Well, it was time, I got tired of using a test rig to power a subwoofer I built years ago so I decided to go with something better.
For a lot of people Parts Express is pretty much the standard for DIY speaker building supplies and their Dayton subwoofer amps have become pretty much the standard for powering home built subs. I decided the biggest of their old school, original series amps, the SA240, 240W one was the way to go. It helped that it was on sale for $99.
Anyway, the amp is available in a couple of different versions, one that supposedly has a flat response curve and the more popular version that has a 6 dB bass boost at 30Hz. I probably would have just ordered the one with the bass boost considering I’m using a sub that has what amounts to a tiny driver (8”) in a huge room (if I remember right it’s something like 19’ x almost 30’), but as luck would have it, they were out of stock. What do you know, I plug it in and decide “Yup, I think I’ll need to play with the bass boost settings.”
Interestingly, Parts Express is quite willing to provide information about choosing component values to change settings, but at the same time a lot of people tend to complain that they are too difficult to modify. For those of you out there debating about the thing, I decided to take a few pictures and put up a quick description so you can decide yourself if this is something you want to get into. Since I’ve build a few speakers and am always changing things and “voicing” different combinations of driver, enclosure, source… combinations, that of course means that I need to over complicate the solution so I don’t have to rip the whole amp apart every time I want to change something.
So here’s a chart of the documented settings for the amp:
Simple enough, change 2 resistors and it does something different, so let’s have at it.
The resistors are located at the center of the pre amp board, the board that has the crossover frequency and gain knobs on it as well as the RCA jacks. To get it out you need to remove the 4 main case screws, disconnect 2 jumpers, one going to the line level jacks and the second going to the main amp board, and then remove the screw that holds it to the back of the face plate, a second screw that comes in through the front of the face plate in the middle of the RCA jacks, the 2 knobs and the nuts and washers holding the pots to the front face. This is all pretty straight forward and if you’re worried about that part than don’t even consider doing the rest. The worst part is that the output wire is glued to its hole in the back of the case limiting the amount of room you have to work around inside.
So once you have it out it looks like this:
I realized that I don’t really have anything that gives you a good sense of scale with these things, well to give you some idea the big monster looking resistors on the bottom left are your typical ½ watt resistors, usually you don’t see them much smaller than this unless you’re talking about surface mounts.
So here are the buggers we’re after, they’re almost right in the middle of the board:
And no, I don’t know who the rocket scientist was that decided to put the capacitor lead right over the top of one of them, and yea, it’s at least as much in the way as it looks in that picture, probably more.
Here is my solution to only having to disassemble this thing once and still be able to tinker with it. Instead of desoldering the resistors and replacing them with the new ones, instead I replaced them with sockets making them easily removable.
Actually, I should give credit to my brother, these were his idea. These are pins used to make IC sockets. I was originally going to use the pins that you use to make computer cable “DB” style ends, which are very similar but taller.
People will give you all sorts of suggestions about the best/neatest way to desolder stuff like this, and I’ll admit, I tried some desoldering braid to start with but got nowhere with it. Not being much of an electronics freak, I didn’t have the typical desoldering irons or even a proper soldering station, so I ended up just flipping the board over in a Tupperware (to give the board that sticks out off of it somewhere to go), grabbing a generic 30watt Weller soldering iron and just heating the pins and pushing them through their holes till the resistors fell out.
To put the pin sockets in I put the socket in the hole and while holding the board in my hand and using my forefinger to push a toothpick up against the socket holding it in position on the board and feeding in solder and the iron with whatever limbs I had left over.
The end result looked sort of like this from the back:
(look towards the center of the board and you’ll see a few solder joints with dark flux around them, the flux was eventually cleaned off before the board was put back in the amp).
From the top you can see the sockets, and you can also see the R35 and R36 labels on the board where the resistors have been removed. You can also see the lead from the previously motioned capacitor carefully bent around the bottom right socket:
And finally a picture with some 1/4watt, radio shack resistors installed (if you know what a normal, small resistor looks like this picture might give you some sense of scale, one set of the pins had to be bent back under the resistor to fit in the socket spacing. I purposely left the leads on the resistors a little longer to allow for easier insertion/removal during testing, and when I finally decide on what values I like I’ll probably put little tape over them or a dab of glue if I have any indication that they might vibrate out (I really doubt it).