VU meter driver assembly

Yep.

Gottcha. Thanks Dave!

:guinness:

Thanks NYD. That makes sense, and I updated my schematic accordingly. Thanks for pointing out the 3.3K resistor -- I actually was using a 10K 20-turn Bourns pot for that, which works great for that.



My next question is for PRR -- if the input resistors could be raised to 1M without any negative effect, was there a specific reason for using the lower 100K value in the original schematic?

Also...
[quote author="PRR"]So since the TL072 has small offset voltage and negligible bias current, use one divider to make around 15V, and two 1Meg input resistors? Same number of parts, and both sides get the same DC voltage (within a couple mV). Input caps could be 0.02uFd, and no output cap.[/quote]

Do you mean like this:



Thanks PRR!

Hi
Two or three years ago I find on my Soundcraft in radio this circuit with TL072
and Sifam AL29 like some of us used for our stuff.

I made board for two VU meter

Voltage is +-16V but have 10ohm and 0,1uF to stabilize and reduce voltage (I mean to 15V)
I have a Corel drawing (too, pdf) and if anyone need it for making board I can send.
Duka

Electro:

Both of those should work great, but the second one (based on PRR's suggestions) is a more elegant design. I would use that one.

Here's another one for the archive, found on the OpAmp Labs site.

[quote author="electronaut"]
Do you mean like this:



Thanks PRR![/quote]

This one looks good, but misses the cap to protect the VU meter.

[quote author="rafafredd"]This one looks good, but misses the cap to protect the VU meter.[/quote]

Hey Raf,

If I understand him correctly, PRR's suggestion of the one divider for both sides was to keep any differential voltage to an absolute minimum, so the output cap should not be needed. If I use 1% resistors, the voltages on either side should be damn near the same.

Hey peoples.

Since this thread was just mentioned in the other VU meter thread, I figured I'd post something I just came up with.

It's a simple emitter follower set up as a voltage buffer, so that I can power the VU meter amp from the Phantom power supply. I haven't tested it extensively, but it works!

Comments welcome, as always!



Oh... since the phantom supply is well-filtered, the filter cap at the divider doesn't have to be a zillion Farads.

> the filter cap at the divider doesn't have to be a zillion Farads.

No, but 6.02EE-23F seems kinda small....

Also the meter-side may be connected to a LINE output, the Phantom side to a Mike Input. No matter how perfect things are matched, "some" signal is going to sneak from output to input. (Input and output are purely human concepts: the electrons go where they want to go.) Sure, the leakage will be small, but I feel better with some clamping. Including something on the "30V" rail.

TL072 supply current is so steady, and load current so small, that you could maybe skip the transistor and just use a series resistor. 5K would be pretty safe, though a high-limit TL072 might need a bit less.

And 0.02uFd versus 22K looks like 400Hz, more "A-weight" than "flat". TLo72 can stand huge grid resistors, change 22K to 2.2 Megs.

[quote author="PRR"]6.02EE-23F seems kinda small....[/quote]

Um... that's 6.23 TIMES 10 to the 23rd. You know, Avogadro's number. A MOLE Farads. Good enough for lightening.

[quote author="PRR"]Also the meter-side may be connected to a LINE output, the Phantom side to a Mike Input. No matter how perfect things are matched, "some" signal is going to sneak from output to input. (Input and output are purely human concepts: the electrons go where they want to go.) Sure, the leakage will be small, but I feel better with some clamping. Including something on the "30V" rail. [/quote]

Now you're freaking me out. I don't want any output sneaking into the input.

So you're saying that a big cap is important there, or are you suggesting something else?

[quote author="PRR"]TL072 supply current is so steady, and load current so small, that you could maybe skip the transistor and just use a series resistor. 5K would be pretty safe, though a high-limit TL072 might need a bit less. [/quote]

Well, my goal in using the transistor in the first place was in hopes of achieving more isolation between the meter circuit and the phantom, so that the electrons would be less inclined to sneak in there. If the emitter follower doesn't help with that than I might as well use the resistor.

I guess I'm a little unclear about the advantages of using the emitter follower as a voltage buffer.

Thanks for your help!

> Um... that's 6.23 TIMES 10 to the 23rd. You know, Avogadro's number. A MOLE Farads.

Ah, yes, you are correct. I must have a negative attitude today.

I don't think UPS will deliver a 6EE23 cap to your porch. And it would take a while to charge-up through the 1Meg resistor. (10^29 seconds, more or less. 30,000,000,000 B-B-BILLion years?)



I guess I'm a little unclear about the advantages of using the emitter follower as a voltage buffer.

Oh, your idea is fine (with a little adjustment in cap-value). I just hate extra parts until I PROVE that I need them. Some folks would call me cheap or lazy.

There is a slim chance that my crude volt-dropper will pop a few TL072s. But they are very cheap. And if they blow, they were just-barely in spec, who needs that?

Well, in an attempt to prove or disprove the advantages of using the emitter follower, I did some experiments this morning. I built the meter amp as shown in PRR's modified schematic above, with a couple of minor changes so I could use the parts I had on hand. The bias resistors were 1 meg instead of 2.2 megs. The filter cap was 22µF, and the other filter cap at the phantom divider was 33µF. I replaced the 3.6K trim pot with a 22K resistor so I could inject +20 dBu of audio (1kHz sine wave) without blowing up my meter. Oh yeah... I didn't have any TL072 opamps on hand so I used an LF353 which is pretty similar.

The power supply was a simple thrown-together rectifier feeding a small-ish pi filter (two 22µ and a 1.8K resistor), producing +48 V with about 50 mV of ripple.

In both cases audio was present at the junction of the four 1 meg resistors and the cap, but was about 63 dB below at about 5 mV RMS. More importantly, there was no measurable audio reaching the 30V rail in either case.

Oddly, on my scope I could barely detect a very slight sign of audio riding on top of my not-very-well-filtered 48V when using the emitter follower, which I did not see when using only the PRR divider. So could the divider with that big cap actually be better than the follower?

In any case, my normal phantom supply is well filtered and regulated, so the regulator action should help attenuate any audio that reaches the output, although it all seems so unlikely that the point is moot.

Thanks PRR for the tip.

I have some VU meters salvaged from a Tascam 80-8 recorder that I want to use for line level monitoring. I don't have any documentation for them, but there isn't any rectifier or other components inside (other than the coil). Would this be the correct implementation for these meters, or is there some more information that I need to know about these meters before I can use them?



Thanks,
Chris

-Bump-

TK,

This thread has links to a good article about rectifierless VU meters as well as another thread about using µA meters as VU meters. Hopefully they will be of some help.

Cheers,

-E.

PRR said:

This is a simple but generally adequate VU meter buffer, for when you really have "line levels":

Click to expand...

In this schematic ought we to use the signal common when we connect the inputs to ground through the 100k resistors? Or will power supply common work fine? Also u1 and u2 can be 2 halves of a TL072?
Thanks, Ben

There's one on my site that I made with a few spare parts lying around:
http://www.axtsystems.com/index.php?option=com_content&view=article&id=69:vumeter&catid=42:vumeter&Itemid=74

and Joe Malone has one here:
http://www.jlmaudio.com/VU%20Buffer.htm

On the PRR version 2 schematic, how would putting a pad on the input, say a -20dB H-pad, affect the source? It would be useful to be able to accommodate signals that are too high as well as ones that are too low by using a trim pot for R-12, and have something like + / - 20dB of range.
Am I correct in thinking the impedance values to put in the pad calculation are 10K input and 200K output? This gives me 584 for H-pad R1 &R3, and 97503 for R2

thanks

How about building the VU buffer from JLM audio