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a few questions bout components
a few questions bout components
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Old 1st December 2005, 03:45 AM   #1
troublecomes is offline troublecomes  Australia
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Default a few questions bout components

Hello. I've just registered to the diy forums. Im quite a novice, my experience is limited to car wiring and a few guitar pedals. So ive got a few most likely pretty dumb questions. Ive had a search for the answers but im running out of time...

Im running out of time because im currently in India but im going home (to western australia) very soon and would like to get some components cheap over here to start my GC lm3875 (dual monoblock).

So my questions are:
-is there much benefit to getting two transformers instead of one with twin secondries? Its still true mono-block with just one right? Assuming the transformer has a high enough VA rating. (Sorry this doesnt sound like the right terminology).

-I noticed on the decibel dungeon site that some designs use two recitfiers per channel, instead of one per channel. It looks to me (i might be wrong i dont know much about this yet) like each of the twin secondries go to a seperate rectifier. Could someone explain the situation for me? Does this make a huge difference? I ask because i was planning to get a brian GT kit (dual mono-block) and it only has two recifier boards in it, one for each channel) and im about to buy some parts and i need to know what transformer to get.

-Also, i want to used a stepped pot for volume, to keep dual mono-block do i need one with dual poles (dont know if this is the correct terminology sorry). Or is this not how it works?

Thanks all, so about the length, the bad (India travel influenced) spelling and lack of terminology/clue.

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Old 1st December 2005, 04:50 AM   #2
neutron7 is offline neutron7  Canada
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the brianGT PSU board are dual rectifier, if you look close you will see there are 8 diodes per board. so you will need a dual secondary transformer for each one, or a single one with higher rating going to both power supplies.

as for volume, i may be wrong but i do not think it is a true monoblock unless there is no volume control. in that case you would have a seperate "preamp" which might not be anything more than the attenuator, or it might have a buffer, input selector or headpphone amp or whatever you like.

if you are planning to put 2 "monoblocks" in one case then you could use the dual gang stepped attenuator in that case anyways.
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Old 1st December 2005, 04:52 AM   #3
enzedone is offline enzedone  New Zealand
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Default Parts warning

I would be really carefull when buying parts overseas. I have done this in the past and then found that they were all copies. And they are very hard to spot. O.k so you will pay more at home, but going through a reputable company, you will at least get quality parts. Buy them if you really want to but be aware of what could happen. Maybe find someone who is local and also knows the industry. Thats just my two cents..
I realized that a major part of my job was to figure out how to use technology control to create economic force, or leverage, such that money and business flowed in Microsoft's direction” — Alex St.John, father of DirectX.
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Old 1st December 2005, 05:23 AM   #4
Anthony Dockril is offline Anthony Dockril
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Hey don't skimp on parts. I've come back to making gainclones after a break...and my experience making an inverted clone was that one resistor change could make or break the amp. In short try Black Gates caps and good quality resistors (Caddocks, Rikens etc). A good amp is like a good meal you need to make the dish using only the best ingredients.

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Old 1st December 2005, 07:33 AM   #5
troublecomes is offline troublecomes  Australia
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Default thanks

Thanks all.
yeah i was not actually looking at buying a bag of crappy resistors or caps, just the transformers. There is a massive electronics market here and im sure i will be able to get one of a reasonable quality. Good advice to talk to a local who knows the industry - ill do that.

Basically though i wanted to know, do i get two transformers, or just one with twin secondries...does it matter?

Not being sure which chip im going to use yet what voltages are ideal, or max VA for one transformer. Found a lot of help on lm3875 but not as much on the others.


lm3875 18-25V ~200VA
lm3886 ? ?
lm4875 ? ?

this is for one transformer for both channels.

Also, if im planning to eventually build a regulated PSU i need about 3V difference, so i should choose one a little on the high side?

Might sound stupid of me to want to get these here, but after a year of travel, i am completely broke and will be back at uni when i get home - but im keen to have a project.

Thanks again.
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Old 1st December 2005, 07:40 AM   #6
Nordic is offline Nordic  South Africa
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Well if they are cheap, stuff your suitcases with them mate... you could always ebay the ones you don't need.
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