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Old 4th August 2012, 06:59 AM   #1
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Default Mosfet source follower regulated supply

Hi,

I am using this mosfet regulated power supply for my tube preamp and I am not sure which values should I choose for RC filter to the gate of mosfet.
RC filter is: resistor from drain to gate and capacitor from gate to ground.

I have tried these three RC combinations:
100k / 1uF
15k / 10uF
1.5k / 100uF

Although all three variations have almost the same cutoff frequency they sound quite a lot different. Why is that?
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Old 4th August 2012, 07:53 AM   #2
juma is offline juma  Germany
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I suppose it's a Q factor that makes a difference. I often use that kind of circuit (MOSFET capacitance multiplier) in class A amps PSU and values that I like are 10k and 470uF
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Old 4th August 2012, 08:00 AM   #3
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Yes it is mosfet capacitance multiplier. What do you mean by Q factor?
I was thinking lower resistor from drain to gate should be better as it allows for faster current draw?
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Old 4th August 2012, 08:25 AM   #4
juma is offline juma  Germany
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Q factor here is a relation between reactive and linear resistance of the filter. The R times C (RC constant) defines the time of the charging of the C. The value of the C, together with the gain (transconductance) of the active element defines the C multiplication factor of this circuit.
There is no significant current draw in the MOSFET's gate circuit - what we want is a filtered voltage at the gate because it defines the output wave form.
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Old 4th August 2012, 08:47 AM   #5
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Quote:
Originally Posted by juma View Post
I suppose it's a Q factor that makes a difference. I often use that kind of circuit (MOSFET capacitance multiplier) in class A amps PSU and values that I like are 10k and 470uF
I was once experimenting with values of 1.5k and 220uF/100uF and 100uF sounded better, more open, more dynamic. I understand what you are saying, but still I dont get it why it sounds so different, am I missing something?
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Old 4th August 2012, 09:04 AM   #6
juma is offline juma  Germany
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I don't know what amp you use with that PSU and what is its current draw. Look at the ripple amplitude in your PS voltage and take your amp's PSRR into account.
It's true that sound depends on PSU but it's, like most of the things, a matter of taste - what you call open and dynamic someone might describe as bright and aggressive...
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Old 4th August 2012, 09:36 AM   #7
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PSU was for class A amplifier so PSU drawed constant current and still there was difference. I was very surprised by the effect of gatet capacitor value.
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Old 4th August 2012, 10:48 AM   #8
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Are all these caps the same brand?
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Old 4th August 2012, 11:27 AM   #9
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Originally Posted by analog_sa View Post
Are all these caps the same brand?
Yes, everything are electrolytics.

I should also say, that amplifier has no overall global feedback, so these changes show up propably more than with GNFB.
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Old 28th August 2012, 09:03 PM   #10
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Default Function of resistors?

Hi,

what is the purpose of the R21/R22 resistors? I have seen this simple mosfet source follower regulated supply also without them. In which case are they needed?
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