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Old 9th June 2008, 05:20 PM   #51
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Quote:
Originally posted by Mooly
Hello Lineup,

It would be an interesting exercise to modify the design for HEXFET's including a vgs multiplier and see whether the sonics are unchanged.
I omitted the vgs multiplier from my first amp and it sounded distorted at low levels, although didnt sound too bad louder.
On the scope I found the negative half of the wave form to be badly distorted which is odd coz it didnt sound that bad.

I later added a vgs multiplier and blew up all the output MOSFETs ! I had put the pot to the wrong end to start with and put far too many volts to the output FET's

I put the pot right and after fitting a new set of MOSFET's I turned the bias up slowly and the negative phase distortion melted away. The amp now sounds great at low levels but also sounds a lot cleaner at louder levels too.

I would definitely recommend the vgs multiplier but be careful when setting it up !!!!
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Old 9th June 2008, 05:53 PM   #52
Mooly is offline Mooly  United Kingdom
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Easily done, always worth using the 100 watt bulb trick for anything like this.
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Old 9th June 2008, 05:58 PM   #53
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Quote:
Originally posted by Mooly
Easily done, always worth using the 100 watt bulb trick for anything like this.

I decided to put a zener across the vgs multiplier in the end to limit the possible volts across it.

The amp is fused but the MOSFETs pop long before the fuse does.
If I lower the fuse they pop if the amp is turned right up !
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Old 9th June 2008, 06:11 PM   #54
AndrewT is offline AndrewT  Scotland
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Originally posted by otto88
Approx how many watts might it generate into a 3 ohm load?
maximum power is I^2 * R / 2.
Reduce the load from 8ohm to 3ohm and you'll get 3/8 of the power.
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Old 9th June 2008, 06:19 PM   #55
Mooly is offline Mooly  United Kingdom
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Do you have the multiplier in thermal contact with the outputs ?
It should be possible to get a quiescent current that is reasonably stable against temperature changes in the outputs. Although I have never built a HEXFET amp, I imagine it can't be to different to a BJT output amp in terms of temperature compensation. You would need to match the multiplier characteristics to the tempco of a pair of HEXFET's which will be around -0.3%/deg C.
Regards Karl
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Old 9th June 2008, 06:24 PM   #56
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Quote:
Originally posted by Mooly
Do you have the multiplier in thermal contact with the outputs ?
It should be possible to get a quiescent current that is reasonably stable against temperature changes in the outputs. Although I have never built a HEXFET amp, I imagine it can't be to different to a BJT output amp in terms of temperature compensation. You would need to match the multiplier characteristics to the tempco of a pair of HEXFET's which will be around -0.3%/deg C.
Regards Karl

I dont have the vgs transistor on the heatsink.
It is on the driver board.
It is not a mountable transistor anyway, its just a MPSA42.

I dont set it up to an exact current, I just tweak the pot until crossover distortion goes. I would have thought this was OK and on the safe side rather than whacking lots of current unnecesarily through the output transistors.
I am using IRFP240/IRFP9240
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Old 9th June 2008, 06:31 PM   #57
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Try epoxying it to one of the outputs, just to try, you should be able to extend it on wires without any stability issues as normally it's bypassed with a cap anyway C-E .
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Old 9th June 2008, 06:39 PM   #58
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Quote:
Originally posted by Mooly
Try epoxying it to one of the outputs, just to try, you should be able to extend it on wires without any stability issues as normally it's bypassed with a cap anyway C-E .

I reckon it would have to be glued onto a MOSFET as the heatsink I use is huge and just never gets warm but the transistors do if worked hard.
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Old 9th June 2008, 06:43 PM   #59
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About much faster...
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Originally posted by peranders

Is this really true?

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Originally posted by Lumba Ogir
peranders,
it is very much true.
Can you give me any numbers? An source follower built with lateral mosfets in a real circuit has a bandwidth of a couple of MHz. How much do you get for a vertical mosfet follower?
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Old 9th June 2008, 06:45 PM   #60
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Hello Andrew,
Think you would have to derate the PSU voltage as well and think about adding Zeners to limit the gate voltage seen under loading. But the CFP output does stand up well to low impedance loads--- no gain droop, but 3 ohms resistive is tough, not suitable for a single pair of FET's for realistic output levels.
And lets not confuse Space, with any more talk of Zeners
Your not thinking of having bash at it Andrew ? It won't disappoint musically
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