how to calibrate with a voltmeter , my 888s

[gould]

im trying to figure out how you calibrate 888s, with a voltmeter, i know its suppose to be 1.223VRMS on a volt meter. i did the sine wave out of my protools at -18 but its not +4 on my console, it like -20, my 888s pins are right inside the 888s, my console is +4, i dont know how to calibate using a voltmeter, where do you put it, where do you get one, anyone tell me the steps

[O.G. Killa]

Originally Posted by gould

im trying to figure out how you calibrate 888s, with a voltmeter, i know its suppose to be 1.223VRMS on a volt meter. i did the sine wave out of my protools at -18 but its not +4 on my console, it like -20, my 888s pins are right inside the 888s, my console is +4, i dont know how to calibate using a voltmeter, where do you put it, where do you get one, anyone tell me the steps

I just double checked...it's 1.23V, or 1.228V... but not 1.223V. Although, that small a difference isn't going to matter...

http://www.multimeterwarehouse.com/d...multimeter.htm

here are a list of voltmeters...usually called "multimeters" since they don't just measure volts. Go for an inexpensive digital one. I have an M838 and it works fine for what I need it for. But you might like something like the M320 since it is "auto-ranging".

http://www.doctronics.co.uk/meter.htm

This link describes how to use one if you need it. Usually the red cable touches pin 2 or pin 3, the black cable touches ground (pin 1).

[TVPostSound]

Go for an inexpensive digital one. I have an M838 and it works fine for what I need it for. But you might like something like the M320 since it is "auto-ranging".

Please don't.

Inexpensive volt meters are only accurate around 60Hz AC, as they are made to measure home voltages.

You need a "True RMS" AC voltmeter.

Once you have the correct voltage, you then need to calibrate your console meters. Set them to 0VU, that will be +4 EOL

Measure across pins 2 & 3 of the 888s XLR

[gould]

hey thanks man, so do i run audio through the 888, and on the back , touch the red and black wires to the xlr output, or just turn the 888 on , and touch the wires to it, i think you gave me a link, forgot, let me see, thanks

[O.G. Killa]

Originally Posted by DIAFXMX

Please don't.

Inexpensive volt meters are only accurate around 60Hz AC, as they are made to measure home voltages.

You need a "True RMS" AC voltmeter.

Once you have the correct voltage, you then need to calibrate your console meters. Set them to 0VU, that will be +4 EOL

Measure across pins 2 & 3 of the 888s XLR

That's cool. I'm no expert on this sort of thing, so please correct me if I'm wrong... But as far as I know, measuring the output of an XLR is DC not AC, so there is no need for an AC voltmeter, although most can measure AC anyway.

I haven't used one in a little while, but I remember I didn't get the correct voltage readings unless I was measuring to ground. I figure this is because measuring Voltage is essentially measuring the voltage differential? Pin 2 and pin 3 should have the same voltage on them since they are both carrying signal.

In any regard, I don't think it will damage the meter if you try pin 2 to pin 3, but from what I remember the last time I did that it read 0, but when I went Pin 2 to ground and pin 3 to ground it read 1.2V.

[O.G. Killa]

Originally Posted by gould

hey thanks man, so do i run audio through the 888, and on the back , touch the red and black wires to the xlr output, or just turn the 888 on , and touch the wires to it, i think you gave me a link, forgot, let me see, thanks

Put a signal generator on a mono aux channel. Set it's output to A1 and set the sig generator to Sine and to -18dB RMS (or whatever dBfs ref you want to use). Measure the output of of A1 with the voltmeter set on 2000mV. Should read 1228 (or 122, or 123 if your meter can only display 3 digits) or something close to that. Adjust the set screw until it reads 1228 or 123. Move on to the next output. Change the output of the aux to A2 and do the same thing over again. Once you get them all set to 1.228 out...

Create a second aux track, set it's output to something other than A1. Set its input to A1 and go into "Calibration mode" from the option menu. The "vol" reading will start to show "Cal" flashing. Plug the sig generator output from A1 out into A1 in. The meter should read -15 (remember RMS of -18 has a peak of -15). If it doesn't, adjust the set screw for the input on the 888 so it reads -15. Once you've done that to all your inputs then your 888 is calibrated... now you can create 8 aux channels, one for each output, put the same sig gen across all of them. Then plug them into teh inputs on your console. Since you already tested the ins and outs, you know for sure that the 888 is at 1.228V. If the outs of the 888 don't show up as 0 VU on any channels on the console, then you need to adjust their input cal. If any of the tape/bus outputs back to protools don't come in at -15 on aux channels, then you need to adjust the console's output cal for those tracks.

That's how I've always done it... if I'm wrong... please correct me!! I'm always open to learning how to do this better.

[TVPostSound]

But as far as I know, measuring the output of an XLR is DC not AC, so there is no need for an AC voltmeter,

Yes it,s AC, you are measuring between cold and hot.

[O.G. Killa]

Originally Posted by DIAFXMX

Yes it,s AC, you are measuring between cold and hot.

Then why can you have "DC Offset" in your recordings?

[O.G. Killa]

Originally Posted by DIAFXMX

Yes it,s AC, you are measuring between cold and hot.

I think you might be incorrect on this one. AC current is alternating. DC current does not alternate. The signal doesn't alternate on pin 2 and pin 3... it is simply a constant out of phase version of pin 2 on pin 3 at all times.

The power supply of each device converts AC current to DC and DC is then passed throughout the device and from one device to the next.

DC current means that the electrons only flow in one direction. That is why you cannot have an "input and output" on the same cable. DC flows from an output to an input. AC flows back and forth at a specific interval.

You have to think of a balanced cable as two unbalanced cables that share a ground. Using your description, how can you get audio on an unbalanced cable since there is no negative terminal? To measure voltage on an unbalanced cable you measure from tip to ground. On a balanced cable you measure tip to ground and/or ring to ground.

[Craig F]

DC offset comes from poorly designed and/or failing amplifier circuits.

Proper calibration is done into a load measuring across pins 2 and 3.
I personally like use a TT or 1/4" B-gauge to banana cable (long) so I can pick the signal at the patchbay but still get to the trim pots.