tape people: question about reference level for calibration tone (XLR's)

[Jonk]

Thanks to everyone who's helped me thus far calibrating my Otari MTR-12.
I have a pretty specific question here.

My MTR-12 manual says this:

"Important Note Regarding Level Measurements:
The MTR-12 has an active balanced output configuration with cross-coupled feedback.
When level is measured across pin 2 and ground or pin 3 and ground, the readings will be 6 dB lower than in balanced mode. If one side of the line is grounded when measuring "unbalanced" across pin 2 or pin 3 to ground, the level then remains the same as in balanced mode"

Either this is a fancy way of saying something I already know about balanced outputs, or it's saying the Otari's balanced outs are a little different.

Here's my question
If I measure the outputs of the MTR-12, and I'm looking for a balanced 0dB level (1.23V) then should I have the volt meter connected like this: black lead to pin 1, red lead to pin 2,
OR like this: black lead to pin 1 and pin 3, redlead to pin 2

(pin 2 and 3 being interechangeable of course)

I've always just measured pin 1 to the negative lead, and the red lead to one of the other pins.
Now I'm wondering about oscillator signal coming out of my mixer too. Maybe I've been calibrating things wrong.

Is the Otari the same as every other XLR balanced out? Or is what they are saying in the manual indicating that there's something else going on that makes it different and I should therefore be hooking up the VOM by grounding one of the xlr pins?

Any help is much appreciated.

Thanks,
Jon

[Rick Sutton]

The way the quote reads to me is that the Otari has a compensating circuit that supplies extra current to one of the output leads if the other output lead is grounded. So to obtain a 1.23 volt reading on your volt meter, ground the unused output lead (as you stated ...Black (Ground) to 1 AND 3, Red (hot) to 2.)
It really helps to have meters on your board or a calibrated meter unit external to the tape machine to keep you on the mark. But even without these you can do this with a VOM. Once you get the tape machine output set, it's meters calibrated and the playback levels (nW/M) set with a calibrated test tape, you can use the tape machines meters for all the rest of your measurements.

[Jonk]

Thanks Rick,

I wonder if this is the case with my mixer too (Yamaha PM1000)

When I measure output voltage on it's main buss out XLR's, it IS higher when I ground the other pin, but I'm not sure if it's 6dB (I mean, I'm not sure what voltage difference 6db is) but I'm guessing that's what it is.

This is starting to make a lot of sense actually, because I was having to pin the meters on my mixer way in the red to get a 1.23 V reading when measuring one of its oscillator tones. I was just kind of ignoring that, which is a problem I have.

Is it just a good practice to ground the other pin anyway, if you're taking a voltage reading?
It's not going to make a difference if the unit doesn't have a circuit that compensates for the other pin, right?

[Rick Sutton]

Jonk, I've been in the recording business for 35 years and I still have to read the manual for each piece of gear to see how it reacts to unbalancing a balanced output or input. I've seen some that you have to float the unused pin, some where you float ground and various forms of tying ground. The best bet is to not unbalance the output and run it into a balanced input that has properly calibrated meters. I use my Otari 54 board for this so I don't have to rely on an unbalanced VOM for tape machine (or other units) calibration. In a pinch I've used the inputs of compressors that have balanced inputs and a good calibrated meter, that will allow you to read input. I also have a Loft-Tech sig gen/level and freq. meter that is very handy. Really though, for day to day calibration, the meters on a pro board/tape machine combination is really the "old school" way of doing it that I'm familiar with....that way you never have to worry about the unbalanced issue when taking readings. When using a properly calibrated board the fader markings are accurate enough so when you get a reading that is over scale you can put the signal through a fader to the meter and drop the fader by 10 dB or so and get a very accurate over scale reading. My board also has a very accurately calibrated oscillator that has both a variable and a fixed +4 output so a lot of guessing is eliminated on the sig gen side.
Sorry I don't have the answers for your specific situation, but if I guessed at it I'd probably mess you up. A good meter that reads balanced lines(either built into a board or stand alone) would really help out. I used to build simple meter drive circuits and a power supply to drive surplus Simpson dB meters to take these readings but you may be able to find what you need on Ebay or from someome here on Gearslutz. Actually, once you verify that your tape machine meters read accurately you can use them to check your boards readings, Once you get one circuit verified, others can be calibrated to it. There are a few tricks and gotchas along the way, but you'll learn as you go. Good luck. Rick

[Jonk]

Thanks again Rick,

If say, I were to do it with my 1176, where would I put the input and output knobs?

[Rick Sutton]

Man, it's been so long since I've done it with anything other than my tape machine or boards meters I don't even remember which pieces of outboard have good metering. I'm starting a few days of mixing and I'll take a look in my rack and try to come up with some ideas for you on how to use the "gonzo" technique of outboard gear metering.
Let's back up just a bit here though. I think I came into this a bit late so let me ask. You do have an alignment tape? With the alignment tape, the Otari manual, your VOM and the information about measuring the balanced out of the Otari you've got everything neccesary to calibrate the meter and output level of the tape machine and give you a meter that will read "0" when presented with a +4 level. It seems to me that this is your calibrated meter unless I'm missing something here. From this meter, using the input switching on the tape deck, you can see what output you're sig gen is presenting to it. Was the tape machine in decent condition when you got it? Chances are that the VU meters are already calibrated to read a +4 input as this is (meter calibration) not something that is messed with once a machine is set up. Don't confuse meter calibration with operating level (the internal level that the machine is putting on the tape measured in nW/m. Regardless of this internal level the machine is still presenting +4 to the outside world) calibration on a tape machine. I know it seems like we're running in circles, but once you know that one meter is accurate ( like on the Otari) then you can check other pieces of gear in your studio and mark the controls if you want to "calibrate" them to a known value. A disclaimer here...I'm not a tech, just a guy that has aligned enough tape machines to feel comfortable doing it, but it's not something I am used to teaching others, in fact, it's not something I do very often anymore in this day and age of Protools. Chances are if I could look around your control room I could see a couple of ways to help you out but at a distance it looks like you've got the ability to get that Otari aligned and that should get you that calibrated meter that you need to answer your questions. Anyway, best of luck....if you want to get together on the phone sometime maybe that would help you ( or confuse you more). Cheers, Rick

[Jay Kahrs]

Rick's got a lot of good advice there. The rule of thumb is that when you go from balanced to unbalanced you'll lose 6dB of gain. I'll usually let pin 3 float and if I encounter problems I'll tie it to pin 2.

And FWIW, I'd concentrate on getting either the tape machine or console meters calibrated before using and/or checking them with the meter on a piece of outboard gear, especially an 1176 because I don’t think I’ve seen two (including my pair of reissues) that match exactly.

[dach]

The outputs on the MTR12 are reversed, pin 3=Hot and 2=cold

Many of the older Otaris were unbalanced at +4 but the MTR10/12/etc is an active balanced output if memory serves... I'm too lazy to get the service book... and too old to have instant recall. I'm sure someone else here can tell you exactly without thinking about it.

for balanced output calibration, tie 2 and 1 to your black lead and 3 to red.

for the 1176 pretty much the same thing applies. You will probably need to get someone to go over the unit & perform Q-bias, GR zero, etc if things are whacked....

once again, my "oldness and lazy disclaimer" applies here as well. I'm sure Jim Williams or someone like him will not have to even think about it... maybe PM him...

Dach

[Jonk]

Okay,

thanks so much everybody for the help.

I'm totally confused now. What am I looking for on the VOM?
a 0dB (1.23V) balanced signal, or a 0dB (.3V) unbalanced signal? Every output I'm measuring is balanced, but if I only hook up a pin 1 ground, and one of the other pins to the red VOM lead, and leave the other pin floating, am I then looking for an unbalanced (.3V) signal?

At this point the Voltage meter really is going to be my best reference tool (ouside of the calibration tape, which I can't use to calibrate my meters right? because the meters need to be calibrated first.)
The VOM is a Fluke and it was calibrated right before I bought it, so I trust it.

Once I can figure out how to hook it up to the xlr outs on my mixer to measure the oscillator tone coming out of the mixer, so I know it's gonna be a balanced 0dB level, then I can calibrate the meters on the Otari and everything will be okay.

This is a balanced to balanced connection here (mixer tone>tape deck) Though the phase is being flipped, which I'll have to take into account and re-solder a couple cables. Forgot that Otaris are usually pin 3 hot.

[Jay Kahrs]

Originally Posted by Jonk

At this point the Voltage meter really is going to be my best reference tool (ouside of the calibration tape, which I can't use to calibrate my meters right? because the meters need to be calibrated first.)

Yup. Without knowing what the meters are saying the MRL is of little use. When printing tones back to the Otari, there's probably an input gain pot in additon to the EQ pots...just another thing to further confuse 'ya.

[Rick Sutton]

Jonk, I'll try one more time to shed some light on this.

1. Do not rewire anything due to the Otari being pin 3 hot. As long as pin 1 is ground and pins 2&3 are consistent from input to output nothing will get phase flipped.
2. Yes you CAN calibrate your Otari meter with the test tape, your meter and the knowledge of how to take an unbalanced reading from the Otari output circuit, you supplied the info in the first post. It's just too difficult to explain it to you in this forum context. On the phone I could talk you through it.
3. Now as to your concern about your oscillator output. You could measure positive to ground and it should be 6dB under +4. On your meter that may not help you so...... unless it's an old analog meter (like a lot of us old guys use) and has a scale calibrated in dB) so............let me ask you this. Is your Yamaha board a balanced +4 mixer with meters? Does it have tape returns that are metered? On my board, anything you patch into a tape return goes directly (without going through any user adjustable volume control other than internal trim pots) to the corresponding track meter and tells me instantly what level that signal is relative to a +4 =0 VU reading....in other words, if you have a pro board with these common functions you can take your oscillator output and patch it into a tape return and read the track meter and it will tell you what the output level of your oscillator is.
4. I said in an earlier post that I would check my outboard and I did. an Aphex compellor and a UREI La22 both have input reading meters that are +4 calibrated and are independent of the input volume control. You probably don't have these in your rack but it illustrates that calibrated meters are common.
This problem isn't as complicated as you may think. You have the tools and the info, just dive in and do it. Hell, even if you get the levels a little off from standard you will probably learn a lot and be able to start understanding some of this stuff.
good luck. rick

[claveslave]

Originally Posted by Jonk

Okay,
At this point the Voltage meter really is going to be my best reference tool (ouside of the calibration tape, which I can't use to calibrate my meters right? because the meters need to be calibrated first.)

Sort of. You should place the machine in all input mode, feed each channel of the machine with a +4dB (1.227 Vac) 500Hz - 1KHz sine wave and adjust the input trimmers to provide the same +4dB at the output. For this to be accurate, you need to tie cold (Pin 2 in your case) to ground in and out. If you want to be really accurate, let the oscillator settle and check it every few tracks that you align. Next, adjust the VU meters to 0VU. They now represent +4dB (1.227 Vac) when they read 0. After turning the machine off and demagnetizing the heads, load the Test Tape. Then, in repro mode and using the machine's meters, calibrate the output level of each track. How you set them up will depend on the level printed on your Test Tape. If it's a 250 nanowebers/M tape, set the output so that the meters read -2VU. If it's a 320 nanowebers/M tape, set the output so that the meters read 0VU. After that is done load the machine with the type of tape you intend to use, apply the 500-1K tone at +4 to the inputs and in record mode but monitoring off the repro head, adjust the record level so that input and output match (0VU).

None of the above has taken into account the azimuth and zenith adjustments of the heads but I assume this has been done. You will also have to set the bias and HF trimmers for the tape you are using and after that if you want it to be the bee's knees, you should go back and repeat all of the above for final minute adjustment. If you're the only one using the machine and it's handled with care and you keep using the same kind of tape, you won't have to touch it again for ages apart from cleaning.

Of course, I may have missed the point of your question but it's an answer to some question out there.

[Jonk]

Okay,

update. In terms of VU meters, I have my boards', my MTR-12's and my Tascam 48 8 track VU's

I realized that if I have a third party VU meter, it doesn't really have to be calibrated correctly, it just has to give me a consistent reading, so what I did was I measured a 1kHz oscillator tone coming out of the mixer, and a 1kHz osc tone coming out of the the MTR-12.

I measured with the VOM, (experimenting with both ways of connecting the VOM , that is, the way Otari reccomends in the manual, stated in my first post, and also the way it was reccomended to me a long time ago: negative lead connected to pin1 positive lead to pin 2)

When I got them both to 1.23V, I then connected them 1 after the other to the same VU input on my Tascam 8 track, to make sure they matched up not only on the VOM, but on a 3rd party VU too.

So, what I found is that with my Mixer outs, ( Yamaha PM1000), I had to ground the other pin when measuring with the volt meter, just like w/ the MTR-12 to make sure both machines matched up on the Volt meter AND a 3rd party VU.

I also found that when I get a 1.23V reading on the volt meter from both the mixer and MTR-12 by measuring this way (negative lead to pin1 and pin3, positive lead to pin 2)
and then I remove the negative lead to only pin 1, leaving 3 floating, there's about a .05 Volt discrepency between the mixer and the MTR-12(mixer is .62 MTR-12 is .67)

Wierd.

So what I know now is that I can measure the tone coming out of the mixer, send it to the Otaris inputs, then measure the Otari's ouputs accurately.

BUT, is it really a 0dB signal? If I'm reading the Otari's manual correctly, then it is.

Just to see what it looked like though, I plugged both the 1.23V 1kHZ tone from the mixer, and the 1.23V 1kHz tone from the MTR 12 into my masterlink and they both registered at -15dB.
Does that sound consistent for a digital reading of an analog 0dB oscillator tone?

[claveslave]

Originally Posted by Jonk

Just to see what it looked like though, I plugged both the 1.23V 1kHZ tone from the mixer, and the 1.23V 1kHz tone from the MTR 12 into my masterlink and they both registered at -15dB.
Does that sound consistent for a digital reading of an analog 0dB oscillator tone?

Bingo! It's not 0dB though. 1.23 V is +4dB. 0.775 V is 0dB. You are trying to set 0VU to represent 1.23 V (+4dB). Your reading on the Masterlink is correct for a +4 dB signal.

[Jonk]

Oh right. That's what I meant to say.

I finished calibrating it today, and channel one is absolutely perfect in every way, but channel 2 for some reason, neither the repro or record eq pots seem to have any effect, which doesn't matter for everything but the Hi Freq record EQ because 100Hz, 1kHz are flat across the board, but at 10kHz I'm getting about 2.5dB less than I should be when recording. And the eq pots seem to have no effect, either when I turn them up or down.

I could re-align the heads and see what happens, but I thought I got them pretty spot on (w/ the scope). that wouldn't solve the problem of stil no being able to adjust the eq on channel 2 though. Hmmmm. I recorded some bright acoustic guitar and I could difinately notice a difference in the high end between ch.1 and 2.

If somebody has the answer to this problem, boy would I be happy. Maybe that should be another topic I don't know.

And thanks Rick, claveslave, Jay, and dach for all the help.

p.s.

If I re-align the heads, does that mean I have to do everything but the input and VU meters again? Biasing too.
I don't mind, I'm starting to kind of quick at it, but I'm wondering if there's anything else that can go before azimuth adjustments in the sequence of events

[claveslave]

Originally Posted by Jonk

If I re-align the heads, does that mean I have to do everything but the input and VU meters again? Biasing too.
I don't mind, I'm starting to kind of quick at it, but I'm wondering if there's anything else that can go before azimuth adjustments in the sequence of events

The only thing you should do before azimuth is the zenith although on a 2 track this wouldn't be causing you to not see any reaction to the HF eq. unless it was completely . Forgive the obvious question but are you sure you were in repro mode when making that adjustment on track 2?

[Jonk]

Yup,

I'm sure. I'm in repro.

It's like the eq has been completely bypassed for that track

[Rick Sutton]

I'd try switching the cards between the two channels and see if the problem follows the cards. Just taking them in and out could clean contacts or show you other symptoms. Make sure the machine is off when pulling or inserting cards.