How to Calibrate the Limiters on a SD MixPre

B

Benjobe

Well-known member
I purchased a Sound Device MixPre from a previous owner who altered the limiter pots. I'd like to re-calibrate those limiter pots but I cannot find any tutorials online (not on the Sound Devices website, not in the MixPre Owners Manual, not anywhere). Does anyone know how I may go about configuring these pots?

Any help will be much appreciated.
 
I'm not that familiar with limiter adj. on the MixPre, but typically with other 'manual' TH adjustments (Shure FP series for instance), one would apply a signal of known amplitude and adjust the limiter threshold so the indicator starts to flicker. (FWIW, the SD factory limiter setting on most SD products is +18dBu)
 
Benjobe said:
I purchased a Sound Device MixPre from a previous owner who altered the limiter pots. I'd like to re-calibrate those limiter pots but I cannot find any tutorials online (not on the Sound Devices website, not in the MixPre Owners Manual, not anywhere). Does anyone know how I may go about configuring these pots?

Any help will be much appreciated.
Click to expand...

Here's the manual for the MixPre-D. And here's the manual for the MixPre.

For the MixPre-D see Limiters, page 5. Read that section all the way to the bottom. For the MixPre, see #7 on page 4.

Classic method is to set limiters to +18dBu. Use the mixer's tone to set the recorder to -20dBfs. Then, when recording, let the mixer's meter just touch the red which starts at +10dBu. This will get you peaks at the recorder in the -10 to -12dBfs range, which is what most people want. At the same time, you'll find it's almost impossible to clip because of the limiters.

First thing most people ask is, "you want me to record to +10? That's nuts!". And it would be, if the mixer were showing you dBfs, but it is not showing you dBfs. It's showing you dBu. It's the electronic equivalent of a Vu meter. Back in the old mag. tape days, different recorders and tapes would saturate out anywhere from +3 to +15 or so on the old Vu meters. The SD mixers hark back to those days, so you have to ride the meters up to +10. And now you know why the red LEDs start at +10 (at least they do on a MixPre-D, I'm not sure about the MixPre because I've never actually used one), and not zero like the digital generation might think they would.
 
Rather than needing help with how to use the limiters, it sounds like the OP is looking to restore their factory calibration after the original owner has mucked around with the internal adjustments. That's probably going to take a copy of the service manual and accurate test equipment, or a trip back to Sound Devices for servicing.
 
Steve House said:
Rather than needing help with how to use the limiters, it sounds like the OP is looking to restore their factory calibration after the original owner has mucked around with the internal adjustments. That's probably going to take a copy of the service manual and accurate test equipment, or a trip back to Sound Devices for servicing.
Click to expand...

Maybe. But that seems far fetched to me. The phrase "altered the limiter pots" implies that the person physically replaced them with something different. Because it's darn near impossible to actually alter a physical potentiometer. You ever see the inside of one? I'm talking tiny little components. And what, exactly, would you alter? Does anyone really think they can come up with a better wiper than the manufacturer? Much less install it after manufacturing is complete? Doesn't make sense.

If indeed someone replaced the pots, the cure is a trip back to SD.

It's more likely what the OP meant was that the person altered the limiter pots settings. And for that, I think I answered correctly. At least, I hope I did. Else, the OP can always ignore my post. Like most everyone else. :shocked:
 
"Altered" would suggest the adjustments were moved away from the factory settings. Whether the "pots" the OP was referring to are some internal adjustment or are the externally accessible Limiter Threshold Level Control pots on the left (input) panel I don't know but in either case the owner's manual doesn't give explicit instructions on how to reset them to activate the limiters at a particular desired level.
 
Yes I would agree with Bruce, the previous owner/user only changed the threshold adj. and not worth having it serviced by SD. Re-adjusting it to +18dB (or whatever one's preference) is relatively easy. If they were not meant to be user adjustable, SD would have 'hidden' them and assumed it would be used by 'knowledgeable' folks.
 
Rick R said:
Yes I would agree with Bruce, the previous owner/user only changed the threshold adj. and not worth having it serviced by SD. Re-adjusting it to +18dB (or whatever one's preference) is relatively easy. If they were not meant to be user adjustable, SD would have 'hidden' them and assumed it would be used by 'knowledgeable' folks.
Click to expand...
Even so, the user's manual doesn't give explicit instructions on how to set the threshold pots to a desired level. Let's say you wanted to have them activate at +16dBu. How would you do that? The manual only states WHAT to adjust, not HOW to adjust it. A note to SD tech support would most likely provide a quick answer.
 
The pots are adjustable the first user just set them differently from the factory default. If all you want to do is go back to the factory default then you open them up all the way. That puts you at where they come from the factory.

It was already talked about that the only way to set the limiters to a different level is with a know signal level.

But in practical terms after playing around with them I never found a good reason to not use them wide open. If you go through the procedure Bruce mentioned you are set. You can check by just screaming or playing something loud enough to kick in the limiter and adjust your recorders input till it's not clipping. For the PMD671 that seems to be where the tone is just below -20 on the meters. So if I send tone and then adjust my recorder inputs till the -20 just lights and then back off till it just goes out I'm good and clip proof.
The MixPre's limiters look at both the incoming and out going signal so you really are basically clip proof with the above set up.
 
Bruce Watson said:
Classic method is to set limiters to +18dBu. Use the mixer's tone to set the recorder to -20dBfs. Then, when recording, let the mixer's meter just touch the red which starts at +10dBu. This will get you peaks at the recorder in the -10 to -12dBfs range, which is what most people want. At the same time, you'll find it's almost impossible to clip because of the limiters.
Click to expand...

Thank you for your detailed guidance. It was very helpful. I found a bars/tone generator in Premiere Pro and played it at full volume on my speakers. Then I set my Rode NTG-3 shotgun mic right in front of the speakers and ran the signal into the SD MixPre. Then I turned the gain all the way up on the SD and adjusted the limiter pots until the signal got under the +12dBu LED. I then output the signal from the SD MixPre into my Panasonic GH4 using a -25dB L Pad Attenuator cable. The Panasonic GH4's audio levels were set to the minimum level. I recorded tests with the limiters on and off. Then I turned off the tone generator and turned the shotgun mic towards myself and recorded some tests of me speaking loudly with the limiters on and off. I found that without the limiters the signal was peaking, but with the limiters on the signal maxxed out around -5dB. I think that this configuration was successful. Thanks again for your help.
 
You must log in or register to reply here.
Back
Top