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richard.C. 3rd August 2011 12:29 PM

Building a simple "soft start" speaker protection circuit
Hi all again,
Ok, i've just introduced this pre-amp to boost signal strengh to my LM3886TF chip amp. In doing so i've got a small "Pop" on power up and a rather strange "crackle pop" two seconds after powering down, which is all coming from the pre-amp stage. Otherwise the pre-amp and amp sounds quiet during operation.

here's the pre-amp, and is powered with a 12VAC-0-12VAC transformer; Wholesale Freeshipping DIY AMF NE5532 stereo preamplifier board #872

I want to build a simple soft start and cut off circuit for the pre-amp and amp transformers (two seperate transformers used). Also to incorporate a signal cutoff from the pre-amp output, to isolate the "crackle pop" sound before it reaches the power amp when powering down. I was going to base the design on this simple soft start circuit i found on a thread on this forum. Probably make some alterations to values and use a muiltiple point relay for what i'll may need to do.

I was thinking, for the signal cutoff during power down i would use the same relay circuit(different points) to act as a isolation switch for the pre-amp signal output.. Hope i'm being followed correctly. Really, i want to take a simple approach to this. I believe this will deal with the issue.

Also will the relay points induce any noise to the signal if i do go that way about it??

davidmuriungi 4th August 2011 01:11 PM

I wonder why you want to use the same relay to soft start the pre-amp and the main amp, at the same time use the same relay for muting the system:confused:
You should not be overly concerned with noise creeping into your system at this time:mad:
While it may seem simple and cheap to use a single relay for all these functions, it is also a disaster waiting to happen. I would recommend that you use multiple relays as this will save you much grief since you are planning to modify the original design. Relays are easy and cheap to obtain. You do no have to use heavy duty relays for this
Take a look at the spst, spdt, dpst, dpdt relay types and where they are best applicable.


Mooly 4th August 2011 01:49 PM

Or use a speaker relay delay that gives say 6 seconds before closing on power up and that opens quickly on power off.

Relays in low level signal paths can be problematic unless of high quality or unless you use them to "short" the output of the preamp to ground to provide the mute. That brings problems of its own though (ensuring the preamp can stabilise when the output is grounded) so unless you are aware of the issues I think a "catch all" speaker delay is the easiest.

Why not run the preamp of the main amp PSU too ? A simple resistor/zener regulator is all thats needed for an opamp.

richard.C. 5th August 2011 01:21 AM

Hi Mooly,
I was also thinking that grounding the signal would be better, i used some 10ohm resistors to ground the signal from the preamp, and it was enough to remove the poppin noises completely. And the first "pop" is instant to the powering up of the preamp, so soft start may be enough to deal with the first "pop" i think, and this also minimise a spike in current through the fuse. And using DPDT relay i can incorporate a signal grounding circuit from the pre-amp output to deal with the second "pop", which is completely removed with that 10ohm shunt to ground.

Whats the speaker relay delay circuit you speak of? is it simple and cost effective?

Also on the dual power supply, A resistor/zener diode can be built to power my preamp? It requires an AC-0-AC supply, can we do it?

Michael Bean 5th August 2011 04:31 AM

Actually, your preamp has its own built in power supply with regulators, so you can apply a DC source as long as there's high enough voltage to overcome rectifier voltage drop and regulator drop out voltage, something like 5 volts or more than the regulated voltage to be sure of proper operation. Also, I agree with Mooly, a single relay with delay to control speakers is all you need, putting relays in the signal path can cause as many problems as they solve. Have a look at this for some ideas: Loudspeaker Protection and Muting


richard.C. 5th August 2011 05:35 AM

Yeah i'm gonna move away from that idea of putting a contact relay switch in the signal path itself, i'm going to try for the ground (short) method with some resistors. I'm going to build and trial my idea, try a few different resistor values, i believe it may be a novel way to deal with the problem. I'll measure and if it works i'll stick with it, if it doesn't i'll take DC detector idea for a test run.

Also about the pre-amp power supply, i have got the second tranny running for it, but how will i go about using my current 25VAC-0-25VAC tranny to drive this. The PSU/rectifier i'm using outputs 36VDC-0-36VDC in to my chip amp, how will i go about powering my 12VAC-0-12VAC pre-amp from this supply?

Michael Bean 5th August 2011 06:11 AM

If it's already working OK with two transformers, you should just leave it as is, I was just commenting that the preamp board you are using could be run from a DC source if desired. As far as the shorting method, I assume you mean between the preamp output and power amp input? It may fix preamp power up noise, but won't address possible power amp noise, that's why I perfer to use relay switching between power amp and speakers, and as a bonus, makes speaker protection possible.


Mooly 5th August 2011 06:36 AM

1 Attachment(s)

Originally Posted by richard.C. (
Also on the dual power supply, A resistor/zener diode can be built to power my preamp? It requires an AC-0-AC supply, can we do it?

Like this... you feed it from the main -/+ supplies to the main amp.

richard.C. 5th August 2011 06:45 AM

Hi Mike,
yeah i was gonna short the signal between the preamp output and amp input. Or i was just thinking of shorting the power supply on the DC side maybe with the same method. Short + to ground and - to ground using resistors to drain the charge in the caps. I'll try that one too.

If there is an easy way to use one tranny to power both i would like to do that. Mooly suggested a zener/resistor circuit, but i wouldn't know how to go about that.


richard.C. 5th August 2011 06:58 AM

Now i do!! :)
So that diagram, thats not from the 240VAC mains is it??? No it's from the secendary AC side of the tranny?? or the DC side of the rectifier? :confused:

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