Mictester Relay Switching Scheme - Still have switch pop

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MullisMan
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Post by MullisMan »

I installed Mictesters relay bypass scheme into an effect that had a lot of popping with the 3PDT switches from Mammoth/4 site. I wanted to cut down on said popping using the relay. I have installed the scheme using a 40106 Schmitt Trigger and the Takamisawa AL5W-K relay but I am still getting a pop when I engage the momentary switch.

This leads me to believe that there is something wrong with my circuit. I'm leaning towards leaky caps in the circuit. When I used the 3PDT switch I had something like 2.5 volts sitting on the switch lug that comes from the PCB, which then connects to the output jack. That's more than enough DC offset to cause switching pop. I can bleed this voltage down with my multimeter, and when I box it up and plug the effect into my chain it's relatively quiet. But after a while, switching on and off causes the popping again. Does this sound like leaky caps? I'm using a charge pump to boost to 18v but i've eliminated that from being the problem. This circuit is also relatively low gain, think Timmy or TS.

Any suggestions? Thanks.

Jeremy
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Post by mictester »

Try high value bleed resistors (like you did with your multimeter). I usually use 2M2 at the input and 100k at the output. I get no "switch pop" whatsoever.
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MullisMan
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Post by MullisMan »

Thanks for the reply Mictester. I have a 1M pulldown resistor at the input and I tried putting a 2.2M resistor to ground at the output as well, no change, still have the popping. My circuit has a 1K resistor in series with the output as is, I had hoped that would take care of most of the problem.
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Post by mictester »

MullisMan wrote:Thanks for the reply Mictester. I have a 1M pulldown resistor at the input and I tried putting a 2.2M resistor to ground at the output as well, no change, still have the popping. My circuit has a 1K resistor in series with the output as is, I had hoped that would take care of most of the problem.
I hope you mean that the resistors are at the input and output of the actual effect circuit, NOT across the input and output socket!
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MullisMan
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Post by MullisMan »

Yes, sorry I wasn't clearer. The pulldown resistors are at the circuit input and output, not the input/output jack. Standard voltage divider to ground. I've tried to alleviate all the sources of popping, and i'm not sure where to turn to next.
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Post by Lucifer »

Try putting a cap across your LED to slow the inrush current which can cause pops.

It has been mentioned elsewhere on this site, so you should be able to find a suitable value - but if not, experiment.
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Post by MullisMan »

I tried Mictester's suggestion and Lucifer's; I placed pulldown resistors at the input and output of my circuit, and I also placed a cap across the LED junction to ground. This still didn't cure the pop, so I took the LED completely out of the circuit for the time being, and also took the charge pump section out of the circuit to see if that changed anything. I rewired my input/output jacks to see if I was forming some sort of ground loop with my PCB and that didn't help either. Still getting this switch pop and i'm going crazy. :evil: The inside of the box is looking like spaghetti from all the rewiring.

This popping doesn't just happen on one of my circuits either, it's happening on all of my builds except for a sweet honey OD clone i built for myself. I can hear a very very faint pop on that one as well if I really turn the volume up, that's with a 3PDT switch.

Any other suggestions?
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Post by vox_boy »

2.5V is crazy high, must be thudding pretty loud! I would definitely change the output capacitor. Especially if it is a Tant, I have had a few leaky ones myself :) I'm rather skeptical about adding the 2.2M on the output, surely this would affect the output Impedance? :hmmm: I would much rather have popping than high output Z! I'm hoping someone alot smarter than me can chime in about this.

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Post by Nocentelli »

vox_boy wrote:I'm rather skeptical about adding the 2.2M on the output, surely this would affect the output Impedance? :hmmm: I would much rather have popping than high output Z! I'm hoping someone alot smarter than me can chime in about this.
R.G. wrote:Let's think about it for a minute.

Pull down resistors are there to pull the leakage current of the input and output capacitors to ground, or so close to ground that you don't hear a click when switching. Feel free to use any value you like that makes pops go away. We've found empirically that values between 10milliohms and 10 meghoms are all pretty effective at stopping clicks, but most people use a good, solid run of the mill 1M since the lower values tend to reduce signal levels too much. For stopping switching clicks, smaller is better. One inch of copper wire is a good preventer of clicks. No signal, but no clicks.

To keep clicking from happening, pulldown resistors have to be significantly less than the leakage of the capacitors. How big is that? We don't know without measuring, because makers of caps only say "it's less than xxxx." Therefore, it's not possible to calculate a priori what the theoretically right value is without knowing the specific cap's leakage.

As to having a bearing on the output impedance of the effect, yes, a pulldown resistor appears in parallel with the Norton equivalent output impedance of the effect, so it lowers the output impedance. It lowers the output voltage as well. How much? You gotta know what the output impedance of the effect is before you put the pulldown on. Generally, the output impedance of the effect is down in the sub-50K region, so 1M and above is comfortably big enough not to load things down much.

And on the input, the same is true. The pulldown is in parallel with the input impedance, and acts like a load on whatever is driving the input. So the previous effect, if it has a pulldown as well, has to drive both its own output pulldown and the input pulldown of the next effect, in addition to the actual input impedance of the effect itself. So from the standpoint of preserving signal and not loading things down by changing the input and output impedances too much, bigger is better.

Having thought through this once or twice, most people sigh, and just use 1M. I like 2.2M. But be creative!!


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