How to Revive Any Active Speaker

A while ago, a friend of mine gave me an old, broken active speaker. It could still power up, but only some weird noises came out of it. Electronics was completely dead, but the speaker drivers were working just fine. I decided to fix it myself, but where to start? I couldn’t find any instructions on how to replace the amplifier with a different one … which is understandable, because it is usually not a preferable way to do it. Most of the time, damaged electronics can be replaced or repaired but that requires quite some skill (and sometimes a deep pocket). I didn’t want to spend too much money, but I didn’t want to throw away perfectly good speaker drivers either.

I am writing this instructable merely to give a possible idea how to repair the speaker rather than throwing it away. I will try to provide as general instructions as I can, so you have a place to start no matter what kind of a speaker system you have.

My aim is to show you some basics on how to “revive” a speaker yourself. With the term “revive” I want to say, that I am not going to explain how to fix the broken amplifier circuit or power supply unit, but instead how to replace it with (possibly) a cheap new one. We will also have to design our own crossover circuit which will split the sounds from a music source and provide the best output for a specific speaker driver.

You could also use this instructable if you are building a new speaker and would like to learn more about active crossover circuits.

This is a relatively cheap fix, but not preferable one if you have professional speaker system. I advise you that you follow this instruction only if you are certain that the speaker circuit is dead and not even a professional could fix it. Many times, only a fragment of a circuit is not working and if you are able to locate it and replace it, I recommend you to fix it that way.

We will try to replace an electronic circuit, which was specially designed for these specific speaker drivers. Substituting it with a new one is a very hard thing to do. But in my opinion, it is still better to lose a bit of the sound quality than throwing the speaker into garbage and buying a new one. If you put some effort into it, you can maybe achieve the same quality as when they were new.

This will not be a straight-forward process, so some basic knowledge in electrical engineering is desirable. If you have never dealt with electronics before, you should ask someone who has for help. But do not get discouraged…you can try and learn many new things along the way (like many of us did) ;)

Since there are so many different loudspeaker designs, I will quickly try to explain some basics, so we can begin on the same page. You can skip this part if you already know this.

A loudspeaker (or a speaker for short) is a device that converts electrical signals (music) into sound waves which we can hear. To do that the basic things it needs are an amplifier, a crossover, and a power supply to power everything up, the drivers and a box to enclosure them.

Speaker drivers come in various shapes and designs, but by far the most common are the ones with a moving coil. When an electricity is applied to the coil, it moves the cone back and forth, which produces sound waves. Because the signal from our sources (phone, CD player, radio, …) is low, they need an amplifier which will amplify the signal and make it louder. It is almost impossible for one driver to effectively produce sound waves over the entire frequency range that humans can hear (20Hz – 20kHz). For low frequency reproduction, drivers are made with large cones and big magnets (usually called bass drivers, woofers, sub woofers, …) and for high frequency they are normally small (called mid-range, high drivers, tweeters, …). This requires the electrical signal to be divided into two or more bands (low, mid and high frequencies) before signal reaches the drivers. That’s where crossover system comes in place. In a three-way speaker system for example, crossover splits the signal up into three parts – high, mid-range and bass.

Roughly spoken, you can divide speaker design into 2 categories (but there are also some other variants):

• Active speakers
• Passive speakers

We divide them in this way because of the signal path. In passive speakers, audio signal goes from its source to an amplifier, then to a crossover and finally to the drivers (see the picture). This crossover network doesn’t need a power source to work, and in that sense, it is considered passive. They are normally located inside a loudspeaker box and are formed with large inductors and capacitors.

Active speakers on the other hand, do the frequency separation before the signal is amplified. While both crossover networks do the same thing, there are some differences. Active crossover works at smaller voltage levels (because it is located before the amplifier) so the circuit can be designed for precision rather than for power handling. But this setup therefore needs separate amplifier for each driver, which can be much more expensive.

Sometimes, you can quickly determine an active speaker box by its power supply input. Everything is stuffed in one box (the power supply, a preamplifier and the crossover, the amplifier and the drivers themselves).

On the other hand, passive speaker box contains only the drivers and the crossover. The amplifier is located in another, usually steel box, and only that box needs power input. But that is NOT always the case. If there is no crossover network between the amplifier electronic and the drivers, then your speaker is most likely an active type.

In this instructable I will focus on the active speaker systems. They are usually called monitors and are typically used on stage for musicians to hear themselves. They can also be used as a sound system for small concerts and parties. You can find active speakers in professional studios (which I think should be fixed by a professional :D). All battery powered speakers are usually the active type, but these are not in the scope of this instructable.

Before you even start following next the steps of this instructable, you can try to locate the problem and maybe fix the broken circuit yourself. An absolute must: check if maybe only the main fuse is blown! :D This is usually a bad sign, because that means something is wrong with a circuit, but you never know. It might work after all.

Second thing that is also a must: check your audio cables and connectors. Maybe only the cable is bad and changing it with other will fix the problem ... or the input connector has lost the contact on the inside of the box. As it turns out, this is an issue more often then you might think. You can also try to test the different sources of music.

If that is not the case, then is time to open the speaker.

WARNING: Unplug the speaker from electricity, so you don't accidentally touch a live wire!

First take a look at the speaker driver: take a multimeter, set it to measure resistance and check the driver’s terminals. If you get the reading around 2, 4, 8 or 16 Ohms, then the driver’s coil should be OK. If you don’t get any reading at all (the circuit is open) that means that the driver’s coil is fried, and you will have to replace it. Also check if the driver has some visible malfunctions… Follow this instructable or this one to see how to replace a speaker driver.

If the drivers are fine, then you are most likely having problems with circuit. Here are some quick steps which can help you locate the problem; but don’t forget that there are people who have done it many times and you can seek their help (especially if they are near you :D).

Grab some instruments (multimeter, oscilloscope, …), locate the faulty component(s) and replace them. It is always helpful to find the circuit schematics. In my experience, first thing that fails are electrolyte capacitors. You can easily find them by “exploded” caps on top (instructable for help).

Amplifier chips and transistors also tend to fail before any other passive component. Look for some burnt marks – they are good indicator for malfunctions. If you don’t have many experience, try to find some help from someone who has done it before. Along the way always beware if the circuit is powered on or not…don’t trust the memory, rather double check. And remember, internet browser is your friend here :)

Only if you or a professional could not repair it in this (the proper) way, then you have "my permission” to proceed and follow this instructions. :D

First, I will talk about a new amplifier. Since I want to make a cheap repair, I decided to buy a pre-assembled amplifier board from eBay (AliExpress, Amazon or similar). There you can find a sea of already assembled boards with different characteristics at relatively low price. Be mindful while choosing one for yourself; grab a piece of paper, write and draw everything on it before you click that buy button :)

But before you start looking for new amp, you must find out as much data about your speaker system as you can. We will replace the broken amplifier board with a new one, which should resemble the original one as much as possible, hence we need some data about the drivers. You may find some values written on the magnet of the speaker or its enclosure, otherwise try to Google your speaker name and find some user manual.

For each driver (low and high) we must find the following values:

Driver's power

Usually, the most important data about the driver is its power handling. Don’t get fooled by the marketing approach “more power means higher loudness”. Driver power rating simply means how much electrical power a driver’s coil can handle before it melts down. So, amplifier power output shouldn’t exceed the power rating of the driver for too much. And to little power from the amplifier will usually make us turn the volume knob until the end and that will make the amplifier clip, which will also damage the drivers.

Another confusing thing is different power handling labels. Peak, max or dynamic power rating (which is usually written all over the speaker box :D) means exactly that: maximum value of power for a momentary exposure before the driver is fried. Rather search for RMS, continuous or average power. A typical data sheet of an amplifier will give us descriptions like “50W continuous power” and this power should match with speaker’s continuous power.

Here is a good description about speaker power ratings: https://geoffthegreygeek.com/understanding-amplif...

Driver's sensitivity

If you are wondering about the speaker’s “loudness”, check for SPL level: sound pressure level in dB unit is a number that represents a driver’s capability to produce a pressure level at distance of 1m at power of 1 W. It directly relates to how loud a speaker gets (speaker with lower sensitivity (90 dB) will sound quieter than higher sensitivity speaker (93 dB) at the same distance and at the same power).

NOTE: “Loudness” of a speaker is a subjective thing. Here I found a nice article on that topic if you want to learn more: https://geoffthegreygeek.com/amplifier-power/

Driver's impedance

It is also very important to consider impedance while selecting a proper amplifier. Both speaker driver and amplifier have their impedance – an electrical resistance measured in Ohms. We will use it to determine compatibility between amp and the driver. Speakers typically have 2, 4, 8 and 12 Ohm ratings, while amplifiers are rated for a specified range. We must find the one that is compatible with our driver. Most amplifiers can output different wattage into different impedance ratings. For example, same amplifier can produce 100 W output on 4 Ohm speaker and 50 W output on 8 Ohm speaker. Be sure to check the specifications while searching for your amp.

If you can’t find any data about your driver, you can measure its resistance (picture) and round the number to one of the closest possible impedance values (if you measure 7.2 Ohm, your impedance is 8 Ohm).

Frequency range

More about that later at crossover design.

The next very important thing to think about before you buy your amplifier is: how will you power it? You can maybe use the power board from your previous amplifier (if it is not damaged). Or maybe use only the transformer toroid from the previous system and make the voltage regulation yourself. Or maybe buy new one …

If you are searching for cheap solution, maybe you can find some old, unused switching power supply around the house. These are in general avoided in speaker systems. Even though they are more efficient, and they weigh less than the whole transformer, they will produce some additional noise, which is not desired (BTW Switching power supplies are used for audio, but they are specially designed, not those we use every day). Note that some boards are designed for dual supply voltage (usually marked as: +- 12 V) and others for single supply (on eBay usually labelled as DC).

Besides the voltage, also check the amperage rating – power supply must be capable to output as much power as amplifier needs. If an amplifier would like to draw more amps than a supply can provide, the voltage will drop which leads to amplifier clipping.

NOTE: Be aware that most assembled amplifiers on eBay can output different power at different supply voltage. Read through description, don’t trust only the values in the title because they could be misleading.

While searching for amplifiers you can find various operating modes:

• Mono – means only one amplifier circuit on a board.
• Stereo – means two “identical” amplifiers on a board (basically 2 mono amp in parallel).
• Bridge – means two amplifiers combined into one, to produce more power than one alone.
• 2.1 – boards with more than one amp circuit on them. They are used in systems for 2 stereo speakers (L + R) plus one additional, usually woofer speaker.
• 5.1 – like 2.1, but they are used for the whole room (front L+R, back L+R and woofer)

If you have a 2-way system, then you can get yourself one stereo amplifier or 2 times mono. Like explained in the beginning, for an active speaker design, you need as many amplifier circuits as there are speaker drivers in your system.

Amplifiers divide in different classes which are based on their mode of operation. You can usually find classes: A, AB and D selling online. I personally don’t hear much of a difference and for fixes like that I tend to pick an amp in class D. They are cheaper and more efficient (they use less power and don’t heat up as much as other classes).

What to choose???

In the end, a general way to go is to choose an amplifier which can output slightly more power that your driver continuous power rating. Why is it good for? Since music is very dynamic flow of waves, you want your amplifier to accomplish that dynamic. Again, be mindful while selecting, so that the driver and the amplifier impedance ratings are matched. Also keep in mind what supply voltage you have available.

What to do if you have different power ratings for each driver? Best option is to buy 2 different amplifiers, each rated for targeted driver. But if you are looking for an easier solution and both drivers have the same impedance, I would personally pick a higher power rating (usually the bass driver) and at the end attenuate signal for the tweeter down so it won’t be damaged. How to do that: look for the note message in the output stage (step 15).

My example

My speaker system has a tweeter with 70 W, and bass with 100W continuous power rating. They both have impedance of 4 Ohms. I have a single supply power adapter with 24 V, 5 A.

In the end, I decided to go with the stereo amplifier rated for 100 W continuous output at 4 Ohms, powered with 24 V single supply.

When you have your amplifier board it is time to build the crossover. While this will not be the best crossover design, it will give a decent frequency band separation and most importantly, it will provide a protection for the tweeter. The design is generic, and you can adapt it for your needs by changing capacitor and resistor values. Before we go into details, let’s first take a look at some basics.

Again, skip this part if you know this:)

The basic functionality of any crossover (active or passive) is to divide the audio spectrum into two (or more) bands, so they can be applied to drivers dedicated for those frequencies. We achieve this with audio filter circuits. Like explained in the beginning, active crossovers are placed before the power amplifier, while the input signal is still low. That's why we can do a lot more than just frequency separation. Advanced designs can also provide some time and phase compensation, balanced input and output stages and so on.

Filters are used to “stop” or “pass” a certain frequency band, hence the possible naming: high-pass, low-pass, band-stop, etc. Low-pass filter will therefore allow low frequencies to pass and will block higher frequencies. A filter’s cutoff frequency is generally (not always) defined at -3 dB point – frequency where the output of a filter drops for 3 dB. Filters are also described by their order (number of reactive elements in a circuit), which determines the amount of filtering they provide or in other words their slope – 1st order have 6 dB/octave slope, 2nd have 12 dB/octave, 3rd have 18 dB/octave and so on. Every filter design will also produce some phase shift which must be taken into account.

NOTE: Besides dividing audio spectrum, a crossover does one very important thing for us: it protects the high frequency drivers. They are not designed for such high oscillations (low frequencies push the driver's cone in and out much more than high frequencies). If you apply the tweeter directly to an amplifier output, without any crossover in between, you will destroy it – so avoid doing it :D

Here are some useful links to learn more about this topic:

The basics of passive filters: https://www.instructables.com/Passive-Filter-Circuits/

The passive crossover example: https://audiojudgement.com/passive-crossovers-speakers-made-easy/

The active filters from Linkiwtz Lab: http://www.linkwitzlab.com/filters.htm#Active

The active filters from Eliott Sound Products: https://sound-au.com/articles/active-filters.htm

Texas Instruments article on quick filter design: https://www.ti.com/lit/an/sloa093/sloa093.pdf

Another Texas Instruments article: https://www2.seas.gwu.edu/~ece121/Spring-11/filterdesign.pdf

If you decide to design active crossover on your own, this book will guide you through every detail: The Design of Active Crossovers by Douglas Self.

OK, so now when we know how crossovers work, how do we make one? First, we must determine the crossover point. Because with this basic crossover we are mainly aiming to protect the tweeter, we must first find its frequency range – a range from where to where a driver can effectively produce music.

This value is somewhat hard to determine on your own so it is a great fortune if you can find it written somewhere. If you find the specifications of your drivers, search for tweeters free air resonant frequency, marked as Fs (peak in the bottom curve in the first example picture). Usually when designing a high pass filter for tweeters, a 2x value of Fs should be the crossover frequency.

If you don’t have that luck, you can try to find it yourself by this (unprofessional) method: Sweep through the whole frequency range and determine at which point the tweeter losses its “loudness”/volume level.

WARNING: When you are testing high frequency drivers (tweeters or such) do not apply them to a low frequency source. Never! It will damage the driver. For example, they will not survive 500 Hz at a high power. .

Use your phones application “frequency sound generator” (pick whichever you like best) as the source. Then connect the high frequency driver to an amplifier and the amplifier to your phone. Now use the application to sweep through the frequencies and try to hear different levels of loudness. Be mindful! Start on high frequencies (6 kHz) and slowly go down, but not too low. When you hear the volume level going down, this is the end of your frequency range.

If you like to visualize stuff, you can use another phone as a microphone and plot the frequency response with application like Spectroid. In the picture of my measurements you can see how I stopped immediately when I saw the drop in an amplitude output.

NOTE: Because the sound will reflect from the surrounding walls and furniture (which will make dips and peaks in the curve), place the phone very close to the driver (few centimeters away) while measuring the response curve. It is also better if you do this outside.

Described methods are all quite inaccurate, but at least we have a place to start :)

When you did the measurements, it is time to pick the crossover frequency. First find where your response curve starts to roll off (drivers start to lose its loudness). Mine does that at around 800-900 Hz. Then go at least one octave up, which means multiply that value with 2 (I get a 1.8 kHz). Just to be safe, I could pick a value of 2 kHz, but because my woofer driver has quite a high range, I decided to go with frequency of 2.3 kHz.

Now when we know what a crossover does, let’s build one. Here is a picture of my final design and I will try to explain what each part does, step by step.

As you know, there is a range of different filter designs which you can use for your crossover. In this instructable I decided to use Linkwitz–Riley 4th order filter with a slope of 24 dB/octave, which is one of the most commonly used in active crossover designs. It also provides a 360-degree offset in phase at all frequencies, so tweeter and bass will not cancel each other.

NOTE: Basic design can be found on this link or this link for example, while in the picture you can see the design for single power supply.

When you have the desired crossover frequency it is time to use it and calculate the values of capacitor and resistors in the schematics. All resistors in this circuit have the same value, same goes for capacitors. Use the formula:

Fp = 1/(2.83 * pi * RC)

Usually a capacitors value is a place to start and with that you then calculate the resistor. For example, if I take a capacitor with value of 22 nF I get:

R = 1/(2.83 * pi * 22nF * 2300 Hz)

Which is 2222 Ohms.

If I use 2.2 kOhm resistor (I have plenty of them) I will get crossover frequency at 2323 Hz which is good enough for me.

Next thing you should do, is to consider how you will power the crossover. For hi-fi applications the preferred way is to use dual power supply rather than single. But since this is not a sophisticated build and I already have a single power supply for my power amplifier, I will also use it to power the crossover circuit.

When building audio filters for single supply, you must bias all the operational amplifiers to the half of the supply voltage. You can notice some differences between filter schematics found online (those are mostly for dual supply) and in the schematics I use to build this crossover.

You can get that half supply voltage with many techniques, but I decided to use a Zener diode because the current usage will not be that big. You can find detailed information on how to use them at this instructable:

But basic steps are:

• Determine what voltage you need for the circuit [I have 24 V supply, so I need 12 V Zener]
• Calculate the circuit current requirements [NE5532 has 2-8 uA of input bias current, while FET TL074 has input current in rates of pA…nothing in comparison with Zener usage.]
• Calculate the series resistance R = (Vsupply – Vdiode) / I [12 V/10 mA = 1.2 kOhm, I had 1 kOhm which is OK]
• A 0.25W resistor should do [10 mA * 12 V == 120 mW]

I added an electrolyte capacitor (CB4) to increase the stability and a ceramic (CB5) 100nF to filter out potential noise.

On the schematics you can see two 100nF capacitors (CB2 and CB3) which are not connected anywhere. I added them later for op-amp supply biasing, and because they are not on the PCB layout, I left them unconnected.

The first stage in the crossover circuit should be a buffer. It provides the low source impedance, which is necessary for the following filters. We can create it with a simple voltage follower.

Stereo to mono converter

Two resistors in the beginning (Rl and Rr) will sum the inputs, so left and right channel are combined in one mono audio source. If you have two separate active speakers and you will use one for left and other for right channel, than you can skip this first part and make two separate crossover systems.

DC blocking (high-pass filter)

Because our crossover is powered with a single supply, we must bias the input signal to the half of the supply voltage, which we can do with high value resistor (R1). A capacitor (C1) blocks the DC voltage preventing it from going back to our source of music. On the other hand, it also prevents DC voltage to enter our crossover from outside (DC is a terrible thing for drivers). Together they act as a high 1st order pass filter and passes al the frequencies above:

Fp = 1/2*pi*R1*C1 = 0.7 Hz

Large resistor value will cause some DC voltage difference between the input terminals of the voltage follower due to input bias current. Instead of NE5532 it would maybe be better to use some FET op-amp. You can fix that with another 22 kOhm resistor in the feed-back loop of the voltage follower.

On the output stage I added 2 potentiometers and with them you can adjust the driver’s volume level separately. If you look at my schematics, potentiometers are made with pin headers (looking like a LEGO block) because I used wires to connect them (instead of mounting them on PCB).

High frequency signal has the attenuator for the tweeter driver. Most high frequency drivers have higher SPL level than low frequency drivers. That means that at the same power input, they produce louder music. We must take that into account, so I added a potentiometer, which can lower that volume down. Potentiometer is a somewhat good compromise, because we can adjust the volume without knowing the specifications of the driver in advance.

NOTE: Here comes that note message from amplifier section in place. If you have an amplifier with power output higher that tweeter power handling, you need an additional resistor (R10). It will attenuate signal down even if potentiometer is fully up (in case somebody turns it fully on, our tweeter will survive). Which value to pick depends on the power ratings of your amplifier and driver, and the value of potentiometer you will use. Use voltage divider formula.

You can try that out later when you finish the build. Short that unknown resistor (R10), turn the amplifier on and play some music. Slowly raise the pot until you start to hear the first distortion and quickly turn the music off (without touching the pot). Measure the resistance of your potentiometer between the middle and the top terminal. If you have a pot of 10 kOhm and you measured 4 kOhm resistance between those two pins, that means you need a resistor which will be in ratio of 4:6 with the potentiometer. In this case you will need 6k66 Ohm resistor for R10. (If you measure 3 kOhm you need to search for ratio 3:7, so resistor should be 4k28 and so on).

And because of the same thing mentioned above, I added a non-inverting amplifier for a small, variable gain for low frequency drivers, so you can raise their volume. The gain is calculated with the equation

A = 1 + Rpot/R11 

If you decide to modify it, keep in mind that too big amplification is not good, because it may cause clipping in our power amplifier (resistor’s value could be the same as potentiometer value, so the max gain will be 2).

NOTE: One additional trick: op-amp in this configuration will amplify the whole spectrum of low frequencies (in my example from 0 Hz to 2.2 kHz). If you want to amplify only the lower frequencies (below 500 Hz), you can add a capacitor and make a so-called “bass boost”. Why is that? Because at low frequencies, C blocks the signal, forcing low frequencies to go through pot instead and they will be amplified. For higher frequencies, capacitor will act like a short circuit, bypassing the potentiometer and by the equation above we get the gain of 1. In that sense, low frequencies will be boosted while higher frequencies will just be passed through without any gain. It is up to you which capacitor to use. You can try and listen which suits you best (I used 100nF for example).

Before passing the signal to the amplifier, we must do one important thing – remove the DC voltage that we biased before. 1st order high pass filter (C10 & R15 for high, C12 & R16 for low signal) solves our problem. Resistor on the output also defines the ground for the capacitor so it doesn’t develop a voltage when left floating – this would result in *thumbps* when connecting something to the crossover.

There is a lot of good programs which can help you draw your schematics. I tend to use Eagle CAD software which is quite intuitive to use and free for hobbyists to use. You can get it at their official site and you can find tutorials and a lot of good tips on their YT channel.

Here are the schematics and the board design.

Which elements to use?

I decided to use TL074 and NE5532 because they are quite cheap, available almost anywhere and are most commonly used in audio circuits (and ofc I had those two lying around :D). You can easily make same circuit with only one type of op-amps.

Metal film resistors are normally used in audio circuits, while capacitor can be polyester, polypropylene, or similar. Avoid using multilayer ceramic, electrolytic and especially tantalum capacitors, anywhere the music signal goes through them – they produce an unwanted noise.

Because I made the PCB at home, I used larger values for the trace's width, so they will transfer better to the copper with toner transfer method. There is also bigger space between them so the etching solution will work better. I will only use through-hole components and because dual layer PCB is a bit harder to make at home, I routed all connections on the bottom side.

I placed the elements in a way, that all the components of a crossover are distinguishable. When you are testing your circuit later, you can easily find where to change the components.

One thing you should keep in mind is also how to position the PCB into speaker box. In my setup I will use two screws to mount it to chassis, so I added 2 drill holes.

NOTE: There is a lot of place for improvement in PCB. It is not intended for professional use, rather for a quick, homemade, and simple repair :)

While you can manufacture your PCB almost anywhere nowadays, I still enjoy doing PCB on my own in my garage. Speakers just sound better this way (just kidding :D).

There are many different ways on how to do that at home, but I stick with the most simple and greenish method: toner transfer and etching in mixture of hydrochloric acid and hydrogen peroxide.

Instructions on the toner transfer method:

https://www.instructables.com/How-to-make-PCB-at-H...

https://www.instructables.com/DIY-PCB-Etching/

And the greenish etching method (instead of ferric chloride mentioned in above links):

https://www.instructables.com/Stop-using-Ferric-Ch...

https://www.instructables.com/Is-the-best-PCB-etch...

If the toner did not transferred too good, you can use some waterproof marker to fix the connections on your own.

After etching, drill the holes and grab a multimeter to check if all traces have good connection (in case our etchant was too strong). And then the best part: solder all the components. Start with soldering the smallest and work your way up to the larger parts. Beware of potential cold joints (soldering tutorial).

It is a good thing to also add some decoupling capacitor for the amplifier chips, as close as possible – I soldered 100 nF right on the bottom side of the board. A value between 10-100 nF is a good choice.

In the end it is advisable to cover the copper traces with solder to protect it from corrosion, and possibly improve connections in case our etchant was too strong.

NOTE: There are some differences between the PCB on the picture (which I used only for testing purposes) and the one in the attached files. The picture is here only as an example.

WARNING: Whenever you are dealing with electricity you must be extremely cautious! And because you might operate with high voltage, which may be lethal, double, or better triple check every step before you proceed. Avoid lose wires, don’t trust quickly made connections, be aware of exposed terminals, … Electricity won’t jump at you, but show it some respect!

Now it is time to test your crossover. Connect it to power supply and be ready to unplug it in case something is burning :D Carefully check the temperature of the Zener resistor with your finger - it shouldn’t get hot. If it does, recalculate its value. Chips should also stay cold, otherwise there must be an error in the circuit.

If you have an oscilloscope at hand, check what is the actual filter's cut-off frequency. Connect crossover input to one oscilloscope channel, and crossover output to another channel (look at the picture). Then use a phone as your source (application from step 10) and swipe through frequencies until you see a difference in input – output voltage in half (output voltage should be ½ lower than input). Cutoff frequency of Linkwitz-Reily 4th order filter is defined at -6dB (not usual -3dB). Make sure that the tweeters are now protected.

NOTE: There will be some difference in the calculated and measured cutoff frequency due to component tolerances. My low pass filter has cutoff frequency at 2470 Hz instead of 2300 Hz.

Next step is to test how a crossover is working with the amplifier and speaker drivers. Double check the connections! The low freq. output of a crossover must go to the bass driver, same goes for the tweeter. Write it down, mark it with a pen, … don’t trust lose wires and exposed connections, use some electrical tape. Before powering it up, turn the volume down on the source (phone) as well on the amplifier (if it has some potentiometer).

If everything is working up to now, you can do the measurement for the tweeter range again (from the step 10) and see how well the crossover actually works. If you don’t like the sound you can always go back, change the crossover frequency a bit up or down … experiment until you are satisfied.

Here I encourage you to use your creativity. Because you most likely have a different speaker box and different components than me, I will only talk about things you should consider the most.

• First let’s talk about wiring the main power supply (120V/230V) to your box. Your box should have a (Euro) connector from before, which you can reuse. Simply connect the wires from that connector to your power supply adapter. Use good Faston connectors with plastic protection. As you know, a fuse on the mains is a must! It will protect you and the equipment. The “ON” button on the mains is also very useful. In the end, you should not be able to touch or see any exposed mains wires!

• A LED indicator is also a useful thing to have. Add it to the output of the power supply, so when you power the speakers, LED will glow. Check this instructable to find out how to calculate the resistor value for your LED.

• Most pre-assembled amplifier boards have a potentiometer for volume adjustment. You can un-solder it and add some wires in between, or you can mount the board to the speaker box in a way, that the knob will look outside (that is what I did). To look nice, you can add our two potentiometers from the crossover in parallel with this one.

• Use firm connectors that will not disconnect that easily, or the one with the screw (PCB terminal block). Rather than using lousy connectors, you can solder the wires directly to the potentiometers, audio input connector and between boards (but solder it when you are 100% sure that this is your final design). In this way they will not disconnect that easily if the box is shaken.

• Keep connections short. Use good, possibly shielded cables. Twist the wires from the amplifier to drivers to minimize magnetic fields.

• Try to avoid making multiple connections of a ground line to the chassis – only one chassis bond point should be made, otherwise you might create a so called “ground loop” which may create “hum” distortions. So, try to insulate the audio input connector with a piece of plastic (look how I did it). And make sure that your PCBs are not touching the chassis while you position them in place – take special care while mounting your own made crossover.

• Our switching power supply will produce some noise, which can be coupled over its ground to or crossover ground – which we want to avoid. To remove some of the high frequency noise we can connect a ferrite bead between supply ground and the crossover ground.
  

• Wire 1 nF ceramic capacitor from the input connector ground to the chassis right at the entry point. This will improve radio frequency immunity.

• Now you will most likely end up with a different arrangement of input connectors, potentiometer knobs, buttons, and LEDs than your speaker system had before. To make it look nicer while also preserving the speaker box characteristics (next paragraph) you can make new cover. I for example used 1mm thick sheet metal and shaped it to fit my speaker layout. I drilled new holes and positioned it with some screws (look at the picture to get the idea).

• As you know, the speaker box dimensions are precisely calculated for each specific driver. We must therefore make sure that the box will remain airtight, so we won’t change the box' characteristics. I for example used hot glue to fill the holes that were left from previous enclosure.

This is, by no means a professional build. Let this instructable be only a beginning point. I wrote it for those who would like to salvage some old broken speakers, and give them a new life. This is just one way of doing it.

There are many other good forums and sites with thorough explanations. I added a few, but the Internet is your best friend here :)

For an even better sound quality one should also take care of some time delay and phase correction, different ground traces in PCB design, maybe use a different power supply, use better amps, add a buffer on the output stage, obtain frequency range with some professional equipment, etc.

If you decide to follow this instructable, I hope you will manage to fix your speakers and enjoy in their music for many more years! And of course, if you have any questions, leave them in the comments below, I will gladly try to help :)