Is there a preferred method for a sub-20kHz low pass filter at the input of the first (CC) tube stage?? I could use a high value gridstopper (e.g. 56k) and utilize the miller capacitance, or could go with 1k and a 100pF cap between anode and grid. Perhaps it doesn't matter, but I'm just curious.
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It all depends on what you're trying to accomplish and why. Grid stoppers are included to de-Q parasitic resonant circuits that lead to instability.
Are you trying to keep out RF as would be the case for a modulator? Roll off the highs to conserve bandwidth? Signal processing?
"...go with 1k and a 100pF cap between anode and grid"
This is done with solid state amps to establish a dominant pole. It's seldom required for hollow state since the OPT already establishes a dominant pole. Also, hollow state, being Hi-Z, doesn't have the BW of solid state. It also doesn't require it since you don't need the same levels of NFB that SS amps require.
As for implementation, there are a lot of choices, both passive and active.
Are you trying to keep out RF as would be the case for a modulator? Roll off the highs to conserve bandwidth? Signal processing?
"...go with 1k and a 100pF cap between anode and grid"
This is done with solid state amps to establish a dominant pole. It's seldom required for hollow state since the OPT already establishes a dominant pole. Also, hollow state, being Hi-Z, doesn't have the BW of solid state. It also doesn't require it since you don't need the same levels of NFB that SS amps require.
As for implementation, there are a lot of choices, both passive and active.
Hi miles, it's all for the sake of experimentation. To see (hear) how a roll off past 15kHz for example actually affects the overall sound. And to keep the 20+ kHz out of the circuitry (if it's even necessary). Having some fun with tubes 
. The project is an OTL SRPP+ headphone amp (already sounds awesome!). A very large gridstopper seems a bit foolish, but I've seen values as large as 100k. I do find the simplicity quite elegant, that's why I was wondering if it's 'not done' or common practice.
. The project is an OTL SRPP+ headphone amp (already sounds awesome!). A very large gridstopper seems a bit foolish, but I've seen values as large as 100k. I do find the simplicity quite elegant, that's why I was wondering if it's 'not done' or common practice.
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^^
For that, you could use a Sallen-Key LPF. Build it as an outboard module and run in with a couple of 9V batteries. It'll be nice and quiet, and be sonically transparent. The link describes a second order Butterworth filter which is a good, general purpose characteristic that mimics a first order, RC filter.
For that, you could use a Sallen-Key LPF. Build it as an outboard module and run in with a couple of 9V batteries. It'll be nice and quiet, and be sonically transparent. The link describes a second order Butterworth filter which is a good, general purpose characteristic that mimics a first order, RC filter.
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