test equipment -- what's worth buying?

[ethyrvalve]

hi all,

I'd like to check to see if my tape machines, outboard gear and mixer are all more-or-less telling me the truth with their VU meters. Would a AC Millivoltmeter be a worthwhile investment do you folks reckon?

I read in the book 'Studio Produces for Recording Engineers' by Sherman Keene that a VTVM does a good job at measuring console noise. Any thoughts?

also, anyone here use an oscilloscope a lot in their studio? I wonder what's the cheapest scope i could get that would be useful for finding out at what level gear clips at (i have a yamaha analog delay that's a bit of pain trying to find the optimal level for), and maybe checking phase with lissajou patterns.

thanks in advance for any tips

[sweetbeats]

I don't have an analog scope, so I've been using a digital multimeter, but you have to make sure it is a true RMS meter, and that it has the bandwidth to support frequencies you want to measure.

Example: I bought a meter off of eBay that has a great feature set, is a true RMS meter and has actually worked great, but it turns out that it is not accurate for atr calibration work because the bandwidth only goes up to 400Hz and then tapers off from there. Won't even accurately read 1kHz.

I picked up a used Fluke 85 meter which does a pretty good job well over 10kHz.

The other caveat of course is that meters like the one I originally purchased and now the Fluke 85 are, of course, unbalanced. So when you are measuring voltages on a balanced line you have to set the voltage at -6dBm under your reference level.

There's a great online calculator for this here.

So, when I'm setting levels on my 58, and I'm using the balanced +4dBm inputs, if I had a balanced meter I would just dial up 1.23 volts RMS right? Well, I don't have a balanced meter so I need to reference the voltage that corresponds to +4dBm minus 6dBm, which is -2dBm because I'm not measuring one leg of the balanced line. Using the calculator above that gives me about 0.615 volts RMS.

Pete, you probably know this already, but I didn't for awhile and it sent me for a loop so I thought I'd share again my experience in the event it benefits others.

[Dr ZEE]

here's a nice lookin' box
http://cgi.ebay.com/NEW-Audio-Studio...mZ230257741475

[ethyrvalve]

hey, i most assuredly did not know that cory . (i'm shamefully ignorant of the electrical and mechanical side of this stuff) Thanks for the tips and thanks for that great calculator link.

And that's a cool looking unit indeed good doctor.

[analogkid]

I found a free downloadable analyzer suite - o-scope, frequency counter etc - but don't know how good it reall is yet. Let me find link and post.....



Of course I can't find the link now :P

Might be worth a google or two.

AK

[sweetbeats]

hey, i most assuredly did not know that cory .

I didn't either until the kind folks (analog freak type) at Tascam explained it to me several times...I was having trouble with my first stumblings through electronic calibration. I was getting some inconsistent answers to some things and no answers to others so I just called Tascam and it was kind of by a stroke of good fortune that the unbalanced meter thing came up. I'm very happy to be helpful.

(i'm shamefully ignorant of the electrical and mechanical side of this stuff)

I feel like a newbie (I believe I am one after all), and I hope to never stop feeling that way...I think I learn and share more with that attitude anyway. Goodness this stuff can go wide and deep...

[James John]

i know this is an old thread, but maybe someone could throw some light on it: 'vtvm meters' (for calibration) stands for vacuum tube volt meter, can't I just use a standard voltmeter to do this? i'm preparing for my first calibration and i'm pretty confused about a lot of stuff. Could someone please explain to me what sweetbeats said about balanced/unbalanced readings?

[evm1024]

Originally Posted by James John

i know this is an old thread, but maybe someone could throw some light on it: 'vtvm meters' (for calibration) stands for vacuum tube volt meter, can't I just use a standard voltmeter to do this? i'm preparing for my first calibration and i'm pretty confused about a lot of stuff. Could someone please explain to me what sweetbeats said about balanced/unbalanced readings?

Yes, you can use most any modern meter. The key here is that it has a high impedence so that it does not load the circuit under test and that it has the bandwidth for the signal that you are measuring.

In olden days meters were just that. A coil of wire connected to a divider circuit. Depending on the parts used the meter its self could load the circuit and cause a voltage drop. The meter would then read wrong - sometime by a lot.

VTVM meters used a tube as a gain stage that offered very low loading of the circuit and thus were much more accurate. Now almost every meter uses some form of gain stage and offers lower loading than VTVM's.

The bandwidth - cheap meters are able to measure 50 or 60 Hz mains without problems but as we go into audio frequencies they loose their accuracy. All meters have a upper limit of frequency where their accuracy becomes unacceptable. And when we are measuring the 10 KHz for a decks high frequency response this becomes very important.

True RMS is important too. RMS is 0.707 times the peak value of a sine wave. It is the AC voltage measured that would do the same work as a DC voltage of the same value. Do the same work means heat a wire for example.

Regards - Ethan (sorry about the long answer)

[trancedental]

Originally Posted by evm1024

cheap meters are able to measure 50 or 60 Hz mains without problems but as we go into audio frequencies they loose their accuracy.

Regards - Ethan (sorry about the long answer)

Cheap meters nowadays are not very reliable in my experience even for reading mains voltages, there sometimes can be a difference of 4-5 VAC when reading the mains voltage with different meters & the cheaper ones are usually giving false readings.

[evm1024]

Balanced vs unbalanced voltages are not different - the references for the measured voltages are (and loads too).

I use unbalanced into and out of my decks so I calibrate them on the unbalanced lines.

The voltage references for unbalanced lines (On my Tascams) is dBv. dBv is a voltage based reference - 0 dBv is 1 volt across a high impedence line (that means no current or power is transferred in effect). Tascam specs a minimum 10K ohm load typically. And the line reference for tascams is (0 VU =) -10 dBv or about 0.316 volt. The point here is that it is a voltage only reference that cannot transfer any power without drastically changing the voltage output.

Moving on to balanced signals - Balanced signals are referenced to dBm which measures voltage into a 600 ohm load. OK so we have voltage and a load which means power. And that is what it really is: 0 dBm is 1mW of power in a 600 ohm load. So 0.775 volts across 600 ohms 1 mW of power. the balanced drivers are able to push that power and all is good. Most balanced lines are have a line level of (0 VU =) +4 dBm or in some cases +8 dBm. 1.22... volts is +4 dBm. Now if you remove the 600 ohm load the line driver will raise its voltage (try pushing in the the clutch of your car when going up hill without taking your foot off the gas!). The voltage raise is not easily figured out. In an ideal world the voltage will rise until 1 mW is sourced. There are limits that will not allow this.

So the bottem line is that unbalanced lines use dBv and need very low loading to read voltages correctly. Balanced lines use dBm and need a 600 ohm load for the voltage to be measured accurately.

How do we get a 600 ohm load? Just take your (good) meter and clip a 600 ohm resistor between the leads. Now you have a 600 ohm load.

Regards, Ethan

[sweetbeats]

James John,

Just to restate what others are saying...a digital multi-meter is fine, but not a cheapy one. Just make sure it is a True RMS meter, and make sure it has a bandwidth (or in other words can accurately measure) voltages in frequencies from (ideally) 20Hz to 20kHz.

The balanced/unbalanced thing...my reel to reel deck has balanced and unbalanced ins and outs. Ideally, if I'm going to use the balanced I/O in operation I should do my calibration using those jacks. Well, if I have a +4 test tone coming to the deck over a balanced line (three conductors) and I measure the voltage with an unbalanced meter (two conductors), I will only be measuring one leg of the balanced signal (ground, and either hot or cold). Because you are only measuring half the signal it is going to read -6 from what you are seeking. Therefore +4dBm is going to read -2dBm, and there are charts that compare dBm to voltage.

If you are using unbalanced ins and outs then you don't need to worry about any of this...just get a decent True RMS multi-meter with adequate bandwidth.

Does that help?

EDIT: Ethan beat me to it!

[evm1024]

Balanced measured from 'hot" to ground.... That is what you were saying.

In balanced lines there is a non-inverting and an inverting wire. These are driven in opposite polarities by the driver amps. for a positive swing input the non-inverting line will go positive and the inverting line will go negative an equal amount.

The "hot" lead in a truly balanced system (like from a transformer) will be the non-inverting lead. Measuring from one lead to ground will give very wrong readings.

As was noted in a balanced system you measure across the non-inverting and the inverting lines. The ground wire is the shield to help reduce the amount of noise that hits the wires.

Because noise is picked up equally by the inverting and non-inverting lines (in an ideal world) the noise cancels its self out. (+ noise plus - noise = 0 noise)

--Ethan

[mshilarious]

One should be a little careful about dBm and dBu, because manufacturers are sometimes not. Here on the Analog board, you might genuinely care about dBm (which technically can be referenced to any stated load, but 600 ohms is usually assumed). But modern gear not necessarily designed for the days of impedance matching is generally spec'ed in dBu (unloaded, as Ethan described).

So if you are trying to measure some signal level where dBu is appropriate, then don't load down that circuit.

The problem is some people, including those who have never seen a tape machine, will use dBm when they really mean dBu. So be careful out there!

[Richard King]

Warning.... expensive stuff about to be shown.... When I used to work in the pro audio business in Minneapolis the techie types used one of these: http://www.slack.com/images/TE/Amber4400.jpg and one of these: http://www.slack.com/images/TE/Amber3501.jpg for setting up tape machines.

[sweetbeats]

One should be a little careful about dBm and dBu, because manufacturers are sometimes not. Here on the Analog board, you might genuinely care about dBm (which technically can be referenced to any stated load, but 600 ohms is usually assumed). But modern gear not necessarily designed for the days of impedance matching is generally spec'ed in dBu (unloaded, as Ethan described).

So if you are trying to measure some signal level where dBu is appropriate, then don't load down that circuit.

The problem is some people, including those who have never seen a tape machine, will use dBm when they really mean dBu. So be careful out there!

Msh, thanks. I have to keep a reference guide handy to recall the diff btn dBm and dBu...I work so much more with dBm on the Tascam stuff because most of the balanced stuff was meant to be 600ohm-loaded, but that has changed over time huh...thanks

Balanced measured from 'hot" to ground.... That is what you were saying.

In balanced lines there is a non-inverting and an inverting wire. These are driven in opposite polarities by the driver amps. for a positive swing input the non-inverting line will go positive and the inverting line will go negative an equal amount.

The "hot" lead in a truly balanced system (like from a transformer) will be the non-inverting lead. Measuring from one lead to ground will give very wrong readings.

As was noted in a balanced system you measure across the non-inverting and the inverting lines. The ground wire is the shield to help reduce the amount of noise that hits the wires.

Because noise is picked up equally by the inverting and non-inverting lines (in an ideal world) the noise cancels its self out. (+ noise plus - noise = 0 noise)

Um...hm...so, when I picked the brains of the folks at Tascam about this they told me that if I was using an unbalanced meter to measure voltage on a balanced line to measure across either hot and ground or cold and ground. Am I understanding you correctly, Ethan, that that is not correct? Or is it not as critical with non-transformer balanced systems like those in every piece of balanced gear I own?

[mshilarious]

Originally Posted by sweetbeats

Um...hm...so, when I picked the brains of the folks at Tascam about this they told me that if I was using an unbalanced meter to measure voltage on a balanced line to measure across either hot and ground or cold and ground. Am I understanding you correctly, Ethan, that that is not correct? Or is it not as critical with non-transformer balanced systems like those in every piece of balanced gear I own?

I think Ethan's right and I have no idea what Tascam meant. Voltage is always a relative measurement.

[James John]

thanks guys, i kind of get it - i think i need to revise some of my physics! are there any decent links you know of that i can read up on?

[evm1024]

Man the only thing that I can think of for balanced meters would be one that has it's negative lead tied to ground. IF you have such a meter you should move it to your car tool box.....

All the meters that I have have balanced input leads. That is to say that the + and - leads are interchangable and will read the correct voltage with the correct polarity. If you put the positive lead to the negative terminal of a battery (and the negative to the positive) the meter will read negative battery voltage.

One place where balanced will come up is when using a scope. Most probes are non-balanced. The ground lead is exactly that - tied to ground. In some Tek scopes there is an CH1 + CH2 mode that you can select that will show balanced signals (Takes 2 scope channels and 2 probes).

There are lots of circuit topologies used to get balanced or psudo balanced signals out of a (pre)amp/driver. But for the most part measuring between pins 2 and 3 of an xlr with a balanced meter (most any modern meter).

Tascam/Teac's dBv is referenced to 1 volt and is common in home/prosumer systems. There is another dBv that is used in cable TV that we can ignore.

dBv is a measurement of pure voltage. Tascam may spec the loadings that they have in the DUT (device under test) but that loading is not part of the measurement system.

dBu is a measurement of pure voltage referenced to 0.775 volts. The load is Unspecified and not part of the measurement system. The engineers came up with the 0.775 volts for a reference because 0.775 volts into a 600 ohm load = 1 mW of power dissapation (the dBm reference).

Reference voltages are selected to relevance to the industry that is using the system...



More fuel to the fire.... Ethan

[Dr ZEE]

Originally Posted by James John

are there any decent links you know of that i can read up on?

After going through this thread again, I'd say this one may do: http://en.wikipedia.org/wiki/Voltage
"voltage" is a difference between "two".
Also, one can't really "measure" AC-voltage. But one can use the energy, 'provided' by AC to "power up" (or, say - "stimulate") a specially designed circuit, the output of which can actually be "measured" and then pronounce such measurement's value as "AC Voltage" of a kind. People do this kind of things all the time .
***********
- "Balanced meters

----the only thing that I can think of for balanced meters would be one that ...>>>> you should move it to your car tool box....."
- YES!, in the trunk together with a Fire Extinguisher

-"....More fuel to the fire.... Ethan"

***********
p.s.
Regards to "folks at Tascam", btw

[James John]

thanks for all the info guys- i'm considerably less confused now!