FX Loop Switcher and Loud Pops

[mjlee]

I'm not a DIYer, but I know there are a lot of smart people here who are. So I'm wondering if anyone has ideas for this situation:

I have 2 fx loops in my pedalboard, and a box with a single footswitch to switch between the two. When I switch the loops, I also want to switch the channel on my amp, which I currently do with the amp footswitch. I'd like a box with a single footswitch to switch both at the same time. These exist, of course, but here's the rub: when I step on my current fx loop switch and amp footswitch at the same time (essentially mimicking what these boxes do) I get an unacceptably loud pop. I believe this occurs because one of the fx loops is very high gain, and one of the amp channels is very high gain. (These are never engaged at the same time - if they were it would tear your head off.) If I turn down the gain on either the high gain fx loop or high gain amp channel, the pop is acceptably quiet.

Is there a way to build a box to switch the fx loop and the amp channel and avoid this pop? I know that if the fx loop switch occurs just a fraction of a second ahead of the amp channel swtich, there is no pop. My ignorant question is, is there a way to create this delay within the switcher box?  I've looked around for a MIDI footswtich that could send a sequence of Program Changes with a delay between, to a hypothetical switcher which receives MIDI, but I haven't found one, and even if I did, it sounds like a messy, expensive solution.

Any thoughts?

[amptramp]

It sounds like your problem could be solved with a relay.  If you have spare contacts available on the footswitch, let it operate a relay that replaces the amplifier footswitch.  The relay takes a fraction of a second to pull in, so the pop should be gone before the channel switches over.  You would have a DPDT relay if you need another set of contacts to operate an indicator for the amplifier.  If you have a DC relay coil, the flywheel diode used around the relay coil will delay the relay dropout, so you can have it both ways - a relay that switches after the footswitch for both turning on and turning off.  The relay can be powered from the ampifier so you do not need any extra relay power in the stompboxes or their power supply.

BTW, welcome to the forum!

[mjlee]

Thanks for the welcome, and thanks for the reply!  I have no experience with these things so would you know anyone that could and would build such a box?  One detail I left out - to avoid ever having the high gain loop and high gain amp channel engaged at the same time, when switching to loop 2 and amp channel 2, the fx loop switch must come first.  But when switching to loop 1 and amp channel 1, the amp channel switch must come first.

[aron]

I can't think of a solution to this. The only thing I could remotely imagine is some sort of sequenced relay switcher. Sounds way too complicated for something that seems simple but is not....

[tysonlt]

A Microcontroller may seem overkill but it could easily do this.

You would use two relays, one for switching loops (I assume one is always active?) and one for the amp. When the switch is detected, you can switch one relay, then pause before switching the next one.

The advantage is that you can play around and configure the pause; you could even control the delay with a pot.

A PIC can be had for $1 - $3. I got a programmer for $7 on eBay. The coding is not too hard, I could help you with that.

[mjlee]

Everyone, thanks for the suggestions. I know nothing about electronics, so I would need someone to build the entire box, including programming the microcontroller, etc. Anyone out there that could and would build this?

BTW, I did find a convoluted solution. I could send MIDI from a MIDI footswitch to a MIDI Solutions Event Controller, a small box which maps MIDI events. That box would send Program Changes with filler MIDI events to create a delay. I would connect this to a MIDI controlled fx and amp switcher, something like the Payne Labs K-Switch. But I'm always wary of complex solutions to simple problems...

[aron]

If I had to try this sort of thing, I would go with an arduino board. It seems like something that the arduino should be able to do quite easily. But I haven't gotten one yet. There's tons of support!

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1196698219

Wow! Source code too!!!

http://www.glacialwanderer.com/hobbyrobotics/?p=9

[tysonlt]

Arduino is a good choice for a beginner. However, PICs are cheeeeeeeeeap!!!

This might be a fun little community project. I could program the PIC, if someone could do the relays... or I have a few 4053's we could use. Can 4053's switch an amp?

[mjlee]

tysonit, that would be fantastic. I would, of course, be happy to pay for a working box.

[tysonlt]

Nah, no paying! Just for parts/postage.

It just sounds like a fun, interesting little project that's fairly simple. It would be a learning experience for me too.

Would you want battery power or could you plug it in to a 9v adapter?

I might knock up a schematic this week.

Anyone else want to help me?

[mjlee]

Wow, that would be amazing if you could come up with a box!  I've been struggling with this for awhile. At minimum you gotta let me buy you a drink!  Uhh.. wait.. you're halfway 'round the world. Hmm...

What I need exactly is a box with jacks for two fx loops, a jack for amp channel switching, in/out jacks, plus a single footswtich which does this:

When pressed, it switches the amp channel, delays for some period, then switches to fx loop 2. When pressed again, it switches to fx loop 1, delays for some period, then switches the amp channel.  Note that the order of the switching reverses on each press.

Two colored LEDs to indicate which loop is selected.  Ideally blue or green for loop 1, and red for loop2.

Ideally it would have a polarity switch, which determines which amp channel goes with which fx loop.  If this isn't possible, then fx loop 1 should go with the amp clean channel, and fx loop 2 with the drive channel.

The amp I'll be switching is a Hot Rod Deluxe.  I dont' know the exact specs for the switch, but I know the following one works with this amp, if that helps. If you need more info about it let me know.

http://www.loop-master.com/product_info.php?cPath=29&products_id=160

I'm not sure how long the delay should be.  250ms?  Someone suggested a pot to give variable delay, but I don't want to make this too difficult for you. Otherwise I guess we would decide on a delay time, and if that turns out to be too short or too long, I send back the unit and you make the change to the code? I dont' know how involved all of that is.

I'll be putting it into a crowded pedalboard, so smaller is better, but I know I can't be picky. Whatever works for you.

No need for a battery, just standard BOSS 9vDC center negative.

Here's an example of exactly what I'm looking for, without the delays of course:

http://www.loop-master.com/product_info.php?cPath=24&products_id=81

Let me know if there's any other info I can give you.

Thanks!!!!

Oh, and if you feel like going crazy and putting on a decal, your name, logo, whatever, please do!

[tysonlt]

Yep, that's what I was thinking. It was me who suggested the pot for delay time I think it is a good idea to be able to change it.

A switch to change polarity is also a good idea I think. Would you want to just switch between clean/drive? Not use the 'More Drive' channel? (I have a HRD and never use that channel).

The rest should be straight-forward. Keep in mind I am only just designing my own switcher so the design will need to be checked by some other forum-ites. Other than that, it should be fun! It may even work

[mjlee]

Good to know you have an HRD.  I never use More Drive either, so yes, I just want to switch between clean and drive.

[tysonlt]

OK, here is a first-pass schematic.



A full size image is available here: http://edieandbup.com.au/mjlee-fx-amp-AB-switch.jpg

The basic operation is explained in the above posts and in the schematic so I won't reiterate it here.

Regarding the schematic, I think I have done the biasing right. Could someone wise and knowledgeable please check over the circuit and point out any heinous errors?

One thing I'm not sure about is whether a relay is required to switch the 18v AC coming from the amp for channel switching. I know very little about AC circuits. Can you just switch it with a normal relay?

Please comment!

[PRR]

> One thing I'm not sure about is whether a relay is required to switch the 18v AC coming from the amp for channel switching.

That is a truly brain-bending switch scheme.

Open-circuit, there is large voltage swinging both ways.

To make something happen, you short a diode across it. One way gives Channel, the other way gives MoDrive.

You may also put an LED in so the pedal indicates which diode-command(s) is/are active.

With a robot foot, relays are certainly safest/simplest.



The LEDs are optional but may be useful in bebugging.

The whole MoDrive side can be omitted if never used.

There's potentially 38VAC and 31mA loose in the cable, though normal operation shows lower average voltage. Relay contacts ought to be at least this much, probably 50V 50mA nearest common rating. The current isn't really AC nor DC. I would not worry too much about that. I would favor some "industry standard" footprint so that 3 years or 30 years out replacement won't be a big headache.

The relay coils, of course, are 5V or 24V or 117V, whatever your logic or coil-drivers like.

I suspect there IS a way to do this without relays. However the reverse polarity (AC) makes simple transistors do a wrong thing. And the relays give complete isolation which is a fine feature.

[mjlee]

You're probably well aware of this, but here's something I found elsewhere that I thought I'd mention: "this AC signal (from the amp footswitch jack) and it's ground that goes to the pedal is very noisy and should be kept away from audio signals."

[tysonlt]

I suspect there IS a way to do this without relays. However the reverse polarity (AC) makes simple transistors do a wrong thing. And the relays give complete isolation which is a fine feature.

Cool, thanks prof. I was laughing furiously at this artful thread (go RG!) : http://www.diystompboxes.com/smfforum/index.php?topic=74885.20

Out of perverse interest, I wonder if reed relays would work here? I like RG's idea of wrapping them in foil for shielding... Do you think it's worth trying them out for audio switching as well? I just like the form factor...

[tysonlt]

... very noisy and should be kept away from audio signals."

I think it should be OK, maybe that part of the circuit needs extra shielding. PRR mentions above that shielding is one of the advantages of a relay.

[amptramp]

You could use a low slew-rate switcher such as the Boss / Ibanez circuit shown here:

http://geofex.com/Article_Folders/bosstech.pdf

which suppresses switch popping by gradually transferring from one source to another.  You can add resistor / capacitor / diode networks at the flip-flop outputs to obtain the correct priority for switch actuation.  No need for programming.

You can also use vactrol (LED / photoresistive) switching, diode switching and CD4007 low slew-rate switching.  For the latter, check the following datasheet figure 1:

http://pdf1.alldatasheet.com/datasheet-pdf/view/11942/ONSEMI/MC14007.html

and eliminate the inverter connections to pins 6, 8 and 13 and drive pins 3 and 10 directly from a flip-flop as presented in the geofex article above.  Note that this site has some Boss pedal schematics scattered all over in various locations but a complete compendium is here:

http://www.generalguitargadgets.com/tech-pages/45-schematics/55-boss-schematics

so you even have the parts values.  Almost any small-signal NPN transistors will do for the flip-flop.  There is nothing critical in these circuits except that for the diode / JFET circuit used in the Boss pedals, the JFET must be low gate-to-source cutoff and the diode must have lower leakage current than the JFET gate.  (One more reason to like the CD4007 / MC14007 circuit.)

[tysonlt]

You could use a low slew-rate switcher such as the Boss / Ibanez circuit

Indeed, that would work. However, as an ex-software developer I much prefer writing code to wiring components, the latter at which I am only a beginner. I just think that writing 'set_pin(SW_PIN); delay_ms(SW_DELAY); set_pin(AMP_PIN)' is much easier than figuring out all those components.

For me anyway. As R.G once said, "one man's fish is another man's poisson", which made me laugh.