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Old 30th November 2013, 01:03 PM   #2641
AndrewT is offline AndrewT  Scotland
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If you were to standardise your amp on 63V capacitors, then you will find that a 40+40Vac transformer will run with a fully biased ClassA amplifier at about +-55Vdc.
500mA bias through each pair of output mosFETs will give a device dissipation of ~27W.

Now multiply up the number of output pairs to meet your ClassA target current.
3pair = 3Apk of ClassA
4pr = 4Apk
5pr = 5Apk
etc.

The ClassAB transient current can be much higher
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Old 30th November 2013, 03:31 PM   #2642
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Quote:
Originally Posted by 6L6 View Post
...
My specific recommendation to you - use 3 devices and bias until the transistor package is 55C with heatsinks 65C. However, either configuration you make will sound fantastic.
6L6, don't you mean transistor package at 65C and sink at 55C?
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Old 30th November 2013, 03:59 PM   #2643
6L6 is offline 6L6  United States
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Lol... Yes.

I just cant type what I think this morning!
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Old 30th November 2013, 05:24 PM   #2644
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Quote:
Originally Posted by AndrewT View Post
If you were to standardise your amp on 63V capacitors, then you will find that a 40+40Vac transformer will run with a fully biased ClassA amplifier at about +-55Vdc.
500mA bias through each pair of output mosFETs will give a device dissipation of ~27W.

Now multiply up the number of output pairs to meet your ClassA target current.
3pair = 3Apk of ClassA
4pr = 4Apk
5pr = 5Apk
etc.

The ClassAB transient current can be much higher
Thanks Andrew for pointing this out. I realize that for a 100w rms into 8 ohm target with 4pairs of devices, my heat dissipation per device will be much greater with 55V rails then 45V rails given the same bias current; so for 4 pairs it would make more sense to stick with around 45V rails.
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Old 2nd December 2013, 02:56 AM   #2645
dazed2 is offline dazed2  Canada
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Nah, go for broke man !
Run at 55V rails! Click the image to open in full size.Click the image to open in full size.
Thats what I'm doing, 40-0-40 output trafo, for hopefully 52-55V rails, 63V caps,
lots of heat sinks, 4 case affair!
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Old 2nd December 2013, 03:50 AM   #2646
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Quote:
Originally Posted by dazed2 View Post
Nah, go for broke man !
Run at 55V rails! Click the image to open in full size.Click the image to open in full size.
Thats what I'm doing, 40-0-40 output trafo, for hopefully 52-55V rails, 63V caps,
lots of heat sinks, 4 case affair!

4 case?? What Class A output are you shooting for? Custom heatsinks? Can't open your images.

Nash
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Old 2nd December 2013, 01:20 PM   #2647
dazed2 is offline dazed2  Canada
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Sorry about the images, I didn't put any in there. I have no idea how they got in there? I can't seem to delete the link either?

Monoblocks with PSU's separate. 4 HeatSink USA 10.080 per channel
1.5kVa trafo's per mono.... just to be sure
going for 1000W room heaters.... I guess I won't be driving the SUV for a while.

Last edited by dazed2; 2nd December 2013 at 01:22 PM.
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Old 8th December 2013, 10:26 PM   #2648
BigE is offline BigE  Canada
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Has anyone considered, or better still built, an F5T with a split power supply? Ie, a lower voltage for the input fets and a higher one for the output transistors?
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Old 8th December 2013, 10:52 PM   #2649
6L6 is offline 6L6  United States
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The entire point of the cascode version is exactly that - keep the higher voltage from the Jfets, and let the outputs get to the sweetspot with the 32V (or more) rails.

The cascode fets act to keep the higher voltage from burning up the small Jfets - they are kind of an active voltage divider, and add very little sonic flavor to the circuit.
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Old 8th December 2013, 11:06 PM   #2650
6L6 is offline 6L6  United States
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Speaking of F5T cascode - (this is a 2pr v3, or probably better described as a v2 cascode...) Here are some teaser photos. I only have one channel installed in the chassis as of now.

Click the image to open in full size.
600VA 24+24V with new diyAudio PSU v3 board, (8) 18,000uF caps.

Click the image to open in full size.
Newest revision PCB, the new front-end will fit on existing holes in the UMS!

Click the image to open in full size.
Biasing. I have both sets of leads reversed on the red meters, hence the negative values... Lol.
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