300-794 amp mod (1 Viewer)

[Benihana]

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]

I'm not sure if there's a filter adjustment that doesn't affect the bass booste but 1db in that range would most likely be very negligable if you could notice it at all. Just be careful not to drop the rumble filter much below the box's tuning frequency (if it's ported) because the driver will unload and bottom out much more easily.

 

[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.