December 3rd, 2011, 06:00 PM
I am trying to figure out through internet research how the whole true bypass thing works, and like researching anything else on the internet, you get a lot of contradiction, and people piping up who have no idea what they are talking about (I know because I have been that person from time to time):grin:
Thanks for any help clarifying a couple of points that I think I have right, but am not sure. The attached schematic for a Morley ABY is claimed in their catalog as being true bypass, but I would say looking at the schematic it is not because the inactive part of the circuit is not taken out of the signal path, but is simply shunted to ground with a resistor, correct?
I have also read that it is unnecessary to use true bypass with an A/B switch, but I can't think of any reason why it would be any less necessary than anything else. Is there any logic to this claim that there is no benefit to an A/B switch being true bypass?
Thanks again for your help.
December 3rd, 2011, 10:34 PM
Hi Ben.. I'm no expert, but I've built several True Bypass Loopers. I'm not gonna try to answer your specific questions.
Here's the link to Beavis Audio's page about Bypass Loopers.. very nice and clear diagrams are included, and I consider them to be a reputable source of such information. The first diagram on this page is the most basic TB scheme possible. I've built it, and it is a solid and affective tool.
Beavis Audio (http://beavisaudio.com/techpages/PedalHacker/index.htm)
December 3rd, 2011, 11:50 PM
It's a deceptive schematic to look at, but it is 'true-bypass.' I think the bit that got you mixed up was the A&B switch. The schematic shows the device in position to send the signal to both 'A' and 'B'. The switch on the left is for allowing both signals to be active at the same time, the switch on the right is for selecting between the two. When the switch on the left is in the 'up' position, the switch on the right will not have any effect on the signal, other than it will flow through the second switch in the opposite direction.
The resistors going to ground are there to prevent a potential loud pop when switching from 'A' to 'B' or vice-versa. The reason there might be a loud pop is because there could be a voltage difference between the two circuits during the switch, which might cause a popping noise when switching.
It also looks different because it doesn't have a 'circuit' to bypass. Which actually brings up a funny point, you could argue that this is NOT true bypass, because it isn't BYPASSING anything! Which is a telling condition for the industry, that this device is being marketed as 'true-bypass' because that's what guitarist everywhere think is the gold-standard of bypassing an effect. It would be more accurate to call this a 'non-buffered interrupter.'
In the most basic, straightforward way I can think of explaining it, true-bypass is complete disconnection of the effect circuit from the signal. To do this, you have to have at least two poles on a switch so that the input and the output of the effect circuit are isolated from the signal path. Triple pole switches are commonly used to have a separate switch for the LED indicator circuit. Hope that helps a little...
December 3rd, 2011, 11:52 PM
You can actually see an example of the LED thing on this circuit, even though the two switches are only double pole. The dotted line connecting the LED circuit and signal circuit is an indication of this.
December 4th, 2011, 02:40 AM
Thanks, chptunes the link is helpful.
Thanks for the response Justin, your explanation of why that resistor is there makes sense, and your explanation of an A/B switch not having a circuit to bypass helps me understand why I read that A/B switches don't need to be true bypass, but at the same time, the point is, as you put it, "true-bypass is complete disconnection of the effect circuit from the signal." Is there any reason why I could not do this with an A/B switch, or any reason why this would be unnecessary?
Granted, you do not have a circuit in the middle of a chain to remove lie you do with a pedal on a true bypass loop, but don't the same principles apply?
December 4th, 2011, 11:35 PM
The schematic supplied in this thread (http://www.tdpri.com/forum/burnt-fingers-diy-effects/301995-building-b-switcher.html) seems to me to be a true bypass circuit, while the circuit we have been discussing does not. But at the same time, this seems redundant because the unused section of the circuit is already physically removed by the switch. Hmmm... kind of confusing.
December 5th, 2011, 01:21 PM
This thread is titles A-B Switch, but the schematic you posted is an A-B-Y switch.
The left two switches are the "Y" switch, meaning both ON, and is shown in the Y position. It turns both lights on and routes the input to both outputs.
The right two switches are the A-B switch, and switches the input to A or B when the Y switch is in the opposite position shown. It will also turn on either LED A or B.
There is nothing "True Bypass" about an A-B or A-B-Y switch. The input is routed to one or the other, or both.
December 5th, 2011, 11:24 PM
Hi, Ben. I think you've pretty well got it figured out now, right? FenderLover is right, there is nothing true bypass about an A/B switch. There's nothing bypassed, that's why you can't do that with this kind of circuit.
Back in the day, Vox used to call the AC amps 'class A,' even though they were 'class AB.' I'd venture a guess that it was because 'A' is obviously better than 'AB,' right? I think this pretty much the same thing as what Morley is doing with this switch, but I'll say, if Vox's white lie helped get those great amps out there, I'm glad they told it.
While technically not true, I don't consider it outright disingenuous because I assume they figure that most people reading 'true bypass' will at best understand that to mean 'not buffered,' which it is not. I guess they had a few options, call it 'mechanically switched,' 'unbuffered,' or (my favorite) just plain old, 'A/B/Y switch'. I can only assume that they wanted to maximize their market share and profit margin per unit, and saw that they could get it with a buzz-word that's misunderstood by a large percentage of consumers in their market. I think that's a shame on two fronts: 1. that they would think that lowly of our understanding, 2. that we would, in general, prove them right.
December 6th, 2011, 01:35 PM
By the way, I do NOT like this schematic. I don't know if it's an actual Morley schematic and correct, or if it was copied wrong.
My issue is with the two 100K resistors to ground at the A and B outputs. When in the 'Y' position, they are in parallel (50K) and seen by the input. Not good. That will only sound good with a buffer in front of it. No one would feed a guitar signal into a 50K load.
I would favor resistors (47K-100K) in series with each output. They would not load the input, and provide isolation between effects at the output, not to mention that it is the proper way to passively mix and split signals. The existing 100K resistors should be removed or changed to 2.2M if you get switching pops.
If you've built this and don't like it - that's why. If it works for you, it's probably because it follows a pedal that has a buffered output.
December 8th, 2011, 03:08 PM
Thanks guys. FenderLover: that is a schematic from Morley's site, and a pedal that I own and don't use (maybe for the reasons you point out), so I know that it is an accurate schematic for their pedal.