LM1875 Speaker POP - NOT caused by switch (it seems) - diyAudio
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Old 1st May 2011, 11:11 PM   #1
diyAudio Member
Join Date: Apr 2011
Default LM1875 Speaker POP - NOT caused by switch (it seems)

Hi there, I'm new to the forum.

Dual Mono built lm1875
160va +-25V toroidal

I've been getting a speaker pop when switching the amp on, so I installed a delay circuit:
ePanorama.net - Audio amplifier output relay delay

Delay works, but there is still a loud pop once the relay engages. The relay circuit is placed in between the output of the amp and the connectors to the speakers.
There is also the pop when I manually connect the speakers, once the amp has stabilised.

Is this something wanting to discharge? I am new to building amps and would appreciate any advice.


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Old 1st May 2011, 11:56 PM   #2
johnr66 is offline johnr66  United States
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Sounds like significant DC on the output. Measure the voltage DC across the output with no input signal and report back. Could be DC getting in to the inputs or a degraded output transistor. Do you have a schematic of your circuit?
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Old 2nd May 2011, 12:03 AM   #3
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Unfortunately I don't - it is very similar to national semi's layout. Perhaps tomorrow I might draw it out.

I'm off to bed here (UK) but I'll get back to you tomorrow on the dc voltage.
What do you think might be causing this dc though?
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Old 2nd May 2011, 11:01 AM   #4
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
get some sleep.
Do not connect a speaker.
Measure the output offset and report.
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 2nd May 2011, 03:59 PM   #5
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Join Date: Feb 2011
if the relay is dual-state (so it connects to one of the pins when not charged, and connects to other pin when charged) You can try to add a resistor to the idle / off state pin.

my english is poor, if it was not clear -can happen- i can post a picture if you like.
i suspect a capacitor discrage like effect kicking Your speakers, the bleeder resistor may solve it.

Try to switch the amp on, connect a peaker. You hear the plop. Now disconnect the speaker, and connect it again. If there is no plop noise, then the bleeder resistor will solve -partialy- the problem. Partialy as it will not cure the origin of the problem, but will heal the effects.
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Old 2nd May 2011, 08:53 PM   #6
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Join Date: Apr 2011
Hi guys - thank you so much for your help.

I did a complete rebuild today that's working but still getting this pop with the sound of what sounds like flux stabilising in the transformer added.

I'm currently drawing up the circuit layout - I am sure there are things that need to be changed. Sorry if I can't get it up tonight, I will try my best but I've been up till the early hours with this thing for several nights now.

What you're saying about using a bleeder resistor makes perfect sense. I will give it a shot amongst other changes.

Thanks guys, and I'll be back with a layout shortly hopefully!
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Old 3rd May 2011, 12:07 AM   #7
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OK upon crossreferencing with national semiconductor's data sheet, it turns out my layout is exactly the same as their typical single supply layout *embarrassed* :

Click the image to open in full size.

Arty I did what you said - I started without speakers connected, then connected them = POP. Disconnected then reconnected = NO POP. So I took a 10k 2w resistor (all i've got atm - can you recommend an appropriate resistance to solder across the capacitors?) and I'm not seeing a change

Meanwhile, I've been looking at carlosfm's snubberized setup. tomorrow i think i will change to that, as my setting has bridge rectifier (mur860's) going straight to capacitors (4700, 2200 need to get another 4700!) without any resistors or 100nf caps.

I am referring to this design:

Does anyone think that'll make a difference?

Thanks again guys
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Old 3rd May 2011, 12:31 AM   #8
Bill_P is offline Bill_P  United States
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Location: New York, the vampire state
The circuit is single supply powered so the 2200uF capacitor charges rapidly at power on through the speaker until the DC voltage across the cap reaches 1/2 Vcc. This produces the turn on pop. Add a 470 Ohm resistor from the negative lead of the 2200uF to ground and connect the speaker delay relay after the 470 Ohm resistor. That way the 2200uF capacitor will charge through the 470 Ohm resistor and reach steady state before the relay connects the speaker.
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Old 3rd May 2011, 12:31 AM   #9
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try use a charge resistor from the output cap to ground. The delayed connected speaker would then find a charged cap and wouldn't feel the need to charge that capacitor through his voicecoil.

edit:Bill was faster than me...
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Old 3rd May 2011, 03:48 AM   #10
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Your are experiencing the same problem of the Dynaco 120. They used a capacitor to coupling the output transistors and the speaker. You can minimize it by adding an GE CL60 between the rectifier and the capacitor. This is a thermistor that will delay the voltage in the power up.
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