Modifying the rumble filter on a PE 250W plate amp 300-794? (1 Viewer)

[Jeff Meininger]

Is it possible / advisable to modify a PE plate amp's rumble filter? I have the model without bass boost, and I want to keep it that way. But as my next sub will be tuned to 20 Hz, I'd kinda like to move the rumble filter up to 20, and perhaps make it 4th order. Too much cone excursion below tuning.

Long, long ago I think I read something that stated you could borrow an op-amp from the plate amp's XO to increase the order of the rumble filter. I can't find any of this info anymore, though.

Can anyone give me any information about this?

Alternatively, I could just buy the Rythmik plate amp that has a setting for a 20 Hz highpass, but it's probably only second-order, right?

Any advice? Thanks!

 

[Ryan Schnacke]

The 300-794's filter is already at 19.4Hz. Moving it to 20 won't make any noticeable difference.

Using Sallen-Key op-amp filters you can cascade op-amp circuits to get any filter order you want but it takes 1 op-amp for every 2 orders. So yes, you'd have to borrow or add an op-amp to do this. If you're trying to get a specific Q and F3 then you'll have to modify both op-amp circuits to achieve this. Or you could do as others have - leave the stock rumble filter and turn the crossover op-amp into a variable low-pass filter using the potentiometer that's already there. You'd want to center the range around 20Hz or so and I'd use a low-Q filter alignment.

If I knew the range of the pot I could work some numbers for you.

 

[Jeff Meininger]

I'm definitely interested. What information would I need to obtain for you to work some numbers? Anything other than the potentiometer specs?

This would be VERY beneficial for people who wanted to tune a shiva sub to like 22 Hz and still be able to watch Lord of the Rings, etc.

EDIT: I'm still interested in the answer because I think it would be helpful for folks who search the archives and find this thread... but it looks like I'm going to buy a rythmik amp after all.

 

[Ryan Schnacke]

Well, you can find schematics for a VERY similar circuit here:

http://www.apexjr.com/Apexsenior.htm

But unfortunately the legibility is not good enough for me to read all the values.

Sallen-Key filters are pretty easy to calculate once you know the formulas. And since I don't use them often enough to have them memorized I just built a spreadsheet that can calculate any 2nd order filter. If I knew what corner frequency and Q value you were after and what size capacitors you're using I could tell you the resistor combination needed to hit that goal.

If you're REALLY interested, I could email the spreadsheet to you (in MS Excel format).