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!
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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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!)
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!)
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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