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True bypass Questionairre

7/5/2004 12:22 AM
8bit Barry True bypass Questionairre
Out of the following pedals which are not true bypass?  
EH 4800 Small Stone Phaser (late70's)  
EH Electric Mistress Deluxe (late 70's)  
Rat Mk1 (80's)  
Boss Heavy Metal Mk1  
Boss LS2 Line Switcher  
Boss SD1 Super Overdrive  
Boss GE7 Eq  
Boss CS3 Compressor  
MArshall Jackhammer  
Ibanez Delay Echo  
Could I tell from looking at the footswich pins or something?  
How do I make them true bypass? Where could I find mod schematics?
7/5/2004 5:38 PM
Mark Hammer

There are several different "levels" of true bypass.  
The highest level of TB would be one where the entire circuit is removed from the signal path and there is nothing between input and output jacks except for a straight wire. Only do-able with relays or multi-pole stompswitches.  
The next highest level would be one where only what feeds the output jack is switched mechanically. Again, only relays or stompswitches. If the input impedance of the circuit is very high, then leaving the circuit input tied to the input jack should provide very little tone-sucking. Unfortunately this is not always the case. As well, there is enough signal sucking by some of these that when people try to convert their pedal to the first level of true bypass using a more complex switch, they find a volume difference between bypass and effect modes; something which would not occur using the less complex switch because the pedal was designed around it. Many of the classic stompswitched pedals of the 60's and 70's and even 80's (e.g., Small Stone) had this type of switching. Of the ones you list, I think only the Rat has true bypass, and *maybe* the Jackhammer.  
Most effects that use solid-state switching (and this would be anything that has a spring-loaded foot-treadle, so Boss, DOD, Yamaha, Rocktek, Arion, etc.) use FETs to mimic switches. In one state, the FET is a very high resistance resistor, and in the other state it functions like a very low-value resistor. You can see an explanation of how this works in the dodswitch.gif document at my site (; scroll down about 3 pages).  
The way in which these FETs are used varies by effect type. For example, almost any type of modulated or time-based effect such as a flanger, chorus, analog-delay, reverb, or phaser, can cancel the effect simply by killing the line carrying the "wet" signal, leaving the rest of the circuit in the signal path at all times. For instance, in the Boss CE-2 chorus pedal, whether the effect is "in" or "out" it is being influenced by 2 op-amps, 1 transistor, 7 capacitors, and 11 resistors at all times. All that gets changed when you kill the effect is that one resistance goes from low to high. In some pedals, a mechanical switch is used instead of a FET. There may be a stompswitch, but the circuit still remains tied to the input and output jacks.  
In the case of distortion, EQ and compression effects, there is often some FET-switching at the input and output of the effect circuit, but the signal still passes through input and output buffers before and after that FET. In essence, the circuit board is NEVER disconnected from either the input or output jacks.  
If you look at the FAQ over at diystompbox ( you should find lots of useful info to address your questions further, including wiring diagrams.
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7/6/2004 8:55 PM
8bit Barry
Thanks Mark  
This theory behind all of this is rather new and a little confusing, but already I have spent too much time reading lots of information, so much so that I am getting anxious to just get with it!!  
What I really do want a simple diagram to say "put this wire here and that one there" as I want to get playing, not learning an electronics course. I am adept with a soldering iron, but these 'idiots guide' sites are few and far between. I found a great TS9 mod at Tonepad "PhotoEssays" and it is this sort of info I want.  
What I want to do is  
GCB95 Wah true bypass (DPDT as I have one spare)  
Electric Mistress true bypass, dont care how  
Small Stone True Bypass, dont care how  
The fuzz central site has some SD1 and TS9 mods, but they are much easier as Boss numbered all the components on the board (thankyou!!). I am definately going to do both of these....  
I just look at a PCB and I dont have a single idea what is going on, could you help me out if I mail you a JPEG of the SS, EM Deluxe and wah I own? If you could make a simple guide I would be eternally greatful.
7/9/2004 8:33 PM
Mark Hammer

Before you get your knickers in a knot, consider the following:  
a) Some pedals are designed around the fact that they are not true bypass, and attempts to MAKE them true bypass can create sonic headaches. The best instance are those casesof E-H pedals where a "true" pypass results in a volume drop, but a *non*-true bypass does not. If you know how to tend to the volume drop, then switching to true bypass is a move in the right direction. If you are a beginner and don't know what to do, then it can be 2 steps forward, one step back.  
b) As "Pricey" Pete Cornish points out at his site, there are plenty of times when non-true bypass actually IS what you want. E.g., if you want to send your signal over a long cable, you actually WANT an always-on buffer stage in there, which true bypass would otherwise eliminate.  
In 2 of the pedals you note (GCB95 and SS), converting to TB is pretty dead easy. These pedals use SPDT switches presently, leaving the input jack always tied to the input on the board and switching only the output. In both cases, the switch takes its signal from either the input jack directly OR from the output on the circuit board. To convert to TB, using a DPDT switch, one set of contacts selects between going to the board or output jack, and the other set of contacts selects between coming from the board or input jack. There are diagrams in the FAQ at and a more targetted explanation of wirng here:  
Do those two pedals and we'll talk about the EM after that.
7/17/2004 11:46 AM
When I look inside these pedals I have no idea what wires are what. I cant find any schematics for a GCB95 wah wah to let me know what I am looking at. The schematic you suggest is all very well, but I dont know what the layout of the original pedal is to make the modification, the primary problem I have with lots of mods. If only someone would get a jpg and put some simple arrows telling you what you're looking at to start with. Once I know which wire is which it should be a breeze.....
7/18/2004 11:24 PM
8bit Barry
I have done the Wah and the Small Stone, which I found on another site thankfully.  
You said you would explain the Electric Mistress...
7/20/2004 7:25 PM
Mark Hammer

There are a few different issues of the DEM, but I'm assuming they all use the same switching. First, some background.  
Whenever you have a pedal that uses a combination of clean and delayed/affected signal to produce the effect, you can "cancel" the effect by simply cancelling or lifting the connection between the affected signal and the mixing stage or point where the "dry" and "wet" are combined. That's true of flangers, chorus pedals, phasers, and analog delays, among others. In pedals with electronic switching, this is usually a single FET that joins the wet signal and mixing stage. With a mechanical stomp switch, you could probably do it with a simple SPST switch, though I don't know anyone who does.  
The tradition for pedals that used mechanical switches was to pursue the same rute used for fuzzes and wahs, etc. That is, a SPDT switch that was wired to the output jack and either took its feed from the output of the effect board OR from the input jack. From what I've seen, that's what the DEM does.  
To convert the pedal to TB, 3 additional things need to be done:  
1) Uses a DPDT switch.  
2) Wire up the input in addition to the output.  
3) "De-pop" the switch.  
So, one set of three contacts will be used to switch where the output jack gets its feed, just as before. One set of contacts will used to direct where the input goes, to the board or the output. I don't know where the input is physically on the board, but it is where the side lug of the existing switch is tied to at the same time as being tied to the input jack. You should see a 0.1uf cap.  
Remember that caps *store charge*. When that charge lacks any opportunity to drain off, it will drain very quickly at the first opportunity provided. Most pedals will have a capacitor at the input to the circuit to prevent DC from coming in and buggering up the works, and a cap at the output to prevent the pedal from sending DC somewhere else. Usually, such caps have a resistor to ground so that they are never in a state where one end of the cap is "free" with no where for the stored charge to drain. When there IS a free end of this type, you will hear that charge-draining as a loud pop when you switch from the circuit to the jack. If you provide a path to ground for the cap, then even if you have switched the effect off, the current stored in the cap will quietly drain off through that resistor, so that you can switch back and forth pop-free.  
The DEM provides a resistor to ground for the *output* cap, but none for the input cap. However, at the moment, that isn't a problem since the cap is never switched out and either your guitar or some other pedal will always provide a path to ground for the cap end tied to the input jack. Since the conversion to TB will result in that cap *sometimes* just "hanging there", we will need to provide a bleed-off resistor for the stored current. A 1M resistor is likely quite sufficient to drain off any lingering current without simultaneously loading down the pedal too much.  
The resistor goes from the free end of the 0.1 cap to ground.  
Is that enough info?
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