need help getting rid of clicks, pops, and hum in design

In my experience, one source of these sounds is DC leaking through input or output caps and causing the two terminals that are to be connected together to be at different DC levels. The sudden connecting of these two terminals causes the DC conditions to suddenly change, which manifests itself as a loud thump or pop in the audio path.

"True bypass" pedals all exhibit this behavior to some extent, so using relays is going to cause the same thing. (You can find plenty of semi-baked solutions to the true bypass popping problem with a quick google search.) I guess this is why Boss and other manufacturers have stuck to their FET switching arrangements throughout the true bypass craze.

I'm yet to find the perfect solution, and I'm beginning to doubt there is one. As I'm about to embark on a similar project, I am very keen to see what others suggest.

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Hum could be coming from inadequate shielding and/or bad grounding,
or from power supply ripple getting into the circuitry.

Clicks and pops are the result of transients created when switching
is done abruptly.

Can you post a schematic of your circuitry?

Thanks for the replies.

I wondered about the DC leakage issue myself, and so have connected 2M resistors from hot to ground on each input and output, which (I think) would drain away any DC that got away from the caps. Now, these are on the in/out jacks, not on the pedals themselves. I don't know if that would make a difference.

Please feel free to ask questions about your project nifty. I'm really happy with my board. It can switch multiple fx and fx combinations in one stomp. It really is cool, and these pops are the only drawback (they're not terrible mind you, just kind of annoying and I want to get it perfect).

Thanks

craze.

Hi John;

Thanks for the reply. I'll post the schematic in blocks if that's OK. I don't have any schematic drawing software, and it's pretty extensive....

Thanks

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Here's the latest bug. I have these 2 2222s as replacements for relays to switch +15v from the amp to ground for amp channel switching and volume boost. With the relays I had in there before, there was a small click; now it is a big click and there's an annoying hum now that wasn't there before - only when the transistor switch is on (i.e., the 15v is grounded). Not quite sure what's going on with that. Oh one other thing - the stomp switch unit that came with the amp is totally clean and quiet.

Any ideas on this one?

Thanks

+15v from amp +15v from amp o o | | | | | | o o 1K | 1K | ___ |/ +5v control ___ |/ ----5v control -|___|------| ----|___|----o--| 2222 | | |> | | |>

| .-. | | .-. | | | | o | | | o | 10k| | | |

10k| | | | '-' | | '-' | | | | | | | | o----o---------------------|------------o----o | | | | | | | | | | === === | +5v | GND GND | | | (amp) | | | __ | | __ | -o| |o- | -o| |o- | -o| |o- |--o| |o- | -o| |o- ------o| |o-o | -o|__|o- | |---o|__|o-| | -o| |o- |-----o| |o-| | -o| |o- ||| -o| |o-o | -o| |o---||||--o| |o-| | -o|__|o----||| -o|__|o-o | || | | | 74hc373 |o--o4049 | | | | | | | o-----o | | | | | === | | GND | | | | | -----------------------------------------------------|

(created by AACircuit v1.28.6 beta 04/19/05

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Ideally, you should get a hold of the schematic for your amp. Have you tested the factory switch with a multimeter to be sure of what it's doing? Are all grounds connected together? How are you sourcing the 5VDC for the logic?

Generally a simple transistor switch just has a 10k from the control voltage in series with the base, no resistor to ground needed.

Are you aware of diystompboxes.com? They have travelled this road many times before, a quick search of the archive might prove fruitful.

nifty

Thanks Nifty

I do have the schem for the amp, and I know the switch is DPST controlling LEDs and the switching to ground. The 10k from base to ground is there to ensure a definite logic state in the absence of a control voltage. The 5VDC for the logic is coming from the PS to an octal latch, through and inverter and onto the transistor.

I'll check that link too.

This is troubling me, tell me if I'm crazy. I think I'm misunderstanding the amps switching arrangement. What is the amp, BTW?

The original switch has LEDs in it right? And it's only power source is the 15V from the amp? If so, it can't be directly switching the 15V to ground AND powering the LEDs at the same time right, it's gotta maintain that 15V while it switches the control lines. Your switching arrangement with the transistor is attempting to drag the 15V down to the saturation voltage of the transistor, the way it's depicted in the schem you posted.

So it's sourced from a seperate power supply? Is it's ground connected to the amps ground?

The amp is a Carvin X100B. I'll try to explain as best I can. The amp has 3 switching possibilities - reverb, channel, and volume boost. I leave the rev on all the time, so I just left it unconnected. All the functions are switched by connecting +15v (from the amp) to ground. So to activate the lead channel, there is a phone plug with +15v on the tip, and the amp's ground on the sleeve. When I stomp a switch (from my pedalboard or the supplied footswitch), the 15v is connected directly to ground, thus switching the effect in or out. The LEDs are also controlled by the same DPST switch. There is a 1k resistor for each LED in the amp, and 15v travels through it to groun when activated, or just floats when it isn't. So in the case of the suppled footswitch, the DPST has 1 pole switching the amp's 15v for the fx switching, and the other pole switches the +!5v for the LED.

Pretty nifty (no pun intended) eh?

inverter

Is this your amps schem?

That seems quite different to what you're describing.

No, this is a later revision of my amp. I can send you the schem if you're interested.

Here's some other info that may or may not be helpful here. I disconnected all the relays and LEDs last nite and there was no detectable noise from switching (obviously I couldn't hear this directly because the relays weren't actually switching, but I have an NS2 that has an LED that displays when the noise gate is active. As soon as any sound goes through the gate the LED goes off, and it stayed lit the whole time). With the pops that I was getting from the transistor switches, I was starting to wonder if they may have been originating somewhere in the switching logic, or maybe the power supply, but that is looking less likely.

You mentioned the 10k resistor to ground at the base as being possibly problematic. Do you think that this could be the cause of either the switching noise or the hum?

Thanks again

Woops sorry nifty I just noticed the rest of this message.

" Your switching arrangement

Yes, if I understand you correctly. Is that what it should be doing?

inverter

Yes. The 5vDC is coming from the pedalboard's PS, the 15v comes directly from the amp. Its ground is connected to the amp's ground, by necessity. I tried like mad to figure out a way to keep the grounds isolated, but it's impossible without doing some revisions to the amp. The amp's ground will be connected to the pedalboard's ground as soon as I connect the pedalboard out to the amp in. This wasn't a problem before I replaced the relays with the transistors, btw.

Thanks

Here's what my relay to pedal connections look like. Is there anything here that may be contributing to pops?

Thanks

Relay to pedal Connections

FX1 FX2 FX3 FX4

Gtr in--------------In Out In Out In Out In Out | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | o /o ---------o /o --------o /o --------o /o / | / | / | / Relay1 / | Relay2 / |Relay3 / |Relay4 / o | o | o | o | | | | | | | | | | | | --------------- Amp out | | | | | | --------| -------| ------|

(created by AACircuit v1.28.6 beta 04/19/05

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The first thing I'd say is that it's considered bad practice to have the inputs of the effects constantly in the chain. Even when an effect is bypassed it's still in the circuit and affecting impedances. The pops you're hearing is due to the slight difference in DC conditions between the input and the output of an effect, not because of the mechanical switching. However, even with a true bypass arrangement, the pops are still a problem, so...

As for the DC leakage resistors, trial and error works for me, I haven't sat down and worked out exactly what the solution is, but different value resistors work for different FX pedals.

To the amp channel switching; The transistors can switch very quickly, in fact faster than the relays. You might try slowing down the rise time of the control signal with an RC network, perhaps try replacing the 10k from base to ground with a largish capacitor so the transistor turns on relatively slowly. I don't think the 10k is to blame, but while you're mucking around, try taking it out of circuit.

Can you post voltage readings of the transistors when switched on and off? That might give us something to go on.

With the relay amp channel switching, the 5V power supply is mechanically isolated from the amps 15V. With the transitors, it's not, it's coupled via the base/collector junction. This might be something to look at, just a stab in the dark.

Thanks nifty;

Good suggestions. The inputs in the chain (although considered bad practice), is, surprisingly, not an issue (unless it is in fact the reason for the pops). I tested this thoroughly before even beginning to build the unit, and there is no audible difference between the effects unit in bypass (i.e., all inputs connected and in the chain) and the guitar plugged directly into the amp. I think this is one of those "tone sucking" paranoias that has been blown out of proportion. I myself was leery of setting things up this way too, and my bass player, who is something of an electronics whiz, also thinks that this would affect the tone, but I can hear no difference. I have considered switching the relays to DPDT and going true bypass, but I thought I'd give the analog switch route a go first. Of course, I'd like to ascertain where the pops are coming from before I do any more work (this is cutting into my practice time too much already!).

So you're suggesting putting resistors across the input and output caps of each effect right? I do have 2M resistors across each in/out JACK where the pedals connect to the board. Is this the same thing or do they need to be at the pedal itself?

I have thought of the RC thing also, and have tried something, but I've been messing around so much I can't remember where I put the cap. I'm pretty sure I tried one from base to collector (got that idea from the geofex site) but it did nothing. I don't see any reason why the transistors should be causing such a problem, but then I don't know all there is to know either. I still marvel that the actual switch that came with the amp is so dead quiet when it switches, and I would've thought the transistors would be as quiet, but...

Your point about the isolation of PS voltages when using relays is also well-taken. I'm sure that the hum problem is a result of that, and discovered the ground loop I was creating last night. The grounds of the amp and pedalboard are going to be connected no matter what, because as soon as I plug the pedalboard into the amp, or the plug to control amp functions, I've connected the grounds. That being said, When I used the relays, there was only 1 connection to the amp's ground - from the output of the pedalboard to the amp's input. The amp control functions was isolated. With the transistors, though, the amp control function ground is connected directly to the pedalboard ground, and I've got a loop. If I keep the control function ground floating, the hum is gone, but it only works if I have the pedalboard out connected to the amp's input, which is a problem for some of the things I do. I'm going to try a 100 ohm resistor between the 2 grounds and see if that solves that problem. If it doesn't I may have to go back to relays.

There is one other thing I should mention. I am using the relay's coil as a resistor for an LED indicator light for each effect. So when the relay engages, it also pulls the LED on. I wonder if that might be causing some of the clicks as well, or contributing.

Do you want me to post any more schems of my wiring? I think I have ruled out the logic section of the board as causing any problems, so I'm going to concentrate on the functional side.

Thanks again

I think you can rule out the logic too, I'm most sus about the power supplies interacting via the transistors (possibly causing the hum) and the DC leakage issue (pops). The only thing I would say is, perhaps you should decouple the logic chips power supply. Small value caps placed as close as possible to the logic chips power pins help reduce the transient loads on the supply.

If you're cool with the non true bypass, that's all that matters. I was just making the point in case you weren't aware of the whole debate.

I agree. True bypass seems to cause more problems than it solves. In the end, there's buffering and FET switching going on inside of most guitar amplifiers and nobody is worried about that! But, use it in a pedal, even after a high quality buffer, and it's the devil incarnate!

I make it a point to suggest to these people that perhaps in all this searching for "transparency" they're forgetting that one good buffer up front might actually IMPROVE their tone. They seem to think that any change in tone from a direct connection between guitar and amp is bad.

For instance, the buffer I use has a 10M input impedance for passive basses. Compared to a direct connection to my amp my basses have much more top end and punch.

Provided they're always in circuit with the pedal I don't see a problem with that. As I said, trial a few different values.

Try that cap from base to emitter (in place of the 10k) leaving the 1k in place.

LEDs have been suspected in the past of doing strange things like that. The fix is the same as what we're talking about with the RC time constant creating a slow turn on.

FX guru RG Keen's article has an excellent discussion of some of these phenomena.

Another article I stumbled across the other day says it more concisely

keep me posted on progress!

nifty

Hi Nifty;

Thanks again for your reply. I just tried a 3.3uF cap to ground, with no improvement. Maybe tomorrow I'll try a 10 and see if that helps. I do have decoupling caps at the logic chips (not all of them mind you - they are pretty close together on the board and I would think that 1 set of decouplers would handle a couple ICs - correct me if I'm wrong).

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Well, some people are pretty strict about it, but I'm just thinking it's one less thing to worry about if they're all over the place!

You know how to calculate the RC time constant?

time (to reach 63% of full charge) = R x C

So, a 1k resistor and a 3.3µF cap is only 3.3ms, but it starts to turn on the transistor once it gets past 0.6V which is only 12% of 5V. You would start to notice the delay at around 10ms so you might be able to go as high as 47µF before it's too noticeable. (alternatively, bump the

1k up to a 10k and use a smaller cap.)

nifty

Hmmm...

I think this transistor idea may have been ill-conceived. I still can't get rid of the ground loop hum, and there is still a pop everytime I switch. However, it is only in one direction, if that makes a difference. Switching from float to ground is quiet, but the other way round gives a loud and clear pop.

May have to just switch this one back to relays...