Benihana

300-794 amp mod

I have this amp from PE, it has no bass boost, and the rumble filter is at 19hz. Is there anyway I can lower this to 14hz without enabling any boost? In the manual it only shows the 14hz rumble filter enabled with 1db of boost at around 20-24hz.

Stephen Hopkins

Ryan Schnacke

With the Sallen-Key filter architecture used on these amps, you can choose any cutoff frequency and boost you want.

For Fcutoff = 14Hz and Q=0.71 (no boost) use R27=160kohms and R26=80kohms (2 160k's in parallel will work for R26).

Benihana

how did you come up with these values? Is there a document I can check out?

Ryan Schnacke

There used to be a schematic on the Apex Jr website (might still be there) for the preamp board of a very similar plate amp. It wasn't hard to find the filter circuit and identify it as a Sallen-Key op-amp filter.

I did this a while back and entered the equations into an Excel spreadsheet that can run them both ways - enter the filter values and calculate R values or enter the R values and calculate filter values.

Benihana

is there anyway you can tell me the values to try to get the filter of 16hz and 18hz with no boost? I tried the sallen-key calculators, but they don't come out right, its hard to use.

Ryan Schnacke

For 18Hz, no boost: R27=125k, R26=62k

I don't think 125k is a standard size. 130k & 62k will get you 17.7Hz.

For 16Hz, no boost: R27=140k, R26=70k

Again, non-standard sizes. 150k & 68k get you 15.8Hz.