Aleph J-X Amp Project

[slr 5000]

hi all
what sort of output can i get out of the aleph j into 4 ohms if i crank up the bias and add extra sinks ?

is 50w at 4ohm possible ? i can also add extra out put devices if needed but would prefer to run the standard 4 per channel

cheers

[BobEllis]

See the spreadsheet here: NEWER Aleph-X calculationsheet AXE-1.2

You can do 50W/4R with 4 output devices and HUGE heat sinks, probably fan cooled. or just push the junction temperature into the shorter life region.

[slr 5000]

hi bob

is that with a standard aleph j with higher bias and bigger sinks ? ie not ajx

thanks in advance regards sheafer

[Bill Fuss]

You could do it by changing the current gain but it would no longer be a J. There is no replacement for displacement (dissipation).

[slr 5000]

displacement as in extra output devices?

like 3x pair per channel instead of 2 pair as standard

could the 2sj109 run the extra pair per channel and support the extra current needed to bias them high enough for the 50-60w in to 4 ohm safely

regards sheafer

[Bill Fuss]

If you add another pair to the output you will also change the current gain somewhat because the ratio will change, but it makes a large bias increase a little easier to deal with. Still, getting 40W out means lots of heat. Just the standard bias using 6 OPTs is around 170W of dissipation per/ch. I'm building one with 6 OPTs and I have paralleled 2sj74s in the front end to eliminate any question of drive.

Another option would be to lower the rail voltage, thereby lowering the dissipation and allowing more bias capability and optimizing it more towards a 4 ohm load.

Whatever you do it will not be a true Aleph J unless you balance the current gain back to 25%.

I'm sure others have their opinions on this.

[pmchoong]

Hi Bob

Thanks for the calculations... will use the 300mm high sink. I have a 500VA, 2 x 18v secondaries...is this transformer good enough for the PSU? Which PSU scheme is better ? What I have in mind for a 12 mosfet per channel is no longer a Aleph J? Do I need to readjust the bias?

[BobEllis]

Originally Posted by Bill Fuss

... There is no replacement for displacement (dissipation).

That's a muscle car reference.

is that with a standard aleph j with higher bias and bigger sinks ? ie not ajx

I assumed you meant to ask about AJ-X given the title of this thread. Just 8 outputs (same load on input as a AJ) is a bit dicey thermally. Plug it in to the spreadsheet. The spreadsheet will work for the AJ thermals, but the power output calculations don't work.

Bill's suggestion of doubling up the input devices is a good way to ensure that you have enough drive to handle the 12 pairs of outputs that would make the thermals work reasonably.

AFAIK, the AC current gain is set by the ratio of two resistors, not the number of output devices. I don't have the schematics handy to identify them, but they are connected to the base of the bipolar in the current source. Therefore, adding outputs won't alter the current gain.

[BobEllis]

Originally Posted by pmchoong

Hi Bob

Thanks for the calculations... will use the 300mm high sink. I have a 500VA, 2 x 18v secondaries...is this transformer good enough for the PSU? Which PSU scheme is better ? What I have in mind for a 12 mosfet per channel is no longer a Aleph J? Do I need to readjust the bias?

If you want to be a strict about it, any change from the original schematic makes it something other than an AJ-X. Different current gains mean different sound character. However, in my mind, it is still an AJ-X whether the current gain is set to 20% or 60%.

The AJ-X isn't as easily scaled as the A-X (if you want to call it easy to scale), due to lower drive current capability of the jfets. Doubling up seems like a reasonable solution, but I have not built the amp.

I'd go for the unregulated CLC - you're generating enough heat already and this amp has reasonable PSU ripple rejection. If you are shooting for 100W per channel output, that transformer is good for a single channel.

[Bill Fuss]

Originally Posted by BobEllis

That's a muscle car reference.



I assumed you meant to ask about AJ-X given the title of this thread. Just 8 outputs (same load on input as a AJ) is a bit dicey thermally. Plug it in to the spreadsheet. The spreadsheet will work for the AJ thermals, but the power output calculations don't work.

Bill's suggestion of doubling up the input devices is a good way to ensure that you have enough drive to handle the 12 pairs of outputs that would make the thermals work reasonably.

AFAIK, the AC current gain is set by the ratio of two resistors, not the number of output devices. I don't have the schematics handy to identify them, but they are connected to the base of the bipolar in the current source. Therefore, adding outputs won't alter the current gain.

The current gain is also affected by the current through the sensing resistors at the output so they are interrelated.