switching box schematics (a/b, etc.)

[flateric]

I've tidied up my passive switch box schematics and pasted them all together, hopefully these may be of use to people who want to have a bash at wielding a soldering iron and put one of these incredibly useful boxes together themselves.

Passive Switch Wiring
9V battery powers the LED indicators thru a 2k resistor
Uses combinations of 3PDT and DPDT switches
X-grounding for noise suppression where appropriate
Where not indicated, jackplug earths are grounded out together

1. A/B switch allows switching between 2 inputs into one output (eg: switch between 2 guitars in a gig without having to unplug) or use in reverse with one input switching between 2 outputs, eg: 1 guitar into either of 2 amps or amp/tuner, or select pedal chain or bypass for example.

2. A/B/Y switch allows AND/OR config: one guitar into either amp A OR amp B OR both at same time.
Note: Can be used in reverse as A/B box but not recommended for 2 inputs simultaneously into one amp - guitars will interfere and cause hum.

3. A/B+X/Y switch allows A or B choice on input and X or Y choice on output, for example, choose one of 2 guitars on stage into one of 2 different amps, 1 of 2 guitars into either amp or tuner (will mute amp)

4. A/B/C switch allows one of 3 guitars into 1 amp or in reverse, 1 input into either of 2 amps or a tuner for example. Switch 1 chooses between A and (B or C). Switch 2 chooses either B or C.

5. Feedback/Loop switch selects to either bypass or route signal thru an effects box with option to blend some of the 'wet' signal back into the effect loop (level set by 250k pot) or leave it 'one-pass'.

[cynical1]

These are really quite useful. I followed your previous postings, but seeing them all together gives me a much better idea of what they do and why.

+1 for the insight...and some very useful boxes.

Happy Trails

Cynical One

[angelodp]

Phase shift and ground loops. How do these designs deal with phase shift and ground loops. And as they are un-buffered, would there not be a tone loss in this type of device. I have a Radial AB/Y boc which sorts out the phase and ground loop issue but it does such a lot of tone from the signal. I have seen the Lehle AB/Y box and have many say this is the best solution. Anybody know what the lehle circuit is like.

[sumgai]

angelo,

As there are no active components in any of these circuits, there can be no phase shift, period. Phase shift is caused by capacitors, inductors, and most methods of amplification, but not by switches, wires, jacks or resistors, which are the only components we see here.

For the same reasons, there is no likelyhood of compromising one's tone. Unless one has very poor soldering skills (thus causing solder joints that have a high resistance), then one's tone is going to be just fine. All other things considered, of course.

Ground loops are defined as multiple paths for signals to return to their source via what is commonly called "ground". Knowing that, one shouldn't automatically assume a ground loop condition will exist, and that it will automatically be harmful, until all of the connections are known. In flateric's diagrams, one would have to assume that there are no other paths for the signal to return to the source, hence, it follows that the ground connections must be complete, in order for the signal to pass from source to amp's input, and back via the ground conductor.

Ground loop conditions might come into existance when effects pedals are placed in the signal path. They may, or may not, add an additional path to ground via the power circuits. This is more likely if the pedal is powered by a three-conductor AC cord, as opposed to a battery or a wall wart, but there are times when even the latter types can cause ground loops. Rare, but possible.

In general, and there could well be exceptions, it's safe to say that any switching box as simple as flateric's designs will not introduce the problems you describe. If they are touted to allegedly cure these ills, then I'd put them in the same category as those products that cure cancer, apheghenious disease, tulargi and male-pattern baldness.... in other words, snake oil. ChrisK would likely add them to this thread.


HTH




sumgai

[angelodp]

ok, then this device is the perfect solution for an amp switching device with no issues. Why in heavens name do people use the expensive Lehle for instance and claim that to be the state of the art device for this purpose??

ange

[ashcatlt]

Now hold up a second, please.

Perhaps none of these schematics will cause phase shifts, tone sucking, or ground loops on their own. However, I think that they very well could help to facilitate these types of effects.

Consider that we are, in fact, using #2 or 3 as a Y, to split one guitar out to two amps.

First, depending on the design of these amps, it is very much possible that one will invert the signal at some point before the speaker output. This could easily cause one to push while the other pulls, and could make mixing them - either acoustically in a room, or electronically by way of microphones - tricky. You could just reverse the leads for one the speaker cabinets in question, or you could add an inverting stage to one of the outputs of the Y.

And we haven't talked about what might be between the Y-box and the amps. Many effects are either time or filter based. Either of these could cause actual phase shifts. I think this is best addressed by making sure that the two signals correlate as little as possible. That is, the two amps should be making drastically different sounds.

What about tone sucking? A passive split will reduce the input impedance seen by the pickups. This will affect the behavior of the big happy filter which we call a guitar rig (guitar, cable, amp). I would make sure the signal was buffered before hitting the Y.

Then you've got the ground loops. I'm seeing two paths between the amp chassis: one through the cable shields and Y chassis, and one through the earth. I don't claim to understand ground loops well enough to eliminate them in my studio, but this seems like it's the classic scenario.


I think we discussed this stuff on various threads aboard which led to this consolidated post.

[angelodp]

Can you recommend a good buffering device ( pedal ) to position in the chain of an AB/Y box.

a

[sumgai]

ash,

Some good points, and some missed ones.


First, you spoke of phase reversal, not phase shift. My answer to angelo adhered strictly to his question. If he mispoke, and should have said "phase reversal", then all of your fourth paragraph would not only be valid, but contextually correct. If we're truly addressing phase shift, then that part of your answer is still correct, though not on point.

I do admit to not actually checking out any of the aforementioned commercial switching boxes to see what they have to say. Perhaps I'm off base here.....

Next, you addressed the addition of effects pedals between the switchers and the amp(s). Again, that's going beyond the original question, but you've made a fair assumption, that the A/B box is kinda superflous if not actually, er.... switching something. So, should we consider the possible effects of those boxes, with or without a switcher? Seems to me that'd be a valid question deserving of it's own thread, no? At this juncture, I don't see how a switcher can exacerbate any phase shifting problems - if they already exist, then a switchbox isn't gonna eliminate them, that's for sure.

Your last few paragraphs are easily addressed like so: Remember how the "true bypass" came about? It was because most of the original effects circuits kept the incoming guitar signal tied to the input of the circuit, right? And the true-bypass actually switched the signal away from the effect circuit, and switched it only go down the bypass route (a straight wire), right? Well, if you'll look again at flateric's diagrams, he's doing exactly that - eliminating any potential for tone-suck by not letting the signal get into the unused output. In fact, he's even grounding the unused output, something that's just about guaranteed to disallow tone-suck, or so I should think. ;D

I will concede that in flateric's designs, the fact that all ground lugs are tied together may, repeat may, cause a ground loop condition to arise. This is remedied by emulating the layout of a true-bypass for the signal's "hot" conductor - one adds yet another switch pole to control the ground connection, and uses either a plastic case for isolation, or specially insulated jacks that cost only about twice as much as regular ¼" jacks. But again, I caution all and sundry to not instantly assume that a ground loop does indeed exist, and that it is automatically bad. Test first, before instituting a fix, that's my advice.

HTH



sumgai

[newey]

Seems like this comes down to whether we're talking about the box itself versus talking about the box in use, in a real world situation.

Adding more cabling adds capacitance to the circuit which can affect tone. The box doesn't cause this in and of itself, but it does facilitate it. You'd have the same thing without the switcher, of course, if running a line of effects, etc. And, as Ash noted, a buffer along the way would solve the problem.

It's sort of like being married to a junkie- you didn't cause the addiction, but you're an "enabler".

[ashcatlt]

Yeah, the tone suck thing comes from parallelizing the two amp inputs. If you choose A or B, it shouldn't be an issue, but when you choose Y...

Apr 23, 2009 3:09:20 GMT -5 angelodp said:

Can you recommend a good buffering device ( pedal ) to position in the chain of an AB/Y box.

a

If you've got any "big name" pedal (Boss, DOD, Digitech) laying around you've got a pefectly acceptable (IMNSHO) buffer. Put it in line somewhere before the split, but after any pedal that depends on direct interaction with the pickups et al*. Make sure it's got a good battery or a wall-wart and leave it bypassed. These are usually not "true bypass", unless you've modded them.

JohnH has a post around here with some pretty simple buffer circuits. One even fits inside a guitar cable.



*These are usually "vintage" type fuzz boxes and the occasional wah.

[angelodp]

Could we get any gut shots and externals of members builds on an ABY, if anyone has built one of these boxes. I opened up a divided by 13 Switch Hazel which is an ABY with a boost and it was gooped!!! So i cannot say much about that circuit, although it works great. I wonder if anyone has a layout for that pedal or the lehle ABY as a comparison.

[sumgai]

Buffering will certainly take care of capacitance issues, and therefore reduce or eliminate tone suck. However, paralleling both output paths at the same time (the 'Y' connection), presumably to two different amps, well the whole point is to get a different sound/tone. At that point, one cannot address phase shift, nor even tone suck, they are part and parcel of the entire tonality.

Which brings up the next issue, namely, how does phase shifting affect overall tone? Good question, let's address that now.

In essence, phase shift is the term used to describe the interaction between current and voltage, within some given component, device, conductor, what-have-you. Without going into the basic workings of a capacitor (there are lots of good websites covering this, google is your friend), the end result is that when a capacitor is conducting AC, it causes voltage to lag behind current. Think of it this way - as the current rises toward it's peak, the voltage peak is not in step with it - it's behind by 90°. That's what we call phase shift - the two items are not in phase. (You did note that this has nothing to do with the polarity phase that we usually talk about when comparing one pickup's phase to another. That's phase reversal, which ash has properly denoted.)

Now, given that, and given the fact that inductors (including your pickup itself) cause voltage peaks to lead current peaks, we can see that throughout the entirety of the signal path, we'll have a multitude of phase shifts, all working to degrade the signal, or so it would seem, eh? Err, no. The fact is, phase shift occurs throughout the frequency spectrum, but like a tone control, it has a "knee", a point in the spectrum where it becomes more noticible. That said, we need only choose component values that will tend to have the least phase shift in the audible portion of the spectrum. Definitely not a cut-and-tried piece of cake, but it is doable.

And when it's all said and done, there will still be artifacts of all that interaction, no matter how hard we try to avoid them. The real-meal-deal though, is that they are part and parcel of what we hear coming out of the speaker. (Hey, there's a speaker coil, does it too introduce phase shift? Yeppers, surely does.) the bottom line is, some part of the overal tonality is composed of phase shifted peaks and valleys, and for all we know, there might be a boatload of complex interactions going on as the different notes are played. In fact, I'd bet on it.

Now, long story short, I'd ignore any drivel about phase shift in one's signal, it's a moot point. It's there, it ain't goin' away, there ain't anything we can do about it, so why sweat it? (Actually, a dedicated engineer/designer can reduce it, but at what cost in time and effort? And to what degree of improvement in the final tonality? aka, how much bang for the buck?) IOW, practicality over worring about things we can't see or easily control, that's my motto.

HTH



sumgai

p.s. For my money, this is one of the most valid 'off-topic' discussions ever seen in the NutzHouse. Thanks to angelodp for asking such on-point questions.

[ashcatlt]

There are folks in the pro audio community who contend that frequency dependent phase shift on single signal is a noticeable, bad thing. There's a company out there which has made a whole lot of money selling products meant to combat these effects.

I personally think that's all hooey.

Where phase shift might be a problem would be when you're trying to combine two copies of a signal which have different amounts of phase shift at any given frequency. This might cause interference which might be objectionable.

Like I said (and I think sumgai agreed), this is best dealt with by making sure the two signals actually sound significantly different. If they don't, then why are we running two seperate effects lines?

I also agree that this is part of what makes it actually sound like two amps. I wouldn't expect any actual phase shift to be noticeable as a problem in this type of Y setup. If it were, one might try a variable all-pass filter on one side. People are making good money selling those as well.

[sumgai]

ash,

A variable all-pass filter? Errr, isn't that just a fancy name for an amplifier? ;D

And yes, we pretty much agree on the GeHooey factor. The proof is simple - not one of those commercial companies has demonstrated, and repeatably so, the actual presence of phase shift, let alone it's effect on a signal. And that's before they get to demonstrate their ability to clean up the signal by removing the phase shift.

Gonna be hard to do, considering that every musical note I've ever heard was a compendium of both the fundamental and a rich variety of harmonics. Sorta makes you scratch your head, wondering why anyone would bother to bother about all this.....

But I can just see it now: "NEW and IMPROVED!! Look at our new doohickey - we can adjust the amount of phase shift in your signal!! Add new tonality that no one else can get!! Be the first on your block, boys and girls!!!!

I'm now leaning more towards the fact that someone, probably in marketing, has mixed up 'phase shift' with 'phase reversal'. The latter can, and does, play hob with one's tonality. It's child's play to treat that condition with a simple buffer that inverts the signal. Add a switch to the buffer to select the output's phase, and you just might actually improve (or screw up) your tone. This happens (for those of you trying to follow along at home) because phase reversal is absolute - every last detail of the signal, no matter what frequecy, is exactly 180° out of phase, compared to the "other" signal. Even from two amps/speaker sources, this is noticible, and thankfully, easily corrected. In fact, if you dig deep enough, you'll find that this is precisely the source of how chorus and flanger circuits work.

(Note: while such boxes might have knobs that are labeled with so many degrees, they are in fact merely inverting the phase over a narrow range of frequencies, not the whole spectrum at once. The range moves up and down the spectrum, which we perceive as a sweeping effect, and that's the secret to their appeal. There are various circuit additions for various reasons, but that's it in a nutshell.)

Oh, and phase shift is one component of IM, or InterModulation distortion. If that number is down in the negligible territory, then I wouldn't worry too much about being able to hear its effects on one's signal.

Been reading up on some Zappa lately, don't know why. I wonder what pithy quotable he would have dropped, had this topic come to his attention.



sumgai

[ashcatlt]

I’m pretty sure we’ve completely abandoned the original point of this thread, and this stuff about phase should probably go somewhere off on its own.

I'm not a moderator, and the discussion here does seem to lead me to address this:

Apr 24, 2009 15:05:43 GMT -5 sumgai said:

A variable all-pass filter? Errr, isn't that just a fancy name for an amplifier? ;D

No. I was talking about an all-pass filter with a variable cutoff frequency. Most of these do not offer any actual gain. I guess it might be considered an amplifier in the same sense that a unity gain buffer is called an amplifier…

An all-pass filter passes all frequencies (surprised?) without any gain or attenuation. What it does is to apply a frequency dependent phase shift. That is, different frequencies coming through seem to be delayed by different amounts. This phase shift is not generally a hard stair-step type of cutoff from 0 to 180 degrees, but rather a fairly gradual slope. In fact, for a one or two pole filter, this slope will cover a significant portion of the audible frequency range.

There are (as I’ve mentioned) those who believe that they can hear the effect of a frequency dependent phase shift in a single given signal. An example of this might come from the fact that the tweeters in your home stereo (or studio monitor) speakers don’t inhabit the same physical space as the woofers. Therefore, there’s some amount of delay between the two. I personally think that this effect is beyond the notice of most casual (and even most critical) listeners. However, there is a very well known company (starts with a B, ends with E, has a B in the middle) which seems to have made a large amount of money off a very carefully designed all-pass filter meant to correct for this effect. It is marketed as adding “clarity”, “punch”, and other terms, which are vaguely descriptive of completely subjective phenomena.

There are also folks out there selling all-pass filters with variable cutoff frequency as “phase correction tools”. These are generally used for mixing two (or more) similar copies of a source that happen to be delayed from one another. One example would be if you were to record a bass guitar through both a DI and a mic in front of the amp. Another might be something like a snare drum being recorded through a close mic and a room mic.

The sound waves take time to travel through the air, significantly more time than the electrons take to flow through the wires. Therefore, the DI is going to hit the recorder a cut hair earlier than the signal which comes from the speakers, through the air, to the mic, and then down the wire. In this instance the entire signal is delayed by the same amount at the distant mic. However, when we compare the two signals, we’ll find that the direct signal will have “moved on” further (relative to the whole period) for higher frequencies than for lower at any given delay time. This creates a frequency dependant phase shift. When we try to combine the two signals some frequencies will be more in (or out of) phase than others. Some will interfere constructively (come out louder) and some destructively (come out quieter). This creates what we call a Comb Filter. On some graphs, with appropriate scaling, the peaks and dips in the frequency response look like the teeth of a comb…

Now, there are two “right ways” to fix the issue:

1) Align the waveforms (Nudging). In the old days of tape this was accomplished by applying a short delay to the earlier signal. The delay time was varied while listening to the mix until it was “as less bad” as possible. Nowadays, in DAWland, we can view the two waveforms and slide one or the other until they line up as perfectly as possible. Interestingly, if you delay one signal far enough to where the earlier no longer sounds much like the later at any given instant, phase ceases to be an issue. It sounds like an echo, rather than a filter.

B) The 3:1 Rule. This is better termed the “At Least 3:1 Suggestion”, and is even better named the “At Least 9db Consideration”. Let’s say we’ve got a snare drum and two mics. One of the mics is up close, about 1’ away. The other mic is further - say 3’. Well, the sound pressure level of the snare should be about 9db greater when it reaches the close mic than when it reaches that distant mic. Assuming that the two mics have equal sensitivity and are fed through identical preamps set for the same amount of gain, mixing these signals should not have much noticeable phase interference, despite the delay in the distant mic. There will still be comb filtering, because of the delay, but because the distant mic is so much quieter than the close mic, the dips and peaks will span somewhat less than 1db (the accepted threshold of noticeable volume change).

1 + ( –1) = 0 but
1 + (-0.25) = close enough to 1

There are some problems with both of these methods:

A) Aligning waveforms changes the delay between the two sources . In the case of a drum kit, the distant mic is likely meant to give a sense of space and ambience, and changing the delay between the direct and reflected versions of the signal can change the sound and feel of the mix. It will tend to make it sound like a smaller room. Of course, if you’re using the distant mic only for ambience, rather than for its contribution to the direct sound, you can often apply the 3:1 Rule.

2) The 3:1 Rule requires that one of the signals be significantly quieter than the other . Doesn’t really matter which, either. You can just as happily crank up the miked track and just mix in a touch of the DI. But what if the 50:50 mix is exactly what you’re looking for? Then you’ve got to nudge.

Or, you could buy one of these all-pass filters, put it on one of the tracks, and twist the knob till it sounds “as less bad” as possible. This seems like a kludge to me…or maybe a hack. I know of some audio engineers who are seemingly intelligent and definitely well-paid who use these things all the time, and swear by them.

I’d like to mention that this all-pass filter thing is how your phase shift pedal works. The signal from the guitar is split. One of the copies goes through a series of all-pass filters, shifting various frequencies by different amounts, and is then mixed back with the original, where it interferes and causes some frequencies to be attenuated while others are accentuated. The cutoff frequency of these filters is modulated by an LFO, which causes the sweeping sound we hear. Some choruses use this same technology, but most flangers nowadays are created by a very short, frequency independent delay.


After all this I’d like to mention that there is an instance where I know I’ve seen - and believe I’ve heard – a noticeable effect from frequency dependent phase shift on a single signal.

Some waveforms are asymmetrical. Maybe it swings further in one direction than the other (tubes and output transformers sometimes do this, as does the Harmonic Perculator). Maybe it spends more time on one side of 0 (the duty cycle, or pulse width, often found tweakable on analog-style synths). Or maybe there’s just more high frequency content riding the lower frequencies when they swing one way. One place this (apparently) happens a lot is in male voices like my own.

An all-pass filter applied to an asymmetrical waveform will redistribute the energy of the overall signal to where it becomes much more symmetrical. This is easily visible in DAWland, where I’m happy to reside. It’s also subtly but definitely audible. It can make a voice sound a bit more full, more musical, and somehow bigger than life. What was that about vaguely descriptive of completely subjective phenomena? You hear this type of thing all the time on the radio. In broadcast processing, it’s called a “Phase Rotator”, but it’s really nothing more than the all-pass filter I’ve been talking about. It comes out like a very subtle form of compression, and can open up headroom.

Okay, I’m done for now…

[JohnH]

Ash - I just want to say, thats really interesting - thanks for posting that!

John

[newey]

Ash said:

I’m pretty sure we’ve completely abandoned the original point of this thread, and this stuff about phase should probably go somewhere off on its own.

I'm not a moderator, and the discussion here does seem to lead me to address this:

We're not completely off topic as Angelodp did raise the question of phase shift.

But Ash, you're point is well taken, so I reposted your thorough explanation as a separate thread In "Reference Articles". Further discussion can be had there, and we can leave flateric's stompboxes out of it.

If others want to copy selections from the prior discussion to there, please do so.

And 1+ to Ash for a good explanation. I know it was good because even I could understand most of it.

[flateric]

I was asked for a 'gut shot' of the boxes - here's my favourite I use all the time gigging, its the A/B + X/Y box. Not really much to see, just a battery, 2 DPDT switches and some LED legs, as thats all they are!

[iloveguitars]

I appologise if i am repeating what others have said above but i haven't read all of the posts.

Put simply - if you connect two amps together through a passive 'true bypass' ab pedal then you are going to create a ground loop which can create hum.

You can get round this by purchasing a pedal with a transformer on one of the outputs such as the lehle pedal mentioned before. This can create a phase shift of 180' depending on how the transformer is wired. Putting a transformer inline can affect the sound, which is where paying the extra money for a quality pedal pays off.

I believe someone also mentioned a buffer. I use a MarkOne Audio SwitchBox which is actually an active ABC pedal. You can plug upto 3 instruments into it and as it is active it is a buffer as well. It doesn't change the sound at all like some of the cheaper pedals too.

Hope this helps

[ashcatlt]

Soon (yeah, soon ) I'm going to build a feedback loop very much like #5 above.  I prefer to save some money on switches, though, so I'm going with a DPDT stomp switch and leaving out the LEDs.  I'm pretty sure I'll be able to tell when it's on. 

Also, I don't really want to mess with a switch for the feedback.  I was thinking that maybe a bigger pot would pretty much kill the feedback when turned all the way down, but then I realized that this is wired as a variable resistor.  Only two lugs are used, like on a tone control.  So I'm planning to use the old "no load" trick - scape or cut the resistive track at the non-connected end.  This way, it will go completely off when turned all the way down.  No, there won't be an LED here, either, but I figure I'll definitely know when this is on, assuming the knob markings don't clue me in.

Unfortunately this does not solve the issue where you can't get a 100% wet signal when feedback is engaged.  I think I'll live, though.

[ashcatlt]

You know, now that I've gone and recommended #2 as a good A/B/Y, I actually went and looked at what it does, and I'm a little confused. I wish I could post just that one here to look at (especially since we're a page away), but it's all one big image...

Aw what the heck!



Anyway, we're looking at #2. When the right hand switch is flipped toward B and the left hand switch is flipped toward the top lugs, you get A+B. Flip the left switch toward the bottom lugs and you get B only, with A shorted to silence the cable antenna.

But if we flip the right hand switch toward A things don't work out so well. With the left switch connecting upwards we get A+B same as before, but flip that switch down and we get A shorted and B is not connected at the other switch, so it's silent but the cable is open.

I don't think that's what we want.

I think a simple solution would be to eliminate the "X" of wires from the left switch and use another pole on the right hand switch to send A or B (whichever is not selected on this switch) to the bottom left lug of that left hand switch.

This one does what Al intended, but doesn't have LEDs:


This one does it differently, with just an on/off for each output, but doesn't short the unused output. That could be fixed by connecting ground to the upper left (on the left switch) and upper right (on the right switch):