Alesis HD24XR - changing internal reference level?

[Matt C]

I've got a couple Alesis HD24XR units that I'm using as A/D/A converters with my DAW.  Right now, a +4dBu analog input corresponds to -15dBFS.  I'm trying to figure out if it's possible to change this by modifying the units somehow (it doesn't seem to be a normal user-adjustable feature).  I'd like to lower it to -18 or -20dBFS for more headroom. 

That said, I am not super familiar with the circuitry in digital converters so I'm not sure if this is possible.  I thought it might be as simple as adjusting an internal trimpot, or it might be impossible.  On top of that, I've only found schematics for the regular HD24 (not the XR version, which I believe uses different converter chips).  If it's helpful, the manual says the converter chips are AKM 5393 and AKM 4393

So I guess my question is, generally speaking, where in a converter's circuit does this level get set? That might help me narrow my search as I dig through the schematics for clues. Thanks

 

[JohnRoberts]

I've got a couple Alesis HD24XR units that I'm using as A/D/A converters with my DAW.  Right now, a +4dBu analog input corresponds to -15dBFS.  I'm trying to figure out if it's possible to change this by modifying the units somehow (it doesn't seem to be a normal user-adjustable feature).  I'd like to lower it to -18 or -20dBFS for more headroom. 

The analog to digital level will likely be set by the hardware design.

You can just run your analog inputs a few dB cooler to get the lower digital level.  If you don't have level trims on the analog path, a simple resistive pad can accomplish that.

That said, I am not super familiar with the circuitry in digital converters so I'm not sure if this is possible.  I thought it might be as simple as adjusting an internal trimpot, or it might be impossible.  On top of that, I've only found schematics for the regular HD24 (not the XR version, which I believe uses different converter chips).  If it's helpful, the manual says the converter chips are AKM 5393 and AKM 4393

So I guess my question is, generally speaking, where in a converter's circuit does this level get set? That might help me narrow my search as I dig through the schematics for clues. Thanks

It is likely set by gain resistors in the input buffer.

I would not mess with this, an external pad will not affect the resale value of the convertor and is easier to tweak when you change your mind.

JR

 

[RuudNL]

Why not simply 2 resistors in the input connector?

 

[Matt C]

Fair enough, I guess a resistive pad is the simple answer I was overlooking here. 

The problem I'm trying to address is this - I've got a 24 track tape machine, and when I transfer tapes into the computer, the tape machine's outputs are hooked directly to the Alesis's inputs, no gain trim in the path.  The tape machine has a max output level of +28dBu, the Alesis has a max input level of +19dBu (which, unless I'm misunderstanding, is limited by the converter's reference level), so if I really slammed the tape, it's easy to clip the converters while transferring. 

But I think you're right there is probably a better way to go about this than changing the converter's reference level.

 

[scott2000]

Fair enough, I guess a resistive pad is the simple answer I was overlooking here. 

The problem I'm trying to address is this - I've got a 24 track tape machine, and when I transfer tapes into the computer, the tape machine's outputs are hooked directly to the Alesis's inputs, no gain trim in the path.  The tape machine has a max output level of +28dBu, the Alesis has a max input level of +19dBu (which, unless I'm misunderstanding, is limited by the converter's reference level), so if I really slammed the tape, it's easy to clip the converters while transferring. 

But I think you're right there is probably a better way to go about this than changing the converter's reference level.

The max input of the Alesis is what it is (+19dbu) and the calibration won't affect that limitation. (Just saw JR point that out)  :-[Unless I'm looking at it wrong...

Anyhow, I think -15dbfs isn't a bad reference and I believe at least the older Mytek converters recommend this as a good level anyhow. Sounds like you are on the right track with addressing the analog levels you are trying to send to the AD.....some kind of variable attenuation would give more flexibility with gain staging??/ ,,,,,,.. This chart is helpful to get your ac voltages in the ballpark if you need a quick reference..... I think it came from one of the members here......

Good luck!