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Old 6th March 2008, 05:45 AM   #1
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Default Preamp snap crackle pop

Hello,

I've recently built my first preamp using boards from Elliot Sound. Currently I have P05a and P88 mounted in a wooden enclosure with an ALPS pot. I decided to use a 16VAC plug pack for input to the psu board. My question is this:

Why do I hear annoying snaps, crackles, and pops when no source is playing? I also hear it while playing source through the preamp, but it is of course less noticeable... sometimes. What is this noise? It is intermittent and not predictable and very unwanted.

Links to the pcb's I'm using.

http://sound.westhost.com/project05a.htm

http://sound.westhost.com/project88.htm

More information and pictures are available on request of course.

Thanks!
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Old 6th March 2008, 05:54 AM   #2
TheMG is offline TheMG  Canada
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My guess would be that you're hearing noise from the AC mains which are spikes that occur when things turn on and off.

Simple test: turn a lamp on and off on the same breaker as the preamp. If you hear clicking/popping in the speaker, then you know your problem is AC noise which is being induced somehow.
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Old 6th March 2008, 07:44 AM   #3
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Thanks. Yes turning on/off a light does produce some crackling noise. Unfortunately I live in an older apartment and my whole apartment is on one circuit. But I have never had "crackling" problems when I was using the old Harmon Kardon amp I've got as an input selector.

Why suddenly can I hear all this noise?
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Old 6th March 2008, 08:30 AM   #4
gootee is online now gootee  United States
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You need a lowpass RF/high-frequency filter on the input.

In Figure 2 of Project 88, the first gain stage, connect a 1000 pF (i.e. 1 nF or .001 uF) capacitor from the positive opamp input to ground. i.e. from between R2L and the + input, to ground (the junction of R4L and R1L). Do the same thing for both channels.

If that's not quite enough, you could try 1500 pF, or about 2200 pF maximum.

There is a thread about the same thing, here:

Eliminate influence of switching noise from light switch in to power supply

Please let us know if that makes a difference.
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Old 6th March 2008, 09:22 AM   #5
Nordic is offline Nordic  South Africa
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Did you make your own boards?
I have, and can't say I noticed it being overtly sensitive to noise...
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Old 6th March 2008, 11:36 AM   #6
AndrewT is offline AndrewT  Scotland
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and read post4 in Gootee's link.

But you can blame the amplifier for having an inadequate PSRR, when combined with the PSU that let's through the offending RF.
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Old 6th March 2008, 12:03 PM   #7
Nordic is offline Nordic  South Africa
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little tip, he does not show the resistor value in his C-R-C section of the 05A schematic, but you can see it clearly in the photo....
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Old 6th March 2008, 12:16 PM   #8
AndrewT is offline AndrewT  Scotland
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If the interference is airborne or cable transmitted, it must first be attenuated at source.
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Old 6th March 2008, 03:52 PM   #9
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Quote:
Originally posted by gootee
You need a lowpass RF/high-frequency filter on the input.

In Figure 2 of Project 88, the first gain stage, connect a 1000 pF (i.e. 1 nF or .001 uF) capacitor from the positive opamp input to ground. i.e. from between R2L and the + input, to ground (the junction of R4L and R1L). Do the same thing for both channels.

If that's not quite enough, you could try 1500 pF, or about 2200 pF maximum.

There is a thread about the same thing, here:

Eliminate influence of switching noise from light switch in to power supply

Please let us know if that makes a difference.

Wow, I never figured it could be RF. So am I hearing lower harmonics of RF signals created by mains wiring then?

Thanks for the link, I wasn't able to find that thread by searching. I will definitely try your suggested solution.

Quote:
Originally posted by Nordic
Did you make your own boards?
I have, and can't say I noticed it being overtly sensitive to noise...
I did not make my own boards. I did notice in the assembly instructions that Rod instructs to install an RF filter if "RF is bad in your area" or something. I never figured I'd need it. How do yo like the boards you made Nordic? Have you made any alterations? Are you using heatsinks on the psu regulators? I'm wondering if I should ...

Thanks for all your responses; I will post when I install the suggested cap.
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Old 6th March 2008, 04:29 PM   #10
gootee is online now gootee  United States
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Quote:
Originally posted by meaghers



Wow, I never figured it could be RF. So am I hearing lower harmonics of RF signals created by mains wiring then?

Thanks for the link, I wasn't able to find that thread by searching. I will definitely try your suggested solution.



I did not make my own boards. I did notice in the assembly instructions that Rod instructs to install an RF filter if "RF is bad in your area" or something. I never figured I'd need it. How do yo like the boards you made Nordic? Have you made any alterations? Are you using heatsinks on the psu regulators? I'm wondering if I should ...

Thanks for all your responses; I will post when I install the suggested cap.
Any sudden electrical event can produce a broad frequency spectrum. (From studying Fourier Transforms, it is clear that there is a sort-of inverse relationship, for "width", between the time domain and the frequency domain. i.e. Something that's abrupt in the time domain must be broad in the frequency domain.)

You are probably hearing the effect that a burst of mixed high frequencies has, after passing through your amplifier. RF can be rectified by semiconductor device junctions, causing a DC shift that can be amplified and end up at the output, where it might sound like a pop or crack, or even a thump, if it was caused by a HF 'burst' type of phenomenon. That's if you're lucky, since it can probably be effectively eliminated with filtering.

A worse problem, in my mind, might be 'regular' NON-bursty RF, because it might not be noticable-enough to make someone even realize there's a problem, so they'd track it down and fix it, as you are attempting to do, now. Instead, it might be insidiously affecting their amplifier's sound-quality, in more-subtle ways.

The moral of the story is: ALWAYS install RF filtering, on every input stage, at the very least. Some texts even recommend using an RF filter on every active input of every opamp, everywhere.

Also, in general it would be wise to remember that NO amplifier has only one set of input pins. That's because EVERY pin or connection is an input, including the output. So one might also need to worry about adding RF filtering, or extra filtering, for the power supply inputs, and maybe even for the output and/or the feedback loop, depending on how the RF is being propagated and received.
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