3PDT Switches vs Solid State/ COMS Switching.

[scuzzphut]

Hello Chaps 

I was wondering what the general consensus is on using IC based relays/switches :

http://www.jaycar.co.uk/images_uploaded/CD4051BC.PDF
http://www.mouser.com/ds/2/405/schs047g-126999.pdf

...... instead of 3PDT.

My thoughts are that these low cost ICs could shave a couple of quid off a build if paired with a SPDT switch and some control logic (or even a momentary pushbutton and a latching circuit)


I see lots of Millenium bypass stuff and just wondered why that hadn't been extended. I'm sure there's a good reason (signal degradation?) but I'd like to hear your views.

[GibsonGM]

It IS interesting, Scuzz.  I think that most don't like going down this avenue because no matter what you do, there will always be SOMETHING in the signal path that you aren't controlling.  A 3PDT is simply 'a wire', if you will....logic switches like this use transistors etc., which can color the tone.
 
Yes, much of that sentiment is just a 'mojo' thing, but people really feel that way!  I know that there WAS a whole lot of truth in it in years past - not so sure if the 'tone sucking' issues are still wholly relevant today...if one would accept a buffer, then one might be convinced that IC switching isn't so bad.

The only reason I personally don't get into IC-based switching is parts count and complexity...don't want to give up that real estate on my boards, and don't want to have to work up another power supply and so on.

[merlinb]

I think board space is the bigger issue here. One chip doesn't sound like much, but when you add all the coupling caps and bias resistors that you also need to make it work, suddenly your board is a lot bigger, compared with the DPDT+MOSFET+diode needed for Millennium bypass.

[Mark Hammer]

As much as I like DPDT and 3PDT for their simplicity, one of the interesting features of CMOS switches is that they can often permit unlatched switching.  THat is, the effect is only in the signal path for as long as you hold your foot down.  Lift your foot off the button/actuator, and you magically go back to bypass mode.  This permits for a variety of studio-type "punch-in" effects.

Consider, how many times do you want a ring modulator always one through the duration of a tune?  Perhaps it is absolutely essential for emphasis in a riff, and stepping twice (once to turn it on, and once again to turn it off) is a little too involved and distracting.  A momentary that lets you put your foot down, pick/strum the notes/chords in question, and then lift your foot off when done, is a far more fluid and less disruptive approach to switching.

And if the mechanical actuation of the CMOS switch is JUST a SPST, then that gives you the freedom to parallel a latching and non-latching footswitch, so you can use whichever you wish.  I've adapted an Echobase with two momentaries.  The basic design uses a 4066 to do the switching (Thanks! ), actuated by a SPST footswitch.  I paralleled a momentary.  If the unit is set to "tails", such that whatever is already in the delay path conitues to spool out, even after switching back to bypass, that gives me the option to step on the momentary, play a couple notes, lift my foot off and have the delay applied ONLY to those few notes and not the rest of what I'm playing.  As I like to joke, it lets you play the solo at the end of "Don't Stop Believing" properly.

So, don't rule out CMOS switching entirely.  If the effect is something you likely would simply leave on - e.g., an overdrive that stays on while you work the wah - then a mechanical bypass, via stompswitch or relay, is an obvious choice.  If the effect is something you can see using in snippets sometimes, then electronic switching can yield some creative and VERY funtional options.

NOTE, however, that I'm talking about CMOS-based switching that does not employ flip-flops to keep the switch in a given state, but rather CMOS switches that require a physical switch to remain open or closed.

[Seljer]

NOTE, however, that I'm talking about CMOS-based switching that does not employ flip-flops to keep the switch in a given state, but rather CMOS switches that require a physical switch to remain open or closed.

of course, even if you've built it with the flipflop, you're just a toggle switch away from being able to have either action of switching as an option

[amptramp]

I wrote up a blurb for myself (which hasn't seen the light of day here or anywhere else), but the main issues are here:

True bypass has some advantages and disadvantages as listed below:

Advantages

1.   Negligible signal distortion in bypassed mode
2.   Exact unity gain in bypassed mode
3.   Negligible frequency response deterioration, connected or bypassed
4.   Minimal noise addition
5.   Minimal interaction with effect circuit
6.   No power required for switch function
7.   No other circuitry required except switch pop reduction resistors

Disadvantages

1.   Expensive 3PDT alternate-action switch required
2.   Introduction of switch pops unless circuit modified
3.   Different behaviour depending on order of contact transfer
4.   Low reliability and life of switch
5.   Difficulty in adding remote switching capability
6.   Cannot be initialized on turn-on
7.   Advantages not necessarily applicable to delay functions

It is a 5-page Word document, so I do not necessarily want to write a novel and put it in a post, but these are the main issues.  I do have methods of getting around some of the disadvantages of electronic "untrue bypass" switching and I might consider sending it to someone like R.G. to see if he wants to include it in his site.

[Mark Hammer]

I think that's a fair summary.  More importantly, it lays out where the advantages and disadvantages lie, not all of which apply in every scenario, so it"s not an A wins and B loses declaration.  One OR the other will be more suitable for some folks in some contexts.

[amptramp]

I have seen one digital all-in-one effect (priced at $420 in a local store) that had a magnificently smooth 3PDT switch on it.  Then I tried out my own and got a relatively non-precision graunch ker krunch click feel from it.  If you can specify a very good switch with smooth repeatable action AND provide supplier quality assurance to make sure no unauthorized production changes are made, you could get an excellent product.  But the switch may get a bit pricey, into the tens of dollars.  There is a reason why Boss and other production manufacturers have added circuitry for SPST pushbutton use and braved the condemnation of true bypass believers - they want the units to outlive the warranty, maybe by enough time that the buyer would consider buying more of their stuff.

[R.G.]

AT's lists are pretty good.

Here are the biggies:
Pro: Hard switches are easier for beginners to understand.
Con: Hard switches all fail sooner or later, no matter how much you pay for them.

Soft Switches (the CD4053 being a good example)
Pro: Estimated lifetime longer than a human's.
Con: Complex, even if cheap and good performance.

On the original question:

I was wondering what the general consensus is on using IC based relays/switches
...... instead of 3PDT.

There is no general consensus.

My thoughts are that these low cost ICs could shave a couple of quid off a build if paired with a SPDT switch and some control logic (or even a momentary pushbutton and a latching circuit)

They can, and do. Um, have you read the geofex stuff on bypassing and CMOS switches?
The Technology of Bypassing and Switching: http://geofex.com/Article_Folders/bypass/bypass.htm
The Technology of Boss and Ibanez Bypassing: http://geofex.com/Article_Folders/bosstech.pdf
The Millenium C(ontinues): http://geofex.com/Article_Folders/Millenium/The%20Next%20Millenium.pdf
Relays for Audio Switching: http://geofex.com/Article_Folders/relays/relays_for_switching_audio_signa.htm
Millenium 2 Bypass Extensions: http://geofex.com/Article_Folders/Mill2extn/mil2plus.htm
Electronic Switching with the CD4053:http://geofex.com/Article_Folders/cd4053/cd4053.htm

I've left off the articles on remote bypassing, programmable bypassing, and so on, which use multiples of the above as basic elements.

I see lots of Millenium bypass stuff and just wondered why that hadn't been extended.

It has.

I'm sure there's a good reason (signal degradation?) but I'd like to hear your views.

The Millenium Bypass causes no signal degradation - it can't. It's never connected to the active portion of the signal path.

The active switch elements themselves - switch contacts, relay contacts, the *wires* to/from the switches, poor setup on semiconductor switches of various kinds, these can cause signal degradation, but it's independent of the method used to make the switch flip.

Give those a read and come back with questions.

[scuzzphut]

RG.  I have not read those articles.  Off to do so now.  Thanks for a great post 

[Mark Hammer]

Oo, oo, shameless self-promotion: http://www.youtube.com/watch?v=xszB3pAGrdw

Note that the same logic regarding "auxiliary" remote switches can be applied to CMOS-switches too.

[kingswayguitar]

Oo, oo, shameless self-promotion: http://www.youtube.com/watch?v=xszB3pAGrdw

Note that the same logic regarding "auxiliary" remote switches can be applied to CMOS-switches too.

omg thats awesome why didnt i think of that