I recently got an Apollo x8 that I’m using with a Heritage Audio OST-8 ADAT and while running some tests, I realized that the level coming into the Apollo converters is about 3dB hotter than the same signal coming through the Heritage unit’s converters. Is this cause for concern? I always assumed pro line level was the same across different gear, but is that not the case? I guess my ultimate concern is that I would ultimately end up using more gain on preamps that are feeding the Heritage converters, which would then have an effect on overall tone

3dBFS hotter in your DAW? In my RME UCX II, I can choose between +13dBu and +19dBu as the level to match 0dBFS. I think many interfaces don't have this option, and instead have a fixed setting that might differ between interfaces.

Yes, 3dBFS hotter in pro tools. I initially discovered this while doing a test to determine the difference in latency between the converters. I ran a tone out of the Apollo, which I split and ran back into each converter. I found out that the Heritage unit is 3 samples behind the Apollo and to my surprise, 3dB quieter

Reference levels can indeed be different.

Also though, how did you route that signal and what was it? Any chance you switched from one channel config to another? Metering position of course matters and I know a lot of people miss that in addition to mono/stereo/pan law.

I ran a tone using the signal generator plugin in Pro Tools from Apollo's Line out 4, which is half normalled to Apollo's line input 4, then patched from the Line out 4 to a Heritage ADC input on the patchbay. I tried several different Hertitage ADC Inputs and had the same result with all of them. I also tried recording a synth with long sustain through a preamp's DI, then ran the output of the preamp to both the Heritage and Apollo Converters with the same result - Apollo signal is always about 3dB hotter.

I know you can change the headroom setting on the Apollo but I’m not sure if that effects the line in gain…

Give it a try and see what happens.

Can't it be because of pan law settings which are usually 3db difference.

If it's bugging you just adjust the gain to move back to the level you want it at and move on.

Just for the sake of clarity though: Where and how are you measuring this? Measuring the output and input buses directly - both being mono?

In other words for example mono Aux with tone generator in PT with output set to line out, and then you bring that back on another mono Aux with input 4 selected, and have a meter on that 'input' Aux - not on a master...?

I was running the tone from an Aux in PT with Line out 4, then splitting at the patchbay and bringing the same signal to both Apollo and Heritage ADC line ins. Bringing each back in on their own audio track and recording. I initially just noticed that the waveforms were different, then checked the meter on each track in PT. The Apollo Console inputs for each track are showing the same thing - 3dB difference in level.

sort of mundane, but is it possible there is something in your patchbay split that is not quite perfect?

IOW did you swap the signals and see that the Heritage is still hotter using the other 'leg'?

Just take a manual for your interfaces and check about maximum output level - easy as it is.

Indeed, there're some that output +23dBu for 0dBFS, and some +18dBu for the same 0dBFS. It is not a problem at all, as you make gain adjustments for each specific situation anyway.

Some instruments - keyboards or grooveboxes - can give "only" +15dBu at full level, and this is not a problem again. The only thing we need to be aware of - does it output really clean signal at maximum level? - usually, it is not. That's why we have a reference levels - like +4dBu for professional equipment, and everything above up to max output level is considered to be a headroom for dynamic signals and peaks.

The deal is not to clip anything unintentionally - that's all we actually need to do.

Ok, problem solved!

After reading through the Apollo manual, there is indeed a setting that allows you to change the line I/O headroom between 20 dBu (default) and 24 dBu. Switching to 24 has everything perfectly matched between the Apollo and heritage converters.

Thanks for the help everyone!

Told you :-)