Pre Amp Behavior? Is this correct?

H

Hricco

New member
Ok guys

Im seriously confused. Need some smart audio folks to help me out if possible. I guess this is a gain staging/Calibration question.

First the Set up:

Presonus RC500 Channel strip into a Focusrite Saffire pro 40 Via an XLR patch bay

So: condenser mic into patch bay input > xlr from patch bay output to XLR input on RC500 Micpre> Line out from pre to the interface. pretty simple. and..finally into protools

SOOOOOO...here is whats going on. I am trying to calibrate the system. using a tone generator, i can send a 1K signal at +4db into the mic pre. and adjust the gain and output so that the VU meter hits 0db

Here is the thing. The pre amp gain, doesn't seem to have an affect on the VU meter (should it?) BUT....when i adjust the output knob up, even slightly the meter pins, i have to back the output know WAAAAY down to almost completely down, to achieve a 0db reading on the VU. in protools tough the signal is very low, so I turn up the interface gain, to get it to -18dbfs on the Waves PAZ and protools meter which is measuring RMS, leading me to my next questions. WHAT meter type in protools is correct fr this process? and tracking in general. Currently im using the SAMPLE PEAK meter, and guessing where -18dbfs is, because its not indicated on that meter, so im shooting for slightly above -20dbfs on that meter.

So should my pre amp be acting like that, the output knob being SOOOO sensitive that the slightest boost pegs the VU meter with the gain knob set around 40?
And what meters should i be using to calibrate correctly? and where is -18dbfs on the sample peak meters

and why am i doing this anyway. Seeing that every source i record is not always going to be a steady 1K test tone, ill have to adjust the pre amp for that specific source anyway right....maybe it a flute, and ill have to turn up the output, or maybe its screaming rock god of a singer and ill have to turn it down....

First and foremost though, is the pre amp supposed to act like that, i thought the GAIN knob would affect the VU meter more than the final output knob...

Help me get my head around this please

Thanks
 
Do you have the meter set for output level instead of gain reduction...?..it's the toggle switch to the right of the Threshold knob.
 
I'm not sure what's going on with your settings. I would start with the Master at 0 and adjust the Gain to get the record level you want. Bypass or zero out the eq and compression. Don't bother with the 1kHz tone.
 
Output BACK H. I don't know PT but all DAWs AFAIK can generate internal tones* and noise. Generate 1kHz at -18dBfs then play that back in PT.
Now measure the line output from the Pro 40. The manual (will look it up in a mo') should then tell you what line INput you need to get -18 in PT.

And from THAT you will know the sort of level (from the gain figure) to inject into the strip. I suspect +4dBu is way too high.

As to the meter to use in PT? Is there a default setting? If not use the one that shows neg 18 from the aformentioned internal tone.

BUT!! Fair play to you for wanting to calibrate! No enough peeps bother to check their system's gain staging IMHO. Good also from t2t to run an RTA like Right Mark and look for low level, C: -70-80dBfs, hums. These can creep up on you as you plug about and move kit around. Oh, and you might have some nasty form of ground loop?

*Good old Audacity can do it.

Dave.
 
Took a bit of searching but I eventually found the Saffire Pro 40 manual and that gives 0dBfs = +16dBu and so -18 = -2dBu or 615mV if you don't have a dB reading voltmeter.

I shall now look for the spec of the strip.

The RC500 can be set as a unity gain device it seems operating at 0dBu so, provided all dynamic funtions are off I don't see why you have a problem?
Just a thought? The inputs are balanced, how are you injecting the tone? Most sig gennys are referenced to chassis.

Dave.
 
Hey Guys

Thanks for all the replies. REALLY appreciate it. So the Channel strip is def zero'd out in terms of the EQ and Compression circuit, and they are taken out of the path via the toggle switch. The meter is not set to show gain reduction, that toggle is off. So the meter is showing the input signal......

Im using a behringer CT100 cable tester, that also has a tone generator on it....XLR out from CT100 Tone Generator set to +4db there is also a -10db tone, and a setting for -50 mic which is a VERY low signal. not sure what that is used for.

The meter type in protools, i use the default which is SamplePeak. For those that are not Protools guys, there are 17 Different meter types in PT. confusing the situation even more. They have the K_meters, RMS, VU, Digital VU, and tons of others. The LINEAR meter seems to have allot of resolution and indicators for db levels, whereas the sample peak has -40db at the bottom.......loooooots of space, then -20db in the middle, with no other indicators of db level... so thats confusing....

i really appreicate the help
 
ecc83 said:
Took a bit of searching but I eventually found the Saffire Pro 40 manual and that gives 0dBfs = +16dBu and so -18 = -2dBu or 615mV if you don't have a dB reading voltmeter.

I shall now look for the spec of the strip.

The RC500 can be set as a unity gain device it seems operating at 0dBu so, provided all dynamic funtions are off I don't see why you have a problem?
Just a thought? The inputs are balanced, how are you injecting the tone? Most sig gennys are referenced to chassis.

Dave.
Click to expand...

And Dave, i'm not following what you said above, 0dBfs = +16dBu and so -18 = -2dBu ? and that The RC500 can be set as a unity gain device it seems operating at 0dBu Can you expand on that for me? why does -18 = -2 ?
 
This may help with what Dave was saying about dBfs > dBU specific to the Sapphire Pro 40 specs....
StudioLive metering - dbFS vs. dbu – Knowledge Base | PreSonus

This is pretty handy for doing conversions (about a little less than halfway down it mentions dBfs).....
dB dBu dBFS dBV to volts audio conversion digital - calculator volt to dBu and dBV dB mW SPL dB decibels 0 dBFS - convert dB volt normal decibels relatioship relation explanation analog audio absolute level true rms convertor converter decibel to dbf

which came from this collection....
Audio calculations in English acoustics calculator convert audio formulas sound dB audio system microphone electro engineering electronics formula sound recording studio useful stuff free audio calculator recording studio acoustic audio engineering s
 
Sorry! Rattled on a bit there.. Any D/A converter, e.g. in an AI, will have an output spec ref 0dBfs, digital max. Most "pro" gear runs at +22dBu for 0dBfs but prosumer kit will be lower and the cheaper stuff MUCH lower!

The Pro 40 is pretty "pro" and so gives a healthy +16dBu for 0dBfs and (if my math is right, don't bet on THIS old, meds ridden brain!)that means 18dB below that is -2dBu?

Attached is neg 18 at 1kHz. Was 16 bits 44.1kHz but I forgot I can't attach .wavs...Not even a second of them! GRRR! I have not therefore re checked that level but will do next in Samplitude. So, stuff that MP3 into PT and find the meter that reads -18dBfs.

Yus, the MP3 also comes out bang on -18.

Dave.
 

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Dave,

Thanks for all the help. and the MP3 ill check that out later and see whats what

you had mentioned earlier that you though a +4dbu was to high when referring to the test tone, could that be why im pegging the VU on the strip at low output settings? GAIN knob set to around 40, the final output knob set just above zero, almost completely counter-clockwise , very small movements to the knob in the clockwise direction, peg the meter. is there a different level tone i should be sending in?....this is all so confusing....it was so much easier on a tascam 4 track cassette machine
 
Hricco said:
I am trying to calibrate the system. using a tone generator, i can send a 1K signal at +4db into the mic pre. and adjust the gain and output so that the VU meter hits 0db
Click to expand...

That seems way too hot. I mean, it's a topped out line signal into a mic preamp...

Or am I not reading that right?
 
cyrano said:
That seems way too hot. I mean, it's a topped out line signal into a mic preamp...

Or am I not reading that right?
Click to expand...


you read that right. So im beginning to think, that the tone generator im using is not the right tool for this job. Any Suggestions, does anyone know what the "-50 MIC" tone setting on this box is for?

I have a second RC500 and two Joe meek VC1Q's being delivered this week as well. I'd Ike to be able to calibrate correctly...

Thanks for all the help by the way, this forum rocks!
 
Hricco said:
you read that right. So im beginning to think, that the tone generator im using is not the right tool for this job. Any Suggestions, does anyone know what the "-50 MIC" tone setting on this box is for?

I have a second RC500 and two Joe meek VC1Q's being delivered this week as well. I'd Ike to be able to calibrate correctly...

Thanks for all the help by the way, this forum rocks!
Click to expand...

The "-50 MIC" I take to mean -50dBV balanced* to feed a microphone input. -50dBV is -47.7dBu or 3.16mV which is about 3 dB hotter than the spec' of an SM57 so an ideal signal for testing. However, I would not trust Behringer to be that accurate. Do you own a didital mutimeter? If not, go and get one pronto.

Pay as much as you can for a good brand (Fluke are great but losta$$$) Avoid anything with bells and whistles you don't need such as temperature measurement and unless you are getting into electronics DIY you don't need complex component testing nor computer integration.

Plug the meter into the cable tester and read the AC signals at the various levels. Not likely any meter will resolve 3mV but it should do 10. In any case you will get an idea of how close the B is to the truth.

*Almost certainly an "impedance" balanced output but that should be ok.

Dave.
 
Did I understand [MENTION=200733]Hricco[/MENTION] is sending +4dBu into the XLR (i.e. microphone) input of the preamp and wondering why it's pegging meters? Shouldn't he set the tone generator to create an audible signal, measure that with a dB *audio* meter at the microphone and when it reaches the spec (94?), *then* start dialing in whatever?
 
keith.rogers said:
Did I understand [MENTION=200733]Hricco[/MENTION] is sending +4dBu into the XLR (i.e. microphone) input of the preamp and wondering why it's pegging meters? Shouldn't he set the tone generator to create an audible signal, measure that with a dB *audio* meter at the microphone and when it reaches the spec (94?), *then* start dialing in whatever?
Click to expand...

Yes he could Keith but now you have introducced two sources of inaccuracy, that of the SPL meter (and if he is going to get one, get one with a C weighting) and the sensitivity of the mic which is rarely better than a +or- 2bB tolerance. Plus all the hassle of getting mic and meter in close proximity PLUS the bloody row!

I am not really sure K' WHY the op wants to "calibrate" his system? Nonetheless I commend him highly for his efforts, too often there is a distain in this audio game for "science". Those that put back of hand to forehead and claim "I am an ARTIST! Cannot be doing with all this technical stuff" forget that they HAVE all this fabulous kit because some people DID bother!

Dave.
 
Hricco said:
...GAIN knob set to around 40, the final output knob set just above zero, almost completely counter-clockwise...
Click to expand...
You're starting at +4dbu, which is apparently already more than -18dbFS on your interface. Then you're adding 40db of gain. That means it's trying to hit +44dbu which is like 24db more than that unit can put out and about 40db more than it really needs to be. I'm not the least bit surprised that you have to keep the Master way down. It's also not particularly surprising that knob is kind of touchy in the low end of its turn. The first half of its turn covers something like 75db of adjustment while the top half spans only like 15db.
 
ashcat_lt said:
You're starting at +4dbu, which is apparently already more than -18dbFS on your interface. Then you're adding 40db of gain. That means it's trying to hit +44dbu which is like 24db more than that unit can put out and about 40db more than it really needs to be.
Click to expand...

And he's got the master almost completely attenuated. I mentioned this earlier but no one seemed to notice.
 
bouldersoundguy said:
And he's got the master almost completely attenuated. I mentioned this earlier but no one seemed to notice.
Click to expand...
I edited my post, but yeah, you'd kind of have to. In fact, you'd mostly just be using the Master to undo the preamp gain. It actually looks to me like this box is basically intended for pounding the preamp into saturation and the cranking it back down to reasonable levels.
 
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