Tweaker's Asylum

• Constructing high pass filters. YM 18:22:41 11/02/99 (1)
  In Reply to: Re: High pass filters? posted by Chuck on November 02, 1999 at 15:07:10:

  Thats exactly what I'm thinking of doing. I'm going to biamp: tubes on top and SS on bottom. The fact that FMODs are car audio parts concerns me a little. On the other hand they have a better frequency selection than their ACI counterparts.

  I'd be willing to construct the high-pass filters myself using superior parts if only I knew what to insert between the male and female RCA connectors. :( Anyone willing to draw a diagram of said animal for a rather non-technical audience (me)?

  YM
  
  

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  • Re: Constructing high pass filters. Jon Risch 18:13:44 11/03/99 (0)
    In Reply to: Constructing high pass filters. posted by YM on November 02, 1999 at 18:22:41:

    The FMOD parts are unlikely to be of the highest quality, and if youa re seeking the best, then a DIY filter is the way to go.

    Teflon caps are the best, followed by the various froms of polypropylene, and styrene (when you can find it) is almost as good as the teflon.

    In terms of brands, everyone has his favorite, but their is no doubt that Hovland MusiCaps, MIT MultiCap,TRT InfiniCap,Reliable's RelCap are all in the upper tier as far as sonic quality is concerned.

    You simply put the cap in series with the hot lead, make sure the two RCA's have a good ground connection, and try to shield the whole shebang. A large piece of copper braid might do the job, slipped over the cap (hopefully with either insulated leads, or pieces of tefon tubing over the leads).

    In terms of assembling a sonically good filter, use a quality insualted wire for the ground wire, and make it no longer than necessary to bridge the gap between the two RCA plugs. Nestle the cap in between the RCA jacks, and using the tubing to insulate if necessary, use the caps own leads to connect to the hot pins. Then use teflon plumbers tape or a good grade of heatshrink (polyolefin or better), insulate the assembly in the middle, and use the copper braid to shield, grounding it at one end only. You may or may not be able to use the RCA plug barrels, if not, wrap lots of tape tightly around the threaded end of the plug, and use heatshrink as a strain relief. The smaller values of caps might fit inside a large barrel, in which case, a small piece of quality coax could be sued to make the connection from the other plug.

    As far as values go, here are some starting points for a 100 Hz - 3 dB point:

    For a typical SS amp with an input impedance of 10k ohms, use a 0.18 uF cap. Input impedance of 50k ohms, 0.027 uF.

    Traditional tube amps can run as high as 1 Meg ohm, then use 0.0015

    Now for the easy adjustments:
    if you want 200 Hz instead, halve the cap value. If the imput impedance is twice as high, halve the cap value. If you want 50 Hz instead, double the cap, input Z half of the examples, double the cap value.

    Caps can be paralleled (placed side by side, and the leads joined at the ends, keeping the same end together) to double the value, and placed in series to halve the value. So if you buy (in pairs for stereo) about three different caps, this will allow you a very wide range of potential crossover points.

    See:
    http://www.capacitors.com/pickcap/pickcap.htm
    for more information on WHY various types of caps perform differently.

    Jon Risch
    
    
    

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