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Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

RE: Input coupling caps as high-pass filter

That's what I do.

The input cap value is computed against the input impedance (resistance) of the mid/tweeter amplifier (300b SET in my case) and creates a 6db per octave high pass filter at 200Hz (my crossover point for my JBL 2231 woofers).

The woofer amp is feed from a active (opamp based) 200Hz low pass filter.

Equation for simple 6db per octave high pass filter,

-3db crossover point in frequency = 159155 divided by (cap value in uf times resistance)

In my case the input impedance (grid resistor of the first stage) is 220k ohm. A .003535uf Russian SSG silver Mica cap in series with the input signal forms a 6db high pass filter with a -3db point of 204Hz (close enough). The cap is installed in the amplifier between the RCA input jack and the 220k grid resistor.

This keeps a lot of bass frequency out of the 300b amp.

Most series feed single ended (gapped) output transformers don't have enough inductance to play bass well and even though the speakers won't respond to the lower frequencies, if those frequencies are allowed to get to the output tube (and the output transformers) all the frequencies will suffer in terms of distortion because the lack of inductance causes the load line to become elliptical.

In the presents of low frequencies all the frequencies will follow that poor elliptical load line.

Blocking those bass frequencies from the amp made a large improvement in the sound of the mids and highs without resorting to adding opamps to the mid/tweeter signal chain.

BTW The Russian SSG cap are the most transparent caps I've ever heard.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"


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  • RE: Input coupling caps as high-pass filter - Tre' 18:54:52 02/02/17 (0)

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