[Fwd: LF: Low Pass Filters]

[Andre Kesteloot]

G0MRF wrote:

> Decided to spend my spare time in the week getting my 4 x 2SK413 amplifier
> boxed and working.  The amplifiers OK but I needed a drive source and a
> filter. All the original testing was done with a signal generator before the
> band was released.
>
> The drive uses a SGS Thopmson audio amp chip. The TDA2030.   11 sheets of data
> are available @  www.st.com  under consumer products data. The performance is
> ideal with the output going smoothly from 0 to 11 Watts with a  24 Volt supply
> or 6W with  15V. The data suggests that the chip should have a 3dB point at
> 140k but when tested there was no sign of any reduction in power.
>
> The low pass filter was a combination of the info given by TONI HB9ASB
> together with data from the ARRL handbook.  Based on a 0.01dB ripple
> chebyshev, if you're looking for a filter to try, the following may be useful
> to record
>
> LPF.      5 element  pi  design  ( capacitive input)
>
> Ideal values.      22n    68.5uH   39n  68.5uH  22n.        based on 160k
> cut-off.
>
> Actual values used.  All caps  1kV  from RS.
>
> (4 x 4n7.)    (76t on T-200-2.)    (2 x 10n + 4 x 4n7.)  (76t on T-200-2)   (4
> x 4n7)
>
> With 76t of 1mm dia wire the toroids are almost full. I've used the remaining
> gap of 10mm to glue the toroid upright onto the PCB.   (Thanks for info Toni)
>
> With the values used, the cut off is a little higher, with low insertion loss
> to 170k.   A rough measurement with a scope, which is far from ideal, gave:
>
>                      200k   -14dB
> 2nd harmonic  272    -26dB
> 3rd  harmonic  408    -42dB.
>