Mod Duo X input impedance for guitar setup

Hi. My main use of the Mod Duo X is as an effect module on my mixer, but every once in a while, I’d like to use it as an amp in a box for an electric guitar setup. I tried it yesterday but was not impressed by how the Mod Duo X responds to guitar volume and picking and was therefore wondering whether input impedance might be the issue. Electric guitars need low impedance inputs to sound right. Any input or advice welcome.
Txs!

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Passive guitars need a high-impedance input. If the input type is incorrect, then you will lose some high-end. Have you tried increasing the input gain on the Duo X?

BTW, I run my passive bass guitars and mandolins into the Duo X and Dwarf and both work fine.

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Hi @Luke101,

As @looperlative pointed above, guitars/basses need high input impedances on the preamp or amp side of things. Mod Duo X has a 1 M Ohm input. And the typical output of a guitar is 30-40 K Ohms maximum, with volume and tone at their maximum.

There’s a lot of room for setting up the proper gain at the input stages. For straight guitar, you can start at 0 dB and then move it down until you get the dynamics to a desirable level without clipping too easily. Digital interfaces have zero ‘flexibility’ when it comes to signal level, so it’s important not to overload the input.

If you use EMG or other active pickups, then it’s a different story.

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My bad. Indeed. High impedance around 1M ohms is the norm for electric guitar amps. So the issue must be at the gain staging. I’ll test several settings until I get the expected result.

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I use passive pickups. I’ll test further based on you inputs. Thanks!

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I did some more testing and setup as suggested for a guitar setup (levels) and must say that now I’m quite impressed by the quality of the simulations. I was already sold for the infinite routing and customisation capabilities. Definitely worth investing more time.

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Once I added some speaker cabinet IRs, I felt that the emulations really came to life.

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I did further testing and found a really annoying issue with the input stage: In a nutshell no matter how low I set the input stage (down even to -12db) i get crackling distortion while none of the leds flash red at all. I get this crackle when I hit a chord really hard with my Godin neck humbucker pickup. When in true bypass I don’t get the crackling. Looks like the input stage is not handling the transients well.
PS for the testing in order to exclude any other devices, I plug my Godin straight in Input 1 and monitor from headphone directly on Mod Duo X.

I’ll investigate further with my oscilloscope and maybe post an example of the crackling noise later when I have more time to dig deeper.

How can I file a support ticket? Didn’t find it on the new Mod Audio site.

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Just added the input meter on a pedalboard where input 1 goes straight to output 1 and 2 and get crackle when hitting a chord hard on my Godin with any of the mic settings while Level meter holds as low as -5,4dBFS. Here again with true bypass on, crackle disappears.
I can’t think of anything else than the input stage not able to handle the transients. If this is the case, then either I have a defective device or Mod Duo X input stage needs a rework to allow this kind of transients.
I guess if by design this should be an issue for any user but mainly those focusing on clean sounds where this issus is a real show stopper.

@jon Adding you in the thread. Hope you can help.

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I did some further diagnosis. Actually, when true bypass is on, crackle is still there! I though it was not because strumming hard does not always result in getting levels over the threshold. This means, the culprit is the the input buffer. As soon as levels goes beyond +2,5 volt peak, input signal is clipped resulting in audible crackles.

Herewith a trace of the issue:

  • Yellow trace = Direct from guitar cable going to Mod Duo X input 1
  • Purple trace = Directly measured at Mod Duo X output 1

input settings = -12 dB

Pedal board = Straight through from Input 1 to output 1 + Level Meter on input showing hold of -5 dBFS. But same clipping when true bypass in on.

You can clearly see blue trace clipping above

@jon can Mod audio share the schematics of the analog input section? Is there a way to adjust input sensitivity? Or to add a -10dB pad with a dip switch or so?

My Godin Xtsa humbuckers is by far not the hottest out there and I’m sure a lot of clipping at input level is occurring to many customers without really noticing due to majority going for high gain/distortion sounds. But still if Mod Audio is to compete on the pro market, these things must be sorted out.

For now I’ll build a -10dB pad based on : HOW TO BUILD AN AUDIO PAD - FM Systems (fmsystems-inc.com) and do some further testing, hopefully with positive results.

Mod Audio support on this issue would be highly appreciated!

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Thank you so much for this investigation. I have noticed this with the open B string on my bass when I really dig in. Even tho the LEDs say it didn’t clip. I guessed it was so short the LED couldn’t keep up!

I’m curious, how would a -10dB pad be different to dropping the input gain on the dwarf an additional -10dB?

Thanks in advance for answering my beginner question!

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Depends on where the clipping is. @Luke101 is saying that the clipping is in the very first stage of analog coming in. In this case, the digital control can’t prevent the clipping. You can turn down the volume knob on your bass to eliminate the clipping, but none of us like to do that.

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A pad reduces voltage out of your guitar (typically added on one of your guitar cable plugs) before it even hits the Mod input section to avoid it to clip. Input sensitivity through the Mod system menu operates after the input section and probably already in the digital domain (to be confirmed by Mod Audio). Even if you drop input to -12dB, it is already too late since the input signal is already distorted.

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My guess as well since doing it before the first analog stage would add cost to the unit.

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Ok. I did some extensive research and came to the conclusion that a simple attenuator pad solution will always color the sound due to cable capacitance and serial resistors acting as a low pass filter.
Best option would be that Mod Audio modifies input section to get more headroom, but that would require a redesign and a new product. Unless they can propose a service to modify the input section with higher headroom for a fee. Another solution would be for Mod Audio to publish a “mod” with schematics and steps to implement, but that would effectively void the warranty.

So no ideal solution here, but definitely something for Mod Audio to tackle asap as a matter of priority.

@jon Can you please route this to the right people at Mod Audio to get some help? I still did not figure out how to file a support ticket…

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Sorry to learn about this issue.

You can write to support@mod.audio

Can you post either here or send on the support email a little video showcasing what’s exactly happening and when? If you write to support please also share the serial number of your device.

Overall, I’m finding it a bit odd, I never experienced that myself and can’t remember users reporting that. It may totally be the case that something is not ok with your unit. What OS version are you running on the MOD Duo X? The latest?

Ok. Your new discovery makes it a bit more odd. As far as I’m aware (and if you are actually using that), the Hardware bypass on the MOD Duo X connects the ins to the outs physically - the same way as if you were putting a cable in between. So the issue should disappear if created by the device (in theory). Isn’t it possible that something on your chain may be causing it? A broken cable for example?

Sorry for the late reply. With the holiday season, we are working less time over this week so I just managed to reach your message now.

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Hi Jon
Let me first apologize for a few errors I made mainly due to a sloppy test setup. I now made a much cleaner setup on my bench and here are some findings:

  • The clipping I showed in previous post was entirely due to sloppy measures on my oscilloscope on a too low voltage range hence consistent “clipping” in the data acquisition.

  • I confirm, there is no distortion when bypass is on. This is good news and confirms in and outs are indeed hardwired when on.

  • I also confirm the transient distortion when bypass is off and signal goes straight through Mod Duo X AD DA conversion (Input 1 softwired to out 1 and 2) and input level set to -12dB. However, looking at the distorted out signal on the scope, I don’t see any clipping. So the distortion must come from another reason. I can’t for now really compare In and out wave form due to AD DA latency preventing to time align them for comparison (I’m sure there is a way to do that on the scope but haven’t found out yet).

As requested, I’ll make a video of the setup and a sound recording straight from the mixer with Bypass on and off to illustrate the issue and send all to support via email.
Thanks for your support.

Luc.

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Thanks for clearing it up @Luke101.

Particularly on this one, I was finding it extremely weird and unlikely. Yet, it’s not impossible that thinks get broken and are not fully working as supposed to.

Please reach out to support and I will gladly take a look at the video and try to help you. I already understood that you have the skills, equipment and knowledge, so I guess I will learn a few things with you on the process :slight_smile:

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@looperlative @Luke101 thanks both for explaining

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Hi Jon. One of the things that mislead me to believe distortion was same in bypass is that I did monitor from Mod Duo X headphone output which I know now is NOTinfluenced by bypass and hence is kept on Mod Duo X output stage ofter D/A convertion no matter bypass on or off.

Still busy doing measures with Scope (confirming visible artifacts probably at source of cracles) and will also do AB recording with Bypass on and off so you can hear the crackle.
(All tests done with input at -12 db, input led always green, output gain to 0db, output leds always green)
Will send results to support later during the weeken and will let you know in this thread.

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