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Old 4th March 2012, 01:22 PM   #921
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Quote:
Originally Posted by a.wayne View Post
Should i ?
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.
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Old 4th March 2012, 01:40 PM   #922
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Quote:
Originally Posted by AndrewT View Post
with very minor correction all the above is OK.
but you are working with ac voltage.
For an equivalent heating effect (power) to the DC equations shown, you must add ac to all your currents and voltages.

From the Vac and Iac you can determine the peak of a sinusoidal waveform using the sqrt(2) factor.

100W into 8r0 requires 40Vpk and 5Apk.

This requires the F5 to output 80Vpp at the speaker terminals and if the load is truly resistive then 5Apk to flow around the test load circuit.

F5 running on +-44Vdc and biased to ~2.6Adc will stay in ClassA all the way to 5Apk.

Useful shortcut to remember.
50W into 8r0 is equivalent to 20Vac
100W into 8r0 is equivalent to 40Vpk
Halving or doubling those voltages, changes the powers by a factor of 4times (+-6dB)
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
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Old 4th March 2012, 01:55 PM   #923
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44V rails will not get you to 100W continuous in 8 Ohm, requires more.
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Old 4th March 2012, 02:51 PM   #924
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Jacco,
Can you explain where the losses are please.
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Old 4th March 2012, 04:14 PM   #925
a.wayne is offline a.wayne  United States
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Quote:
Originally Posted by jacco vermeulen View Post
Depends.
Balanced or not, number of output devices in parallel, rail voltage ?

.
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 ........

Last edited by a.wayne; 4th March 2012 at 04:16 PM.
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Old 4th March 2012, 09:50 PM   #926
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Quote:
Originally Posted by AndrewT View Post
Nyc1 said it, but it is not accurate.
Each half of the balanced amplifier sees half the total load impedance. This effective lower load impedance uses up the ClassA current capability. This is equivalent to saying that the F5T transitions over to ClassAB earlier than a single ended Push Pull ClassA amplifier.

16V Vcc does not permit 16Vpk output at the speaker.
A typical F5 output stage is likely to achieve ~ 4V less than Vcc at the speaker terminals.
Using a balanced pair running on +-16Vdc is likely to result in 24Vpp to 25Vpp at the speaker terminals. Certainly not 32Vpp, not even into a 100k test load.

If 36Vpp at the speaker terminals is required, then expect to use supply rails of ~+-22Vdc
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.
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Old 5th March 2012, 04:03 AM   #927
a.wayne is offline a.wayne  United States
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Quote:
Originally Posted by NYCOne View Post
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.
Agree, hence why the focus is the V3 ... not the V2 ..
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Old 5th March 2012, 07:28 PM   #928
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Quote:
Originally Posted by jacco vermeulen View Post
44V rails will not get you to 100W continuous in 8 Ohm, requires more.
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 ?
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Old 5th March 2012, 07:34 PM   #929
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Pick your output devices first, makes a difference.

(2.5A bias will also make the rails sag a bit more than 1.3A )
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Old 5th March 2012, 07:44 PM   #930
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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)

Last edited by Geofd; 5th March 2012 at 07:46 PM.
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