OT: Fluke 177 DMM problem

[905shmick]

In an effort to dial in my preamps so that all the channels are sitting at -14dbfs = +4dB I've fired up my tone generator with a 1K sine wave with an amplitude of 1.227 Vrms (+4dB). I was initially measuring this with my Fluke 177 but I've now discovered that I'm getting an incorrest RMS reading from it. When I use my syscomp cirguitgear scope to measure I'm gettng an accurate reading.

I've got a -14dB wave file playing from SAW through SAC on SAC input #1.

Here's the difference in Vrms between the 2 units.

Output / CircuitGear / Fluke
+10dB / 2.73 Vrms / 2.141 Vrms
+7dB / 1.94 Vrms / 1.517 Vrms
+4 dB / 1.39 Vrms / 1.074 Vrms
0dB / 0.892 Vrms / 0.678 Vrms

I get a proper reading from the Fluke when measuring household power with it, just not when doing this stuff.

Any ideas?

On a side not I also noticed that SAC shows the -14dB file at about the -11dbfs mark on the SAC input and output meters when set to unity.

[gdougherty]

I have a greenlee unit that measures RMS voltage, but only below 500Hz. Might be something like that where the Fluke doesn't measure accurately across a wide frequency band.

[905shmick]

Just tried a 250 Hz wave @ 1.22 Vrms and the Fluke is reading 1.022 Vrms.

Strange.

[Ogmeister]

I don't have a Fluke meter but have had similar experiences with DVM's.
I use an old HP analog AC Millivolt meter for these types of tests ( great old 70's vintage piece of kit that is accurate). You may be experiencing an impedance problem with the input circuit of the meter and this is causing the circuit under test to lower its output.
Anther thing to take into account is the design specifications of the device under test. I have found that many manufacturers fudge the specs on things.

OR.. You may need to send this device for calibration.

If you have a old school oscilloscope lying around you could monitor the signal and see what is happening as far as distortion or something when the meter is connected to the circuit.

OGO

[905shmick]

The image I posted is both the waveform generator and scope. It's a nice and clean sine wave.

[Ogmeister]

What are the units you are testing?
Do you have schematics for it?
I could try and look at the circuit to catch an idea of what might be going on.

OGO

[905shmick]

The O'scope and waveform generator are Syscomp CircuitGear

The preamps are Behringer ADA8000 units.

Digital card is a RME HDSP 9652

[905shmick]

Something else I need to figure out is why the "1kz -14dbfs" wave file that I downloaded is showing up on the meters in totalmix and SAC as -11dbfs. Perhaps it's not really -14dbfs!

I recorded my own 1khz / -14dbfs file from my tone generator with SAW and it shows up on the meters properly at -14dbfs when playing it back through SAC via SAW.

[Ogmeister]

The tone source that you are using is it a stand alone unit?
I am supposing that you are measuring the output of the signal gear device.
I see that it is able to make these tones.
I use an old school HP audio signal generator to make these tones and have no real experience with using the computer based devices.
My reasoning for this is that I think you can make a computer display any level you want on those graphic meters. It is up to the programmer and how he displays RMS vs PEAK and the true ballistics response.

I have done a transfer test (phase vs frequency) on the Behringer ADA800 and I can tell you that they very greatly from device to device and even on each channel on the same device. What do you expect for 200.00 bucks.
However I have not found any problems that would prevent me from recommending this as a live preamp. Yes it is noisy and has some poor design features (first mod for me would be a better power supply) but I actually believe that the noise that it induces is beneficial to the D/A conversion process and makes it sound a little less digital in my opinion.

For me it is fine as a SAC pre-amp and I have used the meters in SAC as a reference to set my input gain structures.

I have noticed that the outs on this device are not calibrated to what the meters are showing on the SAC masters. This does not worry me as I know what the level truly is and I have set my drive inputs to match this gain structure. All in all it is great for the money.

With most Behringer gear I have used and tested the so called headroom specs or dynamic range is no where near what they claim. On these outputs as you drive them harder you can see the distortion start to creep in way before you should. This is probably due to them stating these specs at a specific frequency like 1k or lower. These preamps can get quite dirty in the upper range. The good news is these distortion harmonics are above most humans ability to hear around 14-18 K. But could drive you crazy in a studio environment.

Still I have an M7 and Ls9 that are going into retirement due to the incredible sound difference in using this SAC configuration. Not to mention the flexibility of such a virtual system.

For a quick way to set gains in the field I use this method.
One tone source (can be any thing that puts out a 1k tone)
One pizeo transducer buzzer (get them at Radio Shack or tear it out of something else).
Appropriate adapters and connectors to in an out on devices under test.

Procedure:

Connect tone source to input stage and adjust the gain to get a reading of +4 on the channel strip. Connect the pizeo buzzer to one of the intended outputs. You should here no sound from this device as you are not creating any DC yet. Tun up the out gain to achieve the desired output levels that match the next device in the chain using the output meters. Now with this set you can drive the input gain up until you hear buzzing from the transducer.
This is the point that the distortion is happening and causing square wave pulses that the buzzer can announce. Back this input gain off till buzzer is silent and now you know just where the distortion point is.
I have found that a lot of LED meters on consoles are conservative in their readings and give you the impression that you are in the red before you are truly there.
Now I just take some tape or what ever and mark the meters or make a mental note of what I found.

In my shop when I accept a new mixer into my rig I always test the gain of it and adjust the meters to match my drive rack. Or adapt the drive rack gains to this console.

OGO

[905shmick]

How do you get to +4 on a digital strip?