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!
 
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.
 
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?
 
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:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=118097

Please let us know if that makes a difference.
 
gootee said:
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:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=118097

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.

Nordic said:
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.
 
meaghers said:



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.
 
Hi, yes I am in the habit of overheatsinking everything...
Got a standard version in a chipamp I made for my mother in law, got some nice panasonic caps in there... I think 2x2200uf per rail, and it sounds decent enough... Oh and using onlu 10nf wima caps as decoupling caps at power pins.

Still not anywhere near as lively soundig as pedja rogic's jfet buffer, but nothing that jumps out inyour face and says, I am horrible...

Personaly, I am useing only passive volume control now... with my amps adjusted with enough gain for my sources.
 
After looking closer, I believe Rod included provisions to include the RF filter caps that we are speaking of on the board. However, he mentions this:

"The schematic no longer includes 100 pF ceramic capacitors across the opamp inputs, and while the PCB has allowance for them, they are optional. They are needed if high level RF interference is a problem in your area. In general, I recommend that they are not used, because they can lead to instability in U1"

What is he speaking of when he says "instability" in the opamp? Should i be concerned about this?

In any case, this is an easy thing to try as there are spots on the pcb for these caps already and I could always remove them if needed. I will see if I can find time to do this later today.
 
You say you used a wooden enclosure? Wood provides limited shielding against squirrels and birds, but I question it's effectiveness against RF and termites. An RF filter on the input may help, but I think you should consider a metal box, followed by some power supply filtering. You can flip lights and such on and off right next to my preamp, and on the same circuit, with no audible clicks or pops at all, and I don't even have an input filter. I do, however, have quite a lot of filtering on the incoming AC line- a Corcom line filter, a clamp on the cord, capacitor bypasses on the rectifiers, and a carefully optimized ground system.
 
meaghers said:
After looking closer, I believe Rod included provisions to include the RF filter caps that we are speaking of on the board. However, he mentions this:

"The schematic no longer includes 100 pF ceramic capacitors across the opamp inputs, and while the PCB has allowance for them, they are optional. They are needed if high level RF interference is a problem in your area. In general, I recommend that they are not used, because they can lead to instability in U1"

What is he speaking of when he says "instability" in the opamp? Should i be concerned about this?

In any case, this is an easy thing to try as there are spots on the pcb for these caps already and I could always remove them if needed. I will see if I can find time to do this later today.

You could try those. But that is not the same as what I meant.

Those would be between the two opamp input pins, whereas what I was suggesting would be between the positive input pins and ground.
 
I've built my latest pre amp with Rods P05 and P97 boards and this combination is the most quiet I have heard . You went cheap with using a "wall wart" supply and that is the best you will get.

Install a line filter,"Corcom", feeding a transformer of the proper size and the results will be amazing to you. There is nothing wrong with Rods boards or designs but the implementation of these will make or break the performance of your pre amp.