Using PWM input to a Si8244BB predriver soic. After the inductor HF cut filter, on the 8 ohm speaker load there is about 1.5V pk-pk 200Kz constant level of switcher waveform, is that normal? what sort of background level would be normal ?
"Class-D" isn't "audio"... the "D" stands for "distortion" >:-} ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
You think you've got it bad. ;) "Inductorless" Class D amps use the speaker inductance and rolloff to smooth out the waveform. No EMI problems, no sir, never, of course not.
Sounds like your filter is just knocking the sharp edges off the switching waveform in order to pass EMI, rather than actually getting rid of the fundamental.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
If the OP were to give us the approximate reactances of the L and C at
200kHz and the supply voltages, we could probably actually tell him if 1.5V pk-pk was a reasonable residual amount of fundamental for a likely two-pole Butterworth having that break frequency...;)
Why? Of course a Class-D amplifier could have better linearity than a Class-A or Class-AB can achieve.
(1) The damping factor sucks the most sour of lemons ;-)
(2) And your "better linearity" claim has the same engineering fact level as would be put forth by a used car salesman.
(3) Class-D pros: low heat, small size, CHEAP, thus manufacturers of consumer grade (aka junk) audio products love it. ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
Certainly, the rail voltages for a pair of powerFETs is + and -80V, C is .68uF 400V, inductor is about 30mm diameter toroid with about 45 turns of about 1.5mm wire
They're good for controlling thermoelectrics, but you have to filter the daylights out of it to avoid switching crap getting into whatever sensitive item is on the cold plate.
I've used capacitance multipliers with parallelled NPN and PNP pass transistors for that. Works fine as long as you don't mind a volt or so of crossover distortion. ;) (You also have to avoid forward-biasing the BC junction of the nominally-off transistor.) It's basically limited by the Early voltage of the transistors.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
Class-D audio systems, such as common TV sets, play hob (pun intended) with hearing aids which also use class-D :-( ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
Sounds like something on the order of 100 uH, depending on the core material. In that case the LC rolloff starts at
f_3dB = 1/(2 pi sqrt(LC)) ~ 19 kHz. It rolls off asymptotically like (f_3dB/f)**2, which at 200 kHz is 40 dB down (1/100 times the amplitude). So 160V p-p ought to roll off to, yeah, about a volt and a half.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
Well, if you stick in a cap multiplier with a 10-ms time constant, you won't have to be bothered by it. ;)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
Using the tool here:
And plugging in 0.68uF for the capacitor, 100uH for the inductor, and a damping factor of 0.707, it gives me an cutoff frequency with an 8 ohm nominal load of about 20kHz.
At 200kHz the response looks to be about -40dB down, or 0.01 voltage loss from a 160V pk-pk fundamental, so your reported 1.5V pk-pk seems about exactly right if those values are correct.
Drat, I'm too slow! ;-)
I've found that common room acoustic solutions such as a piece of carpet behind the TV set (wall mount flat screen) helps. ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
That's reasonable. Floppy materials tend to be really lossy up in the hundreds of kilohertz.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
Your philosophy has problems too. ;)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
Hah, I've got this picture of Jim with a hacked hearing aid. to-93 pac transistors hanging out of his ear. (I'm sure some geek makes transistor ear rings.)
George H.
That is a visual the world is not ready for. ;)
It's probably possible to do DSP with a spread-spectrum sampling clock to get rid of birdies. The math would be a bit of a challenge.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant
Thanks for that,just as well my hearing is not much good at 200KHz, nor my speaker being that responsive at 200KHz either. Other than winding some extra turns on the toroid and paralleling some more C, anything that could have been added ? It would have been done by the manufacturer I suppose, if possible . There is one of those loop-through ferrite lumps on the internal speaker wires, but that obviously does not filter much 200KHz
You may laugh, but I've considered, for non-conversation situations, such as TV viewing or going to the movies, rolling my own fully-analog hearing aid, with good muffs... could care less about power consumption, a daily recharge cycle no problem. Could even have balance, and adjustable equalization ;-)
I designed several analog class-A and one class-AB aid(s) in the mid-70's.
Sometimes miniaturization leads to crap performance. ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
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