Need help with AB box...

[hoodslider204]

Hi everybody... I've been trying to put together my own rendition of the fulltone AB box, but with a DPDT on/on switch (Mode electronics part # 44-123 if it helps). I figured that if I left out the 3rd row on the fulltone switch, where the LEDs are, it would work fine. Boy was I wrong.... no matter what I did, I always got MASSIVE hum!! I then tried connecting it all with a spdt switch, and the same thing happenned. I'm guessing this is a problem with my grounds, and I just don't can't figure it out.

I was using this AB box as' 2 in > 1 out', rather than '1 in > 2 outs'. I don't think it should make a difference, but I may very well be wrong. I'm still VERY new to this stuff.

Thanks so much everybody.

[KMS]

Ground voltage differences between the two amps causes hum or between two different FX also.

You should build this

http://www.generalguitargadgets.com/pdf/ggg_amp_aby.pdf

That file is at GeneralGuitarGadgets under FX/Routers  and go there to read more and find the link to the schemo over on GEO......read all of R.G. Keen's info on spliters.


I built one......6 way spliter using Keen's schemo....it works flawlessly....no hum.

[hoodslider204]

Thanks for the reply, but I don't need an a/b/y box, because I'm running 2 guitars into 1 amp. I'll never need to run both together, since I can only play one at a time (but I'm working on it  ).  I also wouldn't be able to build the a/b/y box because I have no idea how to make boards. I've played around with pedals, like true bypassing my cbg-95, and I've wired guitars a ton, but I'm no good with effects!

I just need a simple schem. that will work with a DPDT on/on. I'd like it not to pop when I switch, because I will be using it onstage. I thought I had it when I tried the fulltone without the 3rd row, but I just can't figure it out.

Thanks again.

[KMS]

Well someone is bound to help you on this forum.  Sorry, I thought the fulltone switch was for ABY, I've never seen it but I have heard about it.  I have not had the problem your having as all my stuff is multi channel. I doubt you will get rid of the pop without some circuitry. I know how to do it with LED/LDRs.

The switching your talking about........one guitar has two wires....the other two additional wires......both into one DPDT.....and then two wires out to the amp.......sounds easy.   Wire one guitar into the top two pins.....wire the other guitar into the bottom two pins......and the amp into the center two pins. Make sure that all the three pins one side have the same polarity lets call it (-) and then all three pins on the right are (+) or else it will be out of phase. Use a high quality stomp switch DPDT and keep the wires shielded right up to the pins.  There shouldn't be any hum, but only one guitar is on at a time.

The LED/LDR trick requires no circuit board.   You would need 4 photo cells from radio shack and 4 LEDs with high light intensity and some 1/4 shrink wrap and four 4.7K or maybe 1K resistors (depends on your LED voltage) and four 220uf electrolytic capacitors and one 9V battery.  This thing eats batteries so a wall wart would be better. 

You actually only need a SPDT to do this trick.......all of it can be soldered to the switch and put inside a 3x3 electrical box.  There will be no pop at all. 

I'll describe this for a SPDT.

Set the photo cells and LEDs up in shrink wrap like this link describes
http://www.generalguitargadgets.com/index.php?option=content&task=view&id=123&Itemid=141

Scroll down to the second picture and look at the set up. 

I just butt the LED against the LDR.

Be sure not to over heat the shrink wrap such that it shrinks down and blocks some of the light going to the LDR (photo cell).

Set up all four of your LED/LDR(s). Be sure to use black caulking to fill the ends of the shrink wrap to keep all light from entering the setup. Tar will also work...anything black and putty-like consistency.

Use this link to determine the polarity of LED(s)

http://www.generalguitargadgets.com/pdf/ggg_intro_diodes.pdf and you will see that the long metal plate inside the LED is the (-) side.

Twist the positive (+) leads on two of the LED's together along with the (+) leads of two of the 220uf electrolytic capacitors.  Do the same for the other two LED(s) with the other two capacitors.  Lets call these two sets A and B.

Solder a 4.7K resistor to the negative (-) side of each LED using a heat sink connect to the LED lead.

Solder the negative (-) lead of each capacitor to the negative (-) lead of each LED also using heat sink.

Twist the negative (-) leads (the loose ends) from the resistors hooked to the LED(s) the same way for each set A and B that you just twisted the pos(+) LED leads.

Hook the positive wire from the battery or wall wart to the center pole of the switch.

Hook the twisted positive section of A set to one of the outside pins on the switch and solder using a heat sink on the LED part of the wire. You might need to add a pigtail lead to the pins on the switch to solder the mass of twisted leads together.

Hook the twisted positive section of B to the other pin on the other side of the switch and solder in the same way as A. Hook all the negative (-) leads from the resistors to the negative (-) side of the battery or wall wart. 

Now comes the tricky part...the LDR(s).  Make sure the shielding on the audio wires comes as close to the solder joints as possible.

You have 4 LDRs with two LDRs on A and two LDRs on B

Lets set it up so A has a right and left LDR (these are really (-) and (+)) and the same for B.

Take one lead from the left LDR on A and twist it to one lead of the left LDR on B. Twist that to the positive (+) to your amp and solder...no heat sink needed for LDR as long as you don't try to make toast out of it.

Take one lead from the right LDR on A and twist it to one lead of the right LDR on B.  Twist that to the negative (-) to your amp.

Note that an LDR has no polarity so just use one lead from each LDR and don't worry about the direction the current flows through the LDR.

Now you will have four leads remaining two from each A and B set up.

Take the right lead from A and twist it to the (-) negative of guitar A...then solder. The same LDR going to negative of the amp.

Take the left lead of A and twist it to the positive (+) of guitar A....then solder......the same LDR going to the positive of the amp.

Do the same thing for B.

Close the box and play on dude....with no pop.

This reduces your signal strength by 200ohms  to 600 ohms depending on the LDR and how bright your LED is...the brighter the LED the less your signal is reduced and the more battery you need.  I have never been able to tell the difference....600 ohms ain't shit.

[LyleCaldwell]

Whoa, that's way more complex than needed.  The Fulltone diagram will work fine, no hum, with a DPDT, provided you're fine not having LEDs.  In that case, ignore the 2.1mm jack and the rightmost column of switch lugs.

Here's the Fulltone plan:  http://fulltone.com/PDfFiles/AB_switcher.pdf

[no one ever]

Whoa, that's way more complex than needed.  The Fulltone diagram will work fine, no hum, with a DPDT, provided you're fine not having LEDs.  In that case, ignore the 2.1mm jack and the rightmost column of switch lugs.

Here's the Fulltone plan:  http://fulltone.com/PDfFiles/AB_switcher.pdf

while yes, it is complicated, i gotta congratulate KMS on the effort put into that post. however simple the fulltone is, it still has hum and gets some crosstalk between the two channels (well, in the exclusive case of a CERTAIN fuzz pedal's voltage control turning in the unused channel, its kinda bad)

[LyleCaldwell]

Nonsense  , though I also applaud KMS's effort.  The Fulltone has no hum and no crosstalk when wired correctly if there is no ground loop between the two amps used.  The unused channel is grounded out.  If you have crosstalk, use shielded cables.  The one area I differ with the Fulltone plan is I prefer to have only the input jack sleeve connected to the enclosure.  All other jacks should be isolated from the ground.

If you have hum at that point then you have a ground loop for which there is a wide array of cures.  But that's beyond the scope of a simple A/B box.

[hoodslider204]

WOW, what a post KSM!!!

That sounds very helpful. I think I had tried that same wiring (minus the leds), but I'll try it again tonight and see what goes on.

You guys are lifesavers!!!!

[KMS]

Lyle.....please don't take this the wrong way.......but I am not addressing the issue of "hum" with the LDR set up.  The LDR setup address the issue of "pop" because there is no arch/spark in the LDR so there is no pop in the audio signal.  hoodslider said he wants to get rid of the pop.

The hum issue is likely a phase problem (wired pos to neg accidentally).   Crosstalk and hum could also be a shielding issue.


And....it is not that complex to do the LED/LDR setup....but it is complex to explain it using only text as you have noted and thanks for the compliment.

[LyleCaldwell]

Lyle.....please don't take this the wrong way.......but I am not addressing the issue of "hum" with the LDR set up.  The LDR setup address the issue of "pop" because there is no arch/spark in the LDR so there is no pop in the audio signal.  hoodslider said he wants to get rid of the pop.

The hum issue is likely a phase problem (wired pos to neg accidentally).   Crosstalk and hum could also be a shielding issue.

And....it is not that complex to do the LED/LDR setup....but it is complex to explain it using only text as you have noted and thanks for the compliment.

Please don't take my "that's too complicated" the wrong way either.  Yours would certainly work and you explained yourself clearly.  I'm just not sure that someone who's having trouble wiring up the Fulltone circuit is ready for your much more complicated circuit.

I've used the Fulltone circuit many times with no hum, no crosstalk, and in the one case where I had popping a 1M resistor took care of it.  And no big current draw.

Actuallly, let me say the one time I had popping that was due to the switching a 1M resistor took care of it.  There were other times when the popping came from dc leakage on a connected effect - true bypass switching is a good way to find out if you have faulty effects.

[hoodslider204]

HI guys, well I put together that simple version (without the LDRs) of the AB box that KMS posted, and I'm glad to say it works... but it hums.  Quite a lot actually, my tube amp is VERY VERY quiet usually, but as soon I plug in the ab box, it starts humming. Now, is there a way that I could ground the guitar which is not being used in this version?

It was the box where a DPDT with both guitars' + on one side, and both - on the other, with the common out (to amp) in the middle. I'm guessing the humming is from cross talk... right? Do I only need to add a jumper between both (-) to cure this? Or would this make matters worse?

It seems I'm starting to understand a bit, cause every now and then, I'll understand something, but a few minutes later, I can't figure it out again! I hope one day I can help somebody the same you guys are helping me!!

[KMS]

I have an idea.....but I have never worked with a set up like this so I might be wrong.

I'm guessing your box is made of metal.

I'm guessing that the ground (-) of your 1/4 inch jacks are connected to the box....if so then your ground is not isolated from one guitar A to the other guitar B and the ground from your amp too. Try using those plastic 1/4 inch jacks that are insulated...maybe that will work.


You could test this idea out before buying the insulted jacks by taking everything apart (out of the box) and laying it out on a piece of cardboard and see if the hum goes away.

[hoodslider204]

Oh man what a headache!!

I'm actually using a plastic box, so the dpdt switch acted as a relay, nothing else, in the signal chain. Which is exactly what it was intended to do... minus the hum.

Here's a picture of the guts of the box.

http://members.shaw.ca/joelcharriere/abbox.jpg

Thanks again!

[hoodslider204]

I was going to go out and buy a metal enclosure, the ones you get in the electrical department at Home Depot, but I'd like to get this sucker working first.

Should I ground anything to the case (once I get a metal case)?

Thanks ,
J

[KMS]

The pic....it is wired correct....except no shielded wire......so your hum could be coming from that.

The metal box will connect all (-) signals together with those jacks.

Try it.

It would be nice to get it working with no hum before change into a new box.  It should work in the plastic box......but you need something with some more mass to it to keep it on the floor.

Also the metal box will shield the those wires....somewhat.

I would get the metal box because you will eventually need that anyway.  Try it just like you have it now in the metal box.......if hum is still present......try the insulated jacks.......the metal box should take care of the shielding issue.  I'm pretty sure that is what your problem is with the plastic box.....those wires are not shielded.

[LyleCaldwell]

Either the lack of shielding or a bad solder joint.

[hoodslider204]

So I just went out and bought a metal housing.

If that doens't work, you say I should buy some shielded cable? (Oy... I got along ways to go)

Thanks, I'll post later to let you know if the metal casing worked.

[LyleCaldwell]

With a metal case, connected to signal ground at only one jack, you won't need shielded cables.

[hoodslider204]

Cool, thanks Lyle!

[KMS]

Do what Lyle said.
It sounds right......

By the way...no offence taken Lyle, and you gave some good tips too.


I know I get hum if I don't connect the ground to the box in one place.....and I have read somewhere only one ground to the box.

I have a box with two different power supplies......I had to connect one ground from each power supply to stop the hum. All the other jacks and so forth are isolated from ground.