Behringer - AB200 Dual A/B Switch - Mods !  [schematic]

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Behringer - AB200 Dual A/B Switch - Mods !

Postby JiM » 25 Jun 2008, 00:47

Hi, fellow cheap-pedal-pimpers !

I've found new uses for this cheap A/B switch, which makes it a really polyvalent tool for your stompboxes chain ...
The stock AB200 is already cool, being dual, relay-based (hence true-bypass) and configurable for latch or momentary. http://behringer.com/AB200/index.cfm
You can use each half as amp or guitar selector, as amp channel footswitch, or as mute for silent tuning ... and if you stomp on both switches you can use it as a true-bypass box or a loop selector.

The first mod is fairly simple, it connects both switches together to ease the last two uses. Just wire a SPST switch between one of the upper pins (facing the jacks) of each tactile switch.

The second mod is also simple on the paper, but a lot more difficult to do. It allows to swap two loops, for example to put your wah before or after the distortion.
Each channel is based on a DPDT relay, 3 jacks, and a semi-discrete latch made with CMOS inverters. The first pole of the relay connects the tip of the IN/OUT jack to the tip of the A or B jack. So far, so good. The second pole connects the tip of the OTHER jack (ie the one not connected to IN/OUT) to ground thru a 100 Ohm resistor. Each channel has its own ground, to avoid noise problems, and only the first channel is used for battery switching.
So we have a total of 4 poles, dual-throw relays in the box, which is all we need for a swap box. http://beavisaudio.com/techpages/PedalH ... itcher.htm

We "just" need to disconnect the grounded contact of each relay, and connect them together with a DPDT switch. Easy for one of them, there's just a copper track to cut, but the other one (acting as battery switch) is connected to the ground plane on both sides of the PCB. So we need to unsolder the relay, which is very difficult as the lead-free solder needs more heat, and the ground planes act as a heatsink ! :cry: I had to add some SnPb solder to lower the fusion point, and use a lot of patience with de-soldering braid. The relay eventually comes off without too much damage, and it's now possible to cut the ground plane around the via. After re-soldering the relay, and wiring both loops together, we now have the most configurable cheap A/B box in the market ;-)
Or the cheapest flexible AB box :mrgreen:
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Re: Behringer AB200 Dual A/B Switch - Mods !

Postby holyoli » 01 Jul 2008, 10:14

Great !

Sometimes it's nice to have the possiblity to swap the order of two distortions or eq + dist etc

Merci !

Olivier
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Re: Behringer AB200 Dual A/B Switch - Mods !

Postby dogier » 18 Feb 2009, 14:05

Jim,
love the idea. I am probably going to do it. But... how would you mod the Behringer ab200 so that

Switches:
A - turns on/off loop "A"
B - turns on/off loop "B"
the added button inverts the combination of switches a and b

do you understand what i mean?

I am a newbie to the diy stomp world so i need some help.

i got the idea from a simular looking pedal on ebay...
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Re: Behringer AB200 Dual A/B Switch - Mods !

Postby JiM » 20 Feb 2009, 23:44

If you omitt the "pedal sync" switch on the schematic above (or let it in open position), you can have either loop on when the channels are of different color, and both in either order when they are the same.
BUT i don't recommend to do this, as the inactive loop is indeed looping itself (output straight into the input) and this can lead to self-oscillation and potential damage to the effects.

Just to explain it better :
  • when A is green and B is green : input -> loop 1 -> loop 2 -> output
  • when A is red and B is red :input -> loop 2 -> loop1 -> output
  • when A is green and B is red : input -> loop 1 -> output, loop 2 is in positive feedback !!!
  • when A is red and B is green : input -> loop 2 -> output, loop 1 is in positive feedback !!!

One not-so-good patch would be to add an inverting buffer in the "cascade loop" path, but depending on you FX chains it can restore the problem if the signal was already inverted ...
A better solution would be to ground the input of the unused loop when (and only when) the channels are not in the same position. This would involve switching the upper part of S5 with some XOR gate.

Independant switching and inverting, or full bypass, are not possible with this setup as the switches are DPDT relays, and both poles are moved together by the elecromagnet :|

The initial mod turns the AB200 into a Wobo inverter or equivalent.
If you want to do exactly what the Wobo double looper does, just buy two AB200. Attach them together and do the "pedal sync" mod on both, not the "cascade loop". Then just stomp on the two switches in the middle, independently or both at the same time (they are close enough).
It will cost you about half the price of a high-end switch, and work with batteries.
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Re: Behringer AB200 Dual A/B Switch - Mods !

Postby andy_roo » 23 Nov 2009, 12:32

Hi Jim, can you offer some advice about the AB200? I have 2 related issues:

1) I have 3 AB200s which I tried to power using an external 9v power supply. When I do this, a serious amount of hum is audible in the A1/B1/Out1 switch of all switches. The A2/B2/Out2 switch operates noise free. It could just be the cheep power supply I'm using (an old PS2-7000 supply). It's fine when using batteries.

2) The other problem is that when I press the A1/B1/Out1 button, a loud 'pop' is audible. The A2/B2/Out2 switch is fine. It does this also when using batteries.

I am concerned that the problem may have started when I powered the switches using the external supply (I don't know for sure, because that was the first time I tried to use these switches so they were untested with batteries). I think I got the polarity wring the first time I plugged it in (same colour cables on the +/- terminals meant I had to guess). I hooked them all up at the same time. Could connecting the power supply with reversed polarity cause these symptoms? Perhaps the pop and hum are normal?

They all still work OK with battery power, just with pop on the left (A1/B1/Out1) switches. FYI, I use balanced input and I am using them to switch equalizers into an amp, not from instrument input.

Any insight greatly appreciated!
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Re: Behringer AB200 Dual A/B Switch - Mods !

Postby JiM » 26 Nov 2009, 00:53

andy_roo wrote:1) [...] a serious amount of hum is audible in the A1/B1/Out1 switch of all switches. The A2/B2/Out2 switch operates noise free. It could just be the cheep power supply I'm using (an old PS2-7000 supply). It's fine when using batteries.
2) The other problem is that when I press the A1/B1/Out1 button, a loud 'pop' is audible. The A2/B2/Out2 switch is fine. It does this also when using batteries.

Both issues have the same cause : the control circuit ground and the audio ground of channel 1 are connected together, whereas the audio ground of channel 2 is completely isolated.
That's deliberate from Behringer, to use the instertion of a jack in channel 1 as a power switch.
I guess that your external power supply is not (well) regulated, and some ripple bleeds into the audio ground. The pops are also caused by the voltage spike when switching a relay coil, again bleeding into audio ground.

There are three things you can do, in order of preference :
  1. Use a better, more powerful, and well regulated power supply.
  2. If you intend to use exclusively an external power supply, you can disconnect the audio ground of channel 1 from the rest of the circuit. That requires cutting a trace on the PCB, maybe on both sides or under the sockets. And it will be always on, unless you add a switch.
  3. Add a resistor and a couple of capacitors as power supply filtering and reservoir, in each stompbox. For example, you may add one 10 to 100µF electrolytic cap, in parallel from +9V to GND, then a few 100's Ohm resistor to the existing 100µF cap. And one 1nF ceramic across the supply pins of the chip.
andy_roo wrote:Could connecting the power supply with reversed polarity cause these symptoms?
That should have fried the CMOS chips, as there is no protection diode except the ones on the relays.
You're lucky.

May i ask how/where you mounted the power jack ? Rear pannel is pretty crowded, and fear i would stomp on it if it's on the front.
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Re: Behringer AB200 Dual A/B Switch - Mods !

Postby mtk4union » 14 Nov 2010, 23:27

I have performed a similar modification of installing a panel mount power jack to what would be the left rear of the unit. I had soldered a 9 volt battery clip to the appropriate terminals on the jack, reversing the wiring color code, since the clip is attached to the original battery clip, and the male and female terminals become reversed. According to the operation manual, the unit is switched on when a cable plug is inserted into either A1 or B1 jack, and will not function if either of these jacks are not utilized. Also, the noise described by a previous contributer is present if the channel 2 jacks are utilized without either of the channel 1 jacks being connected.
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Re: Behringer AB200 Dual A/B Switch - Mods !

Postby Jack-Daniels » 06 Apr 2014, 14:40

Godd Day Folks:

Recently I have had the need to have a FS3X or FS-300 3-Button Switch. I saw the AB200 Switches which matched my existing LG-FCB1010, a modified FCB1010 with the Gordius Mods.

I have modded the AB200 myself but came to a point that I could not figure out what was happening or what I was doing wrong.

To begin, I have tested the two (2) AB200 and wired cables coming out from A1 and A2 on one AB200 and another A1 from another AB200 onto a breadboard and used some diodes to form the circuit as shown attached herewith.

If I may also attach a video showing that the AB200 configured with the attached FS-300 circuit does in fact work!

PROBLEM:
I have now taken this to another level by trying to eliminate all external cables and also removing the requirement of 9VDC batteries by Tapping soldering points on the AB200 circuit board.
- I am using CAT5e to relay all contacts onto an external breadboard where I can easily build my circuit. I have done this on two (2) AB-200
- I have removed and snipped and spliced the 9VDC (black/red conductors to CAT5e Orange Solid/Striped conductors)
- I have tapped "S" (Shield) on "A1" (common to A1 and B1) with the CAT5e Solid Brown conductor
- I have tapped "S" (Shield) on "A2" (common to A2 and B2) with the CAT5e Striped Brown conductor
- I have tapped "C3" to "S4" with CAT5e Solid Green conductor
- I have tapped "C4" to "S3" with CAT5e Striped Green conductor

I am only concerned and need A1 and A2 on the AB200(s) in Non-Latching Mode to get it to function like a FS-300 3-button using two AB200.

On the Bread-board:
- I have installed a 9VDC Battery (for testing and powering my circuit) and have connected it to Solid Orange (-) and Striped Orange (+)
- I have installed a switch to turn the circuit ON/OFF between Solid Orange [9VDC (-)] and Solid Brown ["S" or "Shield" common to A1 and B1]
- I have constructed the circuit as shown in the attached FS-300 schematic and as previously and successfully tested.

ONLY NOW:
- I can get "A1" to function as FS-300 A-button and "A2" to function as FS-300 B-button and switch on/off correctly with no problems
- As soon as I connect "A1" from the second AB200 switch to function as FS-300 C-button, "A1" and "A2" from the other AB200 switch still functions normally but "A1" to function as C-button does not respond. A Diode is placed between AB200-A1 Switch 1 and AB200-A1 Switch 2; and another Diode between AB200-A2 Switch 1 and AB200-A1 Switch 2.

STUMPED!!!

Current Status:
Successful control and normal operations of "A1" (Tip) and "A2" (Ring) in Non-Latching Mode on a TRS Cable.
Addition of 2nd AB200 to circuit for FS-300 C-button (which uses Diodes to produce a Tip+Ring combination on a TRS Cable) does not work! :cry:

Any help is appreciated.
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Re: Behringer AB200 Dual A/B Switch - Mods !  [schematic]

Postby modman » 18 Jul 2014, 13:03

Perhaps this is useful background info for this thread? ;-)
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Re: Behringer AB200 Dual A/B Switch - Mods !

Postby seanvox » 27 Apr 2015, 12:43

Old thread so hopefully someone sees this.

Is it possible to convert this pedal for use as an ABY splitter with each pedal turning on and off an amp like this Morley: http://www.morleypedals.com/daby.html

Any help would be appreciated.

Thanks
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Re: Behringer AB200 Dual A/B Switch - Mods !

Postby JiM » 03 May 2015, 19:53

seanvox wrote:Is it possible to convert this pedal for use as an ABY splitter with each pedal turning on and off an amp like this Morley: http://www.morleypedals.com/daby.html

It is possible to use this pedal as you say (each footswitch controls one amp), either with external wiring or internal mod. The "tricky" part is to feed your guitar signal to both inputs, then use output A1 for one amp and A2 for the other. Beware of the case with both amps muted. Depending on the wiring, you may encounter ground loops. I'd suggest connecting only the tip of input 1 to the tip of input 2.

Note this is not the behaviour of the Morley pedal you link to : one footswitch to select A or B, one footswitch for Y (A and B at the same time).
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