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Author Topic: On/Off Pop Noise Problem  (Read 7692 times)

weatherlght

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On/Off Pop Noise Problem
« on: April 26, 2010, 01:22:10 PM »

Greetings

I have recently built a 25w tda2050 based amplifier. After double checking all routes and pcb connections, the amp fired up flawlessly the first time. Its clean channel is great, without any hums or hisses. However as I continued using, i noticed that the on/off popping sounds were somewhat much more than tolerable and it may not be safe for the speaker even. The pops are loud and when turning off the amp, after the initial loud pop, 2-3 secs later a silent crackling, tearing sound comes off the speaker too! :( I tried to do a we search but I could not find some confirmed reliable basic practical information. Theres supposedly a circuit to prevent this pop in audio amps but best solutions are the simple ones so I would really appriciate if you can point me in the right direction :)

My build is TDA2050 datasheet with a 10uF nonpolar feedback capacitor instead of the 22uF shown at datasheet. Rest is pretty much the same. It does nto have any hum, hiss or oscillation. It runs pretty smooth and quiet only problem is this popping at turn on/off. For power suplly, I use a 35A bridge rectifier, a 15-0-15 tranny and 4700uF filter caps with one additional 100nF ceramic between the +vcc and -vee capacitor tabs. My star ground is right between the power suplly caps and tda2050 (about 2 inches away from both). My star has a direct connection to mains earth and chasis ground goes directly to mains earth as well. My preamp is a marshall mg15 with a suplly of +-15V zener diodes with 220uF and 100nF caps connected with 680R resistors to power suplly. At around 1w output, my measured dc voltages are +-14.9V at preamp and +21.5V, -21.7V (cheap tranny :( ) at powersuplly. This pretty much sums it up as far as I can remember.

There are 3 main things I am concerned about that may cause this pop

- I read some claims that popping is mainly due to arching at on/off switch. Some people suggest a small mains cap at power switch. Can this work? If yes, what type of cap and how to connect it?

- Some guy talked about DC balance. I'm not sure if he meant +21.5V, -21.7V suplly difference or the 22k non inverting to ground dc resistance and 22k inverting to ground (by speaker). Something about DC offset at output. Are these values critical? I used +%5 varying carbon resistors at poweramp. Shall I measure components induvidially and put 2 22k that are pretty much identical down to less than %1 so these 2 22ks are matched?

- I also have a valve amp and theres absulately no audible pop at on/off. It has a kind of slow start (preamp suplly has some 5k resistor + a series choke) Can the timings of preamp and power amp turn on cause this pop? If yes, then shall I delay the preamp, or the power amp by installing some series resistors? I assume the 680R resistors have that purpose but they dont seem to be enough.

Thanks for your help :)
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joecool85

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Re: On/Off Pop Noise Problem
« Reply #1 on: April 26, 2010, 07:15:40 PM »

If you figure it out, let us know.  I have an LM1875 amp that has been doing this since the day I built it.  Only makes a pop noise when I shut it down, but it's relatively loud and can't be too good for anything...plus its bothersome.
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J M Fahey

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Re: On/Off Pop Noise Problem
« Reply #2 on: April 27, 2010, 12:46:13 AM »

Very short answer: it's absolutely normal.
Half-solution #1: place an AC approved capacitor across either the transformer primary or across the switch contacts .
See examples in Peavey and most other commercial amplifiers schematics.
It is not the "death cap" going from chassis to one side of the power line.
2) Full (cheap and effective) solution: add a "standby" switch in series with the speaker "hot" lead.
Turn on sequence: Power switch ->St By .
Turn off sequence: Standby->Power switch.
You'll be blessed with ethereal silence.
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weatherlght

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Re: On/Off Pop Noise Problem
« Reply #3 on: April 27, 2010, 02:29:25 PM »

One of my professors told me that its due to power suplly caps charge/discharge transients and that might just be the reason why valve amps dont have that pop. Valve amps have a series resistor / choke in their power supllies since valve amps draw small currents (500-1000mA). Problem is, with a solidstate amp (it draws like 2amps+) any significant resistance that I'll be puting in series to charge the caps slower will reduce some serious volts  :( I'll need to ask around and come up with a solution.
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iTzALLgoOD

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Re: On/Off Pop Noise Problem
« Reply #4 on: April 28, 2010, 12:20:36 PM »

I like the Standby switch idea. Is it OK to have the power on with the speaker disconnected?  This usually means OT death in a tube amp.
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J M Fahey

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Re: On/Off Pop Noise Problem
« Reply #5 on: April 28, 2010, 12:56:51 PM »

1) Tube amps avoid "On" popping, because they "slow start", 30 seconds to heat filaments, while SS amps are instant on, even before filter caps are charged and DC stabilizes.
An already hot tube amp pops merrily if you use the main switch.
The polyester capacitor across it helps a lot.
2) No load good for SS amps, *specially* while charging big caps.
Just look at the enormous excursion some speaker cones do when switching on.
Neither they nor the amplifier like that.
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Enzo

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Re: On/Off Pop Noise Problem
« Reply #6 on: April 30, 2010, 12:15:24 AM »

There are pops and there are thumps.  Pops are often from the power switch arcing as it opens.  The fully charged amp circuits have enough energy stored to amplify that noise for the brief moment before they go silent.  That has nothing to do with filter cap discharge rates.

Thumps, or that whump sound you sometimes get turning off is the result of everything collapsing. As power rails collapse, the amp goes through a moment of instability.

Tube amps don;t pop at turn on, but they sure as hell pop at turn off.  Very common.

One thing about the thump on a solid state amp.  The highest voltage the amp has to deliver is one of its power rails.  At turn off, those start to drop.  Whatever transient noise that results can be no larger than the loudest peak the amp could make during normal operation.  If your speakers can handle the full power of the amp, then the turn off thump will be fine as well.  To your speaker, it might as well be just another kick drum hit.
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Jack1962

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Re: On/Off Pop Noise Problem
« Reply #7 on: May 18, 2010, 10:59:56 AM »

Enzo if a tube amp :Tube amps don;t pop at turn on, but they sure as hell pop at turn off.  Very common

 then ya better replace some filter caps , or it couls possible be a bad output transformer

I was going to give ya"ll some solutions to this problem but  J M Fahey  is dead on the soultion to the problem lol lol lol
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guitarsan1

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Re: On/Off Pop Noise Problem
« Reply #8 on: October 06, 2013, 05:49:23 PM »

I know this is an old topic, I just read it an found a resource that will help. It's in refference to a post about a "snubber circut" to stop the popping when turning off and on of solid state amps. This is a pdf file with all the tech info and diagram for the solution with electrical values. Hope this helps.
http://www.paktron.com/pdf/Quencharc_QRL.pdf
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Roly

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Re: On/Off Pop Noise Problem
« Reply #9 on: October 06, 2013, 10:31:27 PM »

Quote from: Jack1962
then ya better replace some filter caps , or it couls possible be a bad output transformer

Nope.  It has nothing to do with filter caps, and certainly nothing at all to do with any output transformer.

The cause of turn off popping is, as @Enzo says, momentary arcing at the switch contact when the supply current is interrupted, and as @guitarsan1's PDF post shows, the answer is to "snub" or slow down the interruption of this current using a CR network across the switch contact(s).

This crack or pop at the moment of switch operation is not the same as the thump that often occurs a second or two after switch off when the supply voltages collapse.  This thump can only be prevented by a "speaker protector" type relay arrangement that disconnects the speakers right at switch off - and turn-on thump similarly (but speaker disconnection is only applicable to solid state amps).

I deal with this problem in greater detail here; http://www.ssguitar.com/index.php?topic=2439.msg19970#msg19970
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