Please help me to calibrate meters on Focusrite ISA preamps

[armansrsa]

I have the focusrite ISA 2 preamp going into a line input on the Clarett 4. On the front panel of the ISA there is metering in dbfs. The problem is that this meter shows too low of a signal compared to what I see in my DAW.

There is a meter calibration knob at the back which lets you calibrate the meter somewhat from 16dbu = 0dbfs to 24dbu =0dbfs but it seems that no matter where I set this, the meter on the ISA 2 is too low. I will be peaking in my DW when the dbfs meter on the ISA barely touches -18dbfs.

Focusrite have told me to calibrate the meter which I did but as I just mentioned,the calibration knob does little to change the metering on the ISA 2. something, somewhere is wrong. Anyone here have ideas while I wait for support to get back to me?

 

[ashcat_lt]

So the Clarett seems to have two sets of line inputs. Some have gain knobs and can handle up to +24dbu with the knob all the way down. That’s good headroom, and will put +4dbu nominal line level signals at -20dbfs in your daw - 2db below the -18 “standard”. The other set has no gain control and tops out at +18dbu which is abysmal headroom and will put nominal levels at -14dbFS - 4db hotter than standard and right around “mastering for YouTube” territory. If it was me, I’d use one of the ones with gain control, turn it all the way down, and then set the calibration knob on the ISA all the way up. That should match perfectly and if it doesn’t, idk what to tell you...

...except that the manual says that if you’re doing a factory reset that knob MUST be in the center detent position. Not sure why that would matter but guess there’s maybe some calibration routine that it does so that the calibration knob actually hits the levels it says. If you’ve never done the reset, then you don’t have any way of knowing where that knob was when it was last done, so maybe go ahead and do it right so that you’re starting from a known good position?

 

[armansrsa]

Thanks a lot for your reply. Very much what I was hoping to find
When I purchased the Clarett 4, I did so based on the fact that there were line inputs for accepting signals coming from external preamps. I understood that it was better that the signal pass through a line input without variable gain. Is this true?
Also, why would an interface have added gain applied to the input? Why can't the input be an input where no gain is added since the front inputs offer added variable gain? I thought, the back inputs were the right ones for using with external preamps.
Then, you speak about headroom. You mention that +24dbu is good headroon which is available on the front inputs and then the back inputs are not good because they have +18dbu only and this explains why signals would be 4db hotter going in through the line inputs on the back. I suppose this is because +18dbu = -14dbfs. However, in my case it seems that there is more than a 4dbfs difference so maybe focusrite are not being honest about this? Is there a way I can check this?
Finally, when you say set the "calibration knob on the ISA all the way up" do you mean, set it fully anti clockwise so that 0dbfs = +16dbu?

After writing all this, the thing I am still confused about is that if the calibration knob on the ISA2 is set to 0dbfs = +16dbu then the signal on the meters should match the signal in my DAW because you just said that the line inputs on the back have 18dbu of headroom (which is only 2dbu difference to what I am setting my meters to). How can there be such a discrepancy.

 

[sasquatch]

It's a gain structure/staging thing and knowing the difference between the different metering standards.

G

 

[armansrsa]

THank you for the conversion chart but posting it doesn't really answer the questions here. On the focusrite ISA 2, you can choose 16dbu = 0dbfs up to +24dbu = 0dbfs so if Im turn this knob all the way anti clockwise I am telling the ISA 2 that I want 0dbfs on the meter to be the same as +16dbu. BUT, after doing so, when I record something, if the meter on the ISA 2 is barely touching -18dbfs the signal is already peaking in my DAW. What else does this have to do with conversions?

I found this on a review of the ISA 2

0db" on the meters is absolutely nowhere near 0db on the digital scale, the internal bias is so far off that the meters are unusable. Even accounting for the adjustment knob on the back turned all the way up, the meters internal bias is still set WAY too low. If you're near clipping on this unit's meters, everything else in your chain is being clipped to hell and back. 0db on this unit is like 15db in your DAW, straight distortion.

but I still want to understand this better because maybe this guy doesn't know what he is talking about or maybe he does and focusrite is to blame but then again, why are you posting conversion charts? !

 

[sasquatch]

This review found the complete opposite.

Put up a screenshot of your DAW meters if possible.

G

 

[ashcat_lt]

Finally, when you say set the "calibration knob on the ISA all the way up" do you mean, set it fully anti clockwise so that 0dbfs = +16dbu?

Well, no. I meant set the calibration knob so that 0dbFS = +24dbu which supposedly is the maximum input on your interface with the gain all the way down. This should make the two units match. If it doesn’t, I still don’t know what to tell you. Have you done the proper factory reset on the ISA yet?

TBH, all the stuff about headroom and nominal level is a bit of red herring in your case. It would only really matter if you’re connecting other analog gear with more typical VU meters where 0dbVU = +4dbu. I’d imagine though that you’re just going to plug in a mic and set the gain on the preamp such that the average level is decent and the peaks don’t clip. In that case, the absolute level (in volts, dbu, whatever) matters completely not at all.

I guess the meter on the preamp might be convenient in some situations, but in the end it’s the level in the DAW that matters. I pretty bably wouldn’t worry too much about the calibration. At least like not let it stop you from actually making music. Just ignore the ISA meter and set your gain based on what you’re seeing in the DAW. If it’s too far off, though, it could force you to use less than ideal settings on the preamp - less gain than you might otherwise use just to keep from clipping downstream - especially if you’re really looking to push the preamp for saturation/distortion on purpose.

But wait! Focusrite has that stupid software mixer thing that comes between the interface and your DAW. Probably check to make sure that is set to unity. Also if you’re not using the ASIO drivers in your DAW you’re just plain fucking up, partly because that sometimes puts your OS audio system in the middle too, and if that’s adding gain... Luckily (not!) those OS utilities usually just say like 0-100% with no indication of where unity might really be.

 

[ecc83]

I have to get mine and her indoors dinners soon but it seem to me the problem stems from the fact that the ISA2 is an out and out professional pre amp running pro levels whereas the Claret, excellent though it is, runs more 'prosumer' levels.

If I don't nod off post dinner I shall crunch some decibels otherwise it will be Thursday morning.

Bottom line. Think you need some 10dB LINE XLR attenuators (not '600 Ohm' mic jobs. Line atts' are trickier to find. I would just make 'um)

Dave.

 

[ashcat_lt]

The ISA is supposedly built specifically to accomodate a large range of interfaces and by the numbers it absolutely should allow for calibration to work fine with this interface. This issue here seems to be that the calibration itself is just way off. Of course, I don’t actually have it in front of me, so I can only try to credit the OP with some basic common sense and believe that it’s not just completely user error. You should be able to find a spot on the calibration knob that works. If you really can’t, then something is definitely wrong.

 

[ecc83]

Gahhhhh! Just wrote a VERY carefully worked out piece and when I tried to post it I got some bllx about 'token' ? Not had that for ages.

Bottom line: Clare4 fixed gain input. 0dBfs seems to = +18dBu. Pre amp seems to be 'settable' such that 0dBfs on the meters = +18dBu out.

But, why legend a purely analogue device in dB fs? Where do you get the calibrating signal from?

Dave.

 

[sasquatch]

Gahhhhh! Just wrote a VERY carefully worked out piece and when I tried to post it I got some bllx about 'token' ? Not had that for ages.

Bottom line: Clare4 fixed gain input. 0dBfs seems to = +18dBu. Pre amp seems to be 'settable' such that 0dBfs on the meters = +18dBu out.

But, why legend a purely analogue device in dB fs? Where do you get the calibrating signal from?

Dave.

Also difficult to calibrate if final metering has no unit of measure.

G

 

[ecc83]

Also difficult to calibrate if final metering has no unit of measure.

G
View attachment 105440

Silly Sods. I was going to say before the 'token issue' that even if calibrated such the the pre amp meters agree with the DAW's this might not suit if it is desired to run the pre amp hotter to genrate a bit of 'attitude'?

Dave.

 

[armansrsa]

Well, no. I meant set the calibration knob so that 0dbFS = +24dbu which supposedly is the maximum input on your interface with the gain all the way down.

Are you sure that this is with the gain all the way down? I remember a sales person telling me that "unity gain" on the front line inputs is actually with the knob turned up a bit. Not sure if this is the same thing or not but from what I remember him saying, wouldn't going below unity gain actually be attenuating the signal?

Have you done the proper factory reset on the ISA yet?

Yes. No change.

Just ignore the ISA meter and set your gain based on what you’re seeing in the DAW. If it’s too far off, though, it could force you to use less than ideal settings on the preamp - less gain than you might otherwise use just to keep from clipping downstream - especially if you’re really looking to push the preamp for saturation/distortion on purpose.

Exactly, this is what I may have been doing up until now. I always just look at the level coming into my DAW but then I am not using my preamps as I should be using them as you say.

But wait! Focusrite has that stupid software mixer thing that comes between the interface and your DAW. Probably check to make sure that is set to unity. Also if you’re not using the ASIO drivers in your DAW you’re just plain fucking up, partly because that sometimes puts your OS audio system in the middle too, and if that’s adding gain... Luckily (not!) those OS utilities usually just say like 0-100% with no indication of where unity might really be.

I actually like the software, it is pretty decent IMO And no it is not adding gain from what I can tell.

 

[armansrsa]

ISA preamps are clean but even so, wouldn't using them with less than ideal gain not give you the best results?

 

[armansrsa]

Bottom line: Clare4 fixed gain input. 0dBfs seems to = +18dBu. Pre amp seems to be 'settable' such that 0dBfs on the meters = +18dBu out.

Exactly, which is why I can't understand why the meters can't be set so that I can still use the back fixed inputs! the meter calibration allows for up to 16dbu = 0dbfs, this is even lower than +18dbu. Can you clarify this for me please?

 

[armansrsa]

The ISA is supposedly built specifically to accomodate a large range of interfaces and by the numbers it absolutely should allow for calibration to work fine with this interface. This issue here seems to be that the calibration itself is just way off. Of course, I don’t actually have it in front of me, so I can only try to credit the OP with some basic common sense and believe that it’s not just completely user error. You should be able to find a spot on the calibration knob that works. If you really can’t, then something is definitely wrong.

Exactly! this is what the guy in the review I posted was talking about. It should work with +18dbu.

 

[armansrsa]

So the Clarett seems to have two sets of line inputs. Some have gain knobs and can handle up to +24dbu with the knob all the way down. That’s good headroom, and will put +4dbu nominal line level signals at -20dbfs in your daw - 2db below the -18 “standard”. The other set has no gain control and tops out at +18dbu which is abysmal headroom and will put nominal levels at -14dbFS - 4db hotter than standard and right around “mastering for YouTube” territory. If it was me, I’d use one of the ones with gain control, turn it all the way down, and then set the calibration knob on the ISA all the way up. That should match perfectly and if it doesn’t, idk what to tell you...

I just found a reply from a tech person at focusrite when I was asking about this interface 2 years ago. This is what he says about the front variable inputs:

If you go in to a line level input on one of the combi inputs just set the gain knob to between 2 and 3 as this should be at unity.

so then, setting the gain knob all the way down is actually attenuating isnt it?

 

[ashcat_lt]

so then, setting the gain knob all the way down is actually attenuating isnt it?

Can’t answer that without a schematic. In many cases the “gain” knob doesn’t actually affect the gain of the amp stage at all. It has a fixed gain and anything short of all the way up is really attenuating the signal before turning it back up again.

Then but also consider that +24dbu is something like 26Vp2p but the ADC chip very often runs off of 5V rails and realize that it can’t possibly have as much gain as you think.


Edit to add - When this person is talking about “unity” being around 2 or 3 on the dial, that means like compared to the rear inputs which we’ve already established are short on headroom compared to actually serious studio gear. So is it actually “unity” if it puts you closer to the upper limit than anything else in your chain.

If you want to make absolutely sure that the preamp distorts before the ADC - so your interface never actually hits clipping - you’d set the calibration on the preamp to the +16dbu setting and go into the rear input on the interface and you should have 2db of “wasted bits”. Then you really can slam that preamp into hard saturation if you want/need and/or trust it to be a little more gentle on accidental peaks and know that it’ll never hit 0dbFS.

BUT if that don’t work, idk what to tell ya. Things borked.

 

[ecc83]

Several years ago we had a ZED10 mixer setup to drive an M-A 2496 card.

The card hits 0dBfs at about 1V rms in but the mixer has a maximum output of +21dBu (marked as +16dB on the attached meters FKs why!) Zero 'VU' on the meters is smack in the middle of the range and equates to, I guessed +4dBu out.

Thus we could not use 80% of the mixer's meter scale and it just blinked the bottom couple of LEDs to deliver an average level in the DAW (Samplitude SE8)of -18dB fs. So...I fitted two multiturn 22k presets in the 2496 inputs so I could get ZED and DAW singing from the same song sheet!

If it is desired to drive that pre amp close to its maximum I do not see any alternative to a similar fix?

Dave.

 

[armansrsa]

Can’t answer that without a schematic. In many cases the “gain” knob doesn’t actually affect the gain of the amp stage at all. It has a fixed gain and anything short of all the way up is really attenuating the signal before turning it back up again.

But the guy from Focusrite said that 2 or 3 o clock is unity gain, so what is 1 o clock? That means the signal is being attenuated! Am I wrong? Why do you need a schematic for that?

If you want to make absolutely sure that the preamp distorts before the ADC - so your interface never actually hits clipping - you’d set the calibration on the preamp to the +16dbu setting and go into the rear input on the interface and you should have 2db of “wasted bits”. Then you really can slam that preamp into hard saturation if you want/need and/or trust it to be a little more gentle on accidental peaks and know that it’ll never hit 0dbFS.

After geting some emails back and forth from Focusrite (god they take forever to reply) it turns out that the back inputs are only +10dbu max input. So I ordered 15db attenuators to be able to connect to the back of the Clarett. Will this work?

Also, just to let you know that the Focusrite ISA is really considered a clean preamp isn't it so does it actually matter if I use too little gain? If I am only using 35db of gain from my ISA 2 going into the back of the Clarett currently, if I put an attenuator of 15db in line, will the sound change in some way now that I can get 50db of gain from the Clarett?