Audiophiles and speaker hobbyists are often trying to tweak speakers to sound better by modifying crossover components which is great but the one area where they often do exactly the wrong thing is when upgrading inductors.
In this post I'll focus on coils, DC resistance (DCR) and how to avoid the common misconception that takes upgrades down the wrong path. Here's the myth:
Increasing the wire gauge of an inductor is always a good thing.
In fact, the opposite is usually true. If you don't want to read more I'll give you a better rule of thumb:
Leave your crossover coils alone. Upgrade your caps and resistors instead. Stop sneering at speaker maker's choice in coil wire sizes until you learn more about what you the circuits are doing.
OK, but if you are here that means you NEED to make changes, it's in your blood and leaving coils alone is against everything you believe to be good and true in this world, plus you are dying to try fancy Mundorf or Jantzen foil wrapped coils.
If this is you then here's the more nuanced rule of thumb you should follow:
You CAN change inductor gauges but you must monitor changes to the system impedance, frequency response AND power handling to do so correctly.
A crossover simulator like XSim or Vituix are the only real ways of doing this, and it implies you've taken the time and energy to analyze the entire speaker system's electrical characteristics. For our work we can actually ignore the acoustical domain and assume ideal frequency output to start. You'll need not only a crossover simulator but a measurement tool like Dayton DATS V3 to measure the components. We'll leave the tools alone for now, and focus on the "whys." Of course, the alternate version of that rule is:
Always compensate for lower DCR with additional series resistance. You may ask yourself "what??? I wanted to LOWER the resistance!" Well, there lie the pitfalls.
Much of what I've written here apples to capacitors and Equivalent Series Resistance, except that they are not as power tolerant. Be especially careful when replacing old electrolytics with modern film caps.
Real Coils
Real inductors have three major electrical parameters we care about:
- Inductance
- Resistance (aka DC Resistance or DCR)
- Power handling (Watts)
We often discuss "ideal coils" as having only inductance, zero DCR and infinite power handling to simplify discussions around a topic but in speaker building DCR isn't a parasite, it's a tool.
We also don't discuss enough the fabulous power handling of a coil vs. a resistor. Let's consider a nice sample (picture above) from Jantzen via Parts Express:
- 1 uH
- 0.71 Ohms (i.e. DCR)
- 200 W Power handling
Take a moment to consider just how much power this "little" coil can dissipate. We'll talk more below, but assuming you reduce the DCR to say 0.21 Ohms, you'll need an external 0.5 Ohm resistor, and it would need to have 140 W of power dissipation to be truly equivalent (200 * (0.5/0.71)).
Observation:
When choosing between a thin gauge inductor (high DCR) or a thick gauge inductor (low DCR) with a compensating larger resistor in series the thin gauge inductor always wins, and that's not even considering the great thermal stability and low thermal noise of a coil.
In general though there are many circuits in which 0.3 Ohms or 0.5 Ohms difference would matter much more than most would realize without simulation and measurement.
Crossover Circuits
First thing we must cover is the variety of circuits that a passive crossover may have that involve coils. These include:
- Low pass filters (odd-order filter stages)
- High pass filters (even-order filter stages)
- Baffle step compensation
- Zobel impedance compensation
- Notch filters
We'll go over the consequences of "willy nilly" replacement for some of these below.
Low-Pass Filters
Significantly reducing DCR may make your woofer more efficient, requiring you to adjust other driver padding to compensate. Plus, the DCR can affect the slope, meaning your crossover phase matching will suffer. Honestly though this is probably the safest of the circuits to play with because the ratio of the DCR to driver impedance is usually already quite small. Sure it may sound better, but it's most likely because you just changed the speaker's overall balance. It's not because of the quality of the new coil.
High-Pass Filters
HP filters use coils going to ground, often with an additional R in series with the coil. This is perhaps the 1 place where you can get into the most trouble if you don't correctly compensate for your changes. Too low an R to ground can alter the frequency response of OTHER drivers because the impedance plummets unexpectedly.
The total R to ground is often very small, 1.5 Ohms or less so small changes to DCR are really significant.
Baffle Step
As
Troels Gravesen has noted (sorry lost the exact link), in a baffle step compensation circuit you can sometimes use the DCR in place of an obligatory resistor in these circuits, or (as I've implied) minimize the power handling of your resistors.
Notch Filters
These are terribly fussy, so you shouldn't adjust the DCR without making up for it in the R. A change here can shift the magnitude of the filter, instead of flattening the output you'd make a hole in it.
Simulation
The only way to truly understand how important your changes might be is to fully simulate your crossover circuit including speaker impedance data. Check the frequency response as well as impedance and power charts before deciding to just change an inductor.
Alternatively, always compensate for a loss in DCR with additional R in series.
No comments:
Post a Comment