The Aleph 5 will not double its wattage into a 4 ohm load--the bias isn't quite high enough for that--but that doesn't mean that it won't sound very good indeed.
There will not be any stability problems. The amp won't come unglued.
If you want to raise the bias, it can be done quite simply. If you're still in the stage where you're getting parts together, the first thing you're going to need to attend to is getting more heat sinks. The booger runs hot as it is, and if you increase the bias, it's going to run even hotter. Once you've got your heat sinks together (and made sure your transformer has enough muscle) you'll adjust some resistor values. You can either reduce the value of the Source resistors for the output stage MOSFETs or you can diddle the resistor values in the Aleph current source. The only schematic I've got handy at the moment is the Volksamp 60 (aka the Aleph 60). It's a similar amp, but not identical. On the 60 schematic, it shows 1 ohm 3W resistors at the MOSFET Sources. If you drop those to .91, .82, or .75 ohms, the bias current will increase in proportion. If you increase the value of R19 (remember, this is the 60 schematic, the part number on the Aleph 5 will be different), the bias will increase. You only need to do one of these, not both.
I'd recommend taking the Source resistor route--it's easier to figure out.
Keep an eye on the heat generated by each MOSFET. It's easy to get carried away.
Or, of course, you can just use the amp as it is and enjoy the music...
Grey
There will not be any stability problems. The amp won't come unglued.
If you want to raise the bias, it can be done quite simply. If you're still in the stage where you're getting parts together, the first thing you're going to need to attend to is getting more heat sinks. The booger runs hot as it is, and if you increase the bias, it's going to run even hotter. Once you've got your heat sinks together (and made sure your transformer has enough muscle) you'll adjust some resistor values. You can either reduce the value of the Source resistors for the output stage MOSFETs or you can diddle the resistor values in the Aleph current source. The only schematic I've got handy at the moment is the Volksamp 60 (aka the Aleph 60). It's a similar amp, but not identical. On the 60 schematic, it shows 1 ohm 3W resistors at the MOSFET Sources. If you drop those to .91, .82, or .75 ohms, the bias current will increase in proportion. If you increase the value of R19 (remember, this is the 60 schematic, the part number on the Aleph 5 will be different), the bias will increase. You only need to do one of these, not both.
I'd recommend taking the Source resistor route--it's easier to figure out.
Keep an eye on the heat generated by each MOSFET. It's easy to get carried away.
Or, of course, you can just use the amp as it is and enjoy the music...
Grey
There are at least 3 easy ways to up bias. The r19 trick is good to lower, but it will only raise it by 30% or so on A2. (i'm not sure for A5. save this adjustment for after it's built)
The 2nd way is to decrease source resistors. Run the nombers and make sure you're not trying to pass too much current (and wattage) through your output fets.
The 3rd is to add more pairs of output fets and source resistors. This is my method of choice because your not putting more load on the fets. The only problem here is if you put too many output fets on, you'll limit the upper bandwidth with gate capacitance. but that's not a real problem until you get past 10 pairs or so of IRFP244 ir IRFP240.
The 2nd way is to decrease source resistors. Run the nombers and make sure you're not trying to pass too much current (and wattage) through your output fets.
The 3rd is to add more pairs of output fets and source resistors. This is my method of choice because your not putting more load on the fets. The only problem here is if you put too many output fets on, you'll limit the upper bandwidth with gate capacitance. but that's not a real problem until you get past 10 pairs or so of IRFP244 ir IRFP240.
macka2,
Solid state amps 'automatically' know when they're attached to a given load--no need to 'program' the amp for a given impedance. Yes, it's true that for some tube amps, you need to rewire the output transformer, but solid state amps can do that on the fly.
Dr. ODD,
In the "Aleph Classic" mode, the output resistor array should pretty much be the same as all the Source resistors before it. In this case, if you dropped the Source resistors to, say, .82 ohms, then you'd drop the output resistor array proportionately. All things being equal (are they ever?) the resistors in the output array will match the Source resistors for the Aleph CCS and the output devices combined. Since the Aleph 5 has six 1 ohm resistors (3 for the outputs and three for the CCS), the final resistance in parallel of the six is .167 ohms. Nelson uses four .47 ohms in parallel to equal .112 ohms or so, then balances the difference elsewhere in the circuit.
Brian,
I tend to approach the addition of more MOSFETs with caution, as it changes the tonal qualities of the amp a bit. Adding more bias to the existing MOSFETs will generally make it sound better. Adding more MOSFETs will tend to darken the sound (and costs more). People may or may not like the result. That said, yes, it will work just fine.
Grey
Solid state amps 'automatically' know when they're attached to a given load--no need to 'program' the amp for a given impedance. Yes, it's true that for some tube amps, you need to rewire the output transformer, but solid state amps can do that on the fly.
Dr. ODD,
In the "Aleph Classic" mode, the output resistor array should pretty much be the same as all the Source resistors before it. In this case, if you dropped the Source resistors to, say, .82 ohms, then you'd drop the output resistor array proportionately. All things being equal (are they ever?) the resistors in the output array will match the Source resistors for the Aleph CCS and the output devices combined. Since the Aleph 5 has six 1 ohm resistors (3 for the outputs and three for the CCS), the final resistance in parallel of the six is .167 ohms. Nelson uses four .47 ohms in parallel to equal .112 ohms or so, then balances the difference elsewhere in the circuit.
Brian,
I tend to approach the addition of more MOSFETs with caution, as it changes the tonal qualities of the amp a bit. Adding more bias to the existing MOSFETs will generally make it sound better. Adding more MOSFETs will tend to darken the sound (and costs more). People may or may not like the result. That said, yes, it will work just fine.
Grey
Start pushing too much wattage into the fets without the aid of liquid nitrogen, then tell me which option is more expensive. 2 fets and 2 resistors is less than $5
Yes, you may darken the sound if you go from 3 to 12 fets, but 3 to 4 will not be a noticable change. Add a switch to the gates of the extras (on the output side)and see. Add a switch to the extra on the ccs side and it'll change the bias.
Yes, you may darken the sound if you go from 3 to 12 fets, but 3 to 4 will not be a noticable change. Add a switch to the gates of the extras (on the output side)and see. Add a switch to the extra on the ccs side and it'll change the bias.
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