MOD-UI has strange artifact or crackling sound

Well I’ve got the device up and running and I really enjoy it. I started playing with the MOD-UI interface (which is super intuitive I think) and started creating pedalboard signal chains.

A quick search didn’t reveal a similar topic, but I might have used different search terms.

A question for those using this device in this manner… I notice a strange popping/artifact sound when playing. I would assume that’s the processor, however the temperature is 60*C and I’m only using a minimal portion of the processor.

I have a Pi 4 with 8gb RAM, HiFiBerry DAC + ADS Pro and updated software earlier this week as this is a new build.

MOD-UI Screenshot CPU

Here are some quick sound files I recorded by running the output jack into my Mac/DAW with no post processing.

First file has several effects pedals with moderate settings and my passive fretless bass with soapbar pickups. I believe there was some dirt, reverb, and maybe a little chorus.

Second is pretty clean with only a compressor in the signal chain. But I compressed it pretty heavily to see how it would react with the fretless bass.

The artifact isn’t consistent, and maybe it isn’t even that bad… but if there’s a way to clean that up I’d love a nudge in the appropriate direction. I would like to gig with this device and prior to that I want to play it around the house and do some serious breaking in to make sure I’m comfortable with it’s limitations prior to using it in public. I didn’t see any settings that would let me adjust sampling down from where I was.

Thanks so much!

Just had a late night thought. Wondering if I’m experiencing an impedance type issue. Passive bass straight into the Zynthian. Maybe a preamp will help? I’ll test that out tomorrow and see if it makes a difference.

Hi @chris

I hear a regular splat approximately every three seconds which is what I think you are referring to. First, this isn’t caused by impedance mismatch but it is advisable to buffer the pickups as the input impedance of Zynthian is too low for direct connection of guitar (including bass) pickups. This will give you a better frequency and phase response and hence sound less muddy.

The splat is often caused by xruns which is when the sound system is processing too few or too many audio samples within a period. This is most often caused by the CPU being unable to perform all of the signal processing within its assigned period. Because this is time critical, it may not show as high average CPU usage in monitoring apps like top because the peak processing limitation may occur for just a few milliseconds. Xruns can also occur if the soundcard or its drivers struggle to transfer audio samples at the correct rate.

To help diagnose the issue, show us the audio configuration from webconf and describe the hardware, e.g. is it a standard V4.3 kit? What inputs and outputs are being used? Is “Headphones” enabled in webconf?

The artifact occurs at regular intervals which could be due to a cyclic process but that doesn’t seem to fit with your described configuration which makes me think this may be a timing / sync issue. If something is not synchronised then it will eventually fill or empty it’s buffers and there will be a discontinuity which results in this kind of sound.

[Edit] 60°C is quite hot. Maybe it is getting closer to thermal throttling. A heatsink may help.

@riban ,

Thank you for the reply and information.

Here is my hardware setup:

Raspberry Pi 4 Model B Rev 1.4
HiFiBerry DAC+ADC Pro
Waveshare 7" HDMI display
Zynthian Basic Kit v2

I have it mounted in a Pelican type case that I’ve posted about in another thread.

Here are the audio settings:

I’m not quite sure what to tweak here, but I do recall seeing something mentioned about Headphones in an older post. However I thought that was a non-issue now.

I was wondering about the temperature, but can’t find great information on that. I have some heatsinks arriving today and I also just installed a fan for some more aggressive cooling… however I need to think about airflow to make the active cooling effective (i.e. cross case airflow with inlet and outlet).
[edit] I thought throttling wasn’t an issue until we hit the 70’s… however I’ve absorbed so much information since I started this build I could be mis-remembering.

So I might have a few options for improvement. I can easily address the cooling today and see if that makes a difference. Also, if it’s as simple as unselecting the headphone option I would be greatly pleased.

Thank you again,


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Yes - deselect headphones. This option adds a samplerate converter to feed the headphone output and enables the headphone (onboard sound) driver. Both of these things add significant load to the CPU.

RPi 4 doesn’t throttle until 80C.


I think that did the trick.

I de-selected the headphones and played it for quite a while. No more issues. I also added a tube preamp that I built a while back between the bass and the Zynthian, which really cleaned up the overall sound.

Additionally, my heat sinks arrived and I installed the fan (but forgot to order a guard screen… sigh) and that’s made a huge difference in the temperature management. Now I’m consistently hovering in the upper 40’s. The starting temp was 37 and the highest I’ve seen so far is 52 with me testing out tons of different effects. Way better than the upper 60’s I experienced previously.

Now I just need to sort through all these pedals and see which ones do what I really need them to do… an adventure unto itself.

Thank you again,



That’s great! Thanks for persevering. I tend to avoid fans, preferring passive cooling to minimise noise. I do plan to include a fan in my 19" rack based rig but with a speed controller so that it only runs when required. My thinking is that if it is hot then it is doing something and the audio being produced should mask any fan noise.

You may wish to consider using Zynthian’s native hosting of plugins, maybe after playing with mod-ui to figure out what you want. I suspect there would be less overhead and hence lower risk of xruns. (I haven’t benchmarked this but it seems likely.) Also there are some limitations in Mod-ui that you may find an issue. Mod-ui is very pretty and gratifying to use and Zynthian can learn a lot from its graphical workflow but in essence it does very similar things to Zynthian.

Good luck with your sound design and keep us informed of progress.

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I’m so glad you mentioned the native Zynthian hosting. I didn’t realize I could use those effects in that manner because I always stopped when it tried to assign a MIDI channel and most of my searches veered me towards the MOD-UI interface. I agree that the MOD-UI is visually appealing, but I like the idea of completely managing effects from the Zynthian device and not a browser based interface. I played with a few of the effects and really liked how cleanly everything went together. Now I just need to read up on addressing the gap analysis of what’s available in MOD-UI vs. the Native Zynthian effects. Mainly I need a good octave pedal… just about everything else is covered through the native Zynthian workflows.

I almost didn’t add a fan, but I thought I might as well add it now as opposed to wishing I had added it at a later date. Figured if I was going to build this for stage use then I’d try to add everything I thought I might need. It’s pretty quiet and once I get on stage I wouldn’t even be able to hear it so it doesn’t bother me. I might add an on/off switch, which I’ve seen on several Class D boutique type amps before.

When you do build your rack based rig please let me know! I almost went that route with this build because one of my rigs is rack based and it seemed natural to go that route, but as a neophyte I’m display dependent until I become more comfortable with the workflow.

If/when RPi4’s become available again I’ll probably tackle that as well. I’ve got a few Tillman Preamps laying around that I built up a while back with a true bypass toggle that would be great in this application. I originally built then as on-body preamps for acoustic guitars and mandolins with a battery life of approximately one year. I gave away most to friends but kept one completed preamp and a few small boards. My other hobby is woodworking so I had some mesquite and ebony scrap that I put to good use.

I think that all the plugins available in the Zynthian’s Mod-UI implementation are (or should be) available within the native hosting. You have to enable plugins in webconf.

My 19" rack will have a Behringer 1820 for 8 analogue inputs and outputs and a class D stereo amplifer. I will use the audio inputs for mics and guitar and be able to provide a FoH mix with effects, etc. I will also use synths so I am hoping this box will provide PA, synths, guitar processing and mixing for small gigs and rehearsals for my band. (I just need to put the band together… and the 19" rack!)

Nice bit of woodwork in your pictures. I am quite impressed!

Yes that’s an absolutely great feature.

yes, and imho, seeing some kind of “real or so” pedals could help you to understand how all these plugins are working, while when they are displayed in Zynthian UI, they look so cryptic.

That’s why Mod-UI web interface is so helpfull. Once you have identified the good knob(s), button(s) that works how you like it, using the Zynthian UI for hardware Midi controls assignments is easy.

I agree. And I like your Zynthian box too :+1:

Is this the Tillman Preamp you are talking about :

If yes, how is sound quality improvement when used VS a guitar directly plugged in ZynADAC ?

OK that sounds amazing! Please document that build as I am very interested in how that might turn out. I’m a jazz musician, but we call our stuff “Free Range Jazz” as we often wander outside the lines. I’ve been known to add a few odd effects to the electric bass our upright double bass on occasion… which is the main reason I’m building the Zynthian.

Thanks so much. I have a full woodshop out back and love working with wood.

That’s the one! In my experience I’ve been impressed with it. Very clean buffer/preamp. However I’ve read that some hum buckers and such don’t like the JFET preamps and can add a little noise to the signal chain. However I sometimes wonder if those people are expecting it to be something it isn’t.

I’ll try an post some audio showing the difference shortly.

This is a known issue with JFET preamps. I can’t recall why.

If you are using Testing branch then update and see an improvement in the display of effects chain. It still isn’t perfect and a GUI / block type presentation would be better and we would hope to see that in the future.

I don’t know either… but I’ve watched guitar players argue about it longer than I recall on pedal building forums.

Here’s a quick sample I just recorded… as an aside… I just learned about the built in recorder functionality. How nice is that? Records, uploads to my Mac, and it’s even a WAV file! This crazy Zynthian device…

For the recording, I used my jazz style bass because it’s pretty bright when played passively. First chorus is with the Tillman preamp level at 50%, second chorus (after the long pause for me to toggle it off) is just the bass directly into the Zynthian. I had to do it at 50% so you could hear the bass with the preamp off because the disparity in volume was so great. One of the great things about the Tillman is you can adjust the resistors to tighten up the sound you want with either a set value or potentiometers. I have mine set up to open up the low end slightly and add a little brightness to the top. I don’t remember the resistor values off the top of my head, but they were pretty close to the original schematic.

Only effect I had active was the Calf Compressor with a 50/50 wet/dry blend. Thought some straight up vanilla style blues would be the easiest for me to hit the greatest range of notes in the shortest time.


Hi @chris , thank you for your detailed answer.

Listening to the sound demo with the built in speakers of my laptop let me hear a little difference but not so huge imho. I have to test it later with my monitors.

=> is it worth using a preamp ? They are such a bunch of digital audio effects that it should be possible to have the same result without using a preamp.

Anyway the circuit is really simple and in my opinion it’s good to have an adjustable hardware gain. So I will give a try, this will be added to my pending basket at Mouser :wink:

If your on Zynthian stable, I suggest you to burn another SD card and then to switch to Zynthian Testing for having even more craziest features :slight_smile:

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Some of my Zynthians are on stable others on testing.

In fact, I do not use effect chains a lot. But I will have a look.

You really shouldn’t hear a huge difference. In theory you only want more volume. The only addition this preamp adds (in my opinion) is a little sparkle on the top, which is exactly what I want in a preamp. If I had upped the level to max, there would have been a pretty huge difference in volume level. It’s a simple build that I’ve done on both Vero board and PCB.