The time finally came today when I could put this off no longer. Though just about the cheapest product in its class one can buy, this mixer has a prominent place on my desk, and I can’t leave a tool that has served me so well looking so frumpy. This thing has earned that dirt through years of being lugged into dirty basements, being left in garages and storage units for several more years, and then flown across the country to sit on my desk for what is likely to be several years yet. I was also experiencing some crackling in headphones when moving a few of the poteniometers, and the push button for sending the RCA input to the headphones & control room seemed to not be making proper contact internally. I took about a half an hour to disassemble, clean, and reassemble it, and decided to install a sick audiophile-class upgrade in the process.
Look at that dirty boy
Here I’ve already removed all the screws, and the nuts and washers holding the 1/4” jacks in place. You can see how futile it was, trying to clean this face plate with the circuit board in place.
I kept the screws nearby in a tray, and was careful to note which types went where. There were only three kinds: two types of small machine screws, and one larger type that was only used on the side peices. Of the two smaller screws, the type with more coarse threads were used on the XLR jack and on the two holes on the back where the back plate is held to the black plastic sides.
Here I’ve taken the circuit board all the way out. Look at that grime! This mixer was one of the first pieces of audio equipment I ever bought, around 2006. No submixes or groups, no on-board effects, and knobs instead of sliders for volume. The most frustrating missing feature, however, is the lack of a solo button for the headphone/control room out. When I use this to monitor myself when recording tutorials (using the RCA output into my motherboard’s mic input, don’t shoot me this is my best option right now), I have to turn my monitors off in order to prevent feedback. There’s no separate output bus for the tape out, it’s just a strict clone of the main bus.
I scrubbed the faceplate, put all the knobs in a tupperware container with some baking soda, and cleaned a few areas on the circuit board that appeared to have corrosion with isopropyl.
I also dropped isopropyl directly onto the push switches and poteniometers that were giving me trouble, and wiggled them around. I definitely should have used a higher distillation of isopropyl than I did. The bottle I have right now is 50% water, which gave me a bit of apprehension, but after waiting a while to make sure everything evaporated, the board fired right back up.
I’ve never been a fan of the silver coating on this unit. It tends to look dirty no matter what, and it doesn’t fit in with the usual audio aesthetic of all-black everything. Had I been in possession of a can of black spray paint at the time, I doubt I would have been able to resist the temptation to black this guy out. I also briefly considered just leaving it partially or entirely outside of the case, because frankly this translucent blue circuit board looks baller.
Instead, though, I got it back together and quickly tested it to make sure that bothersome crackling was gone. I lucked out and it was, and somehow just flushing out the push buttons was enough to fix my other issue.
With the bugs worked out, it was now time to turn my attention towards the upgrades I had in mind.
Upgrading the master section
In addition to the other issues I have with this mixer’s design and general craftsmanship, I found myself limited by the maximum torque limits inherent in the original choice of master volume knob.
The stock knob measures just 7/16” at it’s widest point, and tapers to a mere 3/8” at it’s tip, where you are more likely to be gripping it. The knob I’ve chosen to replace it is 15/16”, is square in profile save for the thin “knuckle-saver” shoulder at its base, and is knurled much deeper on the gripping face. The knurling is especially critical, as in high-performance volume operations the limits of the tangential kinetic friction forces can become an issue.
We can calculate just how much additional torque our new knob allows us to exert on its post. Torque is a dot product between a force and a “lever vector.” In other words, the torque supplied by a force is equal to the magnitude of that force vector multiplied by the magnitude of the distance to the fulcrum, multiplied by the cosine of the angle between the two.
If , then (), we can set up our equation, simplify and solve for x:
When instructed to “pump up the volume,” I am now able to exert 2.143 times the amount of force previously attainable in order to do so. But you might be thinking “Thomas, we’ve only calculated the force you can apply on the knob, without taking into account the greater mass and increased moment of inertia of the larger knob!” Fret not.
The moment of inertia , or the resistance of an object to changes in its angular velocity, is calculated as an integral of all discrete parts of the object:
Fortunately, in our situation, the knobs can be approximated as a hollow cylinder, turning our integral into an easily calculated expression:
For the old knob, this by itself is a good approximation, but in the case of the new knob, its actual construction is closer approximated by two hollow cylinders, one inside the other. is simply .
But we don’t actually know the mass of these two cylinders independently, only the total mass of the knob. This isn’t an issue as long as we assume both have the same area density , and given that they both have the same height, the equation is further simplified.
and if we try to find the proportion of the mass accounted for by each cylinder by introducing a relative factor x:
then we can algebraically reduce to:
Plugging in my own numbers, I find that the inner cylinder accounts for 4/10ths of the total mass of the larger knob, and by extrapolation that the other cylinder is 6/10ths. Now we have all the information we need to calculate the moment of inertia for both knobs.
Yes, I’m using “grams per square inch” because I couldn’t find a ruler with metric measurments and didn’t feel like working with tiny fractions of an ounce. Finally, applying Newton’s 2nd law for rotation will reveal our factor , representing how much faster we can accelerate the new knob vs the old with an equal amount of force.
Rearrange Newton’s 2nd law for rotation:
Express one torque as a factor of the other:
And our final ratio looks like this:
The larger knob is actually only capable of being accelerated at 5/6ths the rate of the old one! We probably could have intuited this around the time we discovered our torque was about doubled, and that our moment of inertia was significantly more than doubled. Luckily this is not even kindof true, as the actual construction of the larger knob features a brass fitting where it contacts its post, so much more of the mass is concentrated near the axis of rotation. Anyways, I hope you enjoyed this completely pointless calculation, and perhaps you learned something!