Kit for assembling a speaker protection board

Today I'm reviewing a kit for building a useful device of your own: a speaker protection circuit. I'll explain what it is, why you need one, and how to build it. And, of course, there'll be a few tests.



Many years ago, I bought a Radiotekhnika UKU-020 amplifier at a store called "Young Technician." Yes, at Young Technician, not in the audio section of some department store. It had no case, but it was in perfectly good working order. Back then, stores sold units like this from factories and repair shops.


The inside looked roughly like this. There may have been some minor differences, but nothing significant.
Looking at this photo brings back a wave of nostalgia: the tone control built around a 140UD1, the idea of adding a built-in equalizer in place of the input indicator lights (I even had a front panel ready, blackened and engraved), the struggle to make a case out of "bits and pieces," and, of course, the endless modifications and improvements...


Everything was fine except for one thing: it didn't have a speaker protection board. Not that one had been removed; the amplifier simply hadn't been designed with one in the first place. Around that time, I saw a simple protection circuit in the magazine "For the Radio Amateur." It even included a PCB laid out specifically for this amplifier model, which could be installed in what was practically a factory-provided spot between the transformer and the rear panel.
I built it, it worked, and eventually I sold the amplifier with the board still installed.


So, what exactly is a speaker protection circuit?
It provides the following functions:
1. Protects the speakers from DC voltage of either polarity appearing at the output if the power amplifier's output transistors fail.
2. Delays connecting the speakers to prevent a pop caused by transients in the amplifier.
3. Disconnects the speakers immediately if the power supply fails. The amplifier may still be running from the filter capacitors, but the speakers will already be disconnected. This also protects them from transients.
4. Sometimes, it also protects the amplifier from overload.

All this is needed to protect your speakers. If the output transistors fail, the full supply voltage can appear at the output. The fuses may blow, of course, but not always—and even when they do, it takes them some time.

In the past, you had to build things like this entirely from scratch, as in the example above. These days it's much easier: just buy a ready-made board or a kit and you're all set. Sometimes the protection circuit is already built into the amplifier board, like the ones I've reviewed before.




The kit comes in a plain plastic bag. No instructions, just a pile of components and a PCB.


The kit includes the PCB and all the necessary components, including terminal blocks. The only thing I think they could have included is standoffs for mounting the board. They really skimped...


The PCB is double-sided.


The quality looks fine to me.


There's no schematic in the kit, but the board has all the component values marked on it. I don't think it's possible to get confused.


A few notes on assembly and the components.
1, 2. First, install the resistors and a couple of diodes. The resistor values are 2.2 kΩ, 10 kΩ (2 pcs), 22 kΩ, and 47 kΩ (2 pcs). The diodes are 1N4007.
3, 4. Next, install the LED, bridge rectifier, 7812 voltage regulator, and the uPC1237 protection controller.
5, 6. Then add the capacitors. The 10 and 100 µF capacitors (2 pcs) are marked on the board; the two 47 µF capacitors go in the remaining spots.


The C1237 IC is specifically designed for speaker protection circuits. As far as I know, there aren't many such controllers around, so if a board has something that looks like this, it'll be a C1237.


The datasheet includes a typical application circuit, though it's not the easiest to read.


So I decided to draw the actual schematic for this board. Unfortunately, the manufacturer doesn't include one. There aren't many differences, mostly just the component values. Also, the overload protection input isn't connected to anything.


The kit came with a couple of relays supposedly made by Omron, but to me they look like some kind of outrageous knockoff. Not only did it look as though the contacts had been chiseled out of a chunk of iron, but one relay's contact assembly is crooked, and some of the solder contacts are oxidized...
The resistance of the normally open contacts when closed is about 10 mΩ.


Although the contacts in the photo from the DigiKey website look very similar, they are at least straighter.


They can only be installed in one orientation, but the oxidized contacts make them difficult to solder. On the board, the input and output for one channel are on the left, and those for the other channel are on the right. Personally, I think it would be more convenient to have the inputs on the left and the outputs on the right, but that's a matter of taste.


I mentioned that there are two relays for a reason: boards with just one relay are quite common. It's not exactly a critical issue, but here the relay contacts are connected in parallel, which means less voltage drop and less effect on the signal.


The terminal blocks had their own issues too. They're convenient enough, and you can connect them together to get more contacts in a single assembly. But what were the designers—or the component suppliers—thinking, providing a board and terminal blocks with different pin spacing?


I bent the outer contacts a little, inserted the terminal block, and soldered it in place, but it doesn't feel right. The terminal blocks also stick out slightly beyond the edge of the board, leaving the mounting holes right next to them with almost no clearance. I think they originally planned to use different ones, but they just used whatever was in stock.


And that's it. Assembly took less time than it took me to type the text you've just read. There were no major difficulties, apart from the awkward relay contacts and terminal blocks, but those were quickly dealt with too.


The finished board looks neat.


The terminal blocks are labeled to show what each contact is for. Be sure to connect the input and output correctly.


I soldered without adding flux, using only the flux in the solder, and it worked great. My only complaint was about the relays. All that's left is to wash the board and perhaps coat it with protective varnish.


The board is powered by 10–24 V AC. However, the 7812 regulator has no heatsink (although there's room for one), so I wouldn't recommend supplying more than 15 V.
You can also power the board with DC. In that case, connect the positive supply to the terminal block contact farthest from the corner of the board. The voltage should then be 12–24 V, and the current draw is about 80 mA.

The relays take 2–3 seconds to switch on. Even after a brief loss of power, the delay barely changes.



The total resistance of the input/output paths measured 13 and 14 mΩ.


The minimum DC voltage that triggers the protection and allows it to reset is about 0.6/0.5 V for positive voltage and -1.1/-1.0 V for negative voltage. At low voltages, it resets almost immediately. At higher voltages (5 V and above), the reset delay is 15 seconds for positive voltage and 3 seconds for negative voltage.


And of course, let's check the most important thing: how quickly it can disconnect the speakers when DC voltage appears at the input, depending on the voltage and polarity. I tested one input, since they are effectively connected in parallel. Because the resistors share a single capacitor, there will naturally be some difference, but it isn't significant.

First, I applied about 3–3.5 V.
With positive polarity, the trip time is inconsistent and can reach 0.7 seconds (screenshots 1 and 2).
With negative voltage, things are much better: the trip time is consistent at 0.4 seconds (screenshots 3 and 4).

But there's no need to worry: this voltage is usually not dangerous to your speakers. You will hear a loud pop, of course, but the relay will still disconnect the speakers.


1, 2. At 10 V, the disconnect time is consistent: 0.3 seconds for positive voltage and 0.12 seconds for negative voltage.
3, 4. At 15 V, the trip time gets even shorter: 0.14 and 0.08 seconds for positive and negative voltage, respectively.
5, 6. Raising the voltage further makes no difference: the shutdown time stays the same. Apparently, beyond this point the limit is "hardware-based".
But as you may have noticed, in every case the shutdown time is shorter when a negative voltage is applied than when a positive voltage is applied.


To sum up, I can say that this is a useful kit and, especially importantly, it works once assembled and doesn't require advanced skills. Everything shuts off reliably, there's a turn-on delay, and power is cut off instantly when the supply is disconnected.
Unfortunately, there are some shortcomings. For example, the connectors have a different pin spacing, though that's forgivable since it doesn't significantly affect how well the device works. But the low-quality relays are a real problem, especially since they usually make up a significant part of the cost of a device like this. Ultimately, it's up to you. I'd buy relays like these from a local store, where you're more likely to get good-quality ones, such as Finder rather than Omron, which is more often counterfeited.

The store provided a couple of coupons. They won't help much if you only need to buy one of these boards, but they might come in handy for larger orders.
3000-300: umkapromo300
5000-600: umkapromo600

That's all from me. I hope you found this review useful.