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Chip Amps Amplifiers based on integrated circuits

Composite amplifier: LM3886 + LME49710
Composite amplifier: LM3886 + LME49710
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Old 22nd February 2019, 07:54 PM   #21
simonra is offline simonra  United Kingdom
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Quote:
Originally Posted by jeanlemotan View Post
I did the math like this:
1. The 3886 is configured with a gain of 12 and can reach (Vsupply - 3V) on each rail before clipping. For +/- 28v supply, this means I want to drive it to max +/- 25V. Say 24V to make it round. So a gain of +/- 2V and a gain of 12x will result in +/- 24V swing.
2. The 49710 can swing +/-14V before clipping so to get the +/-2V for the 3886, I have to divide it by 7.
If you want to you can get closer to the rails, also you could use +-17V supplies for the 49710.

As for the GM/PM discussion, I guess my lack of knowledge/terminology shows - you need to break the loop (as you have done) to see the feedback as it will be presented to the inverting input otherwise you're just looking at a virtual ground... hopefully someone else can advise whether your simulation is accurate in that respect.
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Old 22nd February 2019, 09:58 PM   #22
jeanlemotan is offline jeanlemotan
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Here's the schematic in TINA-TI if anyone is interested in playing with it.
There are both parts - the inverting and non-inverting, some speaker simulation and a small input filter.
Attachment 738242

Enjoy
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Old 23rd February 2019, 07:31 AM   #23
jeanlemotan is offline jeanlemotan
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Quote:
Originally Posted by jeanlemotan View Post
Here's the schematic in TINA-TI if anyone is interested in playing with it.
There are both parts - the inverting and non-inverting, some speaker simulation and a small input filter.
Attachment 738242

Enjoy
For anyone downloading this, note that the SPK+ terminal is connected to ground by mistake due to some tests that I was doing.
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Old 23rd February 2019, 08:15 AM   #24
jeanlemotan is offline jeanlemotan
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Quote:
Originally Posted by simonra View Post
If you want to you can get closer to the rails, also you could use +-17V supplies for the 49710.

As for the GM/PM discussion, I guess my lack of knowledge/terminology shows - you need to break the loop (as you have done) to see the feedback as it will be presented to the inverting input otherwise you're just looking at a virtual ground... hopefully someone else can advise whether your simulation is accurate in that respect.
I just did some clipping tests and it's looking promising.

Here is the LM3886 clipping:
Screenshot from 2019-02-23 10-02-31.png

It's obvious that the 49710 is trying to correct the clipping and goes into saturation.
There are some really bad spikes as well in there:
Screenshot from 2019-02-23 10-14-51.png


And here is the 49710 clipping:
Screenshot from 2019-02-23 10-02-34.png

Much nicer. I had to change the 7:1 divider for a 6:1 divider as my math was off.
Now the voltage swing is around +/- 23V out of a 28V supply for the LM3886 so there's enough margin for it before it clips.

Thanks for the idea simonra!
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Old 23rd February 2019, 08:31 AM   #25
jeanlemotan is offline jeanlemotan
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Quote:
Originally Posted by tomchr View Post
PM, GM are simulated open loop.
I don't show any simulations of a composite amp on my website. I show loop simulations of the LM3886.
I meant to say the that I opened the loop like you do for the LM3886.
I later found this https://training.ti.com/ti-precision-labs-op-amps-stability-3 and tried it as well. The results are in post 18

Quote:
Originally Posted by tomchr View Post
No. The PM/GM (which are loop parameters, hence simulated open loop) are just one piece of the puzzle.

I start with the loop analysis, because if the loop isn't stable there's no point in simulating anything else. I then simulate the closed loop response to make sure I don't have excessive peaking and to pick out potential issues that I may have missed. I simulate the transient response to see how the amp behaves when slewing, when it reaches clipping, and on hard clipping. I look at the DC operating point to see the DC offset. I look at the noise performance. And take a quick glance at the THD to see if anything is fundamentally broken.
I simulated the closed loop peaking and transient response as done here: TI Precision Labs - Op Amps: Stability 4 | TI.com Video
They seem ok as far as I can tell (results in post 18)

Quote:
Originally Posted by tomchr View Post
In a production environment I sweep these parameters across supply voltage and temperature as many of these characteristics vary across temperature. I don't recall if the LM3886 macro model takes temperature into account. It might. I know it laughs at the power supply voltage. Nothing power supply related is simulated with the LM3886 macro model.

Once I have a circuit that I feel reasonably confident in, I'll build it. I then correlate reality with simulation. Over time, I've developed a sense of the strengths and weaknesses of the LM3886 model, which allows me to shorten the development time. Basically, I know how far off target to aim in order to hit the target in reality.
This is the part I'm most afraid of - testing it out. I have an oscilloscope (a DS1054) and a signal gen but not a spectrum analyzer nor a lot of spare time. Simulations are easy to run and tweak - type a value there, add a wire there etc - but real bench testing requires a lot of time.
As I said in the first post - even if it works, I really want to know that it works for the right reasons, so I will do bench testing even if it sounds ok.

Quote:
Originally Posted by tomchr View Post
Based on your questions so far, I recommend taking half a step back and brushing up on some stability theory. Chapter 8 in Franco is a good place to start. I'm sure there's a corresponding chapter in Yung. You can find the full references here: References.
In general, I highly recommend Franco for anyone interested in working with opamps and amplifiers in general. The latest edition (3rd) is absurdly expensive, in particular since it hasn't changed in 15+ years, but the 2nd edition is every bit as good (there were very few changes between editions) and can be found on the used market for less.

Tom
I definitely need to get up to speed with this topic indeed. Fortunately it's very interesting as well so makes for some nice reading.
Thanks a lot for the references. I cannot believe how much info you can find today just by asking in the right places.
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Old 23rd February 2019, 02:14 PM   #26
simonra is offline simonra  United Kingdom
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Quote:
Originally Posted by jeanlemotan View Post
This is the part I'm most afraid of - testing it out. I have an oscilloscope (a DS1054) and a signal gen but not a spectrum analyzer nor a lot of spare time. Simulations are easy to run and tweak - type a value there, add a wire there etc - but real bench testing requires a lot of time.
As I said in the first post - even if it works, I really want to know that it works for the right reasons, so I will do bench testing even if it sounds ok.
The problem with a design like this is if you get it right the noise and distortion should be so low they're difficult to measure. I think you can get a long way with a decent sound card, there's lots of information here on how to do that. A cheap analogue scope is pretty useful too.

Quote:
Originally Posted by jeanlemotan View Post
I definitely need to get up to speed with this topic indeed. Fortunately it's very interesting as well so makes for some nice reading.
Thanks a lot for the references. I cannot believe how much info you can find today just by asking in the right places.
My learning process goes:
read -> think I understand something -> build something -> realise I didn't understand properly -> read some more etc.
I think you have to factor in failures... I've got a drawer full of PCBs that didn't work properly (or at all) but I learned a lot more from those than the designs that worked first time.
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Old 23rd February 2019, 08:15 PM   #27
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by jeanlemotan View Post
[...] nor a lot of spare time. Simulations are easy to run and tweak - type a value there, add a wire there etc - but real bench testing requires a lot of time.
Simulations are certainly the fastest and cheapest way to "build" a circuit. Do expect to spend significant time in the lab with a composite amp, though. They do have a nasty way of pushing the parts into operating corners not characterized well by the simulation models. It's more fun to have your amp generate lots of heat in a load resistor than to have it fry your speakers, so take your time there. It's not like you have to meet a set production schedule.

For a more turnkey solution, the basic LM3886 is pretty tough to beat. All the information you need is in the data sheet (and on my website).

Tom
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Old 24th February 2019, 03:31 PM   #28
jeanlemotan is offline jeanlemotan
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Default Schematic, PCB

I started working on the schematic & PCB on EasyEDA.
Here's the link:
amp-LM3886-composite - EasyEDA

pcb1.png

To keep traces short in the feedback loops the PCB gets very interesting, especially due to the paralleled LM3886. I had to flip the second one in each parallel group, so one LM3886 goes on the top layer and the second one on the bottom layer so they will alternate on the PCB.

The PCB is work in progress - I only have the non-inverting part partially done: the 3886's, the 40720 and the 2277 servos. Due to the flipping of the second 3886 the layout got pretty symmetrical which should help to keep the 2 paralleled branches working similarly (I hope).

Still working on routing the power and quiet grounds.
I had to populate both layers (top for signal components and bottom for decoupling mostly).
Also, the components are SMD as I really like to do SMD and they are a bit crammed together.

Most of the parts are from LCSC as they tend to be 2-5x cheaper for passives (metal film for resistors) and ~2x cheaper for chips.
A few resistors are still from Digikey as I couldn't find them on LCSC.
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Old 24th February 2019, 04:22 PM   #29
asuslover is offline asuslover  Romania
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Composite amplifier: LM3886 + LME49710
Do you need servo for each LM3886 or one should be enough for each pair ?
Tom used in his parallel86(lm4780) one servo for both halves of the chipamp.
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Old 24th February 2019, 04:32 PM   #30
jeanlemotan is offline jeanlemotan
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Quote:
Originally Posted by asuslover View Post
Do you need servo for each LM3886 or one should be enough for each pair ?
Tom used in his parallel86(lm4780) one servo for both halves of the chipamp.
I think that's because the 2 halves of the lm4780 are very similar and will have a near identical offset.
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