Signal Routing Pedal

[duffrecords]

My phrase looper (Boss RC-20XL) can be a lot of fun but often I wish I could send its output to another amp.  For example, I'd like to record a bass track, route that signal to an actual bass amp, and then continue playing guitar through my guitar amp, bypassing the loop station.  I'll also sometimes want to route the loop station to the guitar amp, so I need the flexibility to reroute on the fly without unplugging any cables.  I think this could be done with a custom routing box.



I designed a simplified schematic to illustrate my idea but it's flawed and will almost certainly create hum with ground tied between the two amps.  Should I put isolation transformers on the outputs or create buffer circuits?

If there were such a thing as a 5PDT pushbutton switch, I could just mechanically swap the outputs including ground (and, yeah, it would probably pop).  A 4PDT could do it but then I'd have to lose the LED.  Rotary switches can have enough poles but I want to toggle this with my foot.  So aside from mechanical switches, I guess that leaves relays or transistors.  I'd prefer transistors.  Suggestions?

[R.G.]

I very much suggest something like the CD4053 CMOS switches. Used properly, they can do exactly what you want.

See  http://www.geofex.com/article_folders/cd4053/cd4053.htm and http://www.geofex.com/article_folders/juggler/juggler.htm

[duffrecords]

Thanks, I'll give that a try. Would you still recommend transformers to avoid a ground loop with the two amps or would the CMOS switching circuit be sufficient?

[blackieNYC]

Hey RG, what does the 4053 have over the 4066 as seen here? http://www.thetonegod.com/tech/switches/switches.html
The 4066, with 4 independent 1-throw switches can be configured as 2P2T, but no caps bias resistors on every in and out.  I've used the 4066 somewhere, I don't remember it being very clicky. I should check it again.

[PRR]

> what does the 4053 have over the 4066

Look at both.

'66 is single switches.

In most cases you want *two* switches to transition one ON one OFF.

With '66 this means you need a lot of added control inverters, Tone God's "NOT" control lines.

The '405x series has that built in.

Another point IF you fancy bi-polar audio supplies with uni-polar control/logic systems: the '405x series does that natively, the '66 needs jacking and shimming. (Not if you design all 9V stuff.)

[blackieNYC]

The 4069 controller used in the 4066 circuit is about the same as the one at geofex, further down the page.  With another inverter from the chip used for the inverted switch logic. Yes, they are single switches, but there are four - two go on , two go off.  Am I not seeing a larger parts count in the geofex bypass router? Even though the third 4053 switch isn't used, and the output caps are assumed redundant and removed, it seems a high parts count to do the same 2P2T.

[duffrecords]

Using two CD4053 switches, it looks like I would be able to reroute the tip and sleeve of each connection in tandem so that the ground of one amp will never be physically connected to the ground of the other.  Since I'm switching the sleeve conductor as well, does that also need to be biased?  It's not clear to me from the Geofex examples.

[Groovenut]

Using two CD4053 switches, it looks like I would be able to reroute the tip and sleeve of each connection in tandem so that the ground of one amp will never be physically connected to the ground of the other.  Since I'm switching the sleeve conductor as well, does that also need to be biased?  It's not clear to me from the Geofex examples.

As far as I can tell, in the Geofex article RG states each input and output needs to be biased at or near 1/2 of V+ to avoid distortion of the signal. If 2 outputs are connected together, like his article example of the traditional true bypass, only one bias resistor is needed on that I/O.

[duffrecords]

Correct, but in those examples, ground is common everywhere.  In my case, I'm driving two amps simultaneously so the audio ground of each output must be physically separated from each other to avoid a loop.  I think, in order to do this, the sleeve/shield conductor also has to be switched, in addition to the tip.  So what I'm wondering is whether that part also needs to be biased.  It isn't biased in the examples but the return path isn't being switched either because it's common throughout the circuit.

[duffrecords]

I've assembled a variation of R.G. Keen's CD4053 bypass switch on a breadboard and I've got the indicator LEDs switching the way I want but I'm still confused about the audio ground.  To recap, my goal is for this pedal to be able to multiplex two sources (guitar output and looper pedal output) and two amps.  The shield on the cables going to each amp should be isolated from each other to avoid a ground loop.  My reasoning is that the shield would also have to be switched, so I have two CD4053s--one to mux the center conductors and another to mux their shields.  I understand that the signal lines must be biased to half of the CD4053's supply voltage but would the shield lines need to be biased as well?  Would the CD4053s also need to be isolated from each other?  What if one of the amps had a cable with the ground lifted at one end?  Here is a schematic and a breadboard photo.  This is still a work in progress.

http://pasteboard.co/mUv054uEk.png
http://pasteboard.co/mUvFuA9OX.jpg

[R.G.]

You're most likely to run into issues with the two amps. This is because the two amps by their very nature have two different "grounds" in the sense that "ground" is "a reference of truly zero volts AC and DC". The power supplies in the two amps may each leak a certain amount of AC into their own "ground". That makes connecting two amps' "grounds" very prone to hum because one amp's "ground" will be dragging up and down on the other amp's "ground".

So it's likely you'll run into problems, but not from the signal switching. It will come from the two amps. Whether it's a CD4053 or a hard-metal contact switch will make no difference. Switching the shield of one amp won't help either.

It is possible but unlikely that this will work, but it won't with all possible amps. You really need to isolate the signal and ground of at least one amp to sidestep this issue.

[duffrecords]

Thanks!  I think I'll probably end up using the CD4013 and CD4053 according to your bypass circuit, and insert something like a GigRig HumDinger or Ebtech Hum Exterminator between the switch box's output and the bass amp's input.  On the plus side, not needing a second CD4053 to switch the shield will save some space inside an already crowded BB enclosure with 5 phone jacks, a power jack, 2 switches, and some LEDs.

[duffrecords]

I hooked up my prototype to my pedalboard and was surprised to find it not only passed the smoke test but also routes the signal roughly as expected. There is severe hum, which I'm hoping can be eliminated with one of the isolation products I mentioned in my last post. The momentary switches seem to have some contact bounce--maybe some different capacitors can fix that. The strange thing is that the signal bleeds into both amps sometimes. When my guitar is routed to the bass amp the signal is isolated but when routed to the guitar amp, some of it bleeds into the bass amp. When I've got the guitar signal going to the guitar amp and the phrase looper output going to the bass amp and both sources are playing something, the signals are separated. But when I stop the looper, the guitar begins to bleed into the bass amp again. I'm guessing this is due to the audio ground being common between the two amps. Do you think one of those isolation products would remedy this? Would a buffered output help?

Sent from my XT1254 using Tapatalk

[blackieNYC]

RG has the hum eliminator at Geofex, unless the products you mention are already in your possession.  What's up with the leakage?  Do you have a schematic drawn?

[duffrecords]

I'm still working on updating the schematic in Fritzing--I've copied the breadboard connections but the netlist in the schematic view is a mess.  One thing I did notice while checking my work was that I forgot the 47 uF capacitor between the bias voltage (Vr) and ground.  Adding that cap reduced the hum significantly.  It's not completely gone but it made a big difference.

When I disconnect the cable between the switcher and the bass amp, that also cuts the hum dramatically (and I get a shock if I'm not wearing rubber gloves), so there's a difference in potential there.  I'm thinking a transformer placed in line will help.  Maybe the Radial SB-6 would be ideal, although I'm not sure how good its actual frequency response is, down at 41.2 Hz where a bass guitar's low E lives.  That boilerplate 20 Hz to 20 kHz spec is meaningless without tolerance.

http://www.radialeng.com/stagebugsb6.php

I also upgraded the debouncing caps near the momentary switches to .47 uF and they toggle the flip flop much more reliably now.

[duffrecords]

Oh yeah, I forgot to mention I bought an outlet tester and it indicates there's an open ground where the bass amp is plugged in--not good!  I'm going to fix that first before I continue screwing around with this project.

[PRR]

> forgot to mention .... there's an open ground

FIX THAT.

[duffrecords]

The electrician won't be here until later this week but I've got my schematic cleaned up in the meantime.  Do you see any problems with it?

[duffrecords]

In the meantime, I plugged the bass amp into an extension cord I ran to an outlet in another room that has a good ground.  When I disconnect the audio cable between the amp and my switcher pedal, it attenuates some of the hum but not all, so it seems like an in-line transformer should help there.  By disconnecting cables one at a time, I've determined that the majority of the hum comes from the switcher itself, which is just an exposed breadboard at the moment.  Hopefully once it's inside a shielded metal enclosure that will make a big difference.  I just want to make sure I haven't made any glaring mistakes on the schematic before I make it permanent on a PCB.

[macfilipe]

My phrase looper (Boss RC-20XL) can be a lot of fun but often I wish I could send its output to another amp.  For example, I'd like to record a bass track, route that signal to an actual bass amp, and then continue playing guitar through my guitar amp, bypassing the loop station.  I'll also sometimes want to route the loop station to the guitar amp, so I need the flexibility to reroute on the fly without unplugging any cables.  I think this could be done with a custom routing box.

Hi, just registered to reply to this thread - I'm in a similar boat, looking to build a few switching solutions, and very interested in your progress. Being an electronics noob, still finding my way around a breadboard, initially I was thinking about going fully mechanical, but that idea went out the window with the price of switches. Now I'm googling and reading up as much as I can about mechanical vs logic switching, etc, and in the process stumbled upon Geofex (just wow - thank you so much, Mr. R.G.) and now this thread.

I have a couple of questions about the chips: my local dealer only sells the SN74HC4xxx series, be it 4053, 4066, etc. From what think I've gathered so far, they seem to be designed with lower supply voltage in mind, so am I right in thinking I should look elsewhere for the proper CD4xxx?

Thank you, and could you please keep posting your progress?