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Chip Amps Amplifiers based on integrated circuits

Composite amplifier: LM3886 + LME49710
Composite amplifier: LM3886 + LME49710
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Old 21st February 2019, 09:44 AM   #1
jeanlemotan is offline jeanlemotan
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Default Composite amplifier: LM3886 + LME49710

Hi,

After reading tons of threads on this forum and all the info from the Neurochrome website I thought to give it a chance and design a composite amplifier using the LM3886 and the LME49710. I have some hobby level electronics experience and this would be my 4th serious electronics project (2 digital ones before, some RF involved and an electronic load where I learned about op-amps)

My goals are:
- 80-100W range for 8 ohm
- The lowest THD+N I can achieve
- Use the LM3886 instead of going discreete as it's my first amp
- Stability
- I want to design it to specs instead of just ending up with something that works accidentally.


I settled on a 4 chip parallel/bridged config with the same components as the Modulus-686 (if they work there, they will work for me as well .

My first step was to learn the LM3886 so I started simulating it in TINA-TI to get it to be stable and learn the feedback loop, gain and phase margins and stability in general.
This was the easy part as I followed mostly the info here: LM3886 Chip Amp Stability Analysis

The next step was to add a LME49710 in front of it. This was trickier to stabilize as the whole thing tends to oscillate at around 5Mhz.
Getting the right phase and gain margins was a very interesting game of whack-a-mole for around one week.
After experimenting with various compensation components and adding a DC servo, here's what I have:
non-inverting.png

It's the non-inverting part. The input is setup so that I can inject bias voltages (that's why the cap is bypassed) and to measure the settle time of the servo.

The inverting part is this:
inverting.png

Here is the bandwidth of the inverting side. The non-inverting is the same:
bw.png

The noise says around 32uV at 20KHz for a 60V swing, so 0,000053333%? Am I doing this correctly?
noise.png

Step response. I did this with a 1K voltage generator with 1nS rise time:
step.png
step-zoom.png

My questions are:
- Does the schematic and compensation makes sense?
- How can I simulate noise over output power in TINA?
- The servo takes 15-20 seconds to eliminate a bias of 400mV. This is a long time and I don't want to mute the amp for this duration. Is this normal?
- What else could I simulate before I start doing the PCB layout?

[EDIT]
Check post #90 for the result:
Composite amplifier: LM3886 + LME49710

Last edited by jeanlemotan; 20th June 2019 at 10:29 PM. Reason: Adding a link to the post showing the thing built.
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Old 21st February 2019, 09:55 AM   #2
Pano is offline Pano  United States
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Composite amplifier: LM3886 + LME49710
Moved to Chip Amps
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Old 21st February 2019, 03:25 PM   #3
wallyboy is offline wallyboy  Canada
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Looks like something that I would be interested in as well. Any reason you chose the LME49710 instead of the LME49724?
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Old 21st February 2019, 05:23 PM   #4
tomchr is offline tomchr  Canada
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Looks like you're building a composite bridged amp. Sweet! If you're aiming for 80-100 W into 8 Ω, why the 40 V supply? Not that it matters greatly for the simulation, but it does make a rather big difference in reality. If you're bridging the two amp halves, you only need about 26-27 V to get 100 W into 8 Ω.

Also note that if you ever want to drive a 4 Ω load with this amp, you'll need two LM3886es in parallel per side. That opens up another can of worms as LM3886es in parallel don't always play nice. I've started to include circuits in my parallel designs (Modulus-286 and Modulus-686) that prevent the LM3886es from latching up if one supply lags the other on startup.

The THD you simulate in TINA-TI is mostly an indicator that your circuit isn't fundamentally broken. THD is not included in the LM3886 macro model, so the THD results will always be optimistic. I regularly simulate stuff in the 0.000001 % range in TINA.

A composite bridge/parallel amp would not be my first choice as a first amplifier project, fourth overall electronics project. There are quite a few pitfalls and trap doors in such a design and many of these are not covered by the simulation models. At least it looks like you're building two half-circuits, so you can test each half independently, which is a smart move. You can also always bypass the composite loop and test the LM3886s by themselves.

Before you build the circuit, I suggest looking at the open loop response. I'd also look at both small signal and large signal transient response with capacitive load.

Tom
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Modulus-86, 186, 286, 286 Kit, & 686: 40-240W (8Ω) at <-120dB THD.
Neurochrome : : Audio - www.neurochrome.com - Engineering : : Done : : Right

Last edited by tomchr; 21st February 2019 at 05:25 PM.
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Old 21st February 2019, 06:01 PM   #5
wallyboy is offline wallyboy  Canada
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I agree with Tom regarding the power supply voltage. The latch up problem likely will not be an issue if traditional linear supplies are used. An rc delay on the mute pin should help alleviate the possibility of latch up as well. Any issues that are not identified during simulation can be dealt with during the prototype phase. This is normal design practice.
I may jump on here and try some of your simulations as well
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Old 21st February 2019, 06:46 PM   #6
jeanlemotan is offline jeanlemotan
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@wallyboy - no reason actually, I didn't know about the LME49724. I will definitely check it out, thanks for the hint!

@tomchr - first of all thanks a lot for the wealth of info you put on your website and on this forum.
After posting this I went and changed the power supply to 28 as you recommend on your website.
I do intend to parallel 2 LMs on each side but I didn't include them in this simulation as it was slowing down TINA and I saw no benefit to keep them in the simulation at this stage.
My first step was indeed to simulate the open loop response for both the LM3886 and then the composite one. The bare-bone, no compensation composite schematic was resulting instability until I added the C2 3.3p capacitor and R5/R12 divider plus the C17 33p cap.
This took one week of tweaking and TBH I don't understand the need for the divider.
I get an intuitive feeling that high frequencies pass through C17 and get attenuated less than lower freq so this changes the frequency response but I'm not sure if it's the right way to compensate this beast.
Also - I'm not sure what that divider will do to the input impedance of the LM3886 and if the LM49710 can actually drive that.

Since it was my first simulation for Phase and Gain margin, I followed the methodology you posted here (LM3886 Chip Amp Stability Analysis) and targeted PM > 60 and GM < -10dB. I didn't get exactly there, I got something more like PM ~43 deg and GM < -15dB which I considered good enough.


Good to know that the TINA model doesn't include the THD of the LM3886. Where do distortions come from in this simulation then? Passives tolerances? Phase distortion?

I didn't know that the LMs can latch up. What do you mean by one supply lagging the other? Can this happen due to PCB layout issues - like different lengths for the supply tracks for each of the paralleled LM3886?

Cheers!
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Old 21st February 2019, 07:22 PM   #7
simonra is offline simonra  United Kingdom
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I'm pretty new to this too but I may be able to help a bit... I'm sure someone will jump in and correct me if I'm wrong .

I think you need to look at the closed loop response. I think you'll find you have <0˚ phase margin. That means you'll get positive feedback in the high frequencies which will result in oscillation - I think it's possible to get around 80˚margin with this composite.
I'd also look at behaviour at clipping point, your design will clip the 3886 first which will drive the 49710 to try to compensate, the distortion will be pretty nasty... I'd adjust the gain structure. I think you'll find that lower overall gain and lower bandwidth will be easier to design and keep stable.
It may be worth considering how you can accommodate the stability components from the 3886 datasheet.
Lastly I'd suggest you take into account you may need to go through more than one iteration of PCB to get this working... design the layout so that you can get the soldering iron in to change the components around if you need to.

Like I said, I'm no expert but some of that may help.

Last edited by simonra; 21st February 2019 at 07:33 PM.
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Old 21st February 2019, 07:45 PM   #8
wallyboy is offline wallyboy  Canada
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I believe that the latch up issue is primarily encountered when using SMPS. One of the supplies can easily start before the other. With a linear power supply which is fed by a centre tapped transformer, both supplies will power up at the same time.
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Old 21st February 2019, 09:00 PM   #9
jeanlemotan is offline jeanlemotan
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@wallyboy - that makes sense, I forgot that this is dual-supply
So that makes it official - I'll go for a linear PSU.
If you want the TINA-TI files to run simulations, let me know. I'll be glad to send them to you.

@simonra - Actually my first PM/GM were with the loop closed and I was unable to get both right. Either PM was really good but GM was bad or the opposite, and the step response showed this clearly: there were always some oscillations of around 4-5Mhz. It seemed like the universe conspired to make this really unstable.
Then I re-read tomchr's page on stability and I noticed he measured the open loop response. After doing the same I managed to get both the PM and GM under control (it wasn't easy either, but doable) and the oscillations (with the loop closed) were gone.

So it's not clear for me at all: do I have to do the PM > 60 deg (or at least 0 deg) and GM < -10dB (or at least 0 dB) with both the loop closed and open?
That seems impossible

Good point with the clipping. So you're saying that I should choose the inner/outer gain so that the 49710 will clip first?

Regarding the PCBs, I'm prepared for that. Doing the PCB layout is actually super fun for me and I enjoy every second of it. It's also cheap and fast to get them from china (I'm using JLCPCB) so no worries there.
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Old 22nd February 2019, 12:12 AM   #10
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by jeanlemotan View Post
@tomchr - first of all thanks a lot for the wealth of info you put on your website and on this forum.
You're welcome. I'm glad you found it useful.

Quote:
Originally Posted by jeanlemotan View Post
I do intend to parallel 2 LMs on each side but I didn't include them in this simulation as it was slowing down TINA and I saw no benefit to keep them in the simulation at this stage.
Fair enough.

Quote:
Originally Posted by jeanlemotan View Post
My first step was indeed to simulate the open loop response for both the LM3886 and then the composite one.
Good plan.

Quote:
Originally Posted by jeanlemotan View Post
I don't understand the need for the divider.
If you don't need it, why include it?

Quote:
Originally Posted by jeanlemotan View Post
Also - I'm not sure what that divider will do to the input impedance of the LM3886 and if the LM49710 can actually drive that.
The input impedance of a non-inverting LM3886 is high, so the LME49710 is only loaded by the divider. You have 2 kΩ there total. The LME49710 can drive that just fine.

Quote:
Originally Posted by jeanlemotan View Post
Since it was my first simulation for Phase and Gain margin, I followed the methodology you posted here (LM3886 Chip Amp Stability Analysis) and targeted PM > 60 and GM < -10dB.
That's a good place to start. You have to open the loop in the right place for the simulation, though. If opening the loop affects the load impedances in the circuit, you won't get valid results.

Quote:
Originally Posted by jeanlemotan View Post
Where do distortions come from in this simulation then? Passives tolerances?
Have you included the tolerance of the passives in the simulation? How/why would they impact the THD?

You can often see THD in the simulation due to the simulator's choice of time step. You need to make sure you sample exactly one period of the input sine wave with the exact number of samples as you have points in your FFT. Otherwise, you'll get artifacts from discontinuities. I can find a few articles on that on TI's E2E forum.

Quote:
Originally Posted by jeanlemotan View Post
What do you mean by one supply lagging the other?
One supply starting up before the other.

Quote:
Originally Posted by jeanlemotan View Post
there were always some oscillations of around 4-5Mhz. It seemed like the universe conspired to make this really unstable.
Welcome to amplifier design.

Don't forget to measure the response to overdrive (i.e. soft and hard clipping) once you build the amp.

Tom
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Modulus-86, 186, 286, 286 Kit, & 686: 40-240W (8Ω) at <-120dB THD.
Neurochrome : : Audio - www.neurochrome.com - Engineering : : Done : : Right
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