Aleph-X: High-Power Version

[diyman]

Blitz,

Thanx for the info. In fact calculating the value of source resistors as you mentioned should not be a problem as Grey 's formula is very straight forward. In fact, what I'm curious are the values of the OTHER RESISTORS. If you run the circuit at 27.7V(assuming its correct!), it's still more than double the original supply voltage of +/- 12V. If you compare the resistor values of the Aleph 30 which runs at +/- 25V, you will notice that some of the other resistors are slightly different from the original Aleph-X. So if we scale the power supply to +/- 27.7V @ 8.6A, do we just apply the higher voltage with the correct source resistors values only or do we have to scale the other resistors as well?

ckt

[Edwin Dorre]

Originally posted by Blitz
I have read Aleph-X threats now for 8 hours and I have to admit that I am not yet through with everything.

I have lots of the same questions. Perhaps someone with a lot of knowledge of the X design can make a cook book.

As input parameter; X watts at Y ohms, number of fets and a few (or a lot) of calculations for the volts, amps of bias, ohms of resistors, watts per fet etc. comes out (excel sheet?) .

This will save us a lot of reading and like me I can use all the help I can get. The Aleph-X thread is too long and has too much non-aleph-X info in it. For my Aleph 4 I used mark finnish his doc a lot.

Perhaps a volunteer?

Edwin

[Blitz]

Edwin,

This would be great, but I guess that is more than we can expect.

That's why I started this threat: Clarify the open upscaling questions and once this is clear, it easy to make the cals like I started it above. In parallel I am now reading through the Aleph-X-Threat (440 Word-pages) and delete everthing which is only noise / raw information, but does not influence the schematic of the amp. ONce I have a consolidated version of this thrat, I am happy to share this with you.

Best REgards

[Blitz]

From Mr. Pass:

"The XA200 has a CRCRC (cap/thermistor/cap/thermistor/cap)
filter for the power supply. At about an 8 amp bias, the
remaining ripple is about .02 volts."

Interesting...especially the 8 amps...are these per channel or per side ? I guess per side: If he has 32 mosfets in total, 16 per channel, 8 are current sources, 8 are output devices, 2 sides with 4 outputdevices each, each draws 2 A (most of the data comes from the owner's manual), this would mean 8 A bias for each crcrc-filter, meaning that we have 16 A in one channel, meaning that Grey's calcs on current are wrong, and Audiofreak is right.

Any thoughts ?

[Blitz]

Sorry guys, just found this from Mr. Pass on the xa-200:


Posted by Nelson Pass on 09-11-2002 08:00 PM:
"Been covered before. 4 banks of 8 matched devices."

So, a xa-200 has per channel 32 mosfets. Mmmmmh. I hope I understand my mistake now: The output is like a circutron, Therfore the current of the two sides don't add each other...When I have 32 mosfets, it come down to 8 output fets per side...each should than only draw 1 A instead of two. The pass-side says 2 A .. I am getting confused. Any thoughts ?

[paulb]

Originally posted by Blitz
I am now reading through the Aleph-X-Threat (440 Word-pages) and delete everthing which is only noise / raw information, but does not influence the schematic of the amp. ONce I have a consolidated version of this thrat, I am happy to share this with you.

Blitz, good idea. Could you post this on this Wiki?

[wuffwaff]

Ok,

I´ll try.

First it is very important not to confuse effective and peak (x2^0.5) values cause this makes things very complicated.

Lets say you want 200 watts in 8 ohms this means 40 volts and 5 Amps both effective values.
This is independent of class a,b,c,d,t or x.

Now you must calculate the peak values voltage = 40x 1.41=56.56
current = 5 x 1.41 = 7.07A
These are the values an amp must be able to deliver into an 8 ohm load.

Now for an Aleph X:

Since the amp is sort of bridged each output has to deliver half the voltage wich in this case is 56,56/2=28,28volt. To allow for losses driving the fets and over the source resistors the supply voltage has to be about 2 to 3 volts higher so + and - 30 to 31 volts.

The current that has to be delivered goes through both amps so it still is 7.07 amps. But since you have two amps total current is 14.14 amps.
Nice thing about the active current source is that it can actually double it´s output so total bias for an Aleph X is 14,14/2 = 7,07 amps.

This amp will dissipate 7,07 x 31 x 2 = 438 Watts for one channel.

More dissipation will be in the transformers, diodes, chokes etc. so 500-550 watts per channel is not unrealistic.

This means the transformer has to be about 1000-1500VA depending on personal taste. The secondary voltage depends a lot on wich diodes and/or chokes are used but won´t be much less than the supply voltage (so 28-31 volts). It is very important to calculate this before ordering!

I think it equally important to read Pass Labs user manuals very carefully and look at all the specs as nice but not neccessarily precise information.

william

[Blitz]

Thanks for this summary. After now going to numerous threats, I know that this is the coreect way to calculate it and you have even made us aware of other variables in the equasion. Thanks a lot.

Before I now calcualte the new, right values for the different setups (for the lazy bones): What would be your recommendation on the number of mosfets ? Let's take your example: 200 Watts in 8 ohms:

Power Dissipation only in the mosfets would be: 28,3*7,1*2= 400 Watt. When using 16 Mosfets (Let's assume 044Ns), we have 25 Watt / device at 28,3 V and 0,88 A, Correct ?

Now, from a power dissipation point of view this seems to be feasable. I read somewhere that the fets sound better the more current we put through them tough. Some put 2 A amps through them. Is it really the current which make them sound better or the power dissipation ?

Theoretically this example should result in the same values the XA-200 has. Well, still the 2 A through each Mosfets is not feasible yet or will the peak current be 0,88A * 2 ( I thought the factor 2 was already inculded in the equation above )?

Best Regards

[nobody special]

William,
Nice and concise! It's about time someone posted that...
I have been reading the x thread from the beginning, and it took me a while to sort through the calculations on my own... it's sort of a tricky circuit- with the active current sources and the bridged output stage.
One thing that nobody seems to talk about when considering dissipation is how much it will go up under normal operation when the active current sources "kick in". I assume that most listening will be using around 1-2 watts maximum, and that the peak demands will be very short in duration. Any thoughts on this?
I have calculated my heatsinking to have a 19° C temp rise at idle, but I'm wondering how much heat I will really see in actual operation.
My amp will be a 40 watt/ 8 ohm- 12 output devices / channel... max of about 60W- current limited into lower impedances. Can't wait to see some of the higher powered versions! Unfortunately, my heat sinks just can not handle any more, or I would crank the bias up.
-NS

[wuffwaff]

NS,

dissipation in the output fets is at it´s highest when no power is delivered. So actually the amp will cool down when delivering real power (this power is used to heat up your speakers).

With the X the two active current sources will work together in such a way that when one doubles the current, the other one will shut of so total current draw will be constant all of the time independent of input signal and/or load.

william