Grounding IN & OUT

[antonis]

Good evening (local time) from Greece..!!

I want to connect many effects in a singe case (pedal board) using true bypass w LED via 3PDT shiwitces for each effect (including buffer & booster stages) and grounding IN & OUT when bypassed..



I've 3 questions about the above wiring:

1. Am I missing something which could result in a malfunction..??

2. Should I use a small value resistor (100Ω or any other suggested value) from outer plates of coupling capacitors (PCB's IN & OUT) to GND or just use a jumper..??

3. If 1. & 2. is OK, propably there isn't need for pull down resistors...

[antonis]

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[GGBB]

If you are trying to include an image, you need to host the image somewhere on the net like photobucket and then provide a link to it here or use [img] tags to show the picture in your post.

[antonis]

If you are trying to include an image, you need to host the image somewhere on the net like photobucket and then provide a link to it here or use [img] tags to show the picture in your post.

Tnx GGBB...

(i was trying to upload the image directly of my PC..)

[antonis]

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[GGBB]

I'm not an expert, but two hings stand out to me as either a potential problem or a non-necessity.

In bypass mode, you are connecting the circuit input and output together and then connecting to ground through resistance.  I expect that might cause the circuit to feedback or self-oscillate (depends on the resistor value and the circuit) which could introduce noise into the bypass signal or cause a noise blip when you take the circuit out of bypass.  Also, many circuits already have a large pull-down resistor from circuit input to ground for the purpose of preventing switch pop from the input capacitor when coming out of bypass when the circuit input is not grounded.  Adding another in parallel when bypassed seems redundant.  Some pedals also have a pull-down resistor on one side of the output cap - often it is the volume pot.

I think you are better off just directly grounding the circuit input and output:

[antonis]

many circuits already have a large pull-down resistor from circuit input to ground for the purpose of preventing switch pop from the input capacitor when coming out of bypass when the circuit input is not grounded.  Adding another in parallel when bypassed seems redundant.

My 3rd question was about ommiting the pull down resistor..

I think you are better off just directly grounding the circuit input and output:

You are my man..!!!

P.S.
So there is no need for pull down resistors (and not getting lower the input impendance of the circuit..)

[GGBB]

So there is no need for pull down resistors (and not getting lower the input impendance of the circuit..)

I always include them.  Since they are typically 1M or higher, they rarely would have enough of an impact on the input impedance to be a concern.  But every circuit is different - though you can always go to 2M2 or even higher if you really want to be sure.

[antonis]

I always include them. 

Is there any particular reason for doing this..??
(propably not for reducing the  loading of an ouput stage emitter follower buffer at low level signal because you should use much more lower resistor values...)


P.S.
Sorry for insisting but I like to understand why I should include an "useless" item in my design..

[GGBB]

I always include them. 

Is there any particular reason for doing this..??
(propably not for reducing the  loading of an ouput stage emitter follower buffer at low level signal because you should use much more lower resistor values...)


P.S.
Sorry for insisting but I like to understand why I should include an "useless" item in my design..

No need to apologize - this is the whole purpose of this forum.  I asked pretty much the same question here a few years ago.

As I understand it (there are far more qualified folks here)...

With the typical input capacitor, in theory, when the circuit is bypassed such that the input end of the capacitor gets shunted to ground, the capacitor discharges and that point is at 0V.  So when you engage the circuit, there is no voltage offset jump caused when the circuit is inserted into the signal chain.  If there was any charge in the capacitor, then there would be a sudden voltage change in the signal when the circuit is engaged, which is heard as an audible switching pop.  The pull-down resistor allows the capacitor to discharge - albeit less quickly - so that it isn't necessary to ground the circuit input when bypassing the circuit.

An output cap and pull-down resistor would function in the same manner, although in many pedals it is the volume pot that functions as the pull-down resistor.  You don't often see bypass switching that grounds the circuit output - I wonder if that is because it isn't necessary or because most DIY pedal designs originated before affordable 3PDT stomp switches were around (and some before DPDT).  Switching capabilities may also be the reason for the use of input pull-down resistors as well.  Early pedals would have probably had an SPDT that switched the output jack between the input jack and the circuit output (not true bypass and no LED).  An output volume pot would take care of the output cap, the input cap would never get disconnected.  DPDT could either do non-true bypass with an indicator LED or true bypass switching where one end would need a pull-down and that would usually be the output pot.  I suppose that circuits with no vol pot on the output might implement true bypass by grounding the output and using a pull-down on the input.  Or there might be other reasons - the RAT2 added an indicator LED to the original RAT using DPDT and a millennium bypass type of LED circuit which required the pedal circuit output to be connected to the LED circuit when bypassed to turn it off, and therefore couldn't ground the input so it had to use an input pull-down which was not used in the original RAT.  I'm just guessing, but for one reason or another, the input pull-down became commonplace - probably for pop-eliminating reasons.

So it may seem redundant, but nothing is perfect, so sometimes redundancy can be a good thing.  In the off chance that the bypass switch fails on the connection to ground only, you are still protected from pops (but there are other causes for pops).  In any case resistors are cheap, and they don't cause any problems, so if there's room on the PCB, why not.

[antonis]

Thanks for everything GGBB...!!!
(I mean it..!!! - allthough I knew allmost everything that you mentioned..)

I can imagine another reason for excisting pull down resistors to a true bypass system with In & Out grouding and this is that they "do something" during the "dead" time between switch contacts be effectivelly activated (heavy contact) to one or other side...
(but propably is just my imagination..)

[shrubbery]

Interesting stuff.

Can I wire DPDT to avoid noises you mention?  I don't care much for pops that are not very loud.  Is it enough to ground output or I need to do millenium bypass or something simpler (I don't want a LED)?

[GGBB]

Interesting stuff.

Can I wire DPDT to avoid noises you mention?  I don't care much for pops that are not very loud.  Is it enough to ground output or I need to do millenium bypass or something simpler (I don't want a LED)?

Diagnosing and eliminating switching pops could be its own forum - there's already been many great threads about it here.  In my opinion, the large majority of the time you will not encounter any major popping problems with typical 3PDT true-bypass wiring.  There are many variations including:




If you have a pedal that still pops using this setup, and you are 100% sure its that pedal, then and only then try alternatives.

[shrubbery]

Thanks, but I was interested in DPDT wiring.

From those images it seems though that grounding PCB input is preferred and PCB output is left disconnected.  Do you know why?

[antonis]

From those images it seems though that grounding PCB input is preferred and PCB output is left disconnected.  Do you know why?

Propably because if the Input of a circuit stays open (floating) it's possible to "catch" and amplify some unwanted signals which would interefre with the "clean" (bypassed) signal...

P.S.
Excuse me for using wrong terms but I'm just an electronics hobbist..

[GGBB]

Thanks, but I was interested in DPDT wiring.

Drop the LED or http://bit.ly/1wMsQmw.

From those images it seems though that grounding PCB input is preferred and PCB output is left disconnected.  Do you know why?

I covered that already.  In a nutshell, no - not conclusively.

[shrubbery]

Thanks, but I was interested in DPDT wiring.

Drop the LED or http://bit.ly/1wMsQmw.

I already said I don't want LED.  But even without LED with DPDT I can only ground either PCB input or output.

[duck_arse]

the input will usually be high impedance. the output will be low(er) impedance. ground the input when bypassed and there is nothing being picked-up or "effected" to appear at the output.

just ground the input.

[GGBB]

I already said I don't want LED.

Yes, I know.

But even without LED with DPDT I can only ground either PCB input or output.

Correct.