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    Ph-15 filter

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    GabrielG

    Posts : 11
    Join date : 2012-12-26

    Ph-15 filter

    Post by GabrielG on Wed Dec 26, 2012 10:34 am

    Hi, I just completed the ph-15 kit and like it very much.

    The only issue is with rumble noise as there is no filter for low frequency.

    My idea is to reduce the coupling capacitors size in the 3 stages.

    replacing .22 uf by .1 uf and the last cap by .22 uf instead of 1.5, i doubt it will make much difference though, maybe -3 db ad 10hz.


    goddlediddles

    Posts : 19
    Join date : 2012-10-13
    Location : Kerry, Ireland

    Re: Ph-15 filter

    Post by goddlediddles on Wed Dec 26, 2012 12:01 pm

    @ GabrielG
    I personally never liked the idea of a rumble filter as it is a crude solution to the problem and reducing the coupling caps will give you an unknown slope on the LF response which would be very difficult to calculate and may cut away at some of the bass notes that you want to keep; you would want to do your calculations very accurately or you could be changing caps for ever to get it right. I wonder what gram are you using and how is it mounted? I worked during my career with the EMT range of professional broadcast grams and because of their physical layout they never had a hint of rumble; as a matter of fact rumble from the cutting machine was sometimes audible on a good control desk and speakers especially from early vinyls. If you were going to use a filter it could be as simple as as a capacitor in the input to the PH 15 but you would have to be sure of the PH 15's input impedance with the switch in both MM and MC positions. If the impedance is 47K you will be in the Nano-Farad range for the capacitor and you could even make it variable with a switch to suit your needs. There are many Hi-Pass filter design calculators on the web but let's take some examples of the -3dB response with 47k input impedance (resistive) and a slope of 6dBs per octave using a single cap in the input( duplicated for stereo, of course):

    50HZ - 68nF
    68HZ - 50nF
    15Hz - 220nF
    22HZ - 150nF

    The switchable filter should not interfere with the RIAA filter in the pre-amp as it will be just overlayed onto the response in the O/P. Shielding the switch and caps might be a good idea.

    These are only my thoughts and the designers of the PH 15 may completely disagree with my views.







    GabrielG

    Posts : 11
    Join date : 2012-12-26

    Re: Ph-15 filter

    Post by GabrielG on Sat Jan 12, 2013 9:56 pm

    Hi goddle. Nice reply, thank you for taking the time to answer and calculating frequency responses. I read on forums that the cable to your amplifier acts as a inductor of around 150 nf ... I like the ph-15 very much and I am pretty sure the 0.1 capacitors will not alter the musical bass sounds. I read on more forums, and it seems logical to me, that the smaller capacitors the faster and less colored. I am tempted by this new mundorf gold-oil EVO.

    goddlediddles

    Posts : 19
    Join date : 2012-10-13
    Location : Kerry, Ireland

    Re: Ph-15 filter

    Post by goddlediddles on Sun Jan 13, 2013 5:35 am

    Gabriel, the 150 you refer to is 1000 times off the mark; the capacitance of a typical connecting cable to the amp from the tone-arm could be in the region of 150 Picofarads (1000 Picofarads = 1 Nanofarad) and it is a parallel capacitance. The filter capacitor needs to be in series with the incoming audio signal to form a Hi-Pass filter in association with the amp's input impedance but does not have to be physically adjacent to it; it could be in a shielded enclosure along with a range of other capacitors and a switch with 4 RCA sockets for stereo input and output. If you were going to build the filter, maybe do your first trials with inexpensive caps and substitute the Gold/Oil ones once you are satisfied with the range of rumble reduction obtained.

    GabrielG

    Posts : 11
    Join date : 2012-12-26

    Re: Ph-15 filter

    Post by GabrielG on Sun Jan 13, 2013 9:56 am

    Thats a good idea, I already keep a collection of paper in oil capacitors that I am not using.

    tubes4hifi
    Admin

    Posts : 1292
    Join date : 2008-11-30

    Re: Ph-15 filter

    Post by tubes4hifi on Sun Jan 13, 2013 4:39 pm

    I may be wrong but I just did a quick calc, if you use a 0.1uF output coupling cap and a 100K resistor to ground, that should give you
    a low cut filter (-6db) at around 16Hz, or use a 200K to ground to give -6db at 8Hz.

    GabrielG

    Posts : 11
    Join date : 2012-12-26

    Re: Ph-15 filter

    Post by GabrielG on Tue Jan 29, 2013 12:34 am

    I use 0.1 caps and 1 uf output capacitor going to 100k pot= safisfying bass.

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