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Old 29th May 2017, 1:08 pm   #1
Ted Kendall
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Default Leader 186/AC voltage reference

I bought a Leader 186 dual voltmeter a while ago and found that it wasn't in the best of calibration. So, find the manual (which was a job...) and, not unreasonably, the first thing it says is "take your AC reference..."! Well, that's where it stopped, because I have none, apart from using other meters to set the output of the genny, which lands us straight in the Zanzibar fallacy.

So, any ideas for an AC voltage reference (DIY, I'm guessing) which doesn't endanger life, limb or bank account?
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Old 29th May 2017, 1:44 pm   #2
karesz*
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Default Re: Leader 186/AC voltage reference

Hi Ted,
I don't have a manual for that, but I wuld think on a generator/or mains transformer with such 100Hz or even 50 Hz what you can enough exact measure with a properly good (best TRMS) Voltmeter and so "calibrate".
If, as in the next point is wroten, "for frequency response calibration is a generators signal..." enough good, its not to understand why you must have an AC calibrator if an generators output even so exact measurable(nowadays) and knowed... Dont worry, calibrate it please with a siggen and good DVM.
Btw maybe you can mail me your SM than I can see more...
Regards, Karl
ps: I searched for it just, if yours Type is LMV-186AB (ebaman has it too), than I found it in such seconds.

Last edited by karesz*; 29th May 2017 at 1:57 pm.
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Old 29th May 2017, 2:54 pm   #3
Ted Kendall
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Default Re: Leader 186/AC voltage reference

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Originally Posted by karesz* View Post
calibrate it please with a siggen and good DVM.
...but that's my point - how good is your average DVM at audio, unless you have a fancy Fluke, which I don't?
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Old 29th May 2017, 3:05 pm   #4
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Default Re: Leader 186/AC voltage reference

Quote:
Originally Posted by Ted Kendall View Post
(...)
So, any ideas for an AC voltage reference (DIY, I'm guessing) which doesn't endanger life, limb or bank account?
Just an idea : Two transformers from a power supply, let's say both have 12V output. Connect 12 V outputs togetger and you should get AC 230V, separated from mains line.
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Old 29th May 2017, 3:08 pm   #5
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Default Re: Leader 186/AC voltage reference

That would mean you can touch any of the output wires, but only one at a time ))
The AC will also be slightly filtered.
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Old 29th May 2017, 3:15 pm   #6
Ted Kendall
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Default Re: Leader 186/AC voltage reference

What I had in mind was a box that could give me (say) 400Hz or 1kHz at known volts rms (say 1V for the sake of argument) plus or minus nothing that matters, in other words something that doesn't rely on the calibration of other kit. There have been various solutions proposed in these pages for DC, but AC is a touch more complicated...
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Old 29th May 2017, 3:33 pm   #7
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Default Re: Leader 186/AC voltage reference

Just 1V, OK : A function generator board with 8038. They are for less than £3 on ebay. Use your scope to set correct voltage and frequency.
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Old 29th May 2017, 3:38 pm   #8
Ted Kendall
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Default Re: Leader 186/AC voltage reference

Fair enough, but that still relies on the calibration of the 'scope - there are chips that generate DC references, but none I can actually find for sale that do a similar thing with AC.
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Old 29th May 2017, 3:49 pm   #9
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Default Re: Leader 186/AC voltage reference

A board with AD584 will help you to calibrate your scope. Less than £2.5 on ebay.
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Old 29th May 2017, 4:01 pm   #10
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Default Re: Leader 186/AC voltage reference

I haven't come across a single chip AC reference, but that doesn't mean much. I wonder whether an easier approach in reality is to seek another forum member locally who is willing to lend you a 'decent accuracy' DMM - say 5.5 digits. Even if not recently calibrated, most bench DMMs should still be accurate enough over the audio frequency range to work with an uncalibrated audio generator. Unfortunately, I'm nowhere near Herefordshire.
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Old 29th May 2017, 5:34 pm   #11
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Default Re: Leader 186/AC voltage reference

I wouldn't bother too much 10% is more than near enough for audio work. If it reads the mains close to a (any reasonable) DMM I would give it a pass.
 
Old 29th May 2017, 7:33 pm   #12
Ted Kendall
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Default Re: Leader 186/AC voltage reference

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Originally Posted by merlinmaxwell View Post
10% is more than near enough for audio work.
Really? 10% is a decibel, all but, and therefore audible. That's your whole error budget and then some gone in one hit.
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Old 29th May 2017, 8:14 pm   #13
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Default Re: Leader 186/AC voltage reference

With a good scope, DC coupled, use an AA cell to provide your 1,5v appox reference. You DVM in parallel with give exact voltage. Then apply your 1vpp AC voltage and calculate to almost the accuracy of your DVM.
Nothing difficult there.
Les.
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Old 29th May 2017, 8:52 pm   #14
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Default Re: Leader 186/AC voltage reference

Les

That is the way I do it (sort of)

Use DC volt reference (single chip device, or accurate Volt meter to measure any DC source) and set up a scope on DC input to line up trace with graticule line on scope.

This line becomes my known reference.

Apply square-wave, or sine-wave from function generator to scope so the peaks touch referenced scope line.

Then do the maths for RMS (or TRMS) and use this to calibrate AC meter.

Basically using the scope as a "transfer standard" converting DC standard to AC.

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Old 30th May 2017, 1:01 am   #15
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Default Re: Leader 186/AC voltage reference

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..but that's my point - how good is your average DVM at audio
Ted, its a general metrology problem, more exact "a problem": most (to much) people are beliive that what digital is = precision things! SORRY, its only a superstason An old style VTVM has i.e. 2% readiings, the tipical DMM (chinese) 5-10£ ones often 2-5%!...
For us is very important to know the specs of our instruments!

You know only yourself your ones type, please check for his data on your user man-or on internet... We can only say; if its not a 10-20£ type, possibly has an precision at 1-2% (maybe a 1/10th too) up to several KHz. As sombody wrote: at mains frequency is to measure and if you wish to check/calibrate in full 300V scale= please apply an transformator onto the signalgen output Or a low voltage transformator output (for separation from mains potential). If you are at 50-100Hz, are the most mains transformers enough good working for a test , and measure with an good DVM/DMM ro even VTVM. It will be enough precise for an uncalibrated generator!
Otherwise an oscilloscope with internal graticule and good measurement pcatices (parallax problems) can be usable for readings with 1-2% precision.

In a typical ambiance you can not hear a dB difference, then if your setup brings a precision of maybe 2-5%= absolutly OK for alldays audio/HiFi, but if you need/wish more precision=> you must investigate in more professional generatos and level meters scopes etc...
Regards, Karl
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