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Old 3rd June 2008, 07:01 PM   #1
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Default Guitarist goes stereo! Chip advice needed

Hello all. Got fed up with lurking, so first post!

I've had a fair amount of experience building stomp boxes and such, but this is my first time diy'ing an amplifier.

I'm looking at expanding my rig into stereo and too be honest all these chip types are baffling my brain, I'm not sure which is best for what i want. When finished, my rig should look like this, maybe adding a line driver or 2.

Guitar > FX pedal board > 50W Valve amp (with dummy load, volume on 11) > Stereo FX pedal board > 2 channel chip amp > Speakers

The chip amp needs to have enough beans to be heard above a drummer and to be able to handle a saturated power tube tone without sacrificing too much quality. i basically want the existing tone to come through with minimal colouration of signal after it leaves the tube amp.

The LM3886 or LM4780 look to do the job. However the LME49810 has got far more power, is stereo, and (at least National says) is designed for guitar amps.

Am i on the right path with chip choices? is the LME49810 too complex for a relative beginner? are 2x LM3886 boards enough power and worth it for the simplicity?

Thanks in advance, great informative forums btw
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Old 4th June 2008, 05:35 PM   #2
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If you can read a schematic and are good with the soldering iron i would recomend you to build a LME49810 amp with a couple of mosfets. These can go 100W without paralleling and have a more nice clipping than other chipamps, which is a must as you would be playing uncompressed music.

BTW these are the hardest chipamps to build because they have more parts than usual and because the LME49810 has thiny pins that need some expertise to fit on a perfboard. If you plan to buy a kit online this is not a problem for you and you should definitely go that way, but if you plan to go the perfboard way then you may probably want to build something easier (and cheaper) that can be recycled to give some nice computer loudspeakers before to get some confidence and then build the definitive one.

I belive that some forum members like jackinnj are selling those kits online.

I attach a picture of the one that i'm building (it is not finished yet) to give an idea of the complexity.
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File Type: jpg lme49810.jpg (72.6 KB, 234 views)
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Old 5th June 2008, 10:37 AM   #3
sreten is offline sreten  United Kingdom
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Hi,

The Baker clamp on the LME49810 operates at +/- 95V which in real
terms means for "soft clipping" you will need > +/- 100V rails which
in real terms is a very powerful amplifier. FWIW it is not a stereo chip.

Basically do not go there, you cannot sensibly use the feature that
caused you to choose the chip as being suitable for "guitar amplifiers".

For professional use chip amps have reliabililty / shutdown advantages
which need to be weighed against the inherent thermal limitations of
chip amp packaging, which will be exercised far more professionally
than in typical hi-fi use.

I'd suggest a parallel topology. Heatsinking two devices per channel
is always going to be more efficent than one, and will keep the chip
temperatures vs. heatsink temperatures down, which is good for
reliability, and not tripping the temperature dependent chip limiting.

Also do not overdo the power supply. For the LM4780 parallel with
+/- 35V rails will make a reliable amplifier for an 8 ohm driver load.
For a stereo music amplifier you need an around 200VA supply.

/sreten.
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Old 5th June 2008, 05:10 PM   #4
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Quote:
Originally posted by sreten
The Baker clamp on the LME49810 operates at +/- 95V which in real
terms means for "soft clipping" you will need > +/- 100V rails which
in real terms is a very powerful amplifier. FWIW it is not a stereo chip.
Are you sure about that?

The LME49810 is faster and cleaner than his smaller brothers and the lack of short-circuit protection is not a big problem if it's built inside a guitar amp cabinet.

That's the one i would build for me.
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Old 5th June 2008, 06:59 PM   #5
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by ionomolo


Are you sure about that?

Hi,

That what the specification sheet indicates and there is
nothing in the sheet that suggests it does otherwise.

/sreten.
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Old 5th June 2008, 07:44 PM   #6
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I understood the exact opposite "Baker clamp clipping voltage" as the voltage when the baker clamp stopped working (couldn't prevent hard clipping past it).

Sure that national has desiged the chip thinking that people will operate the chip at millimeters from the absolute maximum ratings?

EDIT: The clipflag lights at clipping regardless of the supply voltage!
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Old 6th June 2008, 12:11 AM   #7
teemuk is offline teemuk  Finland
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http://home.mira.net/~gnb/audio/bakerclamp.html
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Old 6th June 2008, 09:46 AM   #8
sreten is offline sreten  United Kingdom
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Hi,

TBH I find the specification of said clamp rather strange, and the
description of its operation in the data sheet extremely limited.

You would expect a description relative to the voltage rails if it was
always operational, and TBH the simplified schematic does not help
much either in establishing a reference point, both clamps appear
to be driven by the output voltage.

Perhaps I'm missing something completely ..... what is Vclip ?

/sreten.
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Old 6th June 2008, 02:49 PM   #9
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Clamping the output to a limit voltage seems both useless (it would be useful only for a certain voltage in the range of operation) and hard (where you take a 97V voltage reference from?). Perhaps we sould write national and ask?
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Old 8th June 2008, 03:58 PM   #10
Minion is offline Minion  Canada
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You might also consider a LM7293 which is a 100w DMOS amp chip, It is a Bit more difficult to build than say a LM3886 as it has more pins and features but many of these features can be disabled and not used....

I recently built a stereo LM7293 amp for Power Monitors or for PA use and it sounds quite good ,Probably as good or better than the LM3886 and the LM7293 Datasheet has a PCB design while not the best is useable and easy to make at home....

Cheers
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