Noise in the loudspeakers when my LM1875 are turned on

[stenak]

I have built a couple of LM1875 chipamps. When I turn the amplifier on and off, I always get noise like a "pistolshot" in my loudspeakers. What is the problem, and what can I do to get rid of the "pistolshot"

 

[ubergeeknz]

It's normal to have a "turn on thump" while the rail voltages and circuit settle. You could eliminate it by adding a delay switch to isolate the loudspeakers for a few seconds.

 

[Sangram]

LM1875 has no mute circuit and requires one for both turn-on and especially turn-off. Else you will hear that pop.

I use the one below

 

[FauxFrench]

You can find such speaker protection boards on Ebay, Amazon etc...
As an example I show you one AIYIMA 12 18V UPC1237 panneau de Protection de haut parleur kit de carte de Protection de haut parleur partie retard de demarrage moniteur DC-in Amplificateur from Electronique on AliExpress . I have NOT tried this one as I do not use one myself.

 

[ubergeeknz]

I've used this one in another application. It seems well made and will do the job although it needs a (small) separate supply I you want to avoid turn-off thump /noises too. It also provides DC offset protection.

UPC1237 Speaker Protection Board AC 12V-24V OMRON Relay for stereo 2 channels Amplifier
AliExpress

I modified mine so that it would (hopefully!) "crowbar" the amp should the relay contacts weld due to high current, protecting the speakers - however that scenario seems unlikely with an lm1875.

The supply transformer I used was this one

PE2820-I Power 3VA 230V/12V Expory resign encapsulated safety isolating transformer PCB Welding transformer power transformer
AliExpress

 

[stenak]

Thanks to all of You!

 

[NanoFarad]

LM1875 has no mute circuit and requires one for both turn-on and especially turn-off. Else you will hear that pop.

I use the one below

Hi, can you tell me more about this circuit? I mean supply voltage and about delay time. I want to use 30-36vAC CT transformer for lm1875.

 

[NareshBrd]

Dont go much above 30 volts unless the chip is genuine.
And drive the speakers throigh a capacitor.

 

[Sangram]

Hi, can you tell me more about this circuit?

I run the one in #3 off the same rail as the amp. 34VCT for 23V rails under load. 24V relay, and delay time is calculated with time constant of R1xC1.

I used, from memory, 47k and 22uF but it could have been different.

 

[NanoFarad]

alright, thnks mod.