Single Ended FET for 60W in 4 ohms - diyAudio
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Old 1st February 2012, 08:22 PM   #1
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Join Date: Jan 2012
Default Single Ended FET for 60W in 4 ohms

Hi All.
I am new to this forum and do not have experience with DIY but I have some basic understanding of electronics. I would like to build a pair of single-ended monoblocks with dissipation of 160 - 210W and power rating of at least 30W in 8 ohms , 60 W in 4 ohm and to reach peak power around 2.7 ohm (90 W). I have speakers are 4 ohm nominal, 2.7 ohm minimal, 87db sensitive. The room is midsized and I prefer low to medium level listening.
I have been thinking about Aleph 2 derivation since Aleph X seams more challenging. Here is my plan that is based on the original schema for Aleph 2 from the service manual
Reduce rail voltage to about 27V. This should give me 30W @ 8 ohm 60 W in 4 ohm providing voltage drop of 5V on 2 FET and 2 source resistors (reasonable assumption?)
Remove one pair of output FET transistors (total of 5 pairs of FET) to accommodate chassis that consider building
Decrease source resistors (R40-R45 and R46-R51) from 1ohm to o.82 ohm to reflect reduction in number of FETs. This should give me bias of 0.68 A per FET or 3.4A per amp ( I guess this is close to Aleph 2).
Reduce R13 (a part of the current source for the input transistors) from 10K to 4.75K to reflect reduction in the rail voltage.
Leave everything else as in the original schema.
The expected dissipation per FET is around 18-19W. So I would use 10 heat sinks (one per FET) of 5.375 by 3 (thermal resistance 1.15) from HeatSink USA to build a chassis. The chassis would have 3 sinks per side, 2 sinks at the front and 2 at back and should be 3 in height.

Can you please give me some feedback and comments on my plan. I am also open for any suggestions. Thanks
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Old 1st February 2012, 10:45 PM   #2
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You might consider one of the HiFi2000 chassis as well; there is a series with heat sinks.
It is not necessary to take separate heatsinks for each FET.
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Old 2nd February 2012, 10:51 AM   #3
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
Originally Posted by pvc67 View Post
Decrease source resistors (R40-R45 and R46-R51) from 1ohm to o.82 ohm to reflect reduction in number of FETs. This should give me bias of 0.68 A per FET or 3.4A per amp ( I guess this is close to Aleph 2).
What is the maximum ClassA output current?

Maximum ClassA output power equals the ClassA current squared times half the nominal speaker impedance.
Pmax = Ipk^2 * Rload / 2

For 3.4Apk, Pmax = 3.4^2 * 4 / 2 = 23W into 4r0
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 3rd February 2012, 02:37 AM   #4
alexcp is offline alexcp  United States
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60W for 87dB/W/m speakers seems to be adequate; see the discussion here.

If you bias your Aleph at 1/2 of its peak current (see comments here), you will get 6.8A peak. You only need 5.5A peak for 60W into 4ohm, and this is the maximum current you can get with 27V rails anyway if you want to keep at least 5V across power FETs. However, with 6.8A peak you will be able to deliver 60W into 2.7ohm as well.

Last edited by alexcp; 3rd February 2012 at 02:39 AM.
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