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Old 17th December 2012, 09:28 PM   #31
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Oh, this thing may also need a thorough test to see if the output inductor handles the heat. That thing gets hot. Anyone ever thought of just using air core inductors for these things? Also, btw, you play guitar? I love guitar. I was extremely pleased when I manufactured a stereo output for the guitar using 3 rather than 2 pickups. So your ears get to hear two separate viewpoints of the strings motion. It adds information and flavor to the sound for sure.

Last edited by mgshightech; 17th December 2012 at 09:31 PM.
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Old 17th December 2012, 10:54 PM   #32
dmills is offline dmills  United Kingdom
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Some of the guys building PWM modulators for class E ham radio AM gear are using air cored magnetics for the output filter (Often 4th order) at design load impedances in the order of a few ohms, so air core is possible (I must introduce that crowd to some of the syncronous class D parts, that IR driver for single supply rail D springs to mind as a cool thing for that use).

Usually output inductors getting hot is a sign that you used the wrong core material, or are running too much flux density, there is a **LOT** of difference between something like a ETD49 with a 2.5 mm gap and a FT47 ferrite ring for example, also be a little careful of the iron powder cores (TXXX-2,6,17 and the like) they are sometimes disturbingly temperature sensitive.

Note that an air core inductor tends to have a **MASSIVE** fringing field that can be somewhat reduced by winding it on a -0 toroidal form (this is non magnetic but forms the solonoid into a closed magnetic path), things like steel or aluminium panels within a few coil diameters will completly trash the Q.

Some of the sendust derived high flux density materials may be worth a look.

Regards, Dan.
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Old 17th December 2012, 11:01 PM   #33
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Quote:
Originally Posted by dmills View Post

Usually output inductors getting hot is a sign that you used the wrong core material, or are running too much flux density, there is a **LOT** of difference between something like a ETD49 with a 2.5 mm gap and a FT47 ferrite ring for example, also be a little careful of the iron powder cores (TXXX-2,6,17 and the like) they are sometimes disturbingly temperature sensitive.


Regards, Dan.
I use a T106-2 with excellent results. It gets barely warm.

On my first attempt at a class d amp I just used a power inductor and this got to 120 degrees C !
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Old 17th December 2012, 11:16 PM   #34
dmills is offline dmills  United Kingdom
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-2 is a fairly low permiabilty iron powder core that is not horrible for this sort of thing, certainally far better then something using a -47 or so core (ferrite).

Have you measured how much switching residual is making it out? I am a little surprised you got enough inductance on a -2 core.

Power inductors of the SMPSU variety are sometimes ungapped high permiability materials and using such will want the datasheets studying carefully.

Regards, Dan.
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Old 17th December 2012, 11:26 PM   #35
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[QUOTE=dmills;3289293
Have you measured how much switching residual is making it out? I am a little surprised you got enough inductance on a -2 core.

Regards, Dan.[/QUOTE]

There is a few volts of residual carrier on the output but the speakers inductance will reject that frequency that anyway.
I wind on the turns around the core twice to get sufficient inductance.
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Old 18th December 2012, 06:00 PM   #36
dmills is offline dmills  United Kingdom
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There is a few volts of residual carrier on the output but the speakers inductance will reject that frequency that anyway.
Thats not an amplifer, thats a HF jammer!

Regards, Dan.
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Old 20th December 2012, 03:51 PM   #37
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I spoke a little too soon. The added stability for the OCP protection because of the swap to 10 ohm transistor drive seems to be mostly showing up only for the low frequency range. The higher frequency range is also improved, but not nearly so much, so I have more searching to do to get the OCP for this unit under control. I have to wonder why the bottom end is delivered so robustly, but mids and highs are still weak.
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Old 5th January 2013, 12:13 PM   #38
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Did you check the temp of IRS2092 when you changed the gate resistor to 10ohms?
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