True Bypass Clipping? Possible?  [SOLVED]

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slammer88
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Post by slammer88 »

I've recently got a Donner DT-1 tuner, which is cheap and functional, and also has a great compact size.

However, I've noticed that despite being True Bypass, whether it's powered or not, it somehow clips my guitar signal a bit and it inevitably affects the other pedals that follow in the chain.

Normally I plug my guitar directly into my sound interface (Behringer UMC202HD) for years and set a safe amount of input gain and play/record. This way, I have no distortions, or digital clippings, or anything. Haven't owned any amp for years as a result of living in a flat with old neighbours. Recorded numerous demos and such, even an entire album!

Yesterday I tried some of my pedals in the chain (as mentioned on the power-related topic), and then I realized something was glitching my signal and affecting the rest. I've removed the pedals from the chain one by one and finally left with the tuner.

Then I've done it like this; guitar -> tuner -> sound interface and regardless of whether the power jack is plugged or not, the signal is going through indeed on bypass mode, but I hear that signal is mildly distorted, crackling. Decreasing the input volume of the sound interface does NOT help, I just hear this on a lower volume by the way, so this eliminates the digital clipping possibility due to overload of the input.

I've disassembled to see the inside with the thoughts of changing the input or output resistors, but the SMD parts are so small that I've instantly regretted my decision. Anyway, I've checked the in & out jacks and they both (tips and gnds) have contact when the pedal is on bypass, and when activated(tuner is open), tips are cut, which seems normal to me. But I just could not understand the reason why it distorts my signal a bit.

On bypass ; (the below values are the same if the unit is powered or not)

Tip to Tip = 0.4ohm
Gnd to Gnd = 0.1ohm
In Tip to Gnd = 0
Out Tip to Gnd = 0

On Tuner mode (active and powered);

Tip to Tip = 0ohm (same when power is unplugged)
Gnd to Gnd = 0.1ohm (same when power is unplugged)
In Tip to Gnd = 0ohm (same when power is unplugged)
Out tip to Gnd = 34-35ohm (this becomes "0L." when I unplug the power)

I've made a test on Cubase for both and the result is as below, also uploading a video now. The signal is clipped asymmetrically as if there's a single diode on the clipping stage of an OD pedal :)

https://ibb.co/4fk65F6

And here's the clip ->

Has anyone had any experience like this before? (distorted signal on True Bypass)

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mauman
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Post by mauman »

Your waveform pic definitely shows asymmetrical clipping, both powered and unpowered, which suggests a passive component that's passing signal in one direction only (which kind defines a diode.) Does your meter have a diode test setting? If so, with the tuner unpowered and unplugged, check tip-ground in both directions (+ lead to tip/- lead to ground, then - lead to tip/+ to ground.) Look for a diode (forward voltage around 0.6 - 0.7V) in one direction but not the other. If you don't have a diode setting on your meter, just repeat the resistance tests you posted, reverse the leads and look for 1k to 10meg in one direction only (resistance reading of a forward biased silicon diode.)

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Post by slammer88 »

I think theres no setting for diode, or maybe it is, theres continuity setting, though.

I did the readings on the lowest resistance setting of my multimeter which is 200ohm, and indeed i opened it up and measures directly over the Jacks.

As stated above, ones i have written 0 ohms are actually reading OL. and it doesnt make any movement on readings.

Now that you mentioned, i just checked again and on red probe on tip and black probe on gnd, i had literally no readings on even until 200m setting on both Jacks.

But when i probe them vice versa(red on gnd, black on tip) , i read different values on each setting on both in and out. Say, 5.2m on 200m setting, 0.68 on 20m setting, 91k on 200k setting etc for both Jacks.

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Post by mauman »

Different readings on different scales because the test current is different, so it sounds like there are one or more diodes present, with the cathode(s) connected to the signal path and anodes connected to ground.

The next step is to find them. It/they could be actual diodes, or could be transistor junctions, either standalone or within an op amp. They would have to be wired directly to the in/out jacks if they're connected during bypass. It's also possible that your "true bypass" is actually buffered, and you're seeing the buffer transistor (or op amp). Even when it's not powered you can read the junction as a diode with your meter.

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Post by slammer88 »

Here are some gutshots


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I've noticed this;

Image

In this image, I marked as I = In, O = Out jack tips. There's a transistor there, and there's also a seemingly missing part (maybe deliberately, but those 2 empty pins are shorted on bypass) that shorts in and out tips together normally. Other SMD's are so small that I can't even read the values, tho.

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Post by slammer88 »

Just for the note that my pickup output resistance seems to be 22.7 ohm on idle and when i hit all the strings hardly, I see peak of around 56-57k. (S.Duncan Blackout Active Pickups)

Nevermind, its the pots resistance.

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Post by mauman »

Looks like you do have components wired to the jacks, which explains your signal traces. My best guess is that your "true bypass" is the type that adds a hardwired link between in and out jacks, but leaves some of the in or out circuit bridged onto that path. You have a solid DC path during bypass, but that bridge (transistor junction probably) acts like a diode and clips your AC signal if it's above the diode's forward voltage threshold (probably around 0.7V for a silicon diode), and that level is easily achievable with a humbucker, and is possible with a single coil. I don't think you can remove it without buggering up your circuit.

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Post by slammer88 »

mauman wrote: 17 Dec 2023, 14:10 Looks like you do have components wired to the jacks, which explains your signal traces. My best guess is that your "true bypass" is the type that adds a hardwired link between in and out jacks, but leaves some of the in or out circuit bridged onto that path. You have a solid DC path during bypass, but that bridge (transistor junction probably) acts like a diode and clips your AC signal if it's above the diode's forward voltage threshold (probably around 0.7V for a silicon diode), and that level is easily achievable with a humbucker, and is possible with a single coil. I don't think you can remove it without buggering up your circuit.
I literally removed the dpdt switch and mounted a 3pdt,made the wiring quickly and saw that same problems persists. Wtf is that transistor anyway really..

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Post by mauman »

That's how you use a DPDT to do true bypass, that transistor turns the LED on/off without requiring a 3rd set of contacts (3PDT). You can Google "Millennium Bypass" for details.

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Post by slammer88 »

mauman wrote: 17 Dec 2023, 15:18 That's how you use a DPDT to do true bypass, that transistor turns the LED on/off without requiring a 3rd set of contacts (3PDT). You can Google "Millennium Bypass" for details.
So this is called Millenium Bypass, I've heard it before but never checked it out what it really was..

The transistor is 1AM, it says its a 2n3904. What I wonder is, would it cause this sort of a problem?

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Post by slammer88 »

By the way, why the article is marked as "solved"? Nothing is solved here? :(

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Post by slammer88 »

mauman wrote: 17 Dec 2023, 14:10 Looks like you do have components wired to the jacks, which explains your signal traces. My best guess is that your "true bypass" is the type that adds a hardwired link between in and out jacks, but leaves some of the in or out circuit bridged onto that path. You have a solid DC path during bypass, but that bridge (transistor junction probably) acts like a diode and clips your AC signal if it's above the diode's forward voltage threshold (probably around 0.7V for a silicon diode), and that level is easily achievable with a humbucker, and is possible with a single coil. I don't think you can remove it without buggering up your circuit.
Can you give some further info about this? Like how can I bypass that? It really bothers me how badly it clips my input signal which is quite present in clean tones :(

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Post by slammer88 »

Here is a video of the issue. Pedal is unpowered btw.


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Post by mauman »

I think you'd need to bypass the pedal completely, since whatever it is, remains in the signal path during the current bypass method. It's a pretty small enclosure, and the bypass switch is mounted on the PCB so that might be tough unless you rehouse it in a larger enclosure to make room for a separate 3PDT bypass switch.

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Post by slammer88 »

Its solved. And it was much easier than i expected. I Just removed the small transistor between in and out, which is a 2n3904 in a sot23 form, and i saw that its working perfectly now. Tuner function is also unaffected. I think it was either damaged or clipping the signal in a bad way for some reason.

Long live the brute force!

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