eliminating low pass filter

for the DIY ST35, the Dynakit and every other PP EL84

eliminating low pass filter

Postby Stunch » Mon Aug 25, 2014 1:36 pm

There was mention on another audio forum about omitting some things from the original ST-35, one of them being the 0.1 uF cap at the input. I'm guessing the DIY version does the same thing, or at least it's an option. Someone pointed out that the 33pF/470R combo after the DC blocking cap was basically a low pass filter to attenuate RF, and that this pair could be removed if there was no concern of picking up RF in the area. For those who like less stuff in the way of the signal, this would probably be a good idea. Are there any concerns you would have with removing the cap and resistor? This would then (only) leave the 47KR resistor is series between the input and the grid.
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Re: eliminating low pass filter

Postby Shannon Parks » Tue Aug 26, 2014 5:04 am

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Re: eliminating low pass filter

Postby paart » Mon Sep 01, 2014 11:15 am

If you remove the .01mfd input cap, be absolutely certain that there is NEVER a DC voltage present at the output from the previous component. This can be a problem if the preamp is either a solid state circuit, or a tube preamp with a cathode follower output. A DC offset occurs most frequently when the preamp is turned on or off. The preamp I’m currently using, initially produced a 22 volt “pulse” at turn on. It was originally designed with a large coupling capacitor after the cathode follower to allow use with low impedance (solid state) power amplifiers.
This issue can and will quickly destroy loudspeakers.
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