Lower Audio Interface Noise Floor

James Freeman

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2,790
This might not sound that impressive but you can lower any audio interface noise floor by 3db if you have two Instrument inputs, this is basically a freebie.

I knew that two identical signals sum to a 6db boost, and two random noise sources of the same volume sum to a louder noise by only 3db.
So I paralleled the two Instrument inputs of my audio interface and it works as expected, I get a lower noise floor by -3db when I sum to mono and lower by -6db to get the original guitar signal level.
Note: Parallel inputs halve the input impedance, if you think 500k is not enough use a buffer pedal.

Interestingly, when using Helix Native I don't have to do anything in my DAW before the plugin, I just selected the L+R Instrument recording inputs in my track and Helix Native automatically lowers the input stereo signal by -6db when I put any mono block in the signal chain like an Amp block, resulting in a -3db lower noise floor.

And of course this little experiment is useless if you already use a Helix/HX, AxeFx/FM or any other Audio Interface with 120dB+ of dynamic range for the Instrument inputs.


Y Adapter Interface.jpg



Helix Native, mono vs paralleled stereo input and a single mono gain block in the chain:
spectrum.png



PS.
If you have an ADC chip with 8 parallel inputs you can achieve 9db lower noise floor than the chip specification.
I guess that is how modelers achieve better than 120dB+ of dynamic range even though the ADC chip is around 110-114dB.
 
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This might not sound that impressive but you can lower any audio interface noise floor by 3db if you have two Instrument inputs, this is basically a freebie.

I knew that two identical signals sum to a 6db boost, and two random noise sources of the same volume sum to a louder noise by only 3db.
So I paralleled the two Instrument inputs of my audio interface and it works as expected, I get a lower noise floor by -3db when I sum to mono and lower by -6db to get the original guitar signal level.
Note: Parallel inputs halve the input impedance, if you think 500k is not enough use a buffer pedal.

Interestingly, when using Helix Native I don't have to do anything in my DAW before the plugin, I just selected the L+R Instrument recording inputs in my track and Helix Native automatically lowers the input stereo signal by -6db when I put any mono block in the signal chain like an Amp block, resulting in a -3db lower noise floor.

And of course this little experiment is useless if you already use a Helix/HX, AxeFx/FM or any other Audio Interface with 120dB+ of dynamic range for the Instrument inputs.


View attachment 4865


Helix Native, mono vs paralleled stereo input and a single mono gain block in the chain:
View attachment 4866


PS.
If you have an ADC chip with 8 parallel inputs you can achieve 9db lower noise floor than the chip specification.
I guess that is how modelers achieve better than 120dB+ of dynamic range even though the ADC chip is around 110-114dB.
Cool to see it working in DIY practice! What are you using to get two identical signals?

And what's going on in that photo? Are you doing parallel two ways, a)parallel into input 1 and b)parallel input 1 and input 2?
 
Yes, paralleling devices is an old trick to improve SNR because correlated signals increase by 6dB but uncorrelated signals (noise) only increase by 3dB.

This was even done in the tube days and you even see it in tube amps today. Both halves of the input buffer will be wired in parallel. The downside, however, is the Miller capacitance is also doubled.

You can do it with op-amps too. Put two or more in parallel and tie their inputs and outputs together.

Some ADCs can parallel the inputs inside the device itself.
 
And what's going on in that photo?
Parallel inputs. Y splitter and a jumper cable.

In the bottom image I've used stereo white noise generator and stereo function generator, the latter allows Dry signal to pass so I can see how both signal and noise behave when goin through Helix Nagive.

Another thing I forgot to mention in the first post is noise sums according to Power law so paralleling inputs for the purpose of improving SNR diminishes returns very quickly.
For example the 'improvement' between 8 and 16 inputs is only 3db, so 8 is probably what manufacturers use and because ADC chips with 8 inputs are common like CS5368 and PCM1863.

EDIT:
Found a 1/4" mono Y splitter in my box of things.
I replaced the image in the first post.

Y adapter.jpg
 
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@Digital Igloo @FractalAudio

You know what I would buy?
I would buy a small box with your patented super quiet Guitar Input and a SPDIF+Optical outputs to be used with other audio interfaces that have digital inputs.

The point is to strip all ADC/DAC stages and USB buffers but retain the specialized super quiet guitar input.
A true upgrade to any old or new audio interface with SPDIF Inputs for recording guitar with the best SNR available on the market.
It should be cheaper than a modeler of course, that's the whole point.

Using my Helix Floor and sending the Guitar input to Digital Output into my SPDIF input on my audio interface then DAW, the signal to noise ratio is unreal, clean 127dB of dynamic range!
Super low latency from the Helix itself, just a single ADC stage and a straight line in the DSP routing chain.
* I also bypass the Volume Knob on Digital Outputs for unity gain.

When using Helix Native with the same amount of gain it is quieter than my tube amps which is already super quiet with DC filament heaters.
Other guitar plugins like Amplitube never sounded better.


Helix Noise Floor.png


-

Unfortunately SPDIF output does not exist on all modelers that have super quiet Guitar Inputs.
HX Stomp and ToneX do not have Digital Outputs at all, Helix LT has an AES port you can use with a Female XLR to RCA adapter or cabe, it's the same signal as SPDIF, I've tested it personally and it works.
FM3 and FM9 have SPDIF output so you can use Fractal's super quiet "Secret Sauce" Guitar Inputs with your audio interface for a much better experience with plugins.
 
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The point is to strip all ADC/DAC stages
Surely it would need an ADC to convert it to digital for SPDIF?

and presumably this would need powering in some way? and being spdif, it opens questions on clocking and meeting any kind of technical requirements for that format.

RME Digiface USB looks useful for combining digital gear together.
 
Surely it would need an ADC to convert it to digital for SPDIF?
and presumably this would need powering in some way? and being spdif, it opens questions on clocking and meeting any kind of technical requirements for that format.
Yes of course, you know what I mean.
All the proper engineering to make it work, clean power, clocks, sampling rates, etc.

Just make it a small and affordable ADC box with one or two (SPDIF is Stereo) super quiet patented Hi-Z Guitar Inputs like we have in the Helix and AxeFx and SPDIF+Optical outputs.
Maybe also an analog Pad, but it worsens slightly the SNR, I never needed a Pad with EMG81 and Helix.
 
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I have to admit, within reason I just don't care about noise floor at all - I've found about -85dB to be the point I start to notice it when recording multi-tracks, and I found that out when I had low-level ground noise coming from switching noise in an ungrounded firewire PCI card's back panel; -85dB over 12 input channels was enough to notice. But that was high frequency, constantly changing noise that drew the ear to it. When I fixed the ground issue (removing powder coating on case) it went down to -95dB and that was enough for me.

If the noise is constant shaped white/ pink noise and in certain contexts, even -70dB isn't really a big deal unless you're compressing a fair bit, it's just something that needs gating/ dealing with in quiet passages & at starts and ends of tracks.

In guitar signals I'm even more lax - hiss from overdrives at -30dB? It's just part of the magic. It's MEGADITHER.
 
Just make it a small and affordable ADC box with one or two (SPDIF is Stereo) super quiet patented Hi-Z Guitar Inputs and SPDIF+Optical outputs.
Maybe also an analog Pad, but it worsens slightly the SNR, I never needed a Pad with EMG81 and Helix.

Right yeah - in my head I was imagining something really small, but I think it would either need a barrel connector hanging out of it or some kind of battery power (no idea if it would be possible to get a low noisefloor/decent performance that way). I suppose this kind of device would again have the input calibration question marks on it depending on what modeller its being used with. I can comfortably clip the HX Stomp input with passive pickups - personally I'd prefer an ADC with higher headroom rather than adding a pad to the guitar's circuit, especially if the goal is to improve the input that's already on the device.

I've been using SPDIF to route my Kemper for years. FM-3 only has SPDIF out though. As much as I'd love an SPDIF input, the FM-3 is still massively overkill for how I use it and an AxeFX III would only be about the rackmount format and better I/O.

One annoying thing about SPDIF is most interfaces only have one or possibly 2 at the most in's and out's for it. It's also not uncommon these days to use seperate A/D and D/A in conjunction with a bigger interface - for instance, many monitor controller's have their own D/A built in and people may have a seperate 2 channel A/D (for printing an analog mix back in with, or 2 "money" channels).

May be tough to sell a guitar-centric input chain in high enough quantities to make it affordable, annoyingly I think its just one of those things where you have to pay an order of magnitude more for a small difference (if its something you care about that much).

My main DI chain (Avalon U5 and RME ADI-2 PRO FS R BE) wasn't cheap, but both devices offer a lot of flexibility in terms of features and routing that overall make it a more economical purchase
 
Right yeah - in my head I was imagining something really small... barrel connector hanging out
Nay, something like a direct box in size and construction, a solid professional unit.

I can comfortably clip the HX Stomp input with passive pickups.
I'd prefer an ADC with higher headroom rather than adding a pad
I believe you have to pad the analog input before ADC to achieve greater digital headroom, the ADC chip already operates at maximum safe supply voltage.
Fractals manual specifically says that adding pads will worsen SNR by the same amount, it is automatically compensated so the models have the same gain with or without pads.
 
I believe you have to pad the analog input before ADC to achieve greater digital headroom, the ADC chip already operates at maximum safe supply voltage.

would these specs be the other way around if a pad was being used for more headroom? I mean, its all in the realm of making bugger all difference but I basically always leave it at +24dBU

Screenshot 2023-03-21 at 11.49.15.png
 
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