the new range of output offset adjustment will be due to a different Vgs of the mosFET.
mosFETs come off the production line with an enormous range of Vgs (the datasheet shows this). This is very different from the Vbe of BJTs.
Farjon,
This is the original amp with no higher than 42V rails, where the single IRFP and C5200 is just sufficient.
If you want a more powerful amp, say 50V or even more, you need to change the dimensions of the driver stages, both for the nmos AND for the npn. Of course, this is a different beast again, so we need a new thread for that........ and of course, to be sure, we need to build it out and test it thoroughly. To be honest, the best option would be to use a 280W nmos, like FQA40N15, and a matched pair of C5200 OR even a single 200W Sanken 2SC3264.
HD
This is the original amp with no higher than 42V rails, where the single IRFP and C5200 is just sufficient.
If you want a more powerful amp, say 50V or even more, you need to change the dimensions of the driver stages, both for the nmos AND for the npn. Of course, this is a different beast again, so we need a new thread for that........ and of course, to be sure, we need to build it out and test it thoroughly. To be honest, the best option would be to use a 280W nmos, like FQA40N15, and a matched pair of C5200 OR even a single 200W Sanken 2SC3264.
HD
Hi Thimios,
My mistake; FQA40N25 is the designation.
Here are two choices at Digikey:
FDA38N30 - Fairchild Semiconductor MOSFET N-CH 300V TO-3
802 - Immediate 31,950 - Factory Stock
$3.45 85 mOhm @ 19A, 10V 5V @ 250µA 60nC @ 10V 2600pF @ 25V
312W Through Hole TO-3P-3, SC-65-3 TO-3PN
FQA40N25 Fairchild Semi MOSFET N-CH 250V 40A TO-3P
234 - Immediate 8,100 - Factory Stock
$3.28 70 mOhm @ 20A, 10V 5V @ 250µA 110nC @ 10V 4000pF @ 25V
280W Through Hole TO-3P-3, SC-65-3 TO-3PN
The problem with these devices is normally the buy qty; you might buy 25, a tube, to get them so I'd suggest looking around. Essentially you want:
280W rating Min 150V Min 35A cont Rdson Min 120milliohms
Max 5000 pF gate Ciss TO3P through hole Single device buy
Hugh
My mistake; FQA40N25 is the designation.
Here are two choices at Digikey:
FDA38N30 - Fairchild Semiconductor MOSFET N-CH 300V TO-3
802 - Immediate 31,950 - Factory Stock
$3.45 85 mOhm @ 19A, 10V 5V @ 250µA 60nC @ 10V 2600pF @ 25V
312W Through Hole TO-3P-3, SC-65-3 TO-3PN
FQA40N25 Fairchild Semi MOSFET N-CH 250V 40A TO-3P
234 - Immediate 8,100 - Factory Stock
$3.28 70 mOhm @ 20A, 10V 5V @ 250µA 110nC @ 10V 4000pF @ 25V
280W Through Hole TO-3P-3, SC-65-3 TO-3PN
The problem with these devices is normally the buy qty; you might buy 25, a tube, to get them so I'd suggest looking around. Essentially you want:
280W rating Min 150V Min 35A cont Rdson Min 120milliohms
Max 5000 pF gate Ciss TO3P through hole Single device buy
Hugh
Hugh,
Thanks for the information. I was thinking to be more 4 ohm load tolerant with a Dacz like layout (TO247/TO264) at 42 V.
So, FQA40N25 and MJL4302...
Rgds.
Thanks for the information. I was thinking to be more 4 ohm load tolerant with a Dacz like layout (TO247/TO264) at 42 V.
So, FQA40N25 and MJL4302...
Rgds.
You bet Dacz! This is a 73A 300V device I believe; this is huge current and voltage rating just check the gate to drain capacitance is no higher than 5nF. If this is too high the mosfet slows at high frequency.
However NP has used a 120A single end IXYS device with local feedback. That's good enough for me.....
HD
However NP has used a 120A single end IXYS device with local feedback. That's good enough for me.....
HD
Hi,
I'm still looking for an answer regarding the expected output power into 8/4 ohm if the amp is powered from +/-35 to +/-42Vdc max. Huge amount of power is not something I crave, I simply want to size the material properly in case my teenager gets a hold of the volume..lol
Bias current will be as mentionned above therefore at idle 84V x 0.15A is about 13W dissipation but what about at max power, should I expect 25W dissipation at 4 ohm with music material ? I'm also helping a friend so want to make sure we will be buying the right stuff 😀
Since I'd like to order some materials like enclosure, power transformers, etc can someone please recommend the proper size for them. Ex for each channel I would require;
1 x 250VA, 60Vct transformer + recommended filtering cap value/rail.
1 x Heatsink of 0.5C/W (I like my heatsink below 45C)
Thanks for the help, much appreciated !!
Eric
Eric, with 8 ohm speakers you can expect about 50W and 80W with 35V and 42V, respectively. Theoretically the numbers double into 4 ohms, and provided your power supply is stiff enough and the wiring are up to the task you'll get pretty close.
Your transformers and heatsinks are fine since you are building these as monoblocks. 4700uF per channel per rail will suffice. It would be a good idea to increase this to 10,000uF (or better yet 2 x 4700uF in parallel) if you're driving 4 ohm speakers but otherwise the improvement will be marginal at best.
Your transformers and heatsinks are fine since you are building these as monoblocks. 4700uF per channel per rail will suffice. It would be a good idea to increase this to 10,000uF (or better yet 2 x 4700uF in parallel) if you're driving 4 ohm speakers but otherwise the improvement will be marginal at best.
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Just work it out.
Pmax = Vpk²/Rload/2
Vpk << Vsupply.
expect Vsupply to be about 3V to 6V below quiescent voltage.
Expect Vpk to be 2V to 5V below Vsupply
eg quiescent voltage to amplifier ±42Vdc
Vdrop on max power is 4V
Vloss through amplifier is 3V
Therefore Vsupply ~ 42 -4V ~ 38Vdc
Vpk ~ 38V-3V ~ 35V
Max P ~ 35²/8/2 ~ 76W into 8r0 dummy load.
It really is that simple.
After you have built a few PSU you will find what Vdrop to expect for your style of PSU building.
Once you have built a few amplifiers you will find what Vloss to expect for the different topologies.
Assemble your PSU and your amplifier.
Lay them out on the bench. Get them working.
Add on all the auxiliary circuits you want/need. Get them working.
Now fold that assembly into a package that will fit the chassis.
Measure the package and only then buy a suitably sized chassis.
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You'll not get pretty close. R.Cordell gave us a target for a really good amplifier that is capable of delivering adequate current. He said allow 180% of the power into the double value load for the maximum power into your rated load.
I can confirm that achieving 180% is not easy. eg build a 4ohms capable amplifier that delivers 100W into 8r0 and you will find that getting 180W into 4r0 is not easily acheivable. 100W/8r=28.28Vac, 180W/4r=26.83Vac, that's a loss of only 0.46dBV into the rated load !
Getting a 150W/4r from a 4ohms rated amplifier that delivers 100W/8r is easy. That's a loss of -1.25dBV
I find that bass is quite sensitive to the PSU build.
I recommend a high pass filter set to 2Hz and use ±20mF per 8ohms channel for good deep bass. This becomes ±40mF for each 4ohms channel.
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Andrew,
I said that he can get "pretty close" to doubling the power into 4R provided the power supply and the rest of the wiring is up to the task. 180% is "pretty close" in my books. What parameters other than PSU and wiring do you suggest he needs to pay attention to?
That 250W transformer is sufficient for a 150W /4R channel and I already recommended doubling the capacitance to account for the increased current demands of 4 ohm operation.
I don't share your view that +/- 20mF of caps are needed per channel for good deep bass. There is no real problem in doing so except that it increases the inrush current and probably necessitates some sort of limiting circuit. It would be good if you could point that out to people when giving this advice.
I said that he can get "pretty close" to doubling the power into 4R provided the power supply and the rest of the wiring is up to the task. 180% is "pretty close" in my books. What parameters other than PSU and wiring do you suggest he needs to pay attention to?
That 250W transformer is sufficient for a 150W /4R channel and I already recommended doubling the capacitance to account for the increased current demands of 4 ohm operation.
I don't share your view that +/- 20mF of caps are needed per channel for good deep bass. There is no real problem in doing so except that it increases the inrush current and probably necessitates some sort of limiting circuit. It would be good if you could point that out to people when giving this advice.
I use a ~200ms delay on a transformer soft start, even down to 160VA.
I typically use ±20mF to ±75mF of smoothing capacitance in my PSUs. I have not found the need to use a slow charge circuit to limit the capacitor charging current.
I typically use ±20mF to ±75mF of smoothing capacitance in my PSUs. I have not found the need to use a slow charge circuit to limit the capacitor charging current.
I don't need to because I don't believe your contention that a slow charge circuit is necessary.
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I expect an amplifier of normal build to easily get to a 150% target, i.e. an amplifier running from a 35-0-35Vac transformer with ±4700uF smoothing to hit the 150W into 4r0 and hit the 100W into 8r0 targets.
I would expect the performance of that amplifier to be quite inferior to an amplifier from the same 35-0-35Vac transformer that meets the two targets of 100W/8r0 and 180W/4r0
I would expect the performance of that amplifier to be quite inferior to an amplifier from the same 35-0-35Vac transformer that meets the two targets of 100W/8r0 and 180W/4r0
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Few here would use a soft start with a 160VA or even 250VA transformer. 200ms is 10 AC cycles, by which time that cap bank, a dead short on power on will holding a decent charge.
A 250VA transformer with +/- 4m7 or 10mF behind it can get away without a soft starter. That same transformer with +/- 20mF or 40mF behind it is probably going to need it.
See my point?
A 250VA transformer with +/- 4m7 or 10mF behind it can get away without a soft starter. That same transformer with +/- 20mF or 40mF behind it is probably going to need it.
See my point?