Depends.
Balanced or not, number of output devices in parallel, rail voltage ?
The diode thing is most beneficial for low rails, and single output devices.
F5 with 24V rails : 10A peak in 1 Ohm => 10V out.
10A times 0R47 = 4.7V
Vgs at 10A out for the Fairchild P-channel is ~6.7V
4.7 +6.7 = 11.4V
24V - 11.4V = 12.6V
The 2.6V difference between 12.6V and 10V is rail sag and voltage slack to keep the output devices from misbehaving.
Add the diodes, and nearly 4.5V gained. Taadaah, as P would say.
Pick higher rails, and peak output current goes up.
Place a number of output devices in parallel (+ JFETs), peak output current goes up.
Beauty of the diode thing for balanced operation is twice cancellation of the voltage drop across the source resistors at high output currents, one of the downsides of running balanced.
Example, suppose i'd go full monty with a high power balanced F5 build.
I could place 4 Violet oldy JFET pairs in parallel (got plenty J72/K147 and J73/K146), running at 12mA Idss each, good for 8 power MOSFETs in parallel (aka 8 times F5).
Suppose i'd raise the rails to 35V, might skip the cascodes, minus ~7.5V for Vgs and a couple of volts slack, good for at least 15A peak output current per MOSFET (conservatively).
In balanced mode, enough for +200W/8 continuous, with 32 MOSFETs effortless in full Class A if i wanted to, and 120A peak output current.
Cost to build for me would be say 1/4th of the regular bill, at the current exchange rate i pay under $4/lb for raw aluminum in any size/thickness (and a cheese folding cheapskate prefers to buy parts by 100s or 1000s)
Last time i counted, i had 900 common cathode diodes left, i wouldn't notice it if i stuck 64 of them in 2 monaurals, can't eat 'm anyway.
Still wouldn't do it for such a build if i can get a clean 120A without them.
For a balanced setup with 30V-35V rails, 2 Blue JFETs in parallel, and 2 MOSFETs per quadrant, there i'd use the diodes.
Andrew,
Sure,
So we are multiplying by 1.414 to give us the RMS value.
Making (as you say) 5A at the output.
With +-44V rails are you assuming a loss of 4V per rail rather thatn 2V or are you making an allowance for headroom?
Andy
Thanks Dutchie,
The amp will not be balanced , currently not running balanced in my line stages and dont plan to switch over as yet, so i might give balanced some thought if going to use in the future.
So , I'm contemplating, SE, 16 outputs/channel/ 45volt rails ........
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I was showing the order of magnitude, I defer to your specific calculations.
My point is a balanced F5 with only 4 output devices (can be done on a pair of reasonable heatsinks) gets you ~128W peak in Class A. One cannot possibly be complaining about 24Vpp (or so). If the goal is Class A, the F5T seems a tough sell.
As my plan is to build the V3 and want 100W into 8 ohms continious, what do you recommend the voltage per rail to be to achive this ?
I have IRFP9240 and IRFP240 Mosfets - same as I used in my F5
I cannot find a source for the FQA devices anyway
Also with these devices (IRFP) would you recommend parralleling the the 2SJ74 and 2SK170
I only have 8 matched pairs of the 2SJ74 so I dnt want to use them if I dont have to ( 2SK I have around 100)
I cannot find a source for the FQA devices anyway
Also with these devices (IRFP) would you recommend parralleling the the 2SJ74 and 2SK170
I only have 8 matched pairs of the 2SJ74 so I dnt want to use them if I dont have to ( 2SK I have around 100)
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200W is 40V peak.
To open shop, Vgs is 2-4 Volts for the IR, usually more than 3V, and 3-5 Volts for the Fairchild, see datasheets.
To bias them requires a bit more, then you have to add for sagging rails.
Then there's the little matter where you'd like the common cathode diodes to kick in. (me, i'd rather do at least 4A out before the doctor with the nitrous oxide enters)
On top of that, transformers usually can't be had with any desired secondary voltages, so the first one that would apply is a 33Vac.
Same curves for the TO220 FQP19N20
The FQA12P20 need about 5.5 V
In the zoomed in 2 controll top and botom and a couple of pairs
I use to match them at 24 V 2A and constant themperature 50C
Tea Bag Could I have same please?
The FQA12P20 need about 5.5 V
In the zoomed in 2 controll top and botom and a couple of pairs
I use to match them at 24 V 2A and constant themperature 50C
Tea Bag Could I have same please?
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No commercials here. But I wanted to note that an effort is being made to get a large # of them available.
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