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Y.A.F5Tv3.B.T. Yet another F5Tv3 Build Thread
Y.A.F5Tv3.B.T. Yet another F5Tv3 Build Thread
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Old 17th October 2017, 07:30 PM   #1
gionag is offline gionag  Italy
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Join Date: Jun 2007
Default Y.A.F5Tv3.B.T. Yet another F5Tv3 Build Thread

Hi guys,
i've finally decided to collect all my "garbage" heavy stuff and make this amp.

I want to go the "hard way", so the rev.3 was choosen.

Dual mono-block, for sure.. I can machine on a proper CNC 3-axis my schizoid delusions, so in the end the amps will appear something like attached pic.

At the moment, i have on my desk 2 heatsinks : 300x400mm h.86, they weight 9.5Kg each, and by the specs, by natural convection, they give a nice 0.18 K/W figure.

I have also two big irons, 1400VA each, with only the primary winding wound with a 15% more of the nominal flux for that geometry. On top of that i have a screen winding and then, when i finally choose the rail voltages i'll bring them to a fellow's company that finalize the windings.

For the filtering bank i am not sure what path to follow... in my mind i have a CLC filtering, by simulation a 1mH coil will give a nice output (compared to a CRC)... any advices on that ? The capacitance will be donated by an array of 4x22mf each rail, for each side.

I need help in taking some decisions about the rail voltages / bias.
I have made some calcs, and they went that way :
Starting from the end i want an increase of 20 degC above room temp.
so, given 0.18k/W is 20 / 0.18 = 111W of thermal power.

The topology is a pushpull, so in best case i will have a power efficenciency of 40%. 111W * 0.4 = 44W of class-A power.

in making a 100W A/B class on 8ohm i will give -> sqrt(100*8) = 28V rms or
39v peak-t-peak.

44W / 39v = 1.12A bias. cut in half and i have 0.56A per bank of N and P.
0.56 / 4 elements = 0.14A each fet.

To that point, how many mistakes i have done ??
If all the things are correct, i will have 27Vrms for the secondaries and 44w of full class-A and capability to arrive at 100w in AB.

Please, correct me if i am doing something stupid about.

I will follow with the developments.
Thanks !
Attached Images
File Type: jpg heatsink_gorini_5314_018.jpg (84.6 KB, 528 views)
File Type: png cabinet.PNG (144.8 KB, 526 views)
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Old 17th October 2017, 08:12 PM   #2
Marra is offline Marra  United Kingdom
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Y.A.F5Tv3.B.T. Yet another F5Tv3 Build Thread
I like the look of your proposed finished amp. Gonna be watching with interest.
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Old 17th October 2017, 08:16 PM   #3
gionag is offline gionag  Italy
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Quote:
Originally Posted by Marra View Post
I like the look of your proposed finished amp. Gonna be watching with interest.
Thanks Marra...

Right now i am looking in confirmations about my calcs for the rail voltages.
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Old 17th October 2017, 08:43 PM   #4
WalterW is offline WalterW  Netherlands
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Y.A.F5Tv3.B.T. Yet another F5Tv3 Build Thread
You should not use the 40% efficiency in your calculation.
Say you can dissipate around 120W each channel and you want 100Wpc class AB.
Use 44 volt rails. That gives a clean 40volt sine wave. Or 100 Watt in 8 Ohm.
120 Watt of heat, divided by 44Volts is around 2.8 Ampere of bias. Divided by 8 N and P MOSFETs is around 340mA bias per MOSFET.
Your amp will stay in class A the first 30 Watts in 8 Ohm, after that it will be class AB.
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Old 17th October 2017, 08:59 PM   #5
gionag is offline gionag  Italy
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Quote:
Originally Posted by WalterW View Post
You should not use the 40% efficiency in your calculation.
Say you can dissipate around 120W each channel and you want 100Wpc class AB.
Use 44 volt rails. That gives a clean 40volt sine wave. Or 100 Watt in 8 Ohm.
120 Watt of heat, divided by 44Volts is around 2.8 Ampere of bias. Divided by 8 N and P MOSFETs is around 340mA bias per MOSFET.
Your amp will stay in class A the first 30 Watts in 8 Ohm, after that it will be class AB.
Can you elaborate a little more about how you derived 44 volt rails ?
if i do sqrt(100*8) = 28v rms = 40v p-t-p, you may have added 4 volt for losses ?
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Old 17th October 2017, 09:21 PM   #6
WalterW is offline WalterW  Netherlands
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Y.A.F5Tv3.B.T. Yet another F5Tv3 Build Thread
Correct, 4 volt for losses. You can't swing to the full 44 volt without added distortion.
So your transformer needs to be around 34Volt AC. 34 x 1.31 ( under load) = 44.5 Volts. Or use 32 volts trafo.
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Pass F5, F5Tv3, ACA, Aleph J, DDDAC1794, DCB1, BA3-preamp, VFET CSX1, Pass F6, Pass M2-HPA, VFET-X balanced, Pass J2
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Old 17th October 2017, 09:42 PM   #7
gionag is offline gionag  Italy
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Quote:
Originally Posted by WalterW View Post
Correct, 4 volt for losses. You can't swing to the full 44 volt without added distortion.
So your transformer needs to be around 34Volt AC. 34 x 1.31 ( under load) = 44.5 Volts. Or use 32 volts trafo.
Oh, i see. I will make 0-34v 0-34v (under load) on the trafo.
I have a huge power reservoir, maybe i will try to increase the biasing as much as possible while keeping an eye on the final temperature of the heatsink.

I have made some thermal simulations on the heatsink, and despite the 0.18Rth with a properly distributed load on the surface i can keep the temperature a little bit (10%) less of the calculated one. So it's worth trying pushing bias a little bit.

for the mosfet i will match 4 by 4 a bunch of irfp240 and irfp9240 (btw, i have read anywhere that irfp240 is a 20a device, but in the datasheet from vishay i read that they are rated for 12a... what i am missing ?)

for the input diff. stage i will use the k170 / j74 combo (7ma Idss matched within 0.1ma).

What do you recommend for the cascode ? i have some trouble in getting the SC4793/SA1837. maybe there are some other viable substitution ?

Last edited by gionag; 17th October 2017 at 10:08 PM.
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Old 18th October 2017, 10:11 AM   #8
WalterW is offline WalterW  Netherlands
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Y.A.F5Tv3.B.T. Yet another F5Tv3 Build Thread
I bought them here: 2SC4793
But don't know if they ship to Italy.
In VFET part2 build, Papa is using ZTX450 and ZTX550.
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Pass F5, F5Tv3, ACA, Aleph J, DDDAC1794, DCB1, BA3-preamp, VFET CSX1, Pass F6, Pass M2-HPA, VFET-X balanced, Pass J2
'If you see me talking to myself, don't worry. I'm getting expert advice'
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Old 18th October 2017, 11:14 AM   #9
gionag is offline gionag  Italy
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Quote:
Originally Posted by WalterW View Post
I bought them here: 2SC4793
But don't know if they ship to Italy.
In VFET part2 build, Papa is using ZTX450 and ZTX550.
I will take a look,
i've committed to the transformer manufacturer a 2x0-34v secondary under load.

the cascode devices needs to be matched ? i don't think so, just for confirmation...
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Old 18th October 2017, 11:18 AM   #10
WalterW is offline WalterW  Netherlands
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Y.A.F5Tv3.B.T. Yet another F5Tv3 Build Thread
Correct, no matching.
__________________
Pass F5, F5Tv3, ACA, Aleph J, DDDAC1794, DCB1, BA3-preamp, VFET CSX1, Pass F6, Pass M2-HPA, VFET-X balanced, Pass J2
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