switching clicks in pedalboard problem - solved!! but why??

WOO! Bet you're happy with yourself after all that. Sounds like the buffer is helping with the DC offsets. Could be that you've lucked out with an op amp that has very low offset. Have you got a series cap in that buffer circuit?

It's really hard to say from afar whether the pickups impedance is part of the problem, but whatever the cause was, the problem must have been caused by switching DC somewhere. Maybe you can compare the buffered vs. non buffered and take some DC voltage readings through the circuit to try and ascertain where it was getting into the relays etc.

niftydog

guitar

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virtually no

Hey good to hear from you again nifty. Actually, (as has happened with other apparent solutions in the past) the system is becoming clicky again. At first there were very few times when it clicked, but now (after only a day or so) it's clicking a lot again. Oh and yes, there are 2 series caps in the buffer (which actually has a gain of 2) - 1 on the in and 1 on the out.

How did your pedalboard project go?

This may have been mentioned before, so bear with me.

Most of the audio amplifiers that I know of have a DC blocking cap at their input (and some even have such at their output). These blocking caps are usually electrolytic, and they usually have some leakage. As a consequence, if you leave the the input open-circuited for any length of time the cap will discharge, leaving a (sometimes substantial) DC offset on the input. When you connect this to an output that has some _other_ offset, a click or pop results.

I would choose some DC reference (ground, maybe?) and put a substantial resistor to it at the output of each switch. Then I'd make sure that that low-impedance buffer from your guitar has its own DC blocking cap that's terminated to the same reference (or I'd build the amplifier to make sure that it naturally centers around that value). Assuming that I sized my resistors right, this would reduce or eliminate clicks from this source.

You can check that this is the problem by dancing on the selector switches: if you're going rapidly from one input to another, they should all tend to go to the same DC value. So if you start exercising your foot in this way and the clicks go away, then you purposely let an effect sit there, turned on but idle, then press the switch and hear a 'pop' -- that's probably your problem.

Good luck -- elusive problems are a bitch.

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It didn't... at least, it didn't go very far. It is getting there slowly, but I think it's more of a winter project because I'm spending all summer fixing my house!

What Tim said is a more detailed explanation of what I was suggesting ages ago. Part of the trick is working out what size resistors are "right"!

I wonder if what you've experienced might have something to do with op amp offset. Look into offset compensation of op amps, often something about it in the datasheet, try to minimise the DC at the op amp output. That in turn should lessen the DC going into the switch network. More buffers on the output might also help, although I'm a bit fuzzy on your circuit specifics now!

niftydog

guitar

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virtually no

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Thanks for the reply Tim.

This has been mentioned before, and I have checked it, but just did again and things are indeed different now. Let me fill you in on what I've done so far. The guitar goes into the pedalboard through a 1/4" jack. This is connected internally to the buffer, which then goes to the 1st output jack on the pedalboard, which connects to the 1st effect input via a 4" cable. The 2nd jack on the board is the output to the 1st effect. I am switching effects using relays. So using this example, the 1st jack (input to 1st effect) is connected to the relay. So if that effect is selected (and thus in the signal path) the relay switches to the output of that effect (2nd jack on the board). So the relay is a SPDT, and switches between the input jack (if bypassed) or ouput jack (if selected). This is repeated for 4 other pedals. Because I used SPDT relays, all the inputs are connected when all the effects are bypassed.

Each of the input/output jacks on the board have a 1.5M resistor from hot to ground. I was hoping that this would be enough to get rid of any DC leakage from the pedals themselves. However, there is no resistor at the input jack on the board from the guitar. Perhaps I should have one here? Also, there is a 5th pedal which is always in the signal chain, and is not connected to any relays. The output of that effect is my out to the amp. When I measure the DC there (not connected to the amp yet), it is over 100mV, then dies down to around 0.5mV. Also, when I connect a cable to the input jack for the guitar (on the pedalboard), there is about 10 mV, which gradually dies down to about 0.8mV This is just connected to the pedalboard; not connected to the guitar on the other end. When I measure DC on any of the jacks from the pedals themselves (through the 4" cable), there is about 0.3 - 0.8mV on any given jack. So I am getting a large difference on the initial input and the final output as compared to the DC on the jacks that are connected to the inner workings of the board.

I hope that is fairly clear. If not, maybe I can post a block diagram or something. A few questions: What is a suitable value for the resistors? I'm assuming the smaller the better, but I'm also assuming there's a point at which the value will be too small and degrade the signal. Is 1.5M too high? How low should I go? I believe the input Z of the pedals ranges from 470K -

1M, depending on the pedal. Also, is the location of the resistors now (on the jacks themselves, about 4" from the actual pedal) suitable, or should it be inside the pedal itself? Oh, and another point of interest - according to my amp's schem, there are no input blocking caps. I haven't poked around inside there for a while, so I don't know if there actually are any, but according to the schematic there are not.

Thanks again.

"right"!

Hey Nifty;

Thanks again for the input. I replied in some detail concerning my findings tonite. What range of values should I be trying? Lower is better, I assume, but how low can I go before the signal starts to putz out?

I know how you feel with the pedalboard project - hard to find time for the really important jobs.....

I really don't have that answer as I don't fully understand the reason why one value is better than another. I'm not sure ANYONE knows to be honest! I beleive there's some info about this on the general guitar gadgets website though.

nifty

I'm not sure if this has come up but the other guy with the audio clicking problem seems like it might be the same issue. Essentially you can't just switch the audio from one state to the other beause the possibility of creating a step which results in an audiable click(because of the inertia of the speaker and it changing in amplitude almost instantaneous(w.r.t to how it generally behaves) you get the gibbs phenomena which you hear as ringing(or a click since it dies very fast).

So if it is the case, which probably can be tested if only it clicks when there is actually audio being heard(+ no DC offset)) then there are many methods to fix it. Essentially you need to make sure you actually switch only when the audio is 0. i.e., you can put a compariator between the audio and ground that checks for 0 dc and use it to switch the circuit.

eg., Compariator Audio ---- |\\ | >---- |\\ Ground ---|/ | \\ | / ------- Transistor Switch -----------|/ Inverter

The switch is whatever your using to signal a switch., It needs to be held long enough for the comparision to work and probably needs some feedback to be reset(so you could use some sample and hold which is reset from the inverter)

the transistor is the thing that actually switches the audio channels.

I'm sure you can figure out the circuit or find it online(I wouldn't use what I have above as I'm sure it won't work but just an outline for something that might work). Theres probably IC's that do this too along with other methods. My guess is that this is your problem as it seems to be overlooked quite a bit. (from my experience but I'm no expert)

You could just actually fade from one to the other. There might be easy ways to do this but your simply taking a*A + (1 - a)*B (a < 1 so you don't get some phase change in some cases).

Jon

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Thanks for the suggestion Jon. I think there must be a DC problem somewhere, because the clicks are there even when the volume is at 0 on the guitar, or when I'm not playing anything (unless it's clicking because of hum and noise).

I'd love to fade from one to the other though. I think that would be the most foolproof way to do it. Unfortunately, I don't know how.....

Thanks

Given the thing you're starting from, a motorized cross-fade pot sounds about right (ie, change from switching "one or more relays" to switching on one or more motorized cross-faders).

The less expensive, less complex approach would be to mechanically couple a normal cross-fader to the pedal, and do the fade with the foot pedal, rather than spending for motors to do it.

Here's another idea:

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Well, you can probably solve the DC problem(or atleast remove it from the equation) by using some coupling capacitors. Its just hard to know the exact problem(Specially since I've not messed with this kinda stuff but). Obviously though the clicks occur at the point of switching and so you probably should start there and make 100% its not from that. You will definately(well, probably ;) get clicks if you do not do the zero axis switching thing I mentioned as this is pretty much a mathematical issue(the op amp could have such a small slew rate that it smooths it out though).

Maybe a way to see if its the audio switching problem I mention or not is simply to make sure the audio on the inputs is 0. What you could do is use a computer and its soundcard to send and record the input and output and then you'll know for sure. You could also test its frequency response by this(there are some wave files online that are made for this. Note that the soundcard itself can cause issues with the frequency so you won't get exactly what you put in).

You just have to spend some time trouble shooting. Obviously if it only clicks when you switch then it has something to do with the switching.

Oh, I just through that it could be inductor kickback too? You should look up this online since I'll do a poor job of explaining it. Its a simple solution and I have no idea if its the cause but its worth a look.

The main problem I see if that you might have both issues going on and you might try and fix one but not realize it fixed it because of the other(although they probably sound different).

I think the first step in trouble shooting is to remove all the unnecessary complications such as the guitar, effects, amp, etc... just work with the barebones components that you know/think where the issue is comming from. This is obviously at the switching. If you remove the guitar and ground the inputs and you still get clicking then it would be very strange indeed. Probably means you have some DC components or really messed something up. If you then use a different source to generate the audio(such as a radio) and it doesn't click then its something to do with the interaction of the guitar and the switching. If it clicks then obviously its probably not the guitar but the switching itself.

Anyways, I'm drawing this out but from my little knowledge I think your 3 main possible causes are DC biases, non-zero axis switching, and/or inductor kickback. I'd work through them in that order. Note that you will have to implement zero axis switching no matter what. If you don't you will always have clicking(unless you get lucky every time you switch). This actually might be the first one to do and fix since it removes that issue from the equation.

Well, essentially you have a summing op amp configuration with variable resistors.

Mathematically the equation is aA + (1-a)B where A and B are your waveforms.

a is your resitance. You know how to amplify and you know how to sum so you just put these together.

ofcourse a must be a variable resister and this is your main issue. (there are digital resistors that might work and would be easily controlled). There might be other ways too do this easy but I'm just giving you the basic idea as I've never actually done anything like this. It should be all that hard but might not be all that easy if you run into some more issues. Crossfading is pretty common so I'm sure you can find a schematic online or even a chip that will do this automatically.

Not that essentially its just as simple as the equation above though. Just two op amp's configured in some amplifying way(some methods mihgt be better than others). This is so you can mix the two signals at different strengths. You need to actually sum them to mix them though.

This solves the non-zero axis switchign problem and also is independent of DC bias and probably inductor kickback too. So this might be the best method to go with since it takes out a whole load of possible issues. Although I think to do it right is more complicated since it could introduce a whole new set of issues(although I can't see any off the top of my head).

Anyways, hope you find the solution. Keep me posted as I'd like to know too cause I also play guitar and will probably do some effects later on and might run into the same issues.

Jon

Jon makes some interesting points, but from my experience the DC bias is the major cause here. tempus fugit already has DC blocking caps installed in places, but all caps have leakage, and that's where the problem lies.

Cross fading is a really nice idea but it quickly becomes very complex in a guitar effects rig. Cross fading is fine for when you've got two sources and one destination, but in this context you have ONE source with several different paths to the one destination - it can become a nightmare. Ideally you want to switch (or fade) both the input AND the output of each effect individually and without a noticeable transition.

Problems arise when you realise that you might have three effects in series. Your precious, weak guitar signal has to pass through three cross fade circuits and three effects circuits. The buffers in the effects pedals are far from ideal, and so if you're not careful you start to loose the definition of the guitars sound - even if it's well buffered at the beginning.

There's ways around the complexity issue, like feeding the guitar into a splitter and permanently connecting it to all the effects inputs. But then along comes crosstalk to ruin your day... :(

It's a big problem that's crying out for the silver bullet solution. Guitarists are sometimes a finicky bunch, and while there's plenty of engineers with solutions, there's always an Achilles heel. Even the expensive pro gear I own with mechanical switching suffers from this same DC popping problem.

niftydog

There have been a fair number of auto fade circuits published over the last three decades. Instead of hard switching they fade up, or down, in a fraction of a second to mask any switching noise.

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Michael A. Terrell
Central Florida

I think that tempus would want to do a long fade so there is a very smooth transition. I'm not sure about his setup but if he has multiple effects would would be nice to be able to mix these at any level and also to fade in between them (essentially a volumn pedal) with a variable time transition.

The reason is, say, if your playing with an effect that is using delay and then you switch to an effect using distortion then it will be completely obvious. If you can fade between the two from a very fast(so that its almost instaneous like you mention above) to something much slower then you have a new "effect" to play with. What would be even cooler is to assign an effect to control the fading delay such as a VCO so you can fade between two(or more) effects to get a whole new realm of sounds. Ofcousre this would essentially be a automated mixer and would need some logic such as a microprocessor + some interface to setup the stuff. Not sure if theres anything like this on the market though. I suppose ultimately it would be easier to accomplish this in software but ADC is not a good thing when dealing with musical purists.

It is a compromise between speed and noise. Some fade both the input and output lines to ground at the zero output point, but in the OLD days transformer coupling avoided the problem. Another way is to use op amps with a split supply so that there is no DC offset to require caps. Switching can even be mechanical, if the inputs and outputs are properly buffered. This is common in commercial audio gear for broadcast work.

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Michael A. Terrell
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Michael A. Terrell wrote: > There have been a fair number of auto fade circuits published over > the last three decades.

I'm sure there have, but few if any designed for guitar rigs such as the ones tempus fugit and I are building. I'd love to see some links if you have any in mind.

Again, guitarists are sometimes difficult to please in this area. Google "true bypass" to see the arguments about tonal purity and you'll get a feel for what I mean!

J> If you can fade between the two from a very fast(so that its almost

I agree that it'd be nice, but you'd want to be able to choose to do a slow fade or an "instant" fade. After all, a sudden change in sounds can ALSO be an effect, and instant switching is what comes on all effects pedals.

This is exactly what I was talking about, it starts sucking you into this big black-hole of circuit complexity, when all you wanted to do in the first instance was turn something on/off!

niftydog

DC offset is always present within the effects themselves because 99.9% of them all run on single supplies. You've still got to deal with that at some point in the circuit.

Buffering every in and out adds to the complexity again, most don't want to put that much time, effort and money into a project like this. Especially given that many who embark on such a project are electronics amateurs. You start with a few simple DPDT foot switches and end up with a microprocessor controlled, DSP driven, GPS equipped nightmare!

niftydog

have a

Right you are nifty. I am not interested in creating any new fx - I just want to switch things on or off. That's one of the beauties of my board - it's set up to switch any combination of fx (that I pre-wired of course) to any other combination of fx. Cross fading probably isnt an option anyway, and if it were, I would use something far simpler, like using a JFET to switch one of the fx to ground, thus removing it from the chain without having to run the signal through any other devices. I discovered this little trick in the same way I discovered that buffering helped (for a while anyway). I just installed a piezo bridge in my Explorer to give me acoustic sounds. I used an analog switch (MAX4622 I think) and connected both input and output to the pole of the switch. The other end of the switch was connected to ground, so when I switched the piezo, say, it was instantly and quietly muted. Curiously, it was not quiet when I tried to switch the magnetic pickups to ground. It was at that point that I put the magnetic pickups thru the buffer amp, and lo and behold, it was quiet, and the piezo was not anymore.

As for the DC, what do you think about the high DC levels on the nodes I mentioned? Remember that neither the input to the pedalboard or the output to the amp was actually connected to anything, it was just a measurement I took at the end of the cable I had plugged in. I wonder if this is some floating DC that would disappear when the cable was actually plugged into its destination. Should I put a resistor at the input? If there is DC there, I don't get how it could be, since the intial entry into the pedalboard is straight from the pickups from the guitar. As for the output, I suppose that could be from a leaky cap in the final effect, but this effect does not switch in and out of the chain, it's always in it. As or the non zero signal switching noise, I'm not too worried about that, as the signal is so much louder than any clicks as to be pretty much unnoticeable. What I really need to get rid of is the clicks that occur when I'm switching to different patches in quiet parts of songs, where I'm not playing for a second or 2. This I presume is back to the DC problem, unless, as I mentioned before, it's coming from the low grade hum and noise that is intrinsic to my pickups/amp.

Thanks again

That is where the buffers come in. Four op amps in a single packageand dierectly couipled to the input or output caps. Since there is no switching the single suppy circuits, the problem goes away. Even better would be to modify the effects unit to a split supply

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Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida