Handling headroom from ADC to DAC? How?


#1

Hi,

I’m using my MOD with an amp so I’m not using it for amp modeling or cabinet emulation so I want to use it primarily for effects. Up until now I’ve been using it with a small but loud Fender Pro Junior tube Amp but I recently added a larger Vibrolux Reverb and and I’m becoming aware that the headroom for signals between the Input Analog to Digital Converters and Output Digital to Analog converters is a bit of an issue (for me).

I fully understand the MOD’s gain adjustment structure and the ability to change gain stages and trim the input and output db, where I have difficulty is once the gains are dialed in to not overload the inputs or the outputs the signal coming out of the MOD is significantly reduced, so much so that the amp must be turned up so much that a lot of extra noise is introduced. Anyone who’s used tube amps knows there’s a ‘sweet spot’ on the dial where you get a nice full tube sound without unwanted distortion (that’s what a tube screamer is for!). So the options for fixing this are (1) turn the gains back up on the MOD… now with a small naturally saturated tube amp you can get away with it and a bit of digital overload just becomes part of the roar and gets masked… but in a larger amp with more clean headroom it just sounds nasty. Or (2) plop a compressor or limiter in your MOD effect chain and club the dynamic range down… possibly OK if you play high gain music and dont want a lot of dynamics response to your picking, but if you’re playing jazz, blues, country, indie or roots rock the magic of a tube amp is the dynamic range…

Recently I hooked up my previous analog stompbox pedalboard and realized it doesn’t attenuate the signal going to the amp (with no effects switched on), it’s much quieter and since the signal path is all analog there are no headroom concerns with the signal path so there is a lot of dynamic range. Obviously my old pedalboard is VERY limited compared to the expansiveness and configurability of the MOD but it really seems to be once the signal goes digital there is not near enough headroom to get it in and out of the MOD and preserve a nice clean strong signal without introducing overload in the internal signal path.

My point is NOT to say one thing is better or worse, I love my stompboxes and I love my MOD and I want to use both as needed… how are other people here handling this signal reduction issue with larger amps? From the developer side of things is there any software way to get more adjustment out of the ADC and DAC converters?


Volume levels input / output
#2

I always wonder witch input gain stage most users used to use?
I use the High Stage almost ever and adjust the input gain to -3db. Just out of curiosity I recently started to play with lower stage settings.


#3

The MOD has a 24 bit Codec, I’m not sure what the practical noise level is, and thats a huge part of the equation, but there should be plenty of headroom in that. Inside the mod is floating point, a full 32 bits giving a whopping 190 dB of dynamic range. Actually, the hardware specs say “DAC/ADC: 104dB Dynamic Range”, should still be plenty.

Keep in mind the in AND out clipping indicators are on the digital side (as shown here), meaning that the input gain affects the clipping level at the ADC, but the output gain has no effect on whether the DAC clips. That is strictly dependent on the digital gain you give through your virtual pedalboard. Theoretically the gain of every mod pedalboard should be around unity (so exactly as loud going out as coming in) and the levels are really controlled by your input and output trims.

You should be able to crank the output gain without any digital clipping.


#4

I could see an issue with boost pedals if you want to boost signal into a real amplifier after the Duo.

If the output is set low for the output and then use a boost plugin, I think there should be enough headroom at the end to boost into an amp.
Clipping a digital AD/DA converters is just another thing we really don’t want to think about…

Just a few years ago everyone was saying you should hit a AD/DA converter as hard as possible without clipping but the converters today are so good and we really don’t need to worry about hitting it hard any more.
My LEDs on the input and outputs are usually just green… no issues. At the loudest they hit yellow. So I have plenty of headroom.


#5

Just in case I wasn’t clear, we don’t want to clip the converters, but once your MOD pedalboard plugins are set to prevent clipping, you should be able to change the output gain setting to any value without touching how hard the DAC is hit. DAC clipping is controlled 100% by the pedalboard and not the duo’s gain settings.

EDIT: for completeness, the ADC clipping is exactly opposite, controlled only by gain settings, not the pedalboard.


#6

Hi,

Thanks for the explanation, much appreciated! Some part of my issue is the use of baritone guitars, those heavy guage strings put out a lot more signal than a standard tuned guitar, often I’m going into the red on the inputs and I’m running on the lowest gain stage with about -9db. I also make a lot of use of the volume control on the guitar so things are looking pretty well in the green or yellow with the guitar volume pot at 65% but then turning up the guitar to full volume for a solo or louder chorus part pushed the MOD into the red. If I turn the gains on the MOD down to prevent any red when the guitar is full throttle it very audibly makes the MOD MUCH quieter than the direct signal from the guitar.

In a perfect world I’d like the MOD to not reduce the direct volume at all (I mean the tonal character and volume I get if the guitar is plugged straight into the amp). And then from there give me a similar db of boost with the tubescreamer or big muff as I get with the overdrive stompbox on my analog pedalboard without being solid red on the output… I don’t go overboard with the gains in the MOD pedalboards I make… most are quite simple and the spatial effects like delay, reverb etc are not being attenuated to boost anything. I guess I have to sit down and have a really good look at all my settings again…


#7

Really I think that should be addressed in the analog realm, before it gets to the duo. It seems a shame that you need additional gear, but if you are hitting the red you’re clipping regardless of the settings of your mod pedalboard. I’ve heard of people using a TS as an attenuator that might work, or you may need to get a clean boost pedal that attenuates.

I suppose the problem is that your guitar is really hot and you want it just as hot coming out of the mod. Unfortunately the mod can’t do that so to really keep a clean signal AND get just as hot coming out you’ll have to attenuate the input before it gets to the mod and boost the output after it. The mod has that built-in but it only has so much oomph to attenuate and boost. With the really hot signal, you’ll need more. But the goal is low input gain, high output gain. You always want to avoid pushing it to red though. Then you are clipping and its not a nice sounding one. Ideally you want the input as high as possible WITHOUT ANY clipping, but putting it too low is better than too high. The output gain really just depends on what you are outputting to. If your amp does well with a hotter signal then crank it, or if it likes a starved signal that makes you crank up the amp, keep the output lower.

I suspect with your analog pedalboard you are getting much more compression than you realize. Because the dynamic range of playing softly with your volume knob down vs loud with the knob up is probably rather large, but your signal chain (even your amp) all tend to add some nice natural compression. With plugging direct into the duo you lose that and perhaps it is larger than the dynamic range of the ADC. If thats the case you’d need a compressor before the duo to do it properly. Really though there are a lot of digital pedals out there that I’ve never heard of this problem with a reverb or strymon stuff so I think there’s bound to be a way around it.

I know the humbucker on my super strat is hot enough to clip the ADC (I usually just keep the volume knob down), maybe I’ll try to play with this a little bit and see what I can get. Unfortunately I don’t have an amp to output to :frowning:


#8

I am enjoying this conversation because I’m learning, thank you all for the detailed messages. Seeing these best practices like ‘low input gain, high output gain’ and ‘theoretically the gain of every mod pedalboard should be around unity’ written out are helpful for noobs like me to read and internalize.


#9

Ugh… I was hoping that wasn’t the answer…:thinking:

I completely understand what you’re saying, however with an analog pedalboard and good clean connections you aren’t getting much significant interference with the signal until you stomp on an effect which will ‘affect’ quality and attributes of the signal (hopefully in a positive musical way). But by running hardware compression/attenuation in front of the MOD all the time the signal will always be bottlenecked in some way…:frowning_face:

Yes absolutely anything in the signal chain will affect the signal, attenuation, compression, natural overdrive, filtering (ie a wah) this occurs in both the digital and analog world. However in the analog world of tube amps its a ‘good’ thing that is pleasing to the ear and can be dialed in at the amp itself because the overload is happening right at the amp frontend and tubes reward you with pushing them hard… ADC/DAC’s don’t.

Just turning up the amp to compensate isn’t as easy as it sounds, the MOD already has considerable buffering noise (which yes can be fixed) and boosting the output gain to compensate for the lowered input signal really exacerbates the buffering noise. Also when people design amps they have a fairly narrow definition of what the input signal impedance should be and much of the ‘voicing’ of the amp and influences of the tubes, tonestacks and other circuitry (capacitors and rectifiers etc etc.) are sort of designed around that, turning something up to ‘6’ with a weaker signal when it was designed to sound nice at ‘4’ with a unattenuated signal doesn’t always give the same tonal results…


#10

well, I’m hoping I’m wrong.

@GMaq Could you record a DI signal of your baritone guitar with the knob down playing softly and some big chords with the volume at 10? We could look at that recording to estimate the dynamic range.

I understand. So with the mod analog output gain maxed you still get too much noise out of the amp? It seems to me the noise problems of the mod are the real thorn of this issue. Ideally you should be able to set the amp at its sweet spot and adjust the mod output gain to get the levels just right. But if the SNR is too poor, whether the level boost comes from the mod’s analog circuitry or the amp it will still be noisy. :frowning:

EDIT: the more I think about it (and reread your post) knowing the dynamic range would be informative, but not really that helpful to fix the problem. I think it comes down to fixing the noise issues on mod.


#11

I wonder if you’ve ever tried to use the GxBottleRocket to boost your signal inside the MOD instead boosting it at the DAC. Even if the GXBottleRocket is placed under “distortion”, it isn’t really a distortion pedal. In the Analogue world, this one is used exactly for this purpose.


For your baritone guitar even GxSVT may suit.

even if if you use your MOD in front of a Amp.


#12

FWIW, I have my input on the Low stage, set to 0dB, and the output maxed out.

All of my patches have the Calf Multiband limiter last in the chain, with the Stereo Gain just before them, nominally set to zero, but if the signal path is a bit hot, I’ll back them off… The red lights on the output come on occasionally, but with the limiter in place I never get any digital clipping, and the point at which the limiting becomes noticeable is well into its active range… It’s without doubt a studio-grade mastering limiter - I would LOVE to have it as a VST/AU plug-in for Mac! My signal has a pretty wide dynamic range - before the Duo I have a whole slew of analog pedals and TWO preamps (one solid state for EQ, one tube for loveliness), but directly before the MOD after all the pedals I have a Korg KP Mini, which steps the level down a little more. Still, I don’t need to boost the signal at the input stage (that seems to be where a lot of the noise is generated with the MOD - I’ve never had the orange lights come on on the input, never mind red…

After the Duo, it goes into an MXR Reverb, which is a little picky about how hot the signal is coming in - some of my signals while not hitting the limits of the duo can cause distortion in the MXR… So I level the signal out relative to everything out in my MOTU Ultralite Soundcard/Mixer (which I’m switching out soon, but that’s another story) - I then go from the MOTU to my stereo amp set up (Aguilar SL112 cabs and TH350 heads)…

Getting the gain structure right took a while, but it’s there now, and I have zero problems with clipping, and the only real background noise issues I have now are single coil pickup related, rather than MOD-related… The noise floor isn’t silent, but it’s not close to problematic…

No idea if that helps, but that’s my experience :slight_smile:


#13

Hi, thanks to all for the explanations and suggestions, I had reverted to my previous analog pedalboard for a while but the nice summer weather has brought in a lot more gigs so I wanted to get back to the DUO with my new amp.

In case anyone in a similar situation comes across this thread I thought I’d post what ended up working for me. I found setting my input gain at -9db on the input keeps my guitar signal from clipping for most cases with guitars in standard E and D tunings, with Baritone guitars tuned to B I found it is just easiest to just roll back the gain on the volume knob a bit otherwise those big strings push things over the top. This addresses things on the Input side of the equation…

On the output side I still struggle a bit with plugins that boost gain pushing things into the red (for instance when I step on the Tubescreamer for a solo I want both the overdrive and a gain boost). I have combined trimming my boost settings, adding a High Pass filter at 250-300hz to roll off some tubby frequencies that don’t really need to go to the amp and lastly using the suggested Calf Limiter to tame any volume spikes before they cause the DAC to digitally distort.

Lastly I have decreased the Output gains by -9db as well, this quiets the ‘buffering’ noise significantly yet still leaves enough gain to drive the front end of the amp at reasonable volume settings. I was surprised how much changing amplifiers required fine-tuning the MOD but am grateful for all the helpful comments here which helped me visualize the signal flow better. As much as I love my ‘old faithful’ analog pedalboard it is so great to be back to the infinite configurablity of the MOD-Duo! :sunglasses:


#14

Tangent, but email your local DAW maker and request LV2 support! :slight_smile: There’s no reason it shouldn’t be available on Mac in LV2 format and more support and exposure will bring more LV2 plugins to the world that could end up on your MOD. :metal:


#15

Hi @brummer

I finally got a chance to try the GxBottleRocket, it’s really cool and has a nice snarl you can’t get with just a clean boost from the ‘Gain’ pedal. When I play hard it just starts to break up very nicely!

Thanks for developing it and suggesting it!


#16

@GMaq

Thanks for your feedback, I’m glad you like it.


#17

Thanks all, great thread, very enlightening.

Where can I test this limiter? I cannot fin it in the plugin shop.


#18

Hi @Nicola
You have to enable Beta plugins in the settings menu on the browser interface (the little cog in the bottom right corner).


#19

Good. Tried. :slight_smile: Thanks.

It can be really useful in certain conditions.

It “adds” something to the sound.

I would keep it in my pedalboard just in case, but cpu is going around 97%, so I’ll leave it in bag for the moment.

Soon I’ll also try some of the beta plugins that now I can see… :slight_smile:


#20

I’ve just started using the single band Calf limiter on boards where I don’t have enough CPU for the multiband :slightly_smiling_face: