VU Input Monitoring?

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Ethan

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Could anyone tell me if this is an appropriate way to monitor input level through a VU:
vu.gif


Thanks!
 
Ethan, this will work, but the lowest-distortion way is to use a unity-gain buffer. Otherwise the switching noise of the diode bridge inside the meter can cause distortion in the signal line.

Note I didn;t say 'best' way. because if simplest is best, and this is for test during "non-record" times, then the way you've shown is the best way! :grin:
 
Thanks Keef as always!

But wouldn't this induce the same amount of distortion that is introduced into the signal with output VU meters that are set up the same way without a buffer?
I suppose rather than creating a buffer to put more schtuff into the signal path, a switch position to take the meter completely out would be optimal ...although this might get annoying, but in theory would be even lesser distortion than adding an opamp to create a buffer eh?

Thanks a whole heeeep!
 
when this subject comes up, are we talking distortion on a meter or distortion on a speaker? I have unbuffered meters throughout my console and across most of my tape decks and on some compressors, Ive never found it to be objectional. Its been suggested that I add buffering to the VU's on my console, but when A/b'ing the meter in the circuit, my last conclusion was bigger fish to fry.

my question-

do all vu's dump the same amount and quality of distortion in the signal path? Does the rectification add a standard fingerprint, or do some VU's "sound" better than others?

eek, did I just say that?

dave
 
[quote author="soundguy"] ..do all vu's dump the same amount and quality of distortion in the signal path? Does the rectification add a standard fingerprint, or do some VU's "sound" better than others?
[/quote]

It completely depends on your source impedance. If your source is very low-Z, it will drive the meter rectifiers without noticable sideeffects. But if it's medium-to-high Z, the meter rectifier will present a disturbing non-linear load - introducing distortion.

Jakob E.
 
Hi
I put on my La instead unnecessary +10db, input VU monitoring. Input trafo is 600ohm primary. What are you thinking about this.
When I record I watch G.R. and out metering.
Duka
 
Ethan,

The circuit as drawn will work fine, and it's the way most VU meters were wired up back in the day. As Jakob points out, the amount of distortion introduced by the rectifiers in the meter depends on the impedance of your signal source. The added distortion will be pretty low with most modern gear with very low output Z.

A common method in the old days was to use a range extender to not only increase the range of the meter, but to provide some isolation between the meter and the line as the extender is switched to higher ranges. VU meter range extenders are no longer available commercially, but here's a DIY design:
28kB PDF

You usually don't need such fine control, so it's quite feasible to build a range extender with fewer positions. It's just a 3900-ohm bridged-T that goes between the meter and the 3600-ohm series resistor.

Oh, by the way, I'm pretty sure Keef is talking about a buffer that bridges the signal line and feeds the VU meter only, not one that's inserted in the main signal path.
 
[quote author="Ethan"]Thanks Keef as always!

But wouldn't this induce the same amount of distortion that is introduced into the signal with output VU meters that are set up the same way without a buffer?
I suppose rather than creating a buffer to put more schtuff into the signal path, a switch position to take the meter completely out would be optimal ...although this might get annoying, but in theory would be even lesser distortion than adding an opamp to create a buffer eh?

Thanks a whole heeeep![/quote]

I may be off base here, but I imagine that using a unity buffer would allow you to 'sample' the signal, not be inserted in the signal path.... that's one of those magical properties of op-amps, they have an ENORMOUS input impedance, I think a tl071/2/4 set up as a unity follower has an input impedance of like 10e12 ohms. So you'd set it up as a tap off your signal line, and 1Tohm shouldn't load the signal appreciably ( or even measurably I'd guess)
 
..sorry I've not been back to reply, but yes Ethan, I was meaning that the buffer would go in the meter path, "sniffing" the signal path without loading it, as opposed to in the signal path itself.

As to the audibility of the meter distortion, it happens a little way up the waveform, and on a Teletronix LA-2a or a UREI 1176 or similar, switching from output metering to gain reduction while listening to a 100Hz tone, I've yet to fins someone who can't hear the 'buzz' that disappears when you switch to GR.

Of course, that's a different tone, it's impedance dependant (will happen a lot less with a 15Ω source impsdance than it will with a 600Ω impedance) and you often don't notice it until it disappears, so it's quite likely that it;s like looking through a dirty window... you don't notice until someone cleans half of it!!!

:green:

Keith
 
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