Passive filter at tube amp input

[WDYSUN]

Dear Mate,

Lynn Olson suggests to extend the bass response of his Ariel loudspeaker by using active filter to the low pass, the ARIEL would be passively filtered with a high pass between the preamp and amp, look at this:

http://www.nutshellhifi.com/ME2txt.html#swactive

Now, my question is how do I design such a filter? How to choose the series cap and the resistor to ground? There is any way to come up with a value? Has somebody experimented with such a thing?

Thanks for you interest.

Cheers
Pierre

[coldcathode]

Pierre,

Assuming that you are going to "dedicate" a tube amp to the high pass channels, it is as simple as either placing or "re-placing" the input coupling cap on the input tube with the correct value for the 80hz F3. You might need to change the grid load resistor to match well with the cap.

here is a calculator

I would need the schematic of you tube input to help further.

But an example would be .0047uF cap with a 390K grid to ground would give about 87hz F3.

A 200K and .01uF would be 79.6hz close enough?

[WDYSUN]

Originally Posted by coldcathode

...
I would need the schematic of you tube input to help further.

Hello,

for the mid-highs I use a Primaluna Prologue 4 (it's a 35Watts PP of EL34).

Thanks
Pierre

[DougL]

To add to the previous comment, do not exceed the maximum cathode resistor in the data sheet. if the value of the grid leak resistor is too small, charge will build up and alter the bias point.

Doug

[coldcathode]

According to the spec's I could find that amp has 100K input impedance.
So a .02uF cap gives you 79.6hz F3.

To make a test get a cheap RCA cable and put the .02 cap in series with the center pin.

[boudy]

Doesn't the output impedance of the preceding device need to be factored in?

[m6tt]

I believe the online calculators assume a fairly low impedance looking into a high impedance, but I am not sure. I have generally assumed the calculators are close-ish for low impedance sources.

[m6tt]

Doug do you mean don't exceed the maximum grid resistance? I don't recall seeing a maximum cathode resistor spec in most data sheets. Also, I believe charge builds up when the grid R is large, i.e. greater than 1.5-5Meg, like grid-leak bias. I may not have totally understood!

[planet10]

You can also just put a PLLXO in front of the amplifier.

The calculator linked above will do the math for a first order...

dave