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Old 21st December 2008, 11:38 PM   #1
krips is offline krips  Canada
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Default Help with 5-channel LM3875

Hi there. I'd like to piece together a 5 channel gainclone (perhaps based on the LM3875 Dual Mono Classic Kit from audiosector. I've looked at the guide posted on this forum and this is the list of parts I think I would need to order. I'm looking for input on what to get and help getting it all ordered and put together .
To get a 5 channel amp:
- Power cable http://www.parts-express.com/pe/show...number=110-442
- Fuse + fuse holder (4A ?)
- Rocker switch http://www.parts-express.com/pe/show...number=060-410
- Transformer http://www.parts-express.com/pe/show...number=122-655
- Full wave bridge rectifier http://www.parts-express.com/pe/show...number=050-060 is this one 'full wave' ?
- Reservoir caps http://www.parts-express.com/pe/psho...Number=027-244 Not really sure on these ones.
- 3 LM3875 kits http://www.audiosector.com/lm3875.shtml

Any help would be greatly appreciated. Thanks.
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Old 22nd December 2008, 12:22 AM   #2
lineup is offline lineup  Sweden
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Quote:
LM3875 Dual Mono Classic Kit
1 PCB set, consisting of 2 amplifier and 2 rectifier boards.
2 LM3875TF
4 22k 0.5w compact carbon resistors
2 220R 0.5w compact carbon resistors
2 680R 0.5w compact metal film resistors
4 1500uF 50v Panasonic FC Capacitors
16 MUR860 OnSemi Diodes
2 10uF Panasonic FC Capacitors
This is http://www.audiosector.com/lm3875.shtml parts, for 2 channels.

Now, there are a number of other (more cheap) components you could buy
if wanting to build 5 x LM3875 amplifiers.
------------------------------------------

For example the 1500uF/50V supply caps.
you could use 2200uF or 1000uF caps, (as 1500uF value is not found everywhere)
Say this one would do fine:
Click the image to open in full size.
2200uF 50V Radial Mini Electrolytic Capacitor


There is no real need to use those fat recovery MUR860 rectifier diodes.
Ordinary silicon diodes OR bridge rectifier rated 6-8 Ampere would do well.
DigiKey USA has got many more components for a low price to select from.
http://www.digikey.com/

For example MR752-BP is one standard 6 Ampere, 200 Volt rectifier diode.
MR752-BP - digikey
You get >10 units for $1.225 a piece

------------
EDIT: .... but as the MU860 diodes are cheaper!!!
you may want to order here:
MUR860 - 8 Ampere diodes
$1.000 a piece, if you buy more than 10 of those.

Regards
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Old 22nd December 2008, 02:11 AM   #3
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The caps probably aren't worth skimping on. They're not a huge cost, and they do affect the sound. Resistors probably less so. Honestly, the easiest way is to just buy the inexpensive kit(s) from Peter Daniel or BrianGT.

If you're doing a 5-channel home theater amp, the LM3886 will probably be a significant improvement. They provide a lot more current, so they will handle the lower impedances that are typical of HT speakers. Plus, you tend to drive HT speakers a lot harder (read: louder) than music speakers, so they'll better cope with that as well.

Good luck! I still haven't had time to finish up my LM3886 and LM4780 HT amps
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Old 22nd December 2008, 02:21 AM   #4
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Also, the caps that you linked to from Parts Express would probably be great coupling caps, though you can go down to 4.7uF instead of 10uF.

You might want to take a look at Antek Transformers here:

http://cgi.ebay.com/ws/eBayISAPI.dll...m=370122753005

(This is a 300VA transformer, but they usually have 25+25 500VA transformers available, and they're cheaper than PE).
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Old 22nd December 2008, 08:40 AM   #5
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Quote:
Originally posted by lineup
Ordinary silicon diodes OR bridge rectifier rated 6-8 Ampere would do well.
DigiKey USA has got many more components for a low price to select from.
http://www.digikey.com/

For example MR752-BP is one standard 6 Ampere, 200 Volt rectifier diode.
MR752-BP - digikey
You get >10 units for $1.225 a piece

------------
EDIT: .... but as the MU860 diodes are cheaper!!!
you may want to order here:
MUR860 - 8 Ampere diodes
$1.000 a piece, if you buy more than 10 of those.
At those prices you can buy a complete 8 A bridge rectifier for the price of a single MUR860 diode.
Or, if you want something more robust, spend the money for two diodes on this.
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Old 22nd December 2008, 11:42 AM   #6
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Quote:
Originally posted by pacificblue


At those prices you can buy a complete 8 A bridge rectifier for the price of a single MUR860 diode.
Or, if you want something more robust, spend the money for two diodes on this.
Hi PacificBlue! I really like your ideas.

I enjoy the sturdy standard rectifiers that can be used along with 4 tiny value caps (per each rectifier unit) when I'm after a bit cleaner DC and/or less interference for my radio.

I think that I would use the bridge rectifier unit for a chip amp style amplifier -or- the MR/MUR specialty diodes for a gainclone style amplifier.

For some reason that I can't find documented, owners of gainclones tend to favor the MUR and other specialty diodes. Why is this?
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Old 22nd December 2008, 04:01 PM   #7
BWRX is offline BWRX  United States
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Quote:
Originally posted by danielwritesbac
For some reason that I can't find documented, owners of gainclones tend to favor the MUR and other specialty diodes. Why is this?
Because someone told them that the discrete diodes sound better.
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Old 22nd December 2008, 06:35 PM   #8
Minion is offline Minion  Canada
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You might also consider useing a Bridged or paralell LM3875 for your subwoofer as the Sub needs a lot more power to get a decent Volume compared to your Fullrange or tweeters amps....
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Old 22nd December 2008, 06:54 PM   #9
lineup is offline lineup  Sweden
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Quote:
Originally posted by pacificblue

At those prices you can buy a complete 8 A bridge rectifier for the price of a single MUR860 diode.
Or, if you want something more robust, spend the money for two diodes on this.
Good option, pacificblue

There will be, in my opinion, no difference in performance.
But using one Bridge Rectifier (including 4 diodes in one package) instead of separate diodes
is much more simple in practical terms.

Just drill one hole.
And fasten your Bridge.
Solder the wires to terminals.
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Old 22nd December 2008, 07:18 PM   #10
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Quote:
Originally posted by danielwritesbac
For some reason that I can't find documented, owners of gainclones tend to favor the MUR and other specialty diodes. Why is this?
The name reveals it. The Gainclone is a clone of Lab47's Gaincard and that uses MUR860. Gaincloners derive from that fact that they must have been chosen for sonic reasons.

The real reason however is probably that you can skip the snubbers on the rectifier, if you use fast-recovery diodes. Standard diodes with snubbers generate about the same amount of noise as fast-recovery diodes without snubbers. While there may be little to no difference from a sonic point of view, Lab47 as a commercial manufacturer has to comply with certain standards on interference. And while a bridge rectifier may be easier to use for a gaincloner, using snubbers on it will be so much more expensive for a commercial manufacturer that the people at Lab47 may have found the MUR860 solution more economic.
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