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February 23, 2010 / consort3

X-max problems?

Updated 2 July 2016

How many times have you modelled a vented box design in Unibox, Winisd or one of the other bass box modelling programs only to find you hit the excursion limits . If you use a high pass filter you can stop the driver flapping  when it ‘unloads’ below the port frequency.  See these for more:

http://www.danmarx.org/blog/?p=491

http://www.subwoofer-builder.com/active-filter.htm

The problem is that to get the best results you have to go active with such an approach, as a passive filter would be difficult and expensive. Actually you can use a single capacitor (see the Piccolata mentioned below) The standard method is described in the subwoofer builder link above.  However I have worked out a way of modifying a standard ‘gainclone’ type amplifier to include a third order high pass filter with no extra op-amps

Why third order is better than second:

http://andyc.diy-audio-engineering.org/vented_box_6.html

I was answering a question on Diyaudio when I realized an equal capacitor value third order Butterworth high pass filter might be possible. Uwe Beis’s excellent website where he has an applet to work out filter values gave a clue but not the answer. Lancaster in his active filter cookbook recommends making the first passive R-C stage of such a filter have a capacitor 10 times larger and the resistor 10 times smaller to avoid loading problems with the second active Sallen & Key stage. However this is a brute force approach.

This filter has compensation built in because the underdamped second stage compensates for the loading of the first stage (which would otherwise be overdamped)

The resistors have to be in a certain ratio so I have prepared a table showing the values to give a –3dB cut off range of 13 to 116 Hz The table uses  E24 resistor values and  E6 capacitor values.

Say you want a filter with a 60Hz cut-off. If you look down the 150n column you find 58.5 the nearest value so the 3 equal capacitors C1,C2,C3 are 150n and the associated resistors are 13k, 5.1k and 91k

In practice R2 is joined to the 1 k,  the 9.1k is taken to the inverting input and a capacitor is put between the inverting input and the junction of R2 and the 1k to lower the offset. In this case the capacitor should be a 1000uF bipolar electrolytic The amp can be TDA2030 LM1875 LM3886 etc

680n 470n 330n 220n 150n 100n R1 R2 R3
12.9 18.7 26.6 39.9 58.5 87.7 13k 5.1k 91k
14.1 20.4 29.1 43.6 63.9 95.9 12k 4.7k 82k
15.4 22.3 31.8 47.6 69.8 104.7 11k 4.3k 75k
17 24.6 35 52.5 77.1 115.6 10k 3.9k 68k

The best definition of the values I could find on-line were these:

http://www.tubecad.com/articles_2001/Tube-Based_Crossovers/Tube-Based_Crossovers.pdf

http://users.ece.gatech.edu/~mleach/ece4435/filtrpot.pdf Pages 30 and 31 cover the Chebychev variety as well

Incidentally, the mleach in the above link is none other than the late great Marshall Leach some of whose work can be found at:

http://users.ece.gatech.edu/~mleach/audiothings.html

http://www.edn.com/design/analog/4363970/Design-second-and-third-order-Sallen-Key-filters-with-one-op-amp

For analysis, try the values you get here:

http://sim.okawa-denshi.jp/en/Sallenkey3Hikeisan.htm

One can pull a similar trick on the equal resistor third order low pass filter as the capacitor ratios are conveniently 220nF 560nF 33nF. Next step down would be 270nF 680nF 39nF followed by 330nF, 820nF and 47nF. Note that for the low pass filter the capacitors and resistors are transposed as in the above link

So, you could make a 2way loudspeaker with a 3Khz 3rd order butterworth crossover with 2 power amps like the TDA2030 and no extra op-amps. Low pass filter would be equal resistors 13K and capacitors 5.6n 15n 820pF High pass filter would be equal capacitors 4.7n with resistors 8.2k 3.3k and 56k.

Reasonably priced close tolerance capacitors are now virtually unobtanium. However Farnell and RS sell a range of Panasonic 2% PPS film capacitors, they are chip types but you could solder wire leads on. Here is a possible source of reasonable price and tolerance caps:
http://www.taydaelectronics.com/capacitors/polyester-mylar-film-capacitors.html

If you like polypropylene capacitors try the Panasonic ECWF series from Farnell or Mouser. It seems counter-intuitive but the 630V Kemet F461 and 800V ECNH8 range from Mouser are cheap, if you can tolerate the size. Tayda now do reasonable polyprops:

http://www.taydaelectronics.com/capacitors/axial-polypropylene-film-capacitor.html

Here is a nice small project where steps have been taken to reduce the excursion using one capacitor as a filter:

http://www.rob-elder.com/LouC/speakers/TB4V/Piccolata.html

The capacitor is a first order filter, if a second or third order active filter were used it might be possible to squeeze even more out amplitude out of the driver.

High pass table for E6 capacitors and  unity gain op amps with good current drive

470n 220n 100n R2 R4 R6
17.1 36.5 80.2 14k 5.6k 100k
18.7 39.9 87.7 13k 5.1k 91k
20.4 43.6 95.9 12k 4.7k 82k
22.3 47.6 104.7 11k 4.3k 75k
24.6 52.5 115.6 10k 3.9k 68k
26.8 57.3 126.1 9.1k 3.6k 62k
29.7 63.5 139.7 8.2k 3.3k 56k
32.8 70 154.1 7.5k 3k 51k
35.8 76.4 168.2 6.8k 2.7k 47k

The high pass filters can be used as rumble filters as well. I thought I knew most of the common Hi-fi forums but this one escaped me until now,  shows there are a lot of vinyl addicts out there:

http://www.vinylengine.com/turntable_forum/index.php

Taling of turntables I noticed a bit of renaissance going on with Connoisseur turntables. Arnold Sugden who created them was a pioneer in recording techniques See pages 12-14 here:

www.thefrms.co.uk/pastbulletins/bulletin139.pdf

More info on Connoisseur tuntables

http://www.pinkfishmedia.net/forum/showthread.php?t=18144

Another pioneer from the 50s golden age was Stanley Kelly of Ribbon tweeter fame. He is not mentioned on Wikipedia so here is a potted biography,  info courtesy Ken Kessler.

stankelly

More on the Kelly Ribbon

http://www.soundfountain.com/amb/ribbon.html

http://hd-audio.orpheusweb.co.uk/

Just found an on line magazine Ken Kessler writes for

http://www.soundstagehifi.com/

I knew KK was into watches and there he is on the BBC4  Nicholas Parsons programme on Breguets Marie Antoinette clock.

I found a new forum when I was researching Manticore turntables, despite its name its fairly discerning:

http://hifisubjectivist.org/portal.php

Talking turntables I just discovered Lencoheaven. I thought idler drive was supposed to be crap! Judging by the number of members every owner in the world is on there. In the UK they were associated with Goldring, I seem to remember. There is a nice entry on Wikipedia which explains why they are still popular.

Vinyl record production problems:

http://www.factmag.com/2015/05/07/pressed-to-the-edge-vinyl/

Talking forums there are a lot of tape addicts out there:
http://www.tapeheads.net/forums.php

Interesting vintage electronics forum:

http://golbornevintageradio.co.uk/forum/index.php

Interesting vintage hi-fi site

http://www.thevintageknob.org/

Headphone forum

https://www.head-case.org/forums/

Subwoofers sometimes need a long port and a tube may be inconvenient. This is a good tool for designing long “L” shaped slot ports but for the metric guys it is in inches!
http://reaudio.com/box.php

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One Comment

Leave a Comment
  1. Drabdediure / Jun 7 2010 2:42 am

    Just want to say what a great blog you got here!
    I’ve been around for quite a lot of time, but finally decided to show my appreciation of your work!

    Thumbs up, and keep it going!

    Cheers
    Christian,Earn Free Vouchers / Cash

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