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Old 18th September 2007, 01:32 AM   #1
muzza is offline muzza  Australia
diyAudio Member
Join Date: Dec 2003
Location: Melbourne
Default Loudspeaker Protector

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

Original thread was Delay turn on circuit for amps

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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Old 21st October 2007, 04:49 AM   #2
muzza is offline muzza  Australia
diyAudio Member
Join Date: Dec 2003
Location: Melbourne
Default 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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Old 22nd October 2007, 02:01 PM   #3
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Send a message via Yahoo to si_jopie0382
Hi Muzza,

Interesting.... Can i buy your pcb with microcontroller too

I've a big problem with inrush current and dc current
i'm sure your circuit will solve my problem


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Old 22nd October 2007, 11:22 PM   #4
muzza is offline muzza  Australia
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Join Date: Dec 2003
Location: Melbourne
Default 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.
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Old 23rd October 2007, 03:20 PM   #5
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Send a message via Yahoo to si_jopie0382
Hi Murray,

Not only Softstart but Speaker protector is i needed too... for long term use

and if i have soft start + speaker protector in one pcbs it would be great!

How much you will cost me for your pcb+PICs

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Old 23rd October 2007, 11:11 PM   #6
muzza is offline muzza  Australia
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Default Protector PCB and PIC

Jopie, have sent you an email.
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Old 24th October 2007, 01:39 AM   #7
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I would be interested in a PCB and PIC as well.
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Old 24th October 2007, 03:25 AM   #8
muzza is offline muzza  Australia
diyAudio Member
Join Date: Dec 2003
Location: Melbourne
Default 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.
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Old 29th October 2007, 08:54 AM   #9
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Join Date: May 2007

I will be a candidate for a pair of these boards and PICs if there is enough interest to commission a couple of extras. Sorry, tried to email to this effect but I don't have the rights to do this.


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Old 29th October 2007, 11:57 AM   #10
muzza is offline muzza  Australia
diyAudio Member
Join Date: Dec 2003
Location: Melbourne
Default 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).
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