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Old 21st February 2014, 02:30 PM
bevensee bevensee is offline
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Join Date: Aug 2011
Location: Albertslund, Denmark
Posts: 17
Default Re: Phono 3 with 5751 valves

Quote:
Originally Posted by Richard View Post
Hi Jan,

Keep reading, familiarity is half the battle, it will all drop into place. Bruce Rozenblit is good too,
http://www.amazon.com/Beginners-Guid.../dp/1882580133

That coupling is a cap into an impedance, known as a high pass filter. It has a slope of 6dB/Octave and its "corner frequency" is expressed halfway round at -3dB point.

The bigger the cap value the lower AC f it will pass into a given impedance (AC resistance of path to ground).

In our case C1/2 0.015uF into R13/14 1M

We can use a formula or a calculator,
http://www.muzique.com/schem/filter.htm

Enter the values and see it is 10.6Hz. You can work out the others.

Play with the calc, double and halve values and you'll see the ohms and cap are equally important.

Here's the PhonoII design article covering exactly this, hope Peter doesn't mind, only part of it and a good advert for other articles on the WD archive CD from the shop so you can check those through too against the designer's text,

Sorry, but I don't get this. Can someone explain?
- R13/14 is not 1M
- I you use 0.1 uF into 1MOhm, cutoff is only 1.6 Hz

I guess my question is: If I want to get rid of the switch, and have a corner freq. of the subfilter around 20 Hz, What value should I use?
What if I want to increase the slope to 12 dB per octave, how do I do that?
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