A simple Relay Bypass

i'm guessing there isn't an equivalent of a CD40106 with less inverters in it in a smaller package? say, a 3x in a DIP-8 or even a dual in a DIP-6?
 
also, i'm sure this has been covered extensively elsewhere (so please feel free to just drop a link), but what is the advantage of using a relay over, say, a CD4053?
 
CD4053 is not a perfect switch. One has to consider on-resistance, off isolation, linearity, crosstalk and charge injection. They can and have been used successfully when applied correctly. They are more reliable than relays, although a relay does not significantly impact the reliability of a guitar pedal. Analog switches require more parts than a relay solution because they do not support true bypass. What Boss & Ibanez do with JFET switches could be done with a CD4053 or another CMOS switch.
 
Chuck D. Bones said:
In theory, yes. And it might work in some cases. Do you ever wonder why Boss & Ibanez put buffers before and after their FET switches?
Click to expand...
i remember reading about that on electric druid's page about designing a tubescreamer-style overdrive pedal, but re-reading it now all it really says is that the FET switches need isolating from the outside world. doesn't go into details.

i think the little SPDT diagrams on the CD4053 circuit i posted above also made me forget that they're FET switches rather than physical ones.
 
okay, so this is my existing bypass switching scheme, with a second tactile (small slide) switch to choose between true bypass or using the circuit's existing input buffer for a buffered bypass:

bypass.png

the 3PDT switch that i had previously been using has been replaced by the relay switches, with the cd40106 relay controller and power section off the page. what i was wondering about was your suggestion of using two of the other 40106 converters to drive another MOSFET switch, to cut the signal after the buffer. as it stands, in buffered bypass mode the signal goes through the whole pedal and only gets cut at the output. i'd ideally like to change this for two reasons: 1) i'm currently working on two relatively high-gain fuzz pedals with clipping LEDs that are incorporated into the pedal artwork, and i want them to stop lighting up when the pedal is in buffered bypassed mode; and 2) i've heard it's generally good practice to switch pedals off at the input, in order to keep noise to a minimum, especially in high-gain pedals.

so i attached a second set of what i assumed were the relevant switching parts of the 40106 to the momentary switch -- i assume the two capacitors are for debouncing the momentary switch, so they don't need to be copied again, but the 100k and two 10k resistors are part of the switching circuit. so that would look like this:

mosfet switch 2.png

(edit: oops! r9 should be 10k, not 100k)
please let me know if i've made any other mistakes here.

incorporating that into the bypass switching circuit i thought would look like this (part of the power section is cut off in the screenshot because it isn't really relevant to the discussion):

mosfet switch 1.png

but then i realized that although the MOSFET is isolated from the input jack by the input buffer, without an output buffer it's potentially exposed to the outside world through the output jack. so that means i could use the second half of the TL072 as an output buffer, and not have a buffered VREF, or else i could place the circuit disconnect switch one stage deeper into the circuit, which means sending the switch signal off the i/o board to drive a MOSFET on the main circuit board. which isn't a deal breaker either.

anyway please let me know if i've made any mistakes in my circuit or my thinking.
 
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okay, after reading r.g. keen's article on CD4053 switching, and particularly the section on using a CD4013 to drive it, i understand your CD40106 circuit much better (although i'm still not clear on the purpose of the 100n capacitor). moreover, the reason i was reading the article in the first place is because i realized that with an output buffer in my circuit, i could use an analog SPDT switch to not only cut the signal from going into the circuit, but also to ground the input of the circuit. which brought me to the CD4053 chip. may as well use one of the other switches on the chip instead of a BS170 to switch the ground on the LED and relay too. and since i can control both switches with a single control signal there's no need to duplicate the 40106 section. again, please let me know if any of my assumptions are false. but here's what it looks like right now:

cd4053.png
 
...and then i realized that if i just made one of my op amp buffers be a dedicated bypass buffer, i wouldn't need anything as complicated as the above...

...so, uh, yeah. good chat everyone!
 
to anyone who actually read through all of the above, i apologize, but if it's useful to anyone here is my design. it has input, output, DC jack, power conditioning, LED and switching on one board, gives the option of true or buffered bypass, grounds the pedal input in bypass mode and bypass buffer in true bypass mode, and leaves the other half of the TL072 available as either an input or VREF buffer:

dedicated bypass buffer.png.png
 
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