Hi there! You can use my L-pad calculator to get the speaker resistor values you need. With it you can:
- Easily compute the resistor Ohms values you need for an L-pad or series resistor for any speaker Ohms or dB attenuation.
- Estimate the power handling needed for resistors and power supplied your speakers.
I’ve also worked hard to provide a great guide to help you get the sound you want with less hassle. Read on and enjoy!
- L-PAD CALCULATOR
- How does an L pad work?
- How many decibels of attenuation do I need?
- Understanding the math behind an L-pad resistor network
- How precise do speaker resistors need to be?
- Resistor value tips
- What power ratings are needed for speaker resistors in an L-pad network?
How does an L pad work?
Tweeter attenuation is the reduction of voltage & power to a tweeter to decrease its volume output. This is usually done with an L-pad fixed value resistor network with carefully chose resistance values or an adjustable L-pad you can buy.
In an L-pad network, a series resistor reduces the voltage the speaker receives. A carefully chosen parallel resistor is connected to the speaker so that the total Ohms load seen by the stereo or amp is the same as the speaker itself.
Speaker sensitivity matching basics
More often than not tweeters have higher efficiency (volume output) than woofers and midrange speakers. Tweeters are more efficient for several reasons, one of which is their smaller cone or dome, lower mass they have to move, and so forth.
Woofers (especially ones with a heavy, stiff cone) require more electromechanical force to move the cone and produce sound. Because of their higher output at the same power level, tweeters can sound too “bright” or “harsh” due to our ears being more sensitive to higher frequency audio signals.
In a way, you can say that in many cases woofer or midrange drivers and tweeters are mismatched. For example, let’s say we’re building a 2-way speaker system and the specs are:
- Woofer driver efficiency: 87 dB @ 1W/1M
- Tweeter driver efficiency: 92 dB @ 1W/1M
That’s a difference of roughly 5 dB depending on how they’re measured. In fact, although 5 dB isn’t a huge difference, and because speakers don’t have a perfectly flat frequency response there are bound to be some “peaks” in the output which you’ll hear and won’t like.
We can reduce the tweeter level output to bring it down to something closer to that of other speakers.
Why not just use a series resistor?
While a series resistor can be used to reduce speaker volume in some cases, simply adding a series resistor will increase the total speaker load seen by a crossover and cause crossover shift, resulting in poor sound.
That’s because a speaker crossover is designed for a specific speaker impedance (Ohms load) to be connected. Increasing the speaker load (total speaker Ohms) affects the frequency behavior of capacitors and inductors in a crossover.
For example, let’s say we have a speaker crossover designed for 8 ohm speakers:
- The normal crossover frequency at 8 ohms is 3,500 Hertz.
- Inserting a series 8 ohm resistor to drop it by 6dB would be a total of 16 ohms. The crossover frequency would drop to 1,750 Hz!
The cutoff frequency will shift (move), meaning there can be terrible gaps in sound where there’s no frequency overlap. The speaker load is inversely proportional to the crossover frequency:
- Increasing the speaker load lowers the cutoff frequency.
- Decreasing the speaker load raises the cutoff frequency.
Using an L-pad resistor network allows reducing the dB output to a speaker while still maintaining the correct speaker load.
Why not buy an off-the-shelf adjustable L-pad instead of building one?
Using an adjustable speaker L-pad is nice but there are some disadvantages:
- It may be overkill. Do just need to drop a tweeter’s output by some dB amount, or do you really need to pay more for an adjustable feature?
- Nearly all L-pads are for 8 ohm speakers, not 4 ohm. It’s nearly impossible to find one for 4 ohm speakers, sadly.
- Building your own resistor network is easy, and can be very affordable.
- Ready-made L-pads can be harder to install and a bit bulky, meaning they’re not always easy to deal with.
How many decibels of attenuation do I need?
As I mentioned above, you can often use specs from a speaker manufacturer as a pretty good guide for where to start and how much to reduce a speaker’s output (by how many decibels).
However, the honest truth is that it’s really whatever works best for you. Personally, I use the speaker sensitivity rating as the dB difference I use then I improve the speaker sound using an equalizer or signal processor.
As a general rule:
- -3dB is a very small amount and is hardly noticeable to your ears. It’s usually not enough.
- -6dB and -9dB are often better choices especially for tweeters that sound too bright.
One of the best things to do is use series resistors to temporarily drop the volume and test to find what you think is best, then build your full L-pad to use every day.
Understanding the math behind an L-pad resistor network
L-pad resistors work together to create a voltage divider, reducing the voltage & power the tweeter or speaker receives. In engineering, the power to a load (in this case the speaker) can be expressed in decibels which is often used for non-linear power such as audio. For example, doubling the power to a speaker does not double the volume – it increases a few decibels. Our ears are non-linear in how we perceive sound.
Decibels are either positive (a gain) or negative (a loss). In our case, we’re after a negative dB result at the speaker, reducing its power and therefore the volume. L-pad resistor values are found by:
- L-pad networks: 2 formulas, based on the power formula, can be solved to find Rseries and Rparallel which can be used to let the amp or stereo still see the original speaker Ohms load.
- Simple series network: Only one formula is needed and can be solved to find the series resistor based on speaker impedance (Z) and dB drop you’d like.
The formulas shown here and used in the calculator depend on specifying the attenuation we want, written as a positive number (no negative sign). Using -dB requires a different set of formulas.
How precise do speaker resistors need to be?
The good news is that you don’t need exact parts values or high-precision resistors for a speaker attenuation network. Certainly, there’s nothing wrong with using more precise (and expensive) parts, but 10% or even 20% parts values are usually fine.
Also, you don’t need to stress out over finding specific parts values – the general rule to follow is to get resistor values fairly close to the calculated parts values.
For example, here’s a question I received from a reader from my article on how to reduce tweeter volume:
The closest I can find are 20 watt, 2.5 Ohm and 20 Ohm resistors – will they be ok?
If we take those values, we’ll have:
- 20 ohms in parallel w/ 8 ohms = 1/(1/20 + 1/8) = 1/(0.05 + 0.125) = 5.71 ohms in parallel.
- 2.5 ohms (series resistor) + 5.71 ohms = 8.21Ω total.
As you can see, it’s extremely close to the 8Ω total we need. It’s fine for real-world use. (In reality, it’ll never be perfectly 8.21 ohms due to parts tolerances, so trying to be super-precise is often a waste of time and money).
Resistor value tips
Here are some helpful tips for building your L-pad network:
- It’s difficult – if not impossible – to find exact resistor values for specific parts values. The best thing to do is use multiple standard resistor values to achieve what you want.
- It’s much easier to shop for standard resistor values and just buy multiples that you can add in series or parallel to get the total Ohms value you need.
- Power resistors are often cheaper in quantities of 2, 4, or 5, and so forth. You’ll often get a better deal buying in quantity.
While it would be nice to be able to find the “perfect” resistor values you want, it’s not practical or cost-effective. In my experience, it’s a lot easier to use a combination of resistor values instead of spending time and effort hunting down specific part values.
You might need a small plastic project enclosure to mount them inside of. Those can be found for a good price from places like eBay, AliExpress, and others.
What power ratings are needed for speaker resistors in an L-pad network?
The truth is that tweeters generally use less power than midrange or woofer drivers. Typically, as a general rule of thumb:
- 10, 15W, or 20W resistors will be sufficient for most L-pad networks. I generally use 15W without problems.
- Higher power ratings than those needed won’t hurt anything and will run cooler but may cost more or take up more space.
- NEVER use standard (1/8 watt, 1/2 watt, 1W) parts. These will run excessively hot at moderate power levels and can suffer damage near or beyond their ratings.
The power applied to an L-pad or series resistor network will be divided among the resistors and the speaker. It’s also largely based on how hard you drive the speakers with your amplifier or stereo. As most people use a moderate, average listening level usually, it’s often not very much power dissipated across each resistor or the speaker.