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February 20, 2019 / consort3

Bandpass filter

Just when you thought that I had exhausted the variations on the theme of single op-amp, equal component, Sallen and Key filters I came across this bandpass filter due to the prolific, sadly late, Reg Williamson. To preserve the symmetry of the pass band Reg reckoned that  the time constants of the series and parallel elements should be as close as possible. Reg was the UK editor of Audio Amateur and is perhaps most famous for his record cleaning exploits.

The first stage Q is made underdamped by means of R11 and R12 and this helps to reduce the loss of the filter which is -3.5dB. Bandwidth is 340 to 3.4k

bandpass2

bandpass1

For an wider audio band filter i.e. 27 to 16kHz make R1-4 2.2k, R5-8 56k, C1-4 100n, C5-8 4.7n, R11 16k and R12 11k. This filter has a gain of almost 6dB.

Another way of implementing a bandpass filter is simply cascading HP and LP components. As shown on the TDA2030A datasheet for a 3 way active loudspeaker with simplified circuit for LTspice analysis:

sgs

The result is not as symmetrical as the Williamson above and has a 2dB loss.

sgs1

The Williamson technique does not work so well with narrow band-pass filters, as the losses increase. A more conventional technique is here.

hiqbandpass2

hiqbandpass1

I got this from the calculator here:

http://www.mh-audio.nl/Calculators/abpf.html

Reg Williamson also designed the Reverse-RIAA filter

revriaa

For the record (pun intended) here is Regs’ recipe for the record cleaner

Reg Williamson Record Cleaner

The following is the recipe using 2 oz of Dupont Elvanol.

Distilled Water (initially) 1.5 cups (400 cc or ml)

Elvanol 2 oz (.25 cup or 56 gm)

Glycerin 2 tsp (8.5 cc)

Kodak Photo-Flo 200 3-4 drops

Alcohol 3 1/3 tbsp (11tsp or 50 ml)

CYASTAT SN 1/2 tsp (2.5 ml)

Elvanol is Dupont powdered PVA and Cyastat is anti-static compound

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