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Old 3rd November 2014, 01:13 AM   #1
dellama is offline dellama  United States
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Default An LM3875 composite amp

Lots of us have built gainclones and most of us have outgrown them. It's time to look toward the future and repurpose those chips into something more SOTA. We have some examples of how successful composite amps can be from the esteemed Mauro Penasa with his Howland pump inspired MyRef and from Tom Christianson's own unique implementation of the LM3886. I'm ready with some teflon sheets, copper foil and glue to start prototyping some designs and I'm hoping to get a crash course with LTSpice along the way. The design goals are for a small, pristine clean amp that can use just about any power supply. Here are the projected specs:
30W/8R 50W/4R per channel with THD+N of 0.0002% from 20Hz-20KHz with full bandwidth of 5Hz-50KHz +/-3dB

Last edited by dellama; 3rd November 2014 at 02:22 AM.
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Old 3rd November 2014, 02:14 AM   #2
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Old 3rd November 2014, 07:33 AM   #3
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You may want to check out my MiniRef 1875:

MiniRef Schematic and PCB layout

It's now at PCB version 1.04, with upgrades to the topology and implementation over the last 2 years, and the audible sonics are comparable with the best nested topologies, even with modest chipamps like the TDA2050. The key features are excellent sonics with inexpensive and commodity parts, providing modest output power in the 15 to 20W range into 8 ohm bookshelf-sized speakers.

For higher powers in the 80W per channel range, I also have a prototype dual parallel LM3886 design called the MyRef X2. It is fully functional and stable, but some further work may be needed to bring the sonics on par with the MiniRef v1.04.

BTW, the layout is critical with most nested topologies - point-to-point or breadboard/perfboard-type layouts may have too many parasitic elements to allow easy optimization for sonics. You may have to lay out a PCB with carefully chosen ground planes to be able to get to the level of the existing designs, some of which have taken years to optimize.


Edit: AFAIK, there isn't an LM3875 composite design in the DIY domain, but that may simply be due to the easy availability of the LM3886, which has comparable specs. If I were to develop a fresh design for 4-ohm loads, I'd probably go with the LM4780, dual parallel monoblock.

Last edited by linuxguru; 3rd November 2014 at 07:41 AM. Reason: addendum
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Old 3rd November 2014, 10:24 AM   #4
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<< I'm hoping to get a crash course with LTSpice >>

YouTube is the place.
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Old 3rd November 2014, 11:45 AM   #5
dellama is offline dellama  United States
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Yeah, youtube has some tutorials but they're not likely to be as helpful as direct input from experienced SPICE users. Hey linuxguru, that's a cute current pump implementation. Do you have any experience with Oscad?
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Old 3rd November 2014, 04:41 PM   #6
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Quote:
Originally Posted by dellama View Post
... Do you have any experience with Oscad?
I've used OrCAD on DOS about 25 years ago, but now all my layouts are done with Eagle.
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Old 3rd November 2014, 05:38 PM   #7
tomchr is offline tomchr  United States
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I'm glad to see I've inspired some thinking on the topic of composite amplifiers. That's definitely the way to go if you want performance.

The MyRef-series is a Howland current pump. If you are interested in the design challenges, including stability challenges, of this type of circuit, I encourage you to read Bob Pease's article on the subject (National Semiconductor (now TI) AN-1515). It's actually a very accessible read for anyone with some electronic design skills.

Note that to get the power specs you list, you need to do an excellent job with the thermal management on the LM3875. In particular with 4 Ω load, the thermal management gets to be a real challenge.
Also keep in mind that the THD at higher frequencies (1 kHz and above) is largely determined by the PCB layout. You need to minimize the inductance of the layout, in particular the inductance in the ground reference for your error correction opamp.
You can find a ton of pointers on my Taming the LM3886 website. I use the LM3886 as an example, but the same math applies to any other chip amplifier.

Should you be curious about the performance of your circuit once you have it built, I'll be happy to measure it for you. I should have an APx525 before the end of the year and intend to offer my test&measurement services to the DIY Audio crowd.

And should you want to take a shortcut and get to a working circuit faster, you can always check out my Modulus-86. Should you prefer the LM3876 rather than the LM3886, you can use the LM3876 in a Modulus-86 board as well. As far as I can tell, the LM3876 is the same IC as the LM3875 except the former offers a mute function and is pinout compatible with the LM3886.

~Tom
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Old 3rd November 2014, 06:07 PM   #8
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by bentsnake View Post
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<< I'm hoping to get a crash course with LTSpice >>

YouTube is the place.
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LTSpice. If your a beginner then click the link in my signature.
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Old 3rd November 2014, 06:30 PM   #9
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Quote:
Originally Posted by tomchr View Post
... As far as I can tell, the LM3876 is the same IC as the LM3875 except the former offers a mute function and is pinout compatible with the LM3886.
I can confirm that the LM3876 is not just an LM3886 with smaller output transistors. It behaves differently, and will not work in a stock MyRef (output sticks to -Vs on power up). This suggests that the small-signal internals of the LM3876 differ from the LM3886. It could well be a modified LM3875 as you conjecture.
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Old 3rd November 2014, 07:30 PM   #10
dellama is offline dellama  United States
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I am going to put this amp together in an unorthodox manner and start with the grounding topology. Tom is right to point out the final distortion numbers are likely to depend on how cleverly we can tie this amp together. While you guys look over the grounding scheme, you should start imagining the components working within it.
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