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Old 6th April 2010, 11:47 PM   #1
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Default A chip-amp to rival Hi-End - design advice

Hi all,

I'm currently in the process of designing a chip amp that I want to sound pretty much as good as it gets. A piece that will rival Hi-End. It is not exactly a "money no object" exercise, but I'm willing to go quite far where expenses can be justified.
My requirements (aside from sonics) include:
- ability to drive big floorstanding speakers well (I don't believe in small speakers)
- power anywhere in the region of 25-150W pc
- it has to be simple to build - I admit I don't have much working knowledge of audio electronics.

So I thought it would be a good idea to ask for advice from some knowledgeable source before I get too far with the project

What have I come up with so far:
- a bi-amped stereo design, based around Peter Daniel's LM3875 kit (DIY Chip Amplifier Kits, PCB's, Components and Information.)
I have built a simple stereo amp based on it before and I was quite pleased with it, its attack, bass control and especially how direct and immediate, "heart-grabbing" the sound is - this is my main criterion when judging the sound. I'm hoping that bi-amping, and perhaps a slight bias into class A will improve things further; perhaps also will enable it to resolve fine details with greater clarity (this is not its strongest side at the minute).
- power supply and pre-amp will be in separate cases
- I intend to use the best resistors / caps I can get hold of (a great shame Black Gates seem to no longer be available)
- four silver-plated copper heatsinks (don't laugh - I have them anyway)
- bare solid core silver wiring throughout pre- and power amp sections. Insulation provided by sandwiching wires between layers of wood that will form the case.
- passive preamp, consisting mainly of a single Noble pot (I can't really justify spending the money on a transformer from Stevens & Billington - and I have the Noble anyway)
- a 500VA +-25V toroid from antrimtransformers.com, with antistatic screen, encapsulated.
- the power amp layout at current stage has all input wires of [u]exactly[u] the same length, output wires are also all of equal length. I'm working on power supplying cables, might come up with a perfectly symmetrical layout for them too.

I have put quite a lot of thought into all this, but I realise I may be going in an entirely wrong direction here or there... So if you have any suggestions, any advice will be greatly appreciated.
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Old 7th April 2010, 03:39 PM   #2
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As far as speakers go, bigger speakers are supposed to be easier to drive than small ones. Given that, it depends entirely on exactly *what* "big floorstanding speakers" you are planning to use. I've used several chipamps from a 110Wpc bridged LM4780 to a 30Wpc single LM3886 to drive my floorstanders (3-way, single amp per channel), and they did it with ease.
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Old 7th April 2010, 03:51 PM   #3
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Thanks for the answer Redshift. At the minute, I'm using Wharfedale Middle Field Monitors (3-speaker, 2.5-way floorstanders, 89dB). I'm actually looking into buying new speakers anyway, and I'll make sure they have good sensitivity.

My main concern is with the actual "core" of the design - that is, the choice of chip and topology, as the array of available options is truly bewildering. I'm sure I won't go far wrong with LM3875s (after all, they started the whole Gaincloning movement) - but this time I'm looking for something capable of putting out a truly exquisite sound, not just "good for the money". I hope this makes sense...
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Old 7th April 2010, 03:56 PM   #4
wahab is offline wahab  Algeria
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LM3875/3886 are old amps designs ,dating from the mid 70s at best,
that were integrated in a single chip.
There s no way for them to compete with a properly designed
discrete power amp..
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Old 7th April 2010, 04:04 PM   #5
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You don't need to design anything--it's already been done, and is now being offerred in a group buy at an amazing price. See all threads involving the Mauro Penasa "My Ref" design, but especially the current Group Buy thread if you're really ready to build an amplifier that easily equals or surpasses MANY expensive commercial designs, both chip-based and discrete.

Peace,
Tom E
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Old 7th April 2010, 04:23 PM   #6
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Wahab - I respectfully disagree. Gaincard from 47Labs was based on LM3875, and, upon release, outperformed pretty much every amp in its price range (which was 1500$ for the basic version, if I remember correctly). In light of that fact, your claim is somewhat unfounded.

Madisonears - I didn't mean actually designing the entire circuit, sorry if I didn't make myself clear. I'm looking for a "ready made" design, preferably a kit, which I can then refine. So far, Peter Daniel's LM3875 kit has come out on top, because it's compact, elegantly designed, and very straightforward. Complex designs don't appeal to me at all, they look messy and lack subtlety.
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Old 7th April 2010, 05:01 PM   #7
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Hi,

Quote:
Originally Posted by wahab View Post
LM3875/3886 are old amps designs ,dating from the mid 70s at best, that were integrated in a single chip.
Funny, there is at least ONE feature in these chips that was first documented in the 1990's by D. Self in his Wireless World article series.

Quote:
Originally Posted by wahab View Post
There s no way for them to compete with a properly designed discrete power amp..
This I may give, if you define "properly designed discrete power amp" as one that outperforms (within the power limits of the NS Chips) the LM3875/3886/4780. I personally would actually agree this, as there is little point for discrete designs that are worse, yet objectively and subjectively a well implemented LM38XX is hard to beat in the 30 - 50W power range.

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Old 7th April 2010, 05:10 PM   #8
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Thumbs up Audiosector!

Quote:
Originally Posted by uncle_leon View Post
Wahab - I respectfully disagree. Gaincard from 47Labs was based on LM3875, and, upon release, outperformed pretty much every amp in its price range (which was 1500$ for the basic version, if I remember correctly). In light of that fact, your claim is somewhat unfounded.

Madisonears - I didn't mean actually designing the entire circuit, sorry if I didn't make myself clear. I'm looking for a "ready made" design, preferably a kit, which I can then refine. So far, Peter Daniel's LM3875 kit has come out on top, because it's compact, elegantly designed, and very straightforward. Complex designs don't appeal to me at all, they look messy and lack subtlety.
Hello uncle_leon, I build the LM3875 kits from Audiosector, they sound really good and the best of all is it is so easy to build them.....that a caveman can do it!. But I would "strongly" recommend to use BIG power supply at least 10,000uf per rail. I know it comes with just 10uf cap, but I would not use them.I say this because I try many kind of caps in the power supply and they sound much better with big caps, you will get a punchy bass. There are many people that say the "small" cap gets faster sound..Not true!. just use alligator clips to test them and you decide if you like with big or small caps.
good luck.
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Old 7th April 2010, 05:15 PM   #9
wahab is offline wahab  Algeria
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Those NS circuits are LIN topologies with a transistor connected
as emitter follower to drive the voltage amplifier stage...
A current mirror is used as load for the single differential input stage..

In this respect, the design is the one that was famed as doug self s
blameless amp, with the big difference that the LMs use the
dreadful quasi complementary arrangement for the output power stage.
Anyway, the current capability is very poor compared to a discrete design,
and therefore, it cant drive difficult loads in a satisfactory way...
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Old 7th April 2010, 05:23 PM   #10
Mooly is offline Mooly  United Kingdom
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An interesting thread... and quite timely as I had been looking along similar lines, out of curiosity really, about designing a high performance amp based around monolithic output IC's.

There are pros and cons regarding IC versus discrete. My initial investigations have been based on the TDA7293 device which allows for parallel "master and slave" operation giving the option for driving very low impedance loads. At this point in time I'm not sure it can rival a top flight discrete design sonically. Technically it can not.

uncle_leon mentions using the best materials etc... which is fine although I'm not a fan of "boutique" parts myself.
Of much more practical importance is getting the wiring correct, particularly the grounds and points of reference for the audio signal. Don't underestimate this... it's where the majority of DIY builds fail.

Good luck with it
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