Sunday, August 13, 2017

Introducing the LM-1S

If you are looking for the simplest possible speaker project that is affordable, uses a traditional box, and provides very good sound for the money this may very well be it. Your total cost should be around $195 for the pair, assuming you have all the tools.

The S here stands for simple. The LM-1S uses the same size cabinet as the LM-1 but foregoes the multi-way / multiple driver design in favor of a single full-range driver. As a result we loose the absolute fidelity to the top three octaves the LM-1 has, but we cut the cost in half. Perhaps the S should stand for Surround instead, as these speakers would make ideal companions to a home theater system with the LM-1/LM-1C at the front. 

Compared to the LM-1, the LM-1S is MUCH cheaper, has only 1 driver and no crossover components at all.

The Italian made 4" FaitalPRO 4FE35 has good frequency extension up to 20 kHz and really quite decent off-axis response.

The treble response seems to be a bit peaky on-axis, but this can be adjusted by turning the speakers away from the listener. Compared to boutique full-range speakers, this little gem is really pretty awesome. With the large diameter cone acting as woofer and tweeter you may even find this speaker is unusually clear at a distance compared to more traditional speakers with normal 3/4" to 1" tweeters.

Ported vs. Sealed Cabinet

Visually, the cabinet is a little large for the driver, but that's how we get the 65 Hz cut off. 7L is actually a compromise volume, with WinISD suggesting twice that.

If you use a sealed cabinet, you could use a micro-cabinet of around 3.5L and get down to about 100 Hz.  If you absolutely know you'll use a subwoofer that's a great way to go, especially with a desktop system.

Specifications:

  • Sensitivity 91 dB / 1m
  • Frequency : 65 - 20 kHz 
  • Minimum impedance: Approximately 8 Ohms.
The sensitivity is unusually high because there is no crossover. We pretty much get the full sensitivity of the woofer. 

Shopping List


Here is a list of materials you'll need:
I specify the Denovo knock-down flat-pack cabinets ($50/pr) because they are much cheaper than the furniture grade Dayton cherry cabinets ($190/pr) but either will work similarly. If you want something very nice looking and don't feel like doing the finish work yourself the Dayton cabinets are worth the extra.


Tools and Supplies:
  • Router
  • Hole cutting jig to cut approximately 3 9/16" hole for driver
  • Drill and bits
  • Wood glue
  • Clamps
  • Soldering iron and solder
  • Wire cutter / wire stripper

If you use the port you'll get an f3 of around 65 Hz, 100 Hz otherwise. Remember not to put the port directly behind the woofer! Not only could it be a tight fit, but you want to minimize high frequencies from exiting. If you never plan to put a tweeter into this speaker, then you could certainly place the port in the front underneath the driver, making this speaker easier to place in tight situations.

I also recommend the use of the 3/4" multi-layer Sonic Barrier products on the internal side walls to reduce cabinet resonances. Build your LM-1S first, and if you think voices are sounding a little "boomy" then I highly suggest adding Sonic Barrier.

Fill the cabinet loosely with Acousta-Stuf before final assembly, making sure to keep the area at around the end of the port clear.

Placement

With no equalization circuit of any kind these speakers will be best with close surface reinforcement. A desktop, bookshelf or entertainment center will work great. Hiding them in a bookshelf would also make them ideal for hidden surround placement. 

If you are using them as part of a HT setup the room calibration circuits will compensate for either stand or shelf use. Just make sure to set them to "small" with a crossover of 80Hz or higher so you don't overload them.

Saturday, August 12, 2017

FaitaPRO 5FE120 vs. Peerless 830991

Bass 


I ran the 5" FaitalPRO 5FE120 vs. the Peerless 5.25" 830991 through WinISD for the smallest Dayton cabinet, 0.25 ft^3 / 7L.

What I was really looking for was evidence of significantly better bass response, but truth is these two drivers are pretty equivalent. In the 7L cabinet, with a port the FaitalPRO has a slightly better f3: 53 Hz vs. 65 Hz, but the -10 dB Points are the same.

In ideal cabinets, the 830991 requires a 13L box, vs. 8L for the FaitalPR0, f3 nearly identical.

At least on paper, the FaitalPRO edges the Peerless just for needing a smaller cabinet to reach ideal responses. But the differences are truly small.

Treble


The really big differences between these two drivers are in the top end. The FaitalPRO is remarkably smooth for 2-3kHz more than the Peerless, making it easier to match with a broader range of tweeters. While the LM-1 required a 2kHz crossover, with this little gem we should be able to get up to 3-4kHz. Many less expensive tweeters are going to be very comfortable crossing over here.

Tuesday, August 8, 2017

The LM-2

I'm listening to the LM-1's on my desktop. They've made fine desktop and shelf surround speakers. But truth is they are shy in the bass. Using a new desktop amp, the bass boost feature helps.

So now I'm thinking about the LM-2. An improved version in the same box. While thinking about designing a 3-way I came across a promising new mid-woofer, the FaitalPRO 5FE120.

It is about the same cost, and efficiency but has some interesting improvements:

  • About double the power handling
  • Better fit in a 0.25 cubic foot cabinet - Deeper bass
  • Even smoother response than the Peerless 830991
  • Physically 1/4" smaller. 
  • Much better high frequency extension
A good match for it may be the SB Acoustics SB Acoustics SB29RDCN-C000-4 N ring radiator tweeter. It is as smooth as the XT25BG in the LM-1, but more importantly, about 32 mm smaller in diameter faceplate.  It is unfortunately about $20 more expensive.

The LM-1 were designed to be on a desk or shelf, but the LM-2's with the increased power handling and bass I will make stand-alone.

Another, novel idea I may incorporate here is to reverse the position of the tweeter and woofer. I'll put the woofer at ear level and offset the tweeter.

Sunday, July 16, 2017

A better tweeter for the LM-1 ... Maybe

A fellow DIYAudio member pointed the NE25VTS tweeter to me.

Has a very useful range, from around 2kHz up through 20.

Advantages over XT25BG60 - The diameter is about half the size. This allows very tight mounting next to the tweeter.

Disadvantage: Frequency response. The XT25BG is unbelievably flat through 30 kHz. The NE25 is more ragged, rising response and not as extended.

If I was re-thinking the LM-1/LM-1C, this tweeter would be a top contender as a replacement,especially with the LM-1C as it would allow much closer positioning of all 3 drivers. If you are looking to design a two-way near-field monitor and must have very close center to center spacing AND a low crossover point then this may be an ideal compromise.

It is certainly not a drop in replacement for the XT25BG however. Using it would require you to measure it in place before attempting the correct crossover for it.

Sunday, April 16, 2017

Focal Mezzo Utopia Be Upgrade Kits

Readers:

I'm considering offering a modern, luxury upgrade for the Focal Mezzo Utopias.  If you would like more information or want to give me some feedback before I launch the project, please visit me here:

Focal Mezzo Utopia Be Upgrade




Saturday, December 17, 2016

Crossover Basics - Impedance

In my first post on the subject of Crossover Basics we talked about the electrical response. How various types of high and low-pass filters changed the voltage that the speaker drivers experience. We glossed over the issue of impedance entirely. Understanding this will help you better grasp what's going on.

Introduction to Speaker Impedance

Anyone who has examined speaker specifications has seen an impedance rating, such as 4 or 8 Ohms. If you see speaker measurements from Stereophile or SoundStage you would have come across charts showing complicated details about impedance and phase. Here is the impedance chart from the ported LM-1. This first chart shows the combined effects of the drivers, crossover components as well as the cabinet and port.

Ignore the thin blue line, and focus on the thick one instead. Notice the chart axis. Across the X (bottom) we have the frequency scale, across Y (left side) we have Ohms.  So the first thing you learn is that the nominal 4 or 8 Ohms you are used to seeing are kind of a summary, and sometimes an outright lie. No speaker is exactly 4 or 8. These are guides to help match speakers and speaker counts to amplifiers. Personally I would rate the LM-1 as an 8 Ohm speaker, though it dips below this above the bass. The triple hump you see is indicative of a 2-way, ported loudspeaker. The two humps at left are caused by the port, while the hump at right, just below 2kHz  is where the high and low-pass filters meet.

You might have picked up on the fact that we are using Ohms to measure impedance (sometimes called Z). What makes impedance different from Ohms is that it is frequency dependent, and it has an angle. This angle is the difference between the voltage and current. We'll ignore this for now and focus on the magnitude in Ohms. Impedance is what we use to understand the effects of capacitors, coils and speaker drivers which are electrically complicated little beasts.

One very important point to note is that even though the tweeter and woofer sections are arranged in parallel, the impedance of the drivers does not actually appear in parallel. That is, a pair of 8 Ohm drivers wired in parallel would normally result in an impedance of 4 ohms, but in this "perfect" system the actual impedance stays very close to 8 Ohms at all frequencies.

Let's talk a little more about the LM-1, above. From left to right, the first two humps are classic indicators of a ported loudspeaker. From the bottom up to around 1,000 Hz the impedance is entirely that of the woofer. The peak around 1,800 Hz occurs where the low and high pass filters overlap, and after about 5kHz, the impedance is entirely a result of the tweeter.

In the sections that follow we'll go over each section in detail.

Effects of High and Low Pass Filters

We'll use the original example of a first-order crossover for all of our discussions.

To refresh your memory, this chart shows us how first order filters behave. The red trace is the electrical signal a tweeter would see, while the yellow is the woofer (1 kHz is actually a very low frequency for most tweeters, let's pretend it's OK). Black is the summed response of the tweeter and woofer. While it is smooth and straight we don't care about it right now.

This is strictly the voltage and electrical view of what would be measured at the inputs of the "ideal" 8 Ohm tweeter. We are, of course, missing the acoustical results, but we cover in the post Crossover Basics - Driver Response.

The question we answer in this post is: "How do we achieve this frequency dependent change in response?" Let's take a look at the original crossover first:

There is a capacitor in series with the tweeter, 20uF. We talked about it "blocking" low frequencies. This is achieved by an increased impedance. That is, as the frequencies drop, the Z (impedance) of C1 goes upwards. Let's take a look at the impedance of the tweeter circuit, with the driver and the driver + capacitor.

Series Voltage and Impedance

Keep this rule in mind: The voltages across each component in a series circuit is proportional to the resistance of each component. We're dealing with impedance, and complicated but for the sake of simplicity we ignore it here. This simple rule-of-thumb is enough to explain the behaviors without delving into reactive impedance calculation.

High-Pass Impedance

In the chart below the red line represents our tweeter impedance. Again, this is theoretically perfect 8 Ohms. No drivers are like that in rea life but we need it simple so we can better see what the high-pass filter is doing.  The blue line however represents the entire tweeter section. That is, C1, S1.




Unfortunately at 100 Hz the impedance is cut off but it's close to 80 Ohms. If C1 is 80Ohms we can then estimate the relative voltages across C1 and R1. It's only an estimate because we're not taking phase into account, but it's going to be very close. So we estimate the voltage at S1 to be 8 / 80 = 0.1 of the total. So at 100 Hz the tweeter gets approximately 10% of the voltage. At 10 kHz Z ~= S1, meaning our tweeter gets nearly all the voltage, and C1 is acting like a short circuit.

With those two points (100 Hz and 10 kHz) in mind, now the following graph should make sense. It plots the voltage across C1 and S1 as the frequency varies. Assume a 4V input voltage.

As we calculated, at 100 Hz the voltage across S1 is about 4V x 0.1 = 0.4V

This shifting of impedance and voltage between the filters and the drivers is similar regardless of the order of the filter. It's also mirror-imaged for the woofer.

From Voltage to Decibels

Let's calculate the dB drop at the crossover point of 1 kHz. The peak tweeter input is 4V. At the crossover point it is approximately 2.8V. So, we calculate using the rule:

dB = 20 log ( Vout / Vin ) = 20 log ( 2.8/4 ) = -3 dB

In other words, at the crossover point the input to the tweeter is 3 dB less than the amplifier output. Simple, right? let's do this again for 100 Hz:
 20 log ( 0.4/4) = 20 log ( 0.1 ) = -20 dB
So at 100 Hz, the voltage to the tweeter is -20 dB below the amplifier's output. A very good thing since tweeters are easily damaged by low frequency signals.

Low-Pass Impedance

The same and complementary effect is happening at the woofer side of this example:



In the woofer side, by around 100 Hz L1 has no meaningful contribution to the impedance magnitude (i.e. Ohms) so nearly 100% of the input voltage is at the woofer. By 10 kHz however the impedance of the coil is approximately 80 Ohms. Estimating 8/80 = 10%.



Additional Exercises

Using XSim, and "ideal" drivers, create 2nd order filters and compare the impedance of the high and low pass sections. Compare the electrical response and impedance curves. Combine them and examine the impedance bump where they meet.

Using a second order filter, examine why the 2nd component works without shorting out the entire speaker. For instance, if a high pass filter, examine why the coil (L) works. What would the impedance be if the coil alone was there, but not the capacitor?

Thursday, December 8, 2016

Crossover Basics - The Zobel

Introduction to the Zobel

A Zobel network is used to flatten a driver's impedance (usually a woofer or mid-range), therefore making the filter (usually low-pass) more effective. A Zobel consists of a capacitor and resistor which are wired in parallel to a driver.

C1,R1 on the left are an example of the Zobel network. Their values are chosen to minimize the impedance rise of a driver above the resonant frequency.


Speaker Impedance

If you aren't quite sure what this means, please visit my blog post on Crossover Basics - Impedance before reading more.

Introduction to Series Circuits

Take a look at a very simple series circuit. It consists of two resistors in series (one after another) with the amplifier. The circuit is closed by the ground points (the downward facing triangles).
As before, this may be something you like to play with, so I encourage you to grab a copy of XSim and try this out along with the Peak Voltage chart.

What's important to understanding here is that the voltage across the resistors will be proportional to the resistance offered.  So

Vr1 = Vin * R1/(R1 + R2)
Vr2 = Vin * R2/(R1 + R2)

And of course:

Vr1 + Vr2 = Vin

That is, the voltage across R1 and R2 must add up to the input voltage.

Think of Ohms as elephants that eat volts. The more elephants, the larger portion of the incoming voltage they eat.  Yes, this is a very silly analogy.  Still, we can do some quick math. The total resistance is 100 Elephants (hah!) or 100 Ohms. R1 has only 10 elephants, so it gets 10% of the incoming voltage, whatever that voltage may be. R2 has 90% of the Elephants, so it takes 90% of the incoming voltage. There's a lot more to circuit analysis, but this is the bare minimum to understanding Zobel networks. Hopefully you'll be intrigued and learn more on your own.

Woofer Impedance

So with a little background under your belt you are now ready to look at a typical woofer. We'll use the LM-1 woofer, the Peerless 830991. You should know that impedance graphs will change once a driver is in the cabinet, especially if the cabinet is ported, so the data I present here will be different than a specification sheet which measures the driver in free-air.

You have heard the term "coil" used interchangeably with "inductor." Which is correct, but you may not have thought about the term "voice coil" in the same context. The voice coil is the part of the speaker that will electrically connect to your amplifier and produces the magnetic force which moves it against the magnetic field of the permanent magnet. We won't get too much into this, but suffice it to say that above the resonant peak the voice coil behaves like that of any other inductor, specifically it has both a resistive element (DC Resistance, or Re) and an inductive element (Le). These combine to give us the woofer's electrical impedance (Z) at any given frequency.

In the chart below we will compare the impedance of the woofer in a sealed cabinet (red) with a woofer that has a Zobel network applied (blue).



Let's ignore what happens below 200 Hz. That's a topic beyond this posting, and it's also far below our likely filter points. From about 200 Hz to 400 Hz the woofer is purely resistant, and we have the minimum around 6.6 Ohms, but what happens to the right? That's correct, suddenly more voltage-eating elephants arrive! The inductive qualities of the voice coil become more and more important and overwhelm the well behaved resistance. Compare the peak difference of the two impedance charts, all the way at the right. The blue line represents a woofer compensated with a Zobel network. The impedance never goes above 7 Ohms, while the normal un-compensated woofer goes to over 30! That's more than 4:1 difference. This increase impedance is going to compete with the low pass filter and make it behave in ways we probably don't want.

For clarity we'll leave behind the LM-1 schematic and create a new one, with two identical woofers and 2nd-order low-pass filters set to 4 kHz. Of course, this is not how a real speaker would be designed, we are just using this to see exactly how a Zobel circuit works. The Zobel consists of C1 and R1. 


The first thing we should do is examine the transfer function of the two filter sections:


As you can see, S2 is behaving like we expect a low pass filter to work. S1 however is having a very difficult time getting to the right slope. At 5kHz the output is almost 10 dB higher than we want it to be. That's a big deal. Eventually the impedance (elephants) on the low pas filter take over, but they don't reach our desired behavior until past 20 kHz, definitely not good enough for us. Let's take a look at the final outcome, below:


How Does a Zobel Work? 



Above we discussed how serial components work in a circuit. You may feel a little tricked because while we learned enough to understand why coil inductance needs to be compensated for, we never talked about how a parallel circuit works, which is what a Zobel is. A parallel circuit has one unique property:

The apparent impedance of a parallel section is never more than the smallest impedance.
If we imagine C1 as a short, then no matter what S2 rises to, the impedance will never go above R1, or 8.2 Ohms. It can be less than that, but never more.  In a parallel circuit you calculate the apparent impedance like so:

Rtotal = 1 / (  (1/R1) + (1/R2) + (and so on and so forth)  )
Things are more complicated because we are actually calculating impedance, but you get the picture.

Let's do some quick, Dr. Leach style of analysis on the components in a Zobel. At very low frequencies, C1 behaves like an open circuit, essentially removing R1 and C1 from meaningful contributions to the system impedance. Remember we mentioned that impedance cannot rise more than the smallest value? So at low frequencies C1 is so large that S2 becomes the limit on impedance. You can see the impedance below 500 Hz or so is barely affected. At high frequencies, C1 rapidly decreases until it evectively becomes a short, putting R1 in parallel with the driver, S2 and limiting the absolute maximum impedance to 8.2 Ohms. In this case we don't reach 8.2 until well-past 20kHz but it would eventually reach that point once the woofer's impedance was high enough. Also past our point of concern. If we can limit the impedance from 6.6 Ohms to 7  Ohms then we have a much more stable impedance curve than before, and that's good enough.

Do I Need a Zobel?

That's a tricky question! So let's examine this woofer and it's output. If you wanted to cross it over at 4kHz I would think the Zobel was mandatory, however if you were going to set your crossover frequency at 2kHz or lower I would say not really. The LM-1 uses it, but the effect of the Zobel is small, and benefits the phase response so I leave it in. It is possible a very similar sounding LM-1 could be built without a Zobel and with different choices in the filters left behind. The best chart to look at to see if a Zobel matters in your circuit is the transfer function chart.

It is very rare, but not unheard of, that a tweeter needs a Zobel because their voice coils are relatively tiny and therefore don't have a lot of inductance. The most common exception to this rule is with ribbon tweeters. The ribbon itself is not inductive but the entire assembly often include matching transformers. Transformers are coils .... and coils are inductive... see where this goes? :)

The real point to the Zobel is to make things better in the area you need the filter to behave at it's best. That's usually up to about -20 to -30 dB. Beyond that if your slope isn't perfect we no longer really care. There's no audible difference between -60 and -67 dB for example.

An important consideration in choosing a Zobel or not is that they are not free. The more parts in a system, the more expensive, the more chances of failures or parts being out of specification. If this is a personal project, no problem, it's all experience. However if you are building for mass production eliminating unnecessary components is the final stage before committing a design to the factory. 

Placement

In most cases, you want to place a Zobel closest to the driver. Put anything else such as padding resistors, filters, etc. before it. While the order of serial components does not matter, the order of parallel components does. Leaving the Zobel last prevents unexpected consequences.

There is a rare exception, when you must equalize a driver (usually a tweeter) by adding inductance. In which case you want the equalizing circuit closest to the tweeter, then the Zobel, so the Zobel can also control the EQ's impedance. I'll write more about this later in a section on handling difficult tweeters. 

The Secret Uses of the Zobel

Many will rely on on-line calculators to determine the right values for a Zobel network, and that's fine, but be aware that the absolute values can be tweaked. The main benefit of this tweaking is to gently nudge the phase charts one way or the other, helping you get near-perfect matching between two drivers you otherwise might not have. This is where having a tool like Xsim to simulate your tweaks comes in super handy.

Exercises

The data for the Peerless 830991 is contained in the LM-1 XSim files here.  Feel free to take it and modify it to help you complete these exercises.

The Peerless 830991 has an Re of around 6.6 and Le of around 0.330mH. Try simulating this in Xsim using a resistor and coil in series. Compare your impedance curve with the red impedance curve, above. What's the biggest difference you see?

Try using an online-calculator to create a Zobel for this driver. Tweak the capacitor and resistor values. Can you do better than the on-line calculator?

Using the complete LM-1 schematics, compare the woofer response with and without the Zobel. Is it a big difference? Can you fix the LM-1 so it no longer needs a Zobel? What difficulties did you encounter?Pay attention to the phase matching as well as the frequency response.