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Old 15th March 2002, 05:41 AM   #1
FBJ is offline FBJ  United States
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Question More Output devices...A75

DRUM ROLL PLEASE........ I have searched the Fourm but nothing!
Another Drum Roll please....... Well has anyone added more output FETs to the PASS A75 amplifier? yes I know " more matching and expense" but someone has to ask this question right. I remember in one of the old TAA issuses Nelson Pass answered a reader who wanted to increase the number of output devices of the PASS A40 and NP said just beef up the power supply. So I wonder................
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Old 15th March 2002, 05:54 AM   #2
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Why would you like to do that?
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Old 15th March 2002, 06:42 AM   #3
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perhaps to maintain the same bias across each fet but increase total bias and therefore allow for usage into lower impedance loads..... just a thought.
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Old 15th March 2002, 06:50 AM   #4
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Well that's a good question..... and...... deserves a good answer.
a good answer?
a good answer?
Oh, all my PASS projects have been low power versions and I like to do one last (well maybe) project so I can get back to a few important things like.....eating better and buying some new clothes, sleeping longer hours instead of working on audio projects to the late hours of the night, new car too.
And I like to devote my time to building speakers. I have no power tools yet. And I won't have the time or money to do both.
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Old 15th March 2002, 06:56 AM   #5
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That didn't answer the question yet.
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Old 16th March 2002, 10:51 PM   #6
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Actually, I would like to second this question....

I am considering building a larger Aleph as an experiment. My design is targeted toward 48 fets per monoblock and the design output will be somewhere around 300 watts... I'm figuring on 4 kva transformers with rail voltages set for a 300 watt target...

My line of thinking goes like this.... If I add more outputs, I can achieve better damping, better performance into low impedance loads, overall better transient current carrying capability, and lower junction temperature per device... Therefore, if I use efficient enough cooling, I should be able to push the amount of bias current up a bit higher while not stressing the devices too much....

Yes, managing the heat will be a factor...

I am fully expecting that the universe will be experiencing "heat death" at some point so I figured what the heck I may as well contribute... In all seriousness, I think the idea of eventually marrying the X front end with the Aleph back end would be pretty cool. I figure this will give me a very solid base to spring from...

Hence, when adding additional output devices what other factors should be considered? At what point will the drivers be unable to have enough muscle to push the output stage?

Any thoughts would be appreciated!!!

Thanks,

Steve
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Old 17th March 2002, 01:50 AM   #7
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Apogee,

300W into what load?
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Old 17th March 2002, 02:23 AM   #8
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While I'm inquiring...

In the case of the larger Aleph, I have been considering two scenarios regarding mounting the outputs:

Scenario #1 - outputs mounted directly to heatsinks.

Scenario #2 - outputs mounted to 1/2 thick plate, which heatsinks are then attached to using either thermal grease or thermal epoxy.

When using the modeling software to calculate dissipation, none of the software allows for the scenario #2 configuration.

Does anyone have experience in this area? I am assuming that by mounting to the plate it actually increases the C/W calc slightly because of the inefficiency of the gap between the plate and the heatsinks. But then there's the increase in mass from the plate.... Am I correct in assuming that by mounting to the plates it will create a more even temperature across each bank of outputs and therefore will maintain more even current sharing due to all devices being at closer temperatures? And yes, all devices will be matched. Does it matter? Thoughts on this???

My Threshold amp has the outputs mounted on aluminum plates which, are then bolted (using grease) to the heatsinks. I have seen enough amps built using either carrier plates or "L" brackets to mount the outputs that I'm curious if it's done strictly for aesthetic or assembly efficiency, or if there is some other reason for it. Clearly, when using an "L" bracket it raises the junction temp so my question is why do the manufacturers do it?

Finally, has anyone actually looked inside the X-Series at how the outputs are mounted? Did NP use plates to carry the outputs or are they mounted right to the heatsinks?

I figured that I'd put this out there as it directly deals with increasing the number of outputs of any circuit (and how to mount them).

Thanks in advance for you thoughts,

Steve
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Old 17th March 2002, 02:25 AM   #9
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300W into 8 ohms...
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Old 17th March 2002, 03:20 AM   #10
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I would think since the bias is adjustable up to the set point, you could keep adding sets till you run out of adjustment.I'm sure you could reach a limit. How many do you want to add. Your already driving 12 pair per channel. If you beef up the heatsink you could match and bias at a higher current and maintain Class A into lower impedance loads. If that is what you want to do this for. If your just into spending money go for it. It cost me about 300 bucks for output devices and I got two matched sets (out of 100 mosfets). I later sold the set I didn't use in the trading post. Hopefully Nelson will be weighing in here soon and we'll find out about driving the extra capacitance or whatever limitations you may have to look for.
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