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Old 22nd April 2003, 06:46 PM   #1
orbital is offline orbital  United Kingdom
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Question Not another gainclone!... transformers

OK, so I've been reading away at these gainclone thread(s) for quite a while, and just when I thought I had analysed pretty much every component to extreme, I remembered this graph (hopefully pic attached below!) from the LM3875 datasheet: (page 9, top right)


Now this has got me wondering about what value transformers to use...

The bottom axis is DC voltage, so dual 25Vac transformer = +/-35vdc supplies (approx)

As supply increases, there is a sharp drop in the 'power' rating on the graph
Does this mean I can't drive a 4Ohm load from +/-35vDC supplies?

Guessing, i'm thinking the graph could mean that as you increase your volume/power output, at about 55W (for 4Ohms) the SPiKe / overvoltage protection kicks in and limits the power output (and lots of distortion!) - In which case it would be implying that it would still be fine with lower volumes.

Or is it simply showing that you CANNOT drive a 4Ohm speaker from +/-35vDC supplies at any level?

The reason i'm asking is i was just about to buy a pair of 25vac toroidals, but could i be making an amp which works fine with 8ohm speakers, but incapable of working with 4 or 6ohm?
(In which case, its down to a 18vac toroidal, which would prevent you from reaching the higher power output levels with 8ohm speakers!
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Old 22nd April 2003, 07:23 PM   #2
Bricolo is offline Bricolo  France
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you'll not be able to run 4 ohm loads at full volume (or full power)

the spike will switch on when you'll reach 45W, below this level you won't have any problem

PS: take care that your loudspeaker's impedance hasn't dips below 4ohm, it they have, spike will switch on before 45W (something like 30W for 2 ohm, still enough!)
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Old 22nd April 2003, 08:06 PM   #3
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Why not go with the higher voltages for 8 ohm loads, and parallel a couple chips for driving lower impedances? Just a suggestion.
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