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LM3886 & LME49720 composite amp
LM3886 & LME49720 composite amp
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Old 29th January 2015, 11:03 AM   #1
IO390 is offline IO390  United Kingdom
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Default LM3886 & LME49720 composite amp

G'day. This is my first post here in the chip amps section, so hi.

As well as my stAMPs, I also use an LM3886 amp for normal listening. I've used the LM3886 in the standard (datasheet) config, then I built a myref-ish thing using the LME49720 and a DC servo. If you're wondering why I'm using the LME49720 as opposed to the LME49710, it's because I use the other half as the integrator for the DC servo. Yes it doesn't have the same level of DC accuracy as an OPA227 for example, but that is totally unnecessary for a power amp IMO.

I was making a PCB order a few weeks ago and had some space left, so I whipped this amp and PCB up in a morning. It's not perfect but it definitely sounds like a notable improvement over either my myref-ish thing and definitely better than the standard LM3886. PCB size is 1.75"x1.75", and the ground plane is split in two places so there is a star ground at the LM3886 ground pin. I left spaces on the PCB for extra compensation related components so that it can be fiddled with and improved. I've attached schematics and a couple pictures of the PCB (note that some decoupling caps are missing from the schematic).

Oh and this isn't a copy of anything. I thought I should mention in case someone thought I had ripped off the Modulus-86 or something, which I haven't.

As for performance, the distortion at 15W RMS, 1KHz into 8 ohms was less than my sound card could measure, which is 0.0003%. I'll get some more graphs and upload them. Of course, distortion levels this low are trivial. DC offset on the output is about 500uV, but it does need about 200mS to settle so I have a relay delay output thingy. Please feel free to take the design and figure out some better values for things, I would do it myself but I don't have the time at the moment.

Anyway, any suggestions/comments/criticisms?
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Old 29th January 2015, 12:10 PM   #2
AudioLapDance is offline AudioLapDance  Canada
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Quote:
Originally Posted by IO390 View Post
... and the ground plane is split in two places so there is a star ground at the LM3886 ground pin. ...
The chip gnd pin is just for the mute circuit, so it doesn't deserve to be 'star' of anything

Speaking of 'stars', looks like the actual data's pointing more towards low-impedance grd plane, especially with a fast controlling opamp. But I'll let the pros explain it.

Watch for power-handling issues with small smt parts ... especially Rf since it gets the full swing of the output, I think it's in the SMTLM3886 thread.

Love your stuff by the way, gotta try me 'un of yer cuuuuuuute stamps!

Cheers,
Jeff
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Old 29th January 2015, 12:20 PM   #3
IO390 is offline IO390  United Kingdom
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Originally Posted by AudioLapDance View Post
The chip gnd pin is just for the mute circuit, so it doesn't deserve to be 'star' of anything

Speaking of 'stars', looks like the actual data's pointing more towards low-impedance grd plane, especially with a fast controlling opamp. But I'll let the pros explain it.

Watch for power-handling issues with small smt parts ... especially Rf since it gets the full swing of the output, I think it's in the SMTLM3886 thread.

Love your stuff by the way, gotta try me 'un of yer cuuuuuuute stamps!

Cheers,
Jeff
Yes, I read something about that on Tom's (I forget his last name) page on the LM3886. As I say, there's a lot of room for improvement here.

The low resistance of Rf etc is fine as I only operate this amp on +/-20V and I don't listen to things very loud. Those values could be simply doubled anyway.
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Old 29th January 2015, 01:11 PM   #4
abraxalito is online now abraxalito  United Kingdom
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How about going over to TL431 shunts in place of the LM317/337s? Reason being the regs you have give a surprisingly high Zout at low output currents. Shunt regs also aren't limited to as low an input-output voltage as the LM regs so punters who like it loud can go louder.
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Old 29th January 2015, 01:15 PM   #5
IO390 is offline IO390  United Kingdom
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Originally Posted by abraxalito View Post
How about going over to TL431 shunts in place of the LM317/337s? Reason being the regs you have give a surprisingly high Zout at low output currents. Shunt regs also aren't limited to as low an input-output voltage as the LM regs so punters who like it loud can go louder.
TL431 seems a bit OTT - I've built 0.05% references with a TL431 before.

But yes, a zener shunt makes sense. If I build another set of boards then I'll incorporate this, thanks for the tip.
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Old 29th January 2015, 01:22 PM   #6
abraxalito is online now abraxalito  United Kingdom
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The cooking grade TL431s are about 2% I seem to recall and under half the price of a TO220 reg. Yes they make accurate ones but I'm not suggesting those. Don't use the Chinese (Hong Kong, 'WS') brand though as they're way too noisy.
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Old 29th January 2015, 01:29 PM   #7
IO390 is offline IO390  United Kingdom
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Originally Posted by abraxalito View Post
The cooking grade TL431s are about 2% I seem to recall and under half the price of a TO220 reg. Yes they make accurate ones but I'm not suggesting those. Don't use the Chinese (Hong Kong, 'WS') brand though as they're way too noisy.
Yes - of course. I forgot that there were different grades.
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Old 1st February 2015, 09:37 PM   #8
papasteack is offline papasteack  France
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Since the psrr of the lme49720 is really good and it will correct all, does regulator is really needed ?
I just finished soldering of a Bato MM board - composite bridged lme49720/lm3886 with trim offset, remote sensing, and a lot of decoupling onboard, so i'm really interested in your project.

Last edited by papasteack; 1st February 2015 at 09:40 PM.
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Old 1st February 2015, 10:13 PM   #9
IO390 is offline IO390  United Kingdom
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That's a good point, these ultra low noise regs are not exactly necessary. The opamp designers have worked on high PSRR so that you don't have to worry so much about the supply...

I've not seen the Bato MM board until now. It seems there is a lot more about these composite amps around than I realised. I really like the idea of having a sense line to the speaker. I've joined diysmps so I can read a bit more about it.
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Old 1st February 2015, 11:10 PM   #10
papasteack is offline papasteack  France
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The full version original thread need to be translated from here, but it s a lot more technical : Bato MM-Amp (LM-most by macolakg) : Dokumentacija It seems that the original forum gone down and all was copied on yu3ma s forum. V1 smd pcb is very cool...

Last edited by papasteack; 1st February 2015 at 11:14 PM.
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