Loudspeaker Protector

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Hello, starting a new thread on my Loudspeaker protection project.

Original thread was http://www.diyaudio.com/forums/showthread.php?s=&threadid=103586&perpage=25&pagenumber=2

Square_wave asked about a de-thump circuit.
There were several responses. I suggested a cct I had been using for years (uses LM338 and 14 BC transistors).... was originally published in Electronics Australia in the early 1990's.

I am starting to build a couple of monoblocks and a protection cct is a manditory inclusion.... was going to use my "old" cct.

The UPC1237/NTE7100 was also nominated but, at the time, this device was not easily accessible locally for me and pricing was very high.

So, this got me thinking and I decided to make this a DIY project.

Several forum members suggested a seperate thread on my project.

The Protector circuit I wanted was to have the following requirements:-
1. Power up delay = 2 sec to the speaker relay (dethump)
2. Power Up Soft Start.
3. Low current switch on function.
4. Power-down = immediate disconnect of speakers (dethump)
5. Fault detection = immediate disconnect of speakers
6. 3 LED indicators - Power On (green), Mute (yellow), Fault (red)
7. The above powered from a small (7VA) toriod transformer.

I decided to use a microcontroller (8 pin device PIC12F675) to control all of these functions.

After testing a whole bunch of circuits for the DC fault detection and the AC detection I settled on a circuit for each which operated reliably (i.e. fast detection) and were interface-able to the PIC.

The function delivered is:-
1. To powerup the amp one turns on the small toriod....this powers the main AC power relay - Relay 1 (this means the front panel On/Off switch only switches 50mA or so).

2. On initial powerup, a 60ohm resistance (3 off 180R in // ) is in series with the main (big) toroid...this is the Soft Start.

3. There is a 100mS delay before Relay 2 is operated shorting out the soft start resistors.

4. Also on power-up the speaker relay (remotely located near the speaker terminals) is held off for 2 secs (Mute LED is on during this time).

5. If AC is disconnected...the PIC will detect this via the ACdetect cct and disconnect the speakers within a few cycles.

6. If DC greater that +/-3v is detected on the speaker leads then the PIC will switch the Speaker relay off and light the Fault LED. Detect time is about 50ms. The Speakers remain disconnected whilst the fault is present.
If the fault condition ceases the Speaker relay will be re-energised after 2 secs (i.e. a 2 sec hysteresis).

The circuits were all breadboarded up, I wrote the PIC code and have tested the functionality... all works well.

I decided to mount all of this on the one PCB. The resultant PCB has all the ccts and small PCB toroid on board and measures 170mm x 50mm (the toroid is 50mm x 50mm).
This mainly because my monoblock cases will be 75mm high so the long, thin PCB will be best fit.

I am having a few of these PCBs made currently.... should have them next week.

I'll do some final testing once the PCBs are built up.

I did this only for myself but if anyone else is interested I can supply info (or preprogrammed PIC) so you can DIY as well.


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First one built

The PCBs arrived and I have built one of the units so far.
Tested this one and took traces (storage CRO) of the switching to ensure all worked as it did on the breadboard.

Works as expected.... It's all good... I'm happy :)

I had the three LEDs (I used red for all in testing) mounted on the PCB... there will be a 6pin header there for wiring to the LEDs at the front panel eventually.

Only item missing is the fuse holders (plus need to buy another toriod for the 2nd board.... will get these during the week some time.

Project complete.


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Protector PCB and PIC

Jopie, no problems.
I had a couple of extra PCBs made and have heaps of PICs.

Would be happy to supply at cost.

Note, that if the Soft Start is the only part you are after, there are other Soft Start kits available out there, notibly Greg Ball (SKA Audio) has a very compact unit and ESP (Elliott Sound Products) has what looks to be a good version as well.... these guys are also in Australia.... may be others as well around Europe.

Have a look at those as well and make a decision based on what would fit and work best for you.

Best Regards.
PCB and PIC cost


I had 5 PCBs made and am using 2 of these for my monoblocks.
The remainder (3) were for "future use" but am happy to pass two(2) these on at cost (Jopie was after one of these) .

Good thing I allowed for use with 240vac as well as 110vac wrt the strapping of transformer primaries :)

The PCBs cost me AU$28 each. The PIC would be AU$4.00 and postage looks to be AU$8.50 for airmail to the US.

Please email me re-payment if you wish to proceed.

Thanks and Regards.
Protector boards

Graeme, pair ? I take that to be two amps (two stereo or a pair of monoblocks) ?

I designed the PCB to handle L + R channels but could, obviously, be used in a mono situation. was also designed handle 2 x 300VA or 1x 500VA transformers. (will also handle 2 x 500VA with alteration to ballast resistors).

Let me know....please confirm.

I'll also check with Salesmonster to see if he is still taking the board.... if not, I guess I could let my spare go as well....I have no definite use for it, it was a "just in case".

Beyond that, happy to commission more boards, or, if someone can get them done cheaper, can supply the gerbers.

Also, if there is more interest, can get a list going. I am transferring the PCBs at cost and PIC chip at cost (pre-programmed for nix).
The Zristic Protector

Nice one, love PIC driven stuff.

PIC noise on audio circuits:-
Have done several PIC ccts within audio and have almost never seen any noise. The only occassion I have seen anything it was theorised to be related to the oscillator section.... earthing the crystal can solved the prob completely...in that case there was a layout issue ith the PIC section adjacent to the gian stages.

In mine I am using the internal osc so no issues.... I just waved the cro across the Speaker inputs on the DC detect to confirm.

Will be interesting as I have redesigned my preamp PCB. Was a collection of PCBs mod'ed over the years. Had an ALPS motorised pot on the main PCB with another little PCB with a PIC decoder/controller. I have it all on one PCB now but have not built it yet.
Did not include optos as previous experience had never required these.
Will be interesting to see how this goes.

Soft Start duration:-
0.7-1.0 seems like a very long time... depends on transformer size, but, a lot of current. All my Googling and searching suggested 100mS.
The other SoftStarts available (incl the ESP one) all use around 100mS.
My testing showed a minimimal glitch on the mains using 100mS, longer did not make much of a difference. Sorter time puts less stress on the resistors as well.
Anyway... that is the beauty of micrcontrollers....easy to alter these sorts of things.

One suggestion:-
Try adding a zener in series with the Speaker Relay EMF diode (Anode of the 1N4148 on one side of the coil, Cathodes of the 1n4148 and zener connected together, anode of the zener onto the other side of the coil) this will speed up relay release by around 40%. The BC548 Vceo is 30v so a 20v zener would be good. Although the higher the zener value the better (faster release).
I like to use DB681's their Vceo is 100v so I use 27v zener (plus the one device will drive at least 6 relays).

Keep up the good work.
Hey Muzza thanx for the email, tried to reply to you but for some reason i cannot send emails just receive. Can you please send me the files and also details of the pcb's you got made so i can try and find out how much the difference in price is if there is any.


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