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Old 6th June 2007, 12:04 AM   #351
ctong is offline ctong  United States
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Mr. Pass,

What is the bias per mosfet in x3 and x5? If the idle dissipation is roughly the same as the power output into 8 ohms, x5 would consume 125*5= 625W at idle! The user manual indicates that the total idle power consumption is only 200W while the service manual suggests that the bias is 1A per mosfet.
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Old 29th June 2007, 09:39 AM   #352
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Hello

I have seen that too. With the given source resistor voltage in the service manual of the X5 the whole dissipation for all channels is 1280W!!!
That cannot be correct.
But when you calculate it back with the given consuption of 200W (this value is right and the same for all amplifiers which housed in this chassis size) you get a bias current of about 150mA through each complemetary pair. That gives a voltage of 70mV over the source resistor. Perhaps it is better to read double this value over both resistors of a comp.pair.
X3: The manual says 40 MOSFETs at all. But that seems to be not correct. I think there are 36 or 48 devices. With 3 comp.pairs for one half of a channel the bias is about 170mA per pair. With 4 comp.pairs in parallel about 130mA.

Greetings
Dirk
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Old 29th June 2007, 10:28 AM   #353
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Default NP says this about output stage bias

In this very thread, only about 2 pages back, NP had something to say about bias in general and I seem to recall that the power dissipation documentation error was addressed at some point too.

Quote:
Originally posted by Nelson Pass


Setting aside the issue of what you can hear, there is a
real measurable difference. There isn't a lot of feedback on
these designs, and the performance depends quite a but on
the linearity (=bias) of the output stage. My rule of thumb for
Class AB X amps is that the idle dissipation should approach the
rated output at 8 ohms.

It is less clear than required bias per device but it must be a good starting point unless you use ridiculous number of devices (I like this approach ) while targeting low output power.

Recommended minimum bias per device can be deduced for IRF240 or whatever is used from this rule of thumb.
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Old 5th November 2007, 10:33 PM   #354
Anvil is offline Anvil  Finland
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Hello

Please comment on my design, I have simulated it and it works ok in simulator. I use many current mirrors to balance both halves, so no need to have many trimmers. Fets should be matched (of course) for optimal performance. DC-offset can be trimmed with R108 and R109.

Anvil
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Old 6th November 2007, 08:28 AM   #355
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Default One comment

Interesting schematic.

I only had time to look at it briefly, but I think you will have trouble operating FET's as Vgs current sources. NP has a variant of current sources on the Aleph which could be adapted which uses Vbe of a transistor to keep the current constant.
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Old 8th November 2007, 07:34 AM   #356
Anvil is offline Anvil  Finland
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Thanks Petter.
What kind of problems? And do you mean those current mirrors, because I have only one constant current source in this amp. I think those current mirrors will work well; 100 ohm degeneration resistors, matching and bolted to the same heatsink should keep currents at the same value. Any other comments, please people, do comment.

Anvil
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Old 21st July 2008, 05:42 AM   #357
Serge66 is offline Serge66  Switzerland
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Default Advice, schematics, pcb ...

Hi Guys,

I have a few questions regarding Pass Aleph-X Amps:

I am now interested to build a Pass Amp delivering 70-80W into 4 ohms. I like the X version. How many pairs of T do I need to achieve that output? I found schematics at http://www.kk-pcb.com/amps.html but no mention of power. 1 pair in the +o/p and 1 pair in the - o/p and 2 pairs as CCS.

Where can I find the schematics for the high power Aleph-X?

From whom on this forum can I buy pcbs?

Thanks for your help.

Serge
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Old 21st July 2008, 09:34 AM   #358
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Default Attempt to answer Q's

Anvil, sorry for the late reply, I don't really monitor this thread any longer since it has been overtaken by the Aleph-X thread and that there is no more activity here to speak of.

My way of thinking about the X- circuit and as I recall, that is the same way as the original patent describes it, is that that the circuit is a number of current sources. above and below the signal transistors. In a way you can say that the high gain is possible due to the current sources. Your circuitry appears different. There is no folded cascode which I would have expected to see at or above M44, so you are using some form of current mirror instead. I am don't know how this will work in practice and it is different from the X topology which I am very familiar with. My initial concern was before I had analyzed this portion of your schematic was more concerned with whether you could get balance in the current sources on bottom (i.e how well would they track). When I build my first unit, I went all out and degenerated all of 20 or so volts across the source resistor in order to get highly predictable current sources out of a FET unit. The problem with FET's is their high variability in Vgs which even with tight matching is "no match" against their BJT counterparts where you can rely on Vbe to be pretty darn on target.

So to cut to the chase, I don't fully understand what you are trying to accomplish and therefore it would be best for me to refrain from commenting further. I do not want to discourage you, so please do build and report back to us.


Serge,
The "original" X circuit is far removed from the Aleph-X circuitry. Aa far as number of transistors needed to generate your desired output power, there really is no clear cut answer. Mr Pass uses many more transistors than strictly necessary. Indeed from the datasheets, most amps can be built with single pairs (per balanced leg). Your design choices, particularly for Aleph type "Class A like" circuitry probably make it even more important to select the cooling capacity of the heatsinks first.

There is at least one separate Aleph-X thread where I have not been contributing so others if you post to relevant threads (this one is about X topology, not Aleph nor Aleph-X topology) you might get more appropriate answers or even find the design you are looking for.

Good luck, guys!

Petter
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Old 21st July 2008, 02:46 PM   #359
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If you would stick to Petters schematics, Mr. Pass indicates that the output power is about the amount of dissipated power.

Furthermore a "bullet proof" dissipation for the IRF TO247 devices would be around 12 - 17W per device. This would indicate that your aim of 60W output power could be reached with 4 devices (ie no parallel fets) in the output stage.

You would need to realize that at lower voltages, the linearity is less. 5V across the fets at maximum output is seen by many as the minimum.

An example:
The X600 (monoblock) dissipates about 600W with 48 fets = ~12,5W per fet. This is maintained across the entire X series amplifiers.

I build an X250 with 20 devices per channel. With the elivated supplies and bias my dissipation is around 350W per channel or ~17W per divice. I use 40 - 0 - 40 for the power stage and 65 - 0 - 65V for the driver stage (with a voltage doubler and regulator)

I like the power and performance however it has a tad less sweetness in the highs and mids as the Aleph 60. I use the X250 now on my Orion+ for the woofers and the power seems just endless. The mid speakers are driven by te A60 and the highs by an A30.
Overall I'm expremely pleased with the system. Especially using a vinyl source :-))
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Old 21st July 2008, 09:56 PM   #360
Serge66 is offline Serge66  Switzerland
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Thank you Petter and Tarasque

Serge
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