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Old 26th August 2014, 08:35 AM   #1
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Default Power supply mosfet vs choke filter

Dear members,

Please take a look at the attached very simple power supply ripple filter.

Key performance figures:
406 V offload voltage
134 mA quiescent current
14 mVpp ripple

These are comparable to a 15H choke (~100R Rdc), yet you can get all parts for the mosfet filter for less than 1, compared to the price of a decent choke.

Am I missing something, or why are chokes still so popular in our tube amp power supplies?

Cheers
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Old 26th August 2014, 11:11 AM   #2
JLH is offline JLH  United States
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Because a choke can actually store energy and a MOSFET cannot.
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Old 26th August 2014, 11:32 AM   #3
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I know, but chokes also have a Rdc, typically 100-200R. Voltage drop in this example would be about the Same...
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Old 26th August 2014, 01:44 PM   #4
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Quote:
Originally Posted by goldenbeer View Post
I know, but chokes also have a Rdc, typically 100-200R. Voltage drop in this example would be about the Same...
Voltage drop is not the point choke store energy and reduce ripple by resisting change in current level . Chokes are big, heavy , expensive and inefficient. However they are very effective at the job . As well as increasing impedance vs freq. they are thus very different across the freq. range than the mosfet. Using mosfets for cap multipliers however does work well.
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Old 26th August 2014, 02:03 PM   #5
DF96 is online now DF96  England
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The energy storage in a choke is incidental to its role as the series element in a low pass filter. Energy storage just happens to be the means by which a choke tries to maintain constant current.

Note that the follower shown in post 1 works in a different way from a choke; it is not a true series element as it has a ground connection via the 1uF capacitor. C2 and R1 are the low pass filter; the MOSFET just buffers the output from this. This circuit, unlike the choke, tries to maintain constant voltage, so it is not a choke replacement but an LC replacement.
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Old 26th August 2014, 02:46 PM   #6
Bigun is offline Bigun  Canada
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Choke filters can also radiate noise into other areas of the amplifier through it's magnetic field.
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Old 26th August 2014, 04:43 PM   #7
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I sometimes use MOSFET "choke filters", but often in my circuits the problem is voltage rating of the MOSFET.

For reliabilitys sake I like to have all the MOSFETs withstand the highest possible voltage that can happen at that position, and in the raw B+ filtering stages that can be 800 volts. It gets expensive.

Of course nowadays I mostly do fully balanced, where PSU filtering becomes an afterthought (there's still a regulator in there for 'final' B+).
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Old 27th August 2014, 12:25 AM   #8
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Search for Mosfet Gyrator - the schematic posted above (as DF96 says) is NOT a simulated inductor supply, it is a simple capacitance multiplier.

The "real" gyrator supply emulates the chokes "resistance to current change" behaviour and is reputed to sound much better than a capacitance multiplier "ripple filter".

Here is one which can operate as a true simulated inductor (link out) or in a mode with reduced simulated inductor behaviour but capacitance multiplier function added (link in) - search "E-Choke" and you will find where you can buy this "off the shelf".

Cheers,
Ian
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Old 27th August 2014, 01:41 AM   #9
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I would also echo Goldenbeer's question.

Whichever way the shown circuit works, he has posted the results, which as said would require a fairly large choke to equate. Heat loss vs. cost and inconvenience of a choke. It would be interesting to consider this alternative for a choke-input filter, where the "free-wheeling" spikes can be considerable as well as the induced interference (as mentioned) effect on nearby components. I had difficulty in getting the gap field far enough from circuitry to get rid of magnetic-induced effects.
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Old 27th August 2014, 09:54 AM   #10
DF96 is online now DF96  England
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You can't use a FET to replace the choke in a choke input filter. A FET can't conduct current in the opposite direction to the applied voltage, as a choke can (for a while).
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