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I bought a used plate amp, look just like the discontinued Parts Express #300-794. It does not have a protective case on the back.

Serial number 0171497

AC Power, 115V, 450w
Fuse 4A, 250V

I trying to figure out how to plug in the filters. WinISD uses this amp as an example to quote

"It's subsonic filter has fc of 27.96 Hz and Q value of 1.5811. Lowpass filter's fc is adjustable, from 56.90 Hz to 195.66 Hz, with fixed Q value of 0.60486. "

MikeP says it has a 18Hz subsonic filter.

Which values should I use?
 

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The default values for the resistors are R26=56K, R27=120K (no boost) according to the documentation.

http://www.parts-express.com/pdf/300-794.pdf

At the bottom of the page:

"As it can be seen in the graph, even with the "no boost" setting, the rumble filter
is still in effect. The low frequency F3 is roughly 19 Hz."

This assumes no modifications were made to the amp by the previous owner. Check the values of the resistors on R26 and R27 to verify.
 

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Discussion Starter #3
Thanks Mike

I did read that line, wasn't quite sure what a rumble filter is, now I know.

Filter settings...

Filter.JPG
 

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Delete one lowpass filter and change the other to 80 hz. Also change the n=2 to n=4 as the lowpass filter is usually set at 80 hz and involves a 4th order crossover.

The SSF (highpass filter) in most plate amps is 4 th order, so also change the n=2 to n=4. Some SSF's are 3rd order but trying to obtain the information from manufactures is difficult at the best of times.
 

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If I'm remembering correctly, changing the Q in WinISD doesn't actually change anything with the high/low pass filters...

I should run through the numbers, but when your lowpass has a Q above .707, that means it has a small peak in the response before it rolls off (same is true for a highpass). I think a Q of 1.4 should be something like +6dB. I usually use the peaking 2nd order highpass filter in WinISD.
 
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