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Old 16th August 2010, 03:12 PM   #11
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Are the electrolytics actually installed the way you have them drawn?
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Old 16th August 2010, 03:56 PM   #12
ervinl is offline ervinl  Indonesia
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1. In this schematic, what if BD139/140 is removed, and directly connect B to E (emitor resistor 220 ohm is out/removed). The voltage should be enough to drive MOSFET (4 volt by adjusting Vbe), and the current is also enough (10mA).

2. What if Vbe is replaced by simple VR without transistor, assuming using lower current / class AB, or Lateral MOSFET?

Just curious why BD139/140 driver is needed - if current (and thermal dissipation) is still proper for MOSFET final -, and why Vbe multiplier is needed to compensate bias (as this will lower sound quality).

Thx,

Ervin L
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Old 17th August 2010, 07:45 AM   #13
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Quote:
Originally Posted by madisonhack View Post
Are the electrolytics actually installed the way you have them drawn?
Not sure. To be right oval shaped pole on the picture should be positive.

Without driver stage this schematic will also work but 10mA is not enough to drive huge input capacitanse of HEXFET devices (more than 1200pF for IRFP 240/9240). I tested without drivers and sound was awful. I did not tested with LMOS devices but probably will work without driver stage.
HEXFET's really need a thermal compensation and to clarify that read please this article:

Using HEXFETs in High Fidelity Audio
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Old 17th August 2010, 09:11 AM   #14
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Quote:
Originally Posted by ervinl View Post
1. In this schematic, what if BD139/140 is removed, and directly connect B to E (emitor resistor 220 ohm is out/removed). The voltage should be enough to drive MOSFET (4 volt by adjusting Vbe), and the current is also enough (10mA).

2. What if Vbe is replaced by simple VR without transistor, assuming using lower current / class AB, or Lateral MOSFET?

Just curious why BD139/140 driver is needed - if current (and thermal dissipation) is still proper for MOSFET final -, and why Vbe multiplier is needed to compensate bias (as this will lower sound quality).

Thx,

Ervin L
#1 - Gate capacitance would load the VAS just enough to greatly increase THD. Some designs bias the VAS at over 15ma and run the mosfets directly. With the drivers , you lose another volt on top of the 5v required to forward bias the MOSFET, but isolate the voltage stage which reduces THD. For me , DRIVERS are essential , you would have a junky , cheap design without !!


#2 , with vertical MOSFET's .. they have the same tempco as a typical BJT. So, they would increase their conduction , creating more heat... more conduction , POP .. thermal runaway !! With a very large heatsink this could take a while and how they were biased would be a major factor. Vertical MOSFETS sound the best and simulate with the best THD at @100ma+.

Attached is my MOSFET setup , which I make use of junkpile IRFP240's , grab any ideas or ask ????'s.
OS
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Last edited by ostripper; 17th August 2010 at 09:14 AM.
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Old 17th August 2010, 11:44 AM   #15
ervinl is offline ervinl  Indonesia
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Quote:
Originally Posted by ostripper View Post
#1 - Gate capacitance would load the VAS just enough to greatly increase THD. Some designs bias the VAS at over 15ma and run the mosfets directly. With the drivers , you lose another volt on top of the 5v required to forward bias the MOSFET, but isolate the voltage stage which reduces THD. For me , DRIVERS are essential , you would have a junky , cheap design without !!


#2 , with vertical MOSFET's .. they have the same tempco as a typical BJT. So, they would increase their conduction , creating more heat... more conduction , POP .. thermal runaway !! With a very large heatsink this could take a while and how they were biased would be a major factor. Vertical MOSFETS sound the best and simulate with the best THD at @100ma+.

Attached is my MOSFET setup , which I make use of junkpile IRFP240's , grab any ideas or ask ????'s.
OS
I use DoZ (3 transistor, 1 CCS, 1 common source, and 1 for CFB) as LTP and VAS, bias at around 15mA (and already tried to 30mA without problem using BC141-161), drive MOSFET directly at +/- 24 volt. So I wonder if I add another BD139/140 like in your schematics it will improve the THD or add another sound signature improvement.

Currently I use IRFP 240/9240, and bias to 500mA each without any problem. Previously, I bias up to 1.5A but at lower voltage, +/- 13.5V also without any problem (run for hours) - just hot transformers :-) . I add 9.2 silence computer fans in chasis (front of heatsink).

I don't use Vbe multiplier as I belive it will degrade sound when hot, while without it, it will have "warmer" sound when being hot (class A bias).

I still use DoZ as it is using CFB, and I believe CFB is still better than VFB/differential amplifier.

Any comment for improvement?

Thx,

Ervin L

Last edited by ervinl; 17th August 2010 at 11:47 AM.
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Old 17th August 2010, 11:51 AM   #16
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by ervinl View Post
............I don't use Vbe multiplier as I believe it will degrade sound..............................
............Any comment for improvement?
design and fit some temperature compensation for the output bias.
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Old 17th August 2010, 01:06 PM   #17
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Quote:
Originally Posted by astankov View Post
I won't touch the feedback network.......not at all.
This type of feedback makes bass stiff , fast and more deep and it is used in other designs so I like it . And as you can see on the schematic it is power supply voltage dependent. If someone is interested i will post values for different supply voltages or the way of calculating the values.

About trimpot for bias I agree that is connected dangerously but this is only in prototype design and will be corrected. And I also can add gate protection.

Thank's again for the suggestions.
Which designs also use this feedback scheme ??? Do you have any literature about it explaining its working ?? I guess the way of calculating the values would help as well.
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Old 17th August 2010, 01:26 PM   #18
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My tips for the output stage ..... Use a smaller value source resistor for irfp9240 around 0.15 ohms, this will help match the transconductance compaired to irfp240.

Use a smaller value gate resistor for the P mosfet, around 60-75 percent of the value of the n mosfet, so in this case a value like from 56 to 82 ohm is good if the n mosfet s gate is 100 ohm, that is if its stable with the 100 ohm. For a good explanation see aplication note AN1645 from National.
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Old 17th August 2010, 01:38 PM   #19
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It is used in the design on the picture. On the text file is the formula for calculating the values and on second picture already calculated values for common supply voltages.

Original source: Audio Club Bulgaria - DIY :: Преглед на тема - Excela
(Sorry in Bulgarian Only)

Schematic is copyrighted. Free for personal use!
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Old 17th August 2010, 03:07 PM   #20
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Interesting thanks, Ill try get some of it translated, anyway the original schematic is how I use those v mosfets in output stages,Triples, just change the output bjts to Fets, install gate resistors, modify temp compensation, bias fets at 100ma, drivers at 50ma, predrivers at 7-10ma and you have a very good Fet outputstage.

Btw you really shouldnt use mje340 or kse340 for temp compensation, they have the worst caracteristics for this use, they have very low HFE and high voltage.
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