Hi,
well you have to start somewhere. Normally you should know how much power you want for your speakers. How much do you have now? Is it enough? When not you should at least double it to have an effect.
If you know the power and the impedance of your speakers you can set the right combination of voltage and bias. After that you can set the amount of fets and heatsinking. Voltage to be set is that at the fets, the other voltage depends on your power supply (C or C-R-C or C-L-C etc.)
William
well you have to start somewhere. Normally you should know how much power you want for your speakers. How much do you have now? Is it enough? When not you should at least double it to have an effect.
If you know the power and the impedance of your speakers you can set the right combination of voltage and bias. After that you can set the amount of fets and heatsinking. Voltage to be set is that at the fets, the other voltage depends on your power supply (C or C-R-C or C-L-C etc.)
William
Hi,
looks not too difficult as a load.
You could try 17V at the fets and 6A of total bias.
At 50% ac-current-gain this would give 56 watts 8 Ohms and 90 at 5 Ohms (being also the max. power output)
You would need 8 fets per channel for 26 watts dissipation per fet and 208 watts / channel.
Transformer voltage depends on the used configuration. Try Duncans PSU designer (there´s a link in the spreadsheat).
Heatsink should be lower than 0,15 °K/W per channel
William
looks not too difficult as a load.
You could try 17V at the fets and 6A of total bias.
At 50% ac-current-gain this would give 56 watts 8 Ohms and 90 at 5 Ohms (being also the max. power output)
You would need 8 fets per channel for 26 watts dissipation per fet and 208 watts / channel.
Transformer voltage depends on the used configuration. Try Duncans PSU designer (there´s a link in the spreadsheat).
Heatsink should be lower than 0,15 °K/W per channel
William
Plug your numbers into the AXE-1.2 Spreadsheet here: http://www.diyaudio.com/forums/showthread.php?postid=552824#post552824
One that meets your power goals at your speaker's impedance - roughly 5 ohms minimum, so tending towards higher current will satisfy that. Wuffwaf gave you one operating point that will meet your goal of >80W across the board.
With 21.7 Volts AT THE FETS you'll get around 90W with 6A bias. at your impedance minimum, and you'll still be above 80W to 9.5 ohms. To the left of the peak you are current limited - to raise power increase the bias. to the right of the peak you are voltage limited, increase rail voltage to increase power.
If you can keep your heatsinks under 50C with active cooling you can use 12 FETs. That's >500W to get rid of, plus the active cooling system's waste heat.
With 21.7 Volts AT THE FETS you'll get around 90W with 6A bias. at your impedance minimum, and you'll still be above 80W to 9.5 ohms. To the left of the peak you are current limited - to raise power increase the bias. to the right of the peak you are voltage limited, increase rail voltage to increase power.
If you can keep your heatsinks under 50C with active cooling you can use 12 FETs. That's >500W to get rid of, plus the active cooling system's waste heat.
" To the left of the peak you are current limited - to raise power increase the bias. to the right of the peak you are voltage limited, increase rail voltage to increase power. " sorry my english isn't very good 
. can you explain this? i think the BIAS current is what i can set on the board using a variable resistor, as well as the rail voltage. any one have any suggestion of what rail voltage and BIAS current i go for?
Angus
. can you explain this? i think the BIAS current is what i can set on the board using a variable resistor, as well as the rail voltage. any one have any suggestion of what rail voltage and BIAS current i go for?Angus
lscangus said:" To the left of the peak you are current limited - to raise power increase the bias. to the right of the peak you are voltage limited, increase rail voltage to increase power. " sorry my english isn't very good
Angus
Did you take a look at the AXE-1.2 spreadsheet? There is a graph displaying the maximum power output for different impedances. There is a peak in the graph where the amplifier has its maximum power. At a lower impedance the amplifier is limited by the amount of current it can supply and with a higher impedance the amplifier is limited by the voltage at the fets.
I'm not sure if this post helps you but what i would do is this:
Take a look at the spreadsheet and fiddle with the numbers of
BIAS and 'voltage at fets'. Raising the 'voltage at fets' will move the peak output towards a higher impedance and raising the bias will move the peak towards a lower impedance (both resulting in more heat to be dissipated).
You can go as crazy as you like on the specs (although there is an upper and lower limmit), but remember you've got to get rid of the heat! And using 2 x 500 Watt class A amplifiers in your listening room will heat up the room considerably.
Regards,
Jacco
Hi,
the impedance of your speaker has a minimum of 5 Ohms so no need to have more power at lower impedances.
An Aleph2 is not an Aleph X and has a bias setting of around 3.3A
There is no default bias setting AND the voltage at the fets can only be changed by changing trafos!!!!! Not by changing resistors or fiddling with a pot!!!
William
the impedance of your speaker has a minimum of 5 Ohms so no need to have more power at lower impedances.
An Aleph2 is not an Aleph X and has a bias setting of around 3.3A
There is no default bias setting AND the voltage at the fets can only be changed by changing trafos!!!!! Not by changing resistors or fiddling with a pot!!!
William
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