC spike on the output.

author: JeanLeMotan
author_rank: Contributor
timestamp: March 05, 2019, 09:04:09 am
content: Quote from: bson on March 05, 2019, 01:42:20 amThe gains are setup with these in mind:- Use the minimal LM3886 stable gain, which according to the datasheet is ~10. I used 11 just to be sure- Have the LM49710 clip first to get a much nicer clip behaviour without saturating it. Check the attachments for the 2 behaviors: LM3886 clipping or the LME49710 doing the clipping.- Use as small resistors in the central divider R6/R25 in the new schematic here:https://easyeda.com/jeanleflambeur/amp-lm3886-composite2- The central divider does 2 things: allows the 49710 to clip first, and helps with stability (the C1 cap across R5)Quote from: bson on March 05, 2019, 01:42:20 amI'm listening to a lot of classical music at medium volume and the dynamic range is very high. Since I don't have the speakers yet (I want to build a set) to know their sensitivity, I wanted the head room.Quote from: bson on March 05, 2019, 01:42:20 amHow is the offset of the LM3886 removed in the feedback? I guess you mean the 49710 feedback, right? I imaging the 49710 replaces the LM3886 offset with its own, right?In the new schematic I have on 49710 per paralleled group so I don't think I can remove the servos anymore...Quote from: bson on March 05, 2019, 01:42:20 amI already have the datasheet mute circuit (cap + resistor). How is that circuit different?Don't remember the time constant I calculated but it was several seconds to make sure the DC servos have time to do smth. Although in my simulations the servos need >15s to completely eliminate the offset... Muting for that long will not e user friendly as it might seem the amp is not powered on.Quote from: bson on March 05, 2019, 01:42:20 amBy all means - go ahead. The schematic and PCB are public domain, have fun with them. Just keep in mind that it comes with no guarantees as I'm not an expert (if that was not already obvious)Check the second schematic and PCB first, I think it's an improvement over the first one.https://easyeda.com/jeanleflambeur/amp-lm3886-composite2Also, the corresponding (simple) PSU:https://easyeda.com/jeanleflambeur/psu-lm3886-composite

author: bson
author_rank: Unknown
timestamp: March 05, 2019, 09:36:42 am
content: Quote from: JeanLeMotan on March 05, 2019, 09:04:09 amYeah, if the LM3886 adds an offset the 49710 will remove it. The offset you get on the output is only the latter's input offset times the total gain.Sorry, didn't see you didn't already have a mute circuit.