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Scarlett 2i2 vs 18i8 gain range

Postby fladd » Tue Dec 26, 2017 10:39 pm

Hi there.

I have a Scarlett 2i2 1st gen and a 18i8 2nd gen. Specs of 2i2 state max gain of 46dB. Specs of Scarlett 18i8 state max gain of 50dB. From this I expected the 18i8 to have more gain (i.e. to amplify a mic more at max gain). However, it is the opposite. The same mic at max gain is louder on the 2i2.

Can someone explain this to me? I apparently do not understand the max gain specs correctly.
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Re: Scarlett 2i2 vs 18i8 gain range

Postby Hugh Robjohns » Wed Dec 27, 2017 12:50 am

This is actually a gain-staging question and I think the main issues here are a confusion over the difference between gain and absolute signal level, and the absence of a specification dor the sensitivities of the A-to-D converters in the two different interfaces.

On the face of it, and assuming the same sound source volume in front of the same mic at the same distance, it would appear that the 18i8 can raise the mic level by 4dB more than the 2i2, in the analogue domain.

However, before you get to use that signal in the computer the analogue signal must be fed into the A-D converter in the interface and converted to a digital signal.... But I don't see any specification of either the reference or maximum peak levels of the converter, so we don't know the precise relationship between the signal levels in the analogue and digital domains.

If i were to hazard a guess, I'd estimate that the 18i8 2nd gen is configured with substantially more headroom than the original 2i2 1st gen which received some bad press for a lack of headroom. If that's the case, that increased headroom will translate as an apparently lower digital signal level, even though the analogue stage appears to provide slightly more input gain.

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Re: Scarlett 2i2 vs 18i8 gain range

Postby fladd » Wed Dec 27, 2017 1:52 am

Thanks a lot for the detailed reply.

If I understand you correctly, then there is no such thing as absolute gain. This seems to have several very important and rather surprising implications:

(a) For a manufacturer to provide, and for a customer to read the maximum gain value (in isolation) is completely uninformative.

(b) Statements such as "an SM7b needs at least 60dB of gain" are entirely meaningless.

(c) There is no a priori way of knowing if an interface/preamp will provide enough amplification for a given microphone.

(d) Comparing the amount of amplification between interfaces/preamps is not possible without actually testing them against each other.
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Re: Scarlett 2i2 vs 18i8 gain range

Postby Hugh Robjohns » Wed Dec 27, 2017 11:46 am

fladd wrote:If I understand you correctly, then there is no such thing as absolute gain.

Yes, there's absolute gain -- how much bigger or smaller the signal is made, overall -- but that is the sum of all gain stages, including that of the digital conversion process. In the case of your two interfaces, I suspect the 2i2 effectively has less analogue gain but more digital gain than the 18i8.

Let me try and explain it with some numerical examples ...

Let's say the output voltage from your microphone is 3mV (0.003V).

If the first preamp boosts that by 46dB the signal level will be raised to 0.6V (in round numbers).

If the second preamp boosts it by 50dB it would produce an analogue output signal level of 0.95V.

...and in the analogue world, the latter is obviously (4dB) louder than the former, as you would expect.

However, let's say that these analogue signals are fed into different A-D converters, and the first converter is set up so that it reaches 0dBFS (clipping level) with an input signal level of 1V. In that case, the input of 0.6V produces a digital output level of -4.4dBFS.

But let's say the second interface is set up with 6dB more headroom so that it delivers 0dBFS only when the input voltage reaches 2.0V. In that case the input signal of 0.95V produces a digital output of -6.5dBFS.

...and consequently the output from the second interface is actually 2dB lower in the digital domain than the that from the first preamp, despite that first model having 4dB less analogue gain.

And the reason is that although it has 4dB less analogue gain, it effectively has 6dB more digital gain (from the conversion alignment) and thus 2dB more gain overall from analogue input to digital output.

This seems to have several very important and rather surprising implications:

(a) For a manufacturer to provide, and for a customer to read the maximum gain value (in isolation) is completely uninformative.

Well, not completely uninformative. It is still useful to know the available analogue gain range. 40dB is about the minimum you can get away with with close-miked sources. 60dB is typically required for quieter sources or less sensitive mics, and 75dB or more for very insensitive mics with distant placements... for example.

(b) Statements such as "an SM7b needs at least 60dB of gain" are entirely meaningless.

Without a context, arguably, yes. Although in the context of a typical analogue preamp/mixer, it's a handy rule of thumb to estimate the amount of gain required in a typical application.

(c) There is no a priori way of knowing if an interface/preamp will provide enough amplification for a given microphone.

I refer you to my earlier answer re typical 40/60/75dB gain range applications.

(d) Comparing the amount of amplification between interfaces/preamps is not possible without actually testing them against each other.

This one is definitely true because, as I said in my previous response, few manufacturers specify the the analogue-digital conversion alignment, and thus the gain through to the digital domain.

Out of interest, what sort of digital level difference are you getting from your two Scarlett interfaces?

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Re: Scarlett 2i2 vs 18i8 gain range

Postby ef37a » Wed Dec 27, 2017 6:23 pm

"Have I not so informed you all thusly?"

Hah! I have posted before that a bare "55dB" figure for the gain of an AI pre amp is pretty meaningless, as Hugh has pointed out, we need to know the input voltage (in mV or dBu) that produces 'X' dBFS in recording software. Once we know that we can make an informed decision how the AI will work with any available microphone. Generally mic mnfctrs are good at giving a sensitivity figure but in any case I think most folks know a 57/58 will put out ~1mV/Pa, most capacitors 10-20dB above that and the Sm7b rather less. The dynamics of other makes are almost all AS sensitive as the 5Os but few more than 6dB hotter.

For a mixer, especially one of "pro" aspirations, a flat gain figure or range is fine since we can reasonably assume it is referenced to an output of +4dBu (and if it was not, Hugh would VERY soon be telling them off!) but as this tread has shown, the gain is given almost universally with no reference.

AI specs also rarely give the line output for say, 0dBFS. However, all this is moot since what we REALLY need is a good idea of how noisy the pre amp is? Lowish gain can be fine IF the noise is still acceptable when we boost digitally. Then again (and Hugh said this recently) Room noise is almost always the limiting factor for any mic/AI combination.

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Re: Scarlett 2i2 vs 18i8 gain range

Postby James Perrett » Wed Dec 27, 2017 8:45 pm

When we are talking Focusrite their biggest weakness is often too much gain at the minimum gain setting. If the 2i2 shows this problem then it seems they still haven't learnt from their earlier mistakes with their older interfaces and some of the Octopre devices.
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Re: Scarlett 2i2 vs 18i8 gain range

Postby ef37a » Wed Dec 27, 2017 9:15 pm

James Perrett wrote:When we are talking Focusrite their biggest weakness is often too much gain at the minimum gain setting. If the 2i2 shows this problem then it seems they still haven't learnt from their earlier mistakes with their older interfaces and some of the Octopre devices.

But it is a tough call "marketingwise" James! I do agree, a wider gain range would be ideal but virtually impossible with the one knob gain control almost all AIs are stuck with. 50db or so is about the limit if you are going to get a smooth, progressive change in gain* over the range of the pot but then the pre amp is "low gain" compared to its competitors, so the Marketing Men push for more max gain and so minimum gets greater.

One solution of course is a pad switch but this adds considerable cost at a very competitive end of the market (the switch and associated resistors probably costs AS much as the whole of the mic pre electronics!) The most elegant solution is digital control.

* A similar psychology is found in some guitar amp designs where the main VC has a "S or Bust" law making the inexperience buyer think "Flipping 'eck! If THIS is how loud it is on 2......?"

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Re: Scarlett 2i2 vs 18i8 gain range

Postby James Perrett » Thu Dec 28, 2017 12:29 am

ef37a wrote:But it is a tough call "marketingwise" James! I do agree, a wider gain range would be ideal but virtually impossible with the one knob gain control almost all AIs are stuck with.

As a point of comparison, the Focusrite in question is currently in my mobile rack sitting between an Audient ASP008 and a Behringer ADA8000. Neither the Audient or the Behringer are troubled by the sort of signals that overload the Focusrite's preamps.
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Re: Scarlett 2i2 vs 18i8 gain range

Postby ef37a » Thu Dec 28, 2017 6:46 am

James Perrett wrote:
ef37a wrote:But it is a tough call "marketingwise" James! I do agree, a wider gain range would be ideal but virtually impossible with the one knob gain control almost all AIs are stuck with.

As a point of comparison, the Focusrite in question is currently in my mobile rack sitting between an Audient ASP008 and a Behringer ADA8000. Neither the Audient or the Behringer are troubled by the sort of signals that overload the Focusrite's preamps.

Both the Audient and the Behringer are mains powered and I know the ADA8K at least has +&-15V supply rails conferring a higher headroom compared to a USB 2.0 'bus' powered device.

The extra 2 watts available from USB 3.0 is something I had hoped AI makers would eagerly grab to improve headroom, phantom power levels and fix feeble headphone sockets on "portable" AIs. They do not seem interested in doing this?

Now that this issue has been aired in the forum, can we expect a headroom test as part of AI and related devices reviews? Yes, I know! There is more gear stacking up these days to be 'looked at' and it IS a busy world but I do yearn many a time for the sort of reviews I used to read in Studio Sound from Mr Ford... Idea! Perhaps for one issue Hugh could write the page 4 leader berating the confusing nature and low information level of specifications?

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Re: Scarlett 2i2 vs 18i8 gain range

Postby Hugh Robjohns » Thu Dec 28, 2017 11:10 am

ef37a wrote:I do yearn many a time for the sort of reviews I used to read in Studio Sound from Mr Ford...

My introduction to the pro-audio world was also guided by Hugh Ford's iconic reviews.... but times change. Very few reviewers have the technical chops, let alone the appropriate test gear, required to do proper technical testing. But even if they had, equipment today is in a different league entirely to that of the 70s. Back then it really was the technical specs differentiated the good from the bad; today the tech specs are all universally so good and so similar that it's very rarely an issue, and the differentiators are much more to do with computer OS support, features/functionality, ergonomics and price.

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Re: Scarlett 2i2 vs 18i8 gain range

Postby ef37a » Thu Dec 28, 2017 1:06 pm

Hugh Robjohns wrote:
ef37a wrote:I do yearn many a time for the sort of reviews I used to read in Studio Sound from Mr Ford...

My introduction to the pro-audio world was also guided by Hugh Ford's iconic reviews.... but times change. Very few reviewers have the technical chops, let alone the appropriate test gear, required to do proper technical testing. But even if they had, equipment today is in a different league entirely to that of the 70s. Back then it really was the technical specs differentiated the good from the bad; today the tech specs are all universally so good and so similar that it's very rarely an issue, and the differentiators are much more to do with computer OS support, features/functionality, ergonomics and price.

H

Well yes, ok Hugh but this whole thread has been about a TECHNICAL problem and one that really is caused by the lack of a good specification that allows the buyer to compare products. I do understand the time constraints and the fact that not many reviewers have AP test gear or the ability to use it! They could however criticize poorly written specifications?

I know the time of HF and Terry Long are no more! (I have not seen a protracted 'field test' in SoS for a very long time!) but I think the reviews could be a bit more rigorous? For instance, blithely passing on adpuff words such a "Class A" circuitry for a product that is SO compact and complex that it MUST use op amps?

There are those that have deep suspicions of ALL reviews (yes! Heaven forfend, even those in SoS!) and say you are all in thrall to the advertisers. Old, old carp I know and I don't believe it but.....?

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Re: Scarlett 2i2 vs 18i8 gain range

Postby Hugh Robjohns » Thu Dec 28, 2017 2:23 pm

ef37a wrote:Well yes, ok Hugh but this whole thread has been about a TECHNICAL problem...

I'm not sure it was a 'technical problem', was it? I thought the OP was just confused that the device with more analogue gain had a slightly lower digital output.

However, I agree that incomplete tech specifications are frustrating (for some of us), and that is something I try continually to raise with manufacturers.... but it is a head-wall-contact situation. Marketing people don't like too many specs.... ;-)

...passing on adpuff words such a "Class A" circuitry for a product that is SO compact and complex that it MUST use op amps?

We need to be careful here about confusing/conflating class-A and discrete transistor circuitry. A designer can squeeze in a lot of discrete circuitry (class-A or otherwise) using SMD technology... and Rupert Neve likes to describe his offset-biased op-amp circuits as operating in class-A... which they do!

There are those that have deep suspicions of ALL reviews...

In the modern world of fake news and internet experts I think that's a wise stance to take, and it's largely pointless me arguing about SOS' editorial independence and review integrity because those who don't want to believe me still wont.

All I can say is that I get a lot of emails people who have made purchase decisions based on my reviews and are pleased about the outcomes. At the end of the day it comes down to trust and track records, I suppose.

Incidentally, I currently have two product reviews pending while the manufacturers have gone away to redesign aspects following my feedback, and one manufacturer has re-written part of its website after I complained that they were making erroneous and misleading statements. I think its much better for everyone if, rather than publish a critical review, the product gets improved following feedback from SOS. In the case of the two products mentioned above, I will reassess the updated models and the review will (hopefully) be a lot more positive as a result. Those who claim 'you're in the pockets of the advertisers' don't appreciate this really important aspect of what we do when all they see are generally positive reviews (although there are negative ones when appropriate, too...).

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Re: Scarlett 2i2 vs 18i8 gain range

Postby James Perrett » Thu Dec 28, 2017 4:01 pm

I just wish that all reviewers were as thorough as you Hugh. There was a recent SOS review of a Focusrite 8 channel preamp/convertor and, given my previous experience, I searched carefully for a reference to the minimum gain spec. There was no mention of it which I found surprising given that it is an issue with at least 3 Focusrite products that I know about and probably more that I don't.
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Re: Scarlett 2i2 vs 18i8 gain range

Postby Hugh Robjohns » Thu Dec 28, 2017 5:25 pm

James Perrett wrote:I just wish that all reviewers were as thorough as you Hugh.

That's very kind, James, but I'm often accused of being too thorough -- in that my raw reviews are often much longer than the page space allows and, I'm told, sometimes repeat information that can be found easily by visiting the manufacturer's website. It's a valid point but, as a reader myself, I like to get everything I need from the one place and if I have to go to a manufacturer's website for half the info I need I'm not going to bother reading the (inadequate) review... But that's just me... ;-)

Thankfully, though, the 'extra-value' content I frequently generate (such as some of the test bench measurements etc) is generally included in the web and tablet versions, if not the paper issues.

There was a recent SOS review of a Focusrite 8 channel preamp/convertor and, given my previous experience, I searched carefully for a reference to the minimum gain spec. There was no mention of it which I found surprising given that it is an issue with at least 3 Focusrite products that I know about and probably more that I don't.

Obviously I can't comment on another reviewer's review content; we each write about the things we see as relevant and important in a given product, and that will be informed by the anticipated use of the product and previous experience -- the latter being a very individual thing, of course.

However, my writing style has evolved over the years as I have been influenced by the writing of others, and I'd like to think other reviewers are influenced by my 'thoroughness' to some degree... It's a question better asked of the section editors I suppose, since review quality/consistency falls to them to set and maintain.

As a general rule, I try to include maximum and minimum input levels for preamps and interfaces because -- like you -- I have experienced the frustration of working with preamps that either didn't have sufficient gain, or couldn't handle a large source signal -- and such specifications are rarely included in the product literature.

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Re: Scarlett 2i2 vs 18i8 gain range

Postby ef37a » Thu Dec 28, 2017 6:25 pm

No Hugh I was not confusing "discrete transistor circuitry" with "class A" (nor cathode bias with class A either as happens a LOT!) .

I was referring to the Radial Key Largo article which said "attest to the quality of the Key Largo's Class-A audio circuitry". Search as I might I can find no such reference in the manual although it does say "audio grade operational amplifiers" . NE's no doubt and 072s? Neither of which have class A output stages. I am aware of the 'offset bias' trick but it compromises headroom I understand and I can see no good reason for it? I cannot recall any review in SoS since I have been reading it (feb 2005, wife brung it me after HA in hospital!) any reviewer ever commenting that a product suffered from low level crossover distortion?

I have EVERY faith in SoS reviews and tell people so all the time! You do your best I know to bring a level of consistency. Shame that you get lumbered mostly with the esoteric and expensive stuff (but SOMEONE'S gotta do it Hugh!) where specs are really so good as to be a non event . The gear that really needs 'sorting out' is down at the 100+ quid region.

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Re: Scarlett 2i2 vs 18i8 gain range

Postby Hugh Robjohns » Thu Dec 28, 2017 7:40 pm

ef37a wrote:I was referring to the Radial Key Largo article which said "attest to the quality of the Key Largo's Class-A audio circuitry". Search as I might I can find no such reference in the manual although it does say "audio grade operational amplifiers" .

Ah. A Bob Thomas review. Dunno about the class-A reference. As you say, nothing on the website about it, so whether it's a faux pas, or Bob was told something separately I don't know. I'll make enquiries in the new year.

I am aware of the 'offset bias' trick but it compromises headroom I understand and I can see no good reason for it?

It does, but not significantly or problematically in most cases if used carefully... At least according to Mr Neve!

I cannot recall any review in SoS ... any reviewer ever commenting that a product suffered from low level crossover distortion?

:-D suffered, or benefitted?

Shame that you get lumbered mostly with the esoteric and expensive stuff...

It's no lumber, really! ;-)

The gear that really needs 'sorting out' is down at the 100+ quid region.

Yes and no... At the budget end of the market the cost restrictions inherently restrict the potential performance too.

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Re: Scarlett 2i2 vs 18i8 gain range

Postby James Perrett » Thu Dec 28, 2017 10:50 pm

Hugh Robjohns wrote:
There was a recent SOS review of a Focusrite 8 channel preamp/convertor and, given my previous experience, I searched carefully for a reference to the minimum gain spec. There was no mention of it which I found surprising given that it is an issue with at least 3 Focusrite products that I know about and probably more that I don't.

Obviously I can't comment on another reviewer's review content; we each write about the things we see as relevant and important in a given product, and that will be informed by the anticipated use of the product and previous experience -- the latter being a very individual thing, of course.

After writing that I went back and searched for the review on the web site. It was Matt's review of the Scarlett 8 Pre that I was thinking of. However, now that I was able to see all the web links in the review I could trace back through Matt's history of the various Focusrite preamps and I see he mentions the issue in a previous review. This doesn't help those of us who mainly read the print edition but at least I can now see that he has covered the problem previously.

Numbers would still be useful though - I seem to remember in the ancient past that some magazines would have 2 sections to a review with a technical evaluation done by another reviewer - although I could see that could get a bit onerous if Hugh got lumbered with that job for every review.
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Re: Scarlett 2i2 vs 18i8 gain range

Postby fladd » Fri Dec 29, 2017 12:29 am

Hugh Robjohns wrote:
fladd wrote:If I understand you correctly, then there is no such thing as absolute gain.

Yes, there's absolute gain -- how much bigger or smaller the signal is made, overall -- but that is the sum of all gain stages, including that of the digital conversion process. In the case of your two interfaces, I suspect the 2i2 effectively has less analogue gain but more digital gain than the 18i8.

Let me try and explain it with some numerical examples ...

Let's say the output voltage from your microphone is 3mV (0.003V).

If the first preamp boosts that by 46dB the signal level will be raised to 0.6V (in round numbers).

If the second preamp boosts it by 50dB it would produce an analogue output signal level of 0.95V.

...and in the analogue world, the latter is obviously (4dB) louder than the former, as you would expect.

However, let's say that these analogue signals are fed into different A-D converters, and the first converter is set up so that it reaches 0dBFS (clipping level) with an input signal level of 1V. In that case, the input of 0.6V produces a digital output level of -4.4dBFS.

But let's say the second interface is set up with 6dB more headroom so that it delivers 0dBFS only when the input voltage reaches 2.0V. In that case the input signal of 0.95V produces a digital output of -6.5dBFS.

...and consequently the output from the second interface is actually 2dB lower in the digital domain than the that from the first preamp, despite that first model having 4dB less analogue gain.

And the reason is that although it has 4dB less analogue gain, it effectively has 6dB more digital gain (from the conversion alignment) and thus 2dB more gain overall from analogue input to digital output.

This seems to have several very important and rather surprising implications:

(a) For a manufacturer to provide, and for a customer to read the maximum gain value (in isolation) is completely uninformative.

Well, not completely uninformative. It is still useful to know the available analogue gain range. 40dB is about the minimum you can get away with with close-miked sources. 60dB is typically required for quieter sources or less sensitive mics, and 75dB or more for very insensitive mics with distant placements... for example.

(b) Statements such as "an SM7b needs at least 60dB of gain" are entirely meaningless.

Without a context, arguably, yes. Although in the context of a typical analogue preamp/mixer, it's a handy rule of thumb to estimate the amount of gain required in a typical application.

(c) There is no a priori way of knowing if an interface/preamp will provide enough amplification for a given microphone.

I refer you to my earlier answer re typical 40/60/75dB gain range applications.

(d) Comparing the amount of amplification between interfaces/preamps is not possible without actually testing them against each other.

This one is definitely true because, as I said in my previous response, few manufacturers specify the the analogue-digital conversion alignment, and thus the gain through to the digital domain.

Out of interest, what sort of digital level difference are you getting from your two Scarlett interfaces?

H

Thanks again for the detailed response.

I just did a measurement with a SM58 and the difference is 10.5 dB. That is, the 18i8 at full gain is 10.5 dB quieter than the 2i2 at full gain, according to Reapers's meters.

Given that this is quite a substantial difference, I went back to the Focusrite website to look at the specs again. The 2i2 has a max input level of 4 dB. The 18i8 has a max input level of 8.5 dB. If both devices had the same amount of gain, you would expect the 18i8 to be 4.5 db quieter than the 2i2. Given that it is 10.5 dB quieter, I have to conclude that the 18i8 actually has 5 dB gain less than the 2i2. This means that one of the two specs seems definitely wrong. The question is of course which one. Either the 18i8 has actually only 41 dB of gain (instead of the stated 46), or the 2i2 has actually 51 dB of gain (instead of the stated 46).

Are my conclusions correct?
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Re: Scarlett 2i2 vs 18i8 gain range

Postby fladd » Fri Dec 29, 2017 1:15 am

I should also mention that the pad is not engaged. Engaging the pad will result in a difference of 20.5dB.
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Re: Scarlett 2i2 vs 18i8 gain range

Postby Hugh Robjohns » Fri Dec 29, 2017 1:41 am

fladd wrote:I just did a measurement with a SM58 and the difference is 10.5 dB. That is, the 18i8 at full gain is 10.5 dB quieter than the 2i2 at full gain, according to Reapers's meters.

Thanks. Yes, a significant difference, I agree.

The 2i2 has a max input level of 4 dB. The 18i8 has a max input level of 8.5 dB. If both devices had the same amount of gain, you would expect the 18i8 to be 4.5 db quieter than the 2i2.

I don't think your thought process holds water. The max input level indicates the available headroom margin of the input stage, but (a) your test signal is (hopefully) nowhere near those maximum input levels...and (b) the max input levels aren't directly related to the available gain. The gain controls the output level, which might reach as much as +24dBu, for example. So while there is 4.5dB difference in the input headroom, that doesn't relate to the overall system gain in any way.

I have to conclude that the 18i8 actually has 5 dB gain less than the 2i2.

No, the 18i8 has about 11dB less overall gain -- you've already proved that. But since it has 4dB more analogue gain (apparently) it must have a digital conversion alignment which is around 15dB lower, or in other words, around 15dB more digital headroom.

This means that one of the two specs seems definitely wrong.

No, it means the spec is incomplete.

Either the 18i8 has actually only 41 dB of gain (instead of the stated 46), or the 2i2 has actually 51 dB of gain (instead of the stated 46).

Well, it is entirely possible that the actual analogue gain ranges don't match the stated values in the control knobs... but I'd be surprised if that was the case.

What's really needed here is a proper measurement of the actual overall gain from analogue input to digital output. It would be easy to achieve if you have a calibrated analogue test oscillator....
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