A little confused about the Tonepad Small Clone stereo mod.

[skiraly017]

I used the search function a few different way but could not find an awswer to my question.

If I'm reading the Tonepad layout correctly, the Out pad of the stereo add-on board connects to the sleeve of out jack #2. If that's the case...

1) Is this "true stereo" or just adding a second out?
2) Wouldn't the second out jack always be "live"?
3) Would the effect be truly bypassed when disengaged?

Analogman references two options for true bypass for his stereo chorus in his website. Either a 4PDT switch or a 3PDT with a relay. I'm curious and confused as to how the Tonepad build handles true bypass. Thanks.

[reb]

I bought a 4PDT from SBE for this purpose, but haven't thought about implimenting it yet.   Any ideas?

[Mark Hammer]

If you look at the schematic for the mod, you'll see that IC1b looks pretty much exactly like the existing output, which is probably why it confused you.  Look at IC1a and you'll see that it is configured as a unity-gain inverting stage (100k input, 100k feedback = x1).  What means is that when this other inverted version of the delay signal is added to dry in the usual way, it will be a dry-minus-wet (difference) as opposed to dry-plus-wet (sum) signal.  This is how stereo is achieved in a number of pedals.  Personally, I think it is problematic because when fed to two identical amps, the effect can cancel "in the air" as peaks from one output and notches from the other sum acoustically.  That being said, nothing requires you to use both outputs at the same time, and this add-on provides a choice of tones.

How to bypass/cancel it.  Let's put the emphasis on "cancel".  The vast majority of commercial chorus pedals that employ FET switching use a single FET at the functionally equivalent location of point D on that diagram.  That is, a FET is interposed between the output of the last filter section after the BBD, and the point where the wet signal meets the mixer stage.  Once you cancel the wet signal, the effect simply disappears.  All the buffers remain in place, but the effect itself is gone.

So, the question becomes one of "How desperate are you to have 'true bypass' AND stereo?".  If you're fine with having the input and output buffers remain in place, then all you need is a DPDT stompswitch to both cancel the wet signal and engage/disengage a status indicator LED.  In theory, the input and output buffers should contribute nothing.  In theory.  In practice, you may get lucky, or you may not.  Note that the 6k8/.01uf network in each buffer is there to provide a pre-emphasis/deemphasis effect; a poor man's Dolby, if you will.  The network has the effect of adding a treble boost on the input, which is complemented by a corresponding treble cut on the output.  If they are mismatched networks (and tolerances do exist), then you might end up with "cancel" resulting in a bit of treble loss (more de-emphasis than pre-emphasis) or treble boost (more pre than de) than you would get if the unit were completely bypassed by straight wire.  In the grand scheme of things, you'd rather have a bit of treble cut to ward off accumulated hiss in the signal chain, but that's just my own perogative.

If you feel squeamish about having anything less than true bypass, then you'll need to use a 3PDT or the recently available 4PDT for those who want stereo, true bypass, AND status indicators.  Just remember that all those Boss, Arion, and other chorus pedals that people have gushed over for years, have ALL used one lousy stinking measly FET to cancel the effect, leaving the input and output buffers (and all their pre-emphasis and de-emphasis) in place.  Of course this has all had a horrific effect on the history of music and the recording careers of your favourite performers.

I shouldn't sound so snarky, I suppose.  If your rig is a couple of pedals then the wet-lift kind of effect cancellation is more than good enough.  If your rig is 10 pedals in series, with a buffer in the lead-off position, then I can see where you might want to maintain TB across the pedalboard.  A quarter db, 3/8db there, and eventually you end up with measurable hiss and audible treble loss.  Just remember that there ARE many situations where the audible consequences of the "single-FET solution" are negligible.  That's why all those companies could use it all these years.  If your circumstance is equally benign, then you can accomplish what you want at low cost, without the complexity and cost of wiring up a 4PDT.

[skiraly017]

If you look at the schematic for the mod, you'll see that IC1b looks pretty much exactly like the existing output, which is probably why it confused you.  Look at IC1a and you'll see that it is configured as a unity-gain inverting stage (100k input, 100k feedback = x1).  What means is that when this other inverted version of the delay signal is added to dry in the usual way, it will be a dry-minus-wet (difference) as opposed to dry-plus-wet (sum) signal.  This is how stereo is achieved in a number of pedals.  Personally, I think it is problematic because when fed to two identical amps, the effect can cancel "in the air" as peaks from one output and notches from the other sum acoustically.  That being said, nothing requires you to use both outputs at the same time, and this add-on provides a choice of tones.

Fairly understanding things up to here. I did notice that the add-on output and existing output looked the same.

How to bypass/cancel it.  Let's put the emphasis on "cancel".  The vast majority of commercial chorus pedals that employ FET switching use a single FET at the functionally equivalent location of point D on that diagram.  That is, a FET is interposed between the output of the last filter section after the BBD, and the point where the wet signal meets the mixer stage.  Once you cancel the wet signal, the effect simply disappears.  All the buffers remain in place, but the effect itself is gone.

Still with you.

So, the question becomes one of "How desperate are you to have 'true bypass' AND stereo?".  If you're fine with having the input and output buffers remain in place, then all you need is a DPDT stompswitch to both cancel the wet signal and engage/disengage a status indicator LED.  In theory, the input and output buffers should contribute nothing.  In theory.  In practice, you may get lucky, or you may not.  Note that the 6k8/.01uf network in each buffer is there to provide a pre-emphasis/deemphasis effect; a poor man's Dolby, if you will.  The network has the effect of adding a treble boost on the input, which is complemented by a corresponding treble cut on the output.  If they are mismatched networks (and tolerances do exist), then you might end up with "cancel" resulting in a bit of treble loss (more de-emphasis than pre-emphasis) or treble boost (more pre than de) than you would get if the unit were completely bypassed by straight wire.  In the grand scheme of things, you'd rather have a bit of treble cut to ward off accumulated hiss in the signal chain, but that's just my own perogative.

If you feel squeamish about having anything less than true bypass, then you'll need to use a 3PDT or the recently available 4PDT for those who want stereo, true bypass, AND status indicators.  Just remember that all those Boss, Arion, and other chorus pedals that people have gushed over for years, have ALL used one lousy stinking measly FET to cancel the effect, leaving the input and output buffers (and all their pre-emphasis and de-emphasis) in place.  Of course this has all had a horrific effect on the history of music and the recording careers of your favourite performers.

I shouldn't sound so snarky, I suppose.  If your rig is a couple of pedals then the wet-lift kind of effect cancellation is more than good enough.  If your rig is 10 pedals in series, with a buffer in the lead-off position, then I can see where you might want to maintain TB across the pedalboard.  A quarter db, 3/8db there, and eventually you end up with measurable hiss and audible treble loss.  Just remember that there ARE many situations where the audible consequences of the "single-FET solution" are negligible.  That's why all those companies could use it all these years.  If your circumstance is equally benign, then you can accomplish what you want at low cost, without the complexity and cost of wiring up a 4PDT.

This were I start to get to get lost. So as laid out by Tonepad...if built with the stereo mod and a 3PDT is used with a LED then the pedal will not be true bypass? Thanks.

[Mark Hammer]

yes and no (gawd, why does he keep DOING THAT!!!! )

Let's take inventory:
- you need one set of contacts (#1) to switch the input jack from going to the circuit board to going to the output
- you need one set of contacts (#2) to switch power to a status LED

- you need one set of contacts (#3) to switch the output jack between the output of the board and the feed from the input jack

That's 3 sets of contacts, or a 3PDT.  If you are building a mono version, that's exactly enough contacts to get you true bypass with indicator LED.  If you are building a stereo version with true bypass, then you need a fourth set of contacts to replicate what #3 did, only on the additional output.  That's a 4PDT.  When put in the bypass mode, the input jack will be hardwired to both output jacks with no isolation between them.

If you were to use a DPDT switch, with one set of contacts providing power to the status LED, and the 2nd set making or breaking the connection between the wet signal and the two mixing stages, you would have two electronically isolated outputs, suitable for feeding to two ampliers without interaction.  This is electronically analogous to what normally gets done with FET switching, only you'd be using a physical switch instead of an electronic one, and your foot instead of a flip flop.

Clearer now?

[skiraly017]

Clearer now?

Yes, thanks!

[puretube]

I used the search function a few different way but could not find an awswer to my question.



1) Is this "true stereo" or just adding a second out?
...

that is the question!

if it were true stereo, it`d have 2 inputs + 2 outputs...
for this you`d need a 4PDT for truely bypassing the signalpath;
& either an additional so-called "millenium" LED indicator switching,
or a 5PDT for the LED.;

or a relay solution...