Pre-amp board glued in on plate amp. Please help with bass boost/fumble filter mod! (1 Viewer)

[Jim A. Banville]

I removed the nuts and screws holding the pre-amp board in place on my 250 watt Parts Express amp, but it appears that the board is glued to the rubber gasket, which itself is glued to the main faceplate :frowning: I don't want to damage it by trying to force it. Can someone post a pic of the solder side of the pre-amp board showing the points where the resistors that need to be modified are at? Can I then use a multimeter at those points show in the picture to determine the values of the resistors R26 and R27? The instructions that came with the amp say that the default resistors are R26=56K and R27=120K, for NO BOOST. I'd like to verify those values and add another set of resistors to the solder side of the board to set it up for a Fc at around 20Hz and the smallest boost possible. R26 @ 39K and R27 @ 150K results in a 1 dB boost at 31-15Hz and a Fc at 21Hz, which is close enough. Ron-P said Adire told him that they add a resistor and a capacitor to their 250 watt plate amps to get the desired result, but I'm not sure what the original values are on their pre-modded amps. Help!
thanks
Jim

 

[Jedd]

Jim, did you remove 3 nuts from under phase/volume/something black knobs?

 

[Jim A. Banville]

Yes. I removed the nuts under those knobs along with the screws everywhere except for the power cord connector and voltage switch, per the instructions.

 

[Jedd]

Should be 7 screws and 3 nuts and 3 washers(at least )
Maybe I could do a photo on weekend, or maybe someone has schematic here. (I remember someone posted very detailed photos how to do mod, you may try to search)

 

[Patrick Sun]

I had glue on mine as well, I used a small flat head screwdriver and just slowly chiseled the glue from the foam surround for the preamp section.

 

[Jim A. Banville]

Okay, I ran an Exacto knife around the edge of the foam, and the board became separated from the plate! I made a visual (color bands) and measured (multi-meter) reading of my resisitors. They are R26=56K and R27=120K, which is no boost. Does this mean there is no rumble filter? The paper with my amp says that for a 1dB boost at 20-24Hz and a rumble filter at 14Hz, R26=56K and R27=220K. I am building a 12" Shiva vented sub and I don't want it to get damaged by trying to produce ultra low bass. I can either go with a 1 dB boost between 20-24Hz with an Fc at 17Hz, or I can go with a 1 db boost between 31-35Hz and an Fc at 21Hz. It seems like it would be a whole lot easier to add some resistors on the solder side of the board to get the final values, rather than removing the originals (the added benefit is that if I every sell the amp, the new owner can just clip out the added resisitors to get a flat response). Can someone tell me what value resistors to add to the original resisitors to get the values for the 17Hz and 21Hz rumble filters referred to above?

thanks!

Jim

 

[Patrick Sun]

You're better off just removing the original resistors, and then putting in the new resistors on the other side of the circuit board. The resistor values on the PE pdf write-up on the bass boost for the 300-794 plate amp are common values found in a good electronics store (i.e. not Radio Shack). Once you remove the resistors, you won't have to open up the preamp section again, just de-solder/solder in new resistors to try new configuration as often as you like.