Multi amp switcher

[Ritchie200]

I've done several searches here for AB boxes and really the only thing that came up was in the old archives and it referenced a schematic from the Fulltone site - long gone.

I would like to build a switcher that will take the raw guitar signal and be able to go to 4 different amps, each with their own effects.  Only want one at a time.  It would be nice to be able to step on a switch and have the current one drop out as soon as the new one is selected.  However, for the sake of simplicity, four switches and having to stomp twice would be ok too.  My question is, what about pops and noise?  Do I just short out the inputs of the unused amps?  Put a load in there?  I need this operation to be as quiet as possible.

Thanks guys!
Jim

[SILVERGUN]

Before this gets moved, never to be seen again...
http://www.voodoolab.com/ampselector.htm

I know you want to build one...so do I

[G._Hoffman]

R.G. Keen has a bunch of articles about switchers on GeoFex.  The easiest way to do one-of-n (up to 8) switching would probably be an octal latch driving your relays.  I made a switcher for my Leslie with a 74HC573, like so:



You can do the same basic thing for up to 8 switches.  R.G.'s site has a good description of what's actually going on, if you need to know.  The output of the 4050 (or you could use an octal buffer, but you know, I didn't have one when I did this thing) goes to drive the relay.

(Actually, these days, if you have any programing skills, you could just use a microcontroller, be it a PIC, an Arduino, or a Raspberry Pi, and then you don't have to bother with hardware debouncing - software debouncing is pretty easy...but a CMOS or TTL latch is pretty straight forward, as such things go!  Don't neglect the switch debouncing!)

Pops are always going to be an issue, unless you go with a transistor switching setup.  The big problem is always going to be the transitions.  You can improve it a bit by slowing down the impulse which trips the relays (basically a cap across the relay coil).  That gets rid of any noise from the trigger signal, but it doesn't deal with the problem of the amps loosing their ground reference so quickly, which is just going to be a problem. 

You left out the biggest issue, though.  While you are playing, the pops will largely be drowned out by the music.  A much bigger issue is ground loops.  The only safe and certain way to do it is to transformer isolate at least three of the amps.  Anything else is always going to have the potential to come back and bite you - it will work in one venue, but when you get to the next it will be humming like mad.  With transformer isolation, you can be sure it will be right, every night.


Gabriel

[MakerDP]

There is an excellent and very simple design in Merlin Blencowe's guitar preamp book (section 13.17) using a CD40175 logic chip. It uses four momentary switches, four diodes, five resistors and two caps on the channel selection side and whatever switching method you choose on the signal switching side... relays, jfets, whatever you are comfortable with. The nice thing about the momentary switches and this logic chip is that only one press is needed to switch channels... just select the channel you want and it routes it.

Like previous poster said, ground loops may be an issue. I agree you may need transformer isolation on each output. I have read that people have used the ones you find in those cheap Radio Shack 1/4 to XLR "barrel converters" with great success if you have some of those laying around.

[SILVERGUN]

There is an excellent and very simple design in Merlin Blencowe's guitar preamp book (section 13.17)

I have the first edition of this book (2009) and it's not in there...there is no section 13

[Ed_Chambley]

Tidbit.  Whirlwind DI boxes have very nice buffers and transformers.  Some have Jensen Transformers, but overkill for guitar.  The switches and inside components purchased separately will easily reach the $100 territory.  I watched ebay to get some use ones for a similar project and ended up spending less than $90 including shipping.

It was for a different purpose but after looking at RG Keen site if I were to build one I would harvest the parts.  Even the cheapest DI box they sell ($39 new) has a quite nice isolation tranny.

Doesn't RG Keen design and engineer for Voodoo Lab?  Looking at his ABY schematic it seems you could double the AB section, just have to determine a way to not overload your PUPS.

The radial amp switcher I have only does 2 amps, but I got it used for $40.  Seems it may be easy to combine 2 into one unit.  I know the guys that custom build them charge a lot.  Seems much more profitable than amp building would ever be.  Only because the wholesale pricing on the parts has to be less than all the parts to build a champ and the get more for the amp switcher than someone will pay for a nice built Champ.

[MakerDP]

There is an excellent and very simple design in Merlin Blencowe's guitar preamp book (section 13.17)

I have the first edition of this book (2009) and it's not in there...there is no section 13

Ah bummer. It's in the chapter on switching, don't know where or even if that chapter is in the 1st Edition. Time to buy 2nd Ed? 

There are some similar designs in TUT as well. Whichever book has switching stuff. Isn't there a Sigma project that does amp switching in one of the TUT books?

[Ritchie200]

First, thank you guys!


SG, I saw that one.  It has the adjustable level controls for each channel.  Eh, more crap in the signal.  Lot's of $$$ for something that seems so simple! (see Ed!)


Gabriel, I actually thought about an arduino setup!  Haha!  Then I got thinking that a stomp switch could do the same thing with less electronics to go wrong!  Yes isolation was brought up by Jojo in one of our discussions off line as well.


Ed, thanks, yes I think I can dig up some stuff around here to get it done.  Hmm were you stating a business plan?


MakerDP, that might be a great idea!  I've used those barrel converters while recording for that very reason, didn't even make the correlation!


THEN, Jojo sent me this link...
http://www.loop-master.com/abcdef-strip-p-365.html
I'd sure like to see the insides of this.  Not a bad price for a 6 out!  Can only use one channel at a time per description.  Does that mean the old channel automatically drops out when you punch up a new one?  Need to send an email or make a call.


Thanks again guys!
Jim

[G._Hoffman]

Doesn't RG Keen design and engineer for Voodoo Lab? 

Visual Sound.  It's his retirement job.  He used to do industrial electronics, I believe.


Gabriel

[G._Hoffman]

Gabriel, I actually thought about an arduino setup!  Haha!  Then I got thinking that a stomp switch could do the same thing with less electronics to go wrong!  Yes isolation was brought up by Jojo in one of our discussions off line as well.

There used to be a lot of mechanical one-of-n switch banks - not really anything built to be stomped on, as far as I know.  And given how easy it is to do such things cheaply with a microcontroller, I'd bet there aren't a lot of them being made anymore.


Gabriel

[SoundmasterG]

Weber used to make the Fourhead switcher, which switches 4 amps to a speaker load, one at a time. I have the last one made, and I sent it back to TA awhile back so he could blueprint it, since Ted made it originally. I don't know if they were intending on reproducing it or modifying it, but he may be able to make you up one if you wanted and asked nicely? It isn't fancy and doesn't use automated switching or anything like that....just push switches for each amp, and you switch the one you want to the speaker.

Greg

[Ed_Chambley]

First, thank you guys!


SG, I saw that one.  It has the adjustable level controls for each channel.  Eh, more crap in the signal.  Lot's of $$$ for something that seems so simple! (see Ed!)


Gabriel, I actually thought about an arduino setup!  Haha!  Then I got thinking that a stomp switch could do the same thing with less electronics to go wrong!  Yes isolation was brought up by Jojo in one of our discussions off line as well.


Ed, thanks, yes I think I can dig up some stuff around here to get it done.  Hmm were you stating a business plan?


MakerDP, that might be a great idea!  I've used those barrel converters while recording for that very reason, didn't even make the correlation!


THEN, Jojo sent me this link...
http://www.loop-master.com/abcdef-strip-p-365.html
I'd sure like to see the insides of this.  Not a bad price for a 6 out!  Can only use one channel at a time per description.  Does that mean the old channel automatically drops out when you punch up a new one?  Need to send an email or make a call.


Thanks again guys!
Jim

Jim, Hardly a business plan.   Since I am a business man it is difficult to think any other way.


I am in the process of slowly reducing my responsibilities at work and really awaiting my wife to retire from UPS.  She is younger than I and I married up, I promise.  As I begin my move into the Western NC mountains, I have already fell into a new business plan.  I plan to get up and go have breakfast with all the other old farts and complain about the sad state of the nation while basking in the wonderful wilderness and extremely nice people that the good Lord has provided.


Oh yea, I have already been fixing radios for the antique dealers in the area.  If I live a long eventually I will be the only one who can fix them.


Now get off my grass you kids.

[Willabe]