Adding a ground lift to an ABY box (?)

[mr_sooty]

OK, so firstly, I don't even know what a phase inverter is. But here's the thing. I run an ABY box on my board. It's been modded (thanks to you guys), to take two seperate inputs, and send them individually to two seperate outputs.

I just got a spanking new Fender 65 Deluxe Reverb Reissue amp. The thing with these amps is that there's a normal channel and a bright channel (sperate inputs, not switching). The Reverb and Vibrato only operates on the bright channel, but I've found that you can run a patch cable to link the channels, and bleed some reverb back over to the normal channel.

The only problem is that the channels are out of phase. This means they kind of override each other, so you get heaps of one and little of the other, unless you juggle it just right.

I've found a mod for the amp to fix all this, but I don't really want to mod the amp when it's like, 1 day old. I'll void the warrantee.

SO.... I was wondering if it's possible to build a phase inverter into my ABY box, so that when both channels are on, it corrects the phasing? As usual, I know nothing. I don't even know what phases are. I know what phasers are, but that's about all. Pleasde speak slowly and clearly.

Here's the amp:

Here's the ABY box:

And here the wiring diagram I drew up of the box:

Except that the red wire between the two switches has been removed to accomplish aforementioned mod.

Any ideas? Cheers.


Beers?

[ashcatlt]

What has led you to believe that the two channels are out of phase? You say that they "override each other". If they were in fact out of phase, by which we would almost certainly mean inverted polarity, we'd expect them to cancel each other out in whole or in part. In other words, if you turn one up, and then turn up the second, the overall volume should decrease.

I'm not saying you're wrong, but I wonder if you've got a source to which you can point for this information.

I wouldn't be terribly surprised if the two channels were less than perfectly phase coherent accross the entire spectrum. A severe example of this would sound like a phase-shifter pedal stuck in one place, maybe a hollow or thin sound. Is there a dramatic change to the overall tonality as you bring up the volume on one of the channels?

I've jumped the two channels of a number of amps like this on different occassions. In my case, it was used to push the power amp section harder. I don't ever recall having any symptoms of polarity or phase issues.

If you're running the master volume at or above the overdrive threshold, the amp might not get much louder if you turn up the second channel, it'll just get crunchier.

I remember your box well. Glad that worked out. It would be possible (and relatively easy) to invert the polarity inside that box, but it'll require some active circuitry. Don't let that scare you too much. A dual opamp, couple resistors and capacitors is all it really takes (<$5 before shipping). I'd get a small piece of perfboard, but I've seen people use scraps of cardboard. I highly recomend a socket for the opamp, though.

Why does it need active circuitry? Because somebody a long time ago decided that our guitars and amps should share one wire for both signal return and all EM shielding and chassis grounds. In a balanced mic cable, all you'd have to do is swap the + and - wires, just like we do with any "phase" (actually polarity) switch you might find in a guitar scheme. Unfortunately, with a guitar, that puts all the noise we're trying to send to ground onto the hot output, and sends it on to get amplified. So what is left to us is to kind of take a copy of the signal carried on the hot wire and flip that copy over. The copying part requires a battery to do well.

Please feel free to request definitions for any big words.

[mr_sooty]

Nov 20, 2008 22:50:08 GMT -5 ashcatlt said:

What has led you to believe that the two channels are out of phase?

The internet, basically. When I first dicovered this jumpering thing I was like, cool! Then I read lots of people saying that you 'can't' do this on Fender Blackface amps (and reissues) because the channels are out of phase. They recommended getting a pedal like this:

www.sweetwater.com/store/detail/BigShotABY/

...to correct this issue.

Nov 20, 2008 22:50:08 GMT -5 ashcatlt said:

You say that they "override each other". If they were in fact out of phase, by which we would almost certainly mean inverted polarity, we'd expect them to cancel each other out in whole or in part. In other words, if you turn one up, and then turn up the second, the overall volume should decrease.

That's kind of what seems to happen somethimes. But it seems that you can sort of balance them. It's kind of hard to explain, but mixing the channels is not as simple as just mixing them as you would on a desk. If you turn one up just a little bit, , the other will almost disappear, but if you back the first one off again, you can find a balance where they kind of manage to work together.

Adjusting them sort of reminds me of two magnets, you know where you push two same polarity magnets together and they fight each other, but if you push them slightly out of alignment you can get past that opposing force. It's kind of like that I guess.

But the tone never sounds like a phase pedal stuck in the middle or whatever, like you describe. I can actually get pretty good tones if I get the balance right.

Hmm, I guess the problem with contemplating modding the ABY box again is that I don't want to undo what we did last time, because that works great for the stereo setup.

[mr_sooty]

Ok, let's forget the phase inverter for now (topic title modified). What about a ground lift, like the one that's also on that Radial pedal I linked?

When I run two amps from my AB box I get ground loop hum, a common issue in this scenario. I know in America they sometimes plug one of the amps into a ground lifter so that one of the amps acts as the ground for the other. Apparently in my country this is incredibly dangerous, because our power supply differs from yours. But (again, apparently) the ground lift in boxes like the Radial one do something different and aren't dangerous to use.

Anything I can do to my ABY box to incorporate this kind of ground lift?

[ashcatlt]

They're using a transformer. This allows one actually to invert and ground lift in a passive manner. I consider it to be less than ideal, partly because transformers are not nearly as transparent as a unity gain opamp buffer.

Of course, some folks like the way that transformers fail to perform...

Before we can do any kind of ground lifting, one of your jacks must have it's threads insulated from the box. Could be as easy as drilling the hole a little bigger and slipping a short piece of vinyl tubing over your existing jack.

[mr_sooty]

Oh right. Is there much involved after that?

[JohnH]

This schematic does seem to confirm the phase reversing inside the amp:
(see page 6)

www.schematicheaven.com/fenderamps/65_deluxe_reverb_manual.pdf

Both channels get all the same input components, until the vibrato direct signal goes through an extra inverting stage V4b, before linking up with the normal channel to go to the power amp. It means that when one channel is going up, the other is going down, and if they are in balance, there will be a null, but a bit of adjustment on either channel and it will sound again. Is that how it works?

I wouldnt want to mess with a shiny new amp

If you want to persue this, I reckon you could build a simple electronic invertor - my version would be one transistor (a JFET) and a few resistors and caps. Its low cost and no risk.

John

[mr_sooty]

Nov 21, 2008 5:11:55 GMT -5 JohnH said:

It means that when one channel is going up, the other is going down, and if they are in balance, there will be a null, but a bit of adjustment on either channel and it will sound again. Is that how it works?

Yeah, that's the deal.

I wouldnt want to mess with a shiny new amp

Yeah, kinda wish I had a second hand one now. I do have a simple mod that should fix this, but yeah, not really prepared to mod it yet.

If you want to persue this, I reckon you could build a simple electronic invertor - my version would be one transistor (a JFET) and a few resistors and caps. Its low cost and no risk.

Would this be an external thing? I'm listening. Please go on.

[JohnH]

Yes, it would be a fully external device, inline with the cable with which you patch the channels. The active part would be on a small board, very much like the one used in this:

johannburkard.de/blog/music/effects/JFET-Preamp-Splitter.html

This following article about a similar circuit is really good, but has a lot of maths twaddle. Just scroll down until you see a circuit board

www.runoffgroove.com/fetzervalve.html

I built one of these into a guitar and it worked well

I could do you a diagram for the circuit board. The parts for the board are very cheap. I could even (for a fellow Kiwi - I am one when the All Blacks play, I'm English when its cricket however) put the bits from my spares box into an envelope and send them to NZ, or even solder up the board.

John

[mr_sooty]

Oh wow, looks good. I'm having a few other issues with my 2 day old amp, so I'll probably hold off for now, but I'll let you know for sure. Thanks man.