VU meter buffer

[PRR]

> the meter slams to full scale until the output cap (100uf) discharges.

I like the other way. The meter is true floating. We don't actually need a differential amplifier. Wire two identical followers and strap the 3K9 and meter across their outputs. No output cap needed. The inputs can be biased with 100K to any simple splitter. Don't use a ton of capacitance on the splitter: noise here is rejected, and a huge cap will hold the input pins up after the main +24V has collapsed. Inputs brought in with 0.1uFd caps. And while I hate cap-matching, this is one case where better than 10% match may improve the pin-slam problem.

There will always be some meter bounce as the supply rail falls to a volt and different stages and sides of the amplifiers turn-off at different points.

 

[JohnRoberts]

This may not be as much of an issue with bipolar supplies.

Another issue that I have encountered when driving true VU meters from modest SE supplies is that premature clipping of transients due to limited rail voltage can cause noticeable under reporting of VU levels. Driving the meter differentially from two opamps will pick up 2x the signal swing, often enough to deliver useful readings.

I personally consider VU meters archaic. You essentially had to mentally translate for the type of program being metered. But the customer is always right so I would usually add a clip LED somewhere nearby to indicate true headroom.

Note: VU meters still rawk for outdoor use in bright sunlight but having to operate under those conditions is not showing much love for the mixer or the mixer.

JR

 

[NewYorkDave]

One minor niggle: the series resistor for a VU meter should be 3.6K, not 3.9K. Better yet, use 3K plus a 1K trimmer.

 

[bcarso]

Unfortunately my current mirror has good image rejection.

 

[NewYorkDave]

I posted it elsewhere, but in case you missed it, here's the original spec as published in 1939:

PDF

I like VU meters, archaic or not. But adding some sort of peak indicator is definitely useful.

 

[deveng]

Thanks for all the input on this circuit. I think it will be usefull to many once all the bugs are fixed. It's amazing when you're building something and you think it's going to just work.....and it doesn't do what you think! I designed my pcb (single ended, split supply) from a National Semi application note. I "always" breadboard circuits first, but broke my own rule on this one. Turned out to be very similiar to Brian's circuit so it was easy to modify.

Anyway, I really like PRR and Mediatech's spin on this and since it's only a couple of cut traces and changes away from what I have now, I'll modify mine and let you know how it works.

Jeff

 

[Brian Roth]

[quote author="NewYorkDave"]One minor niggle: the series resistor for a VU meter should be 3.6K, not 3.9K. Better yet, use 3K plus a 1K trimmer.[/quote]

Belated reply...I've been up to my kiester on some projects.

I had to go digging back through some discussions from the Ampex mailing list to refresh my memories, and I found the pertinent info from 2002 in my files. Jay McKnight, Dale Manquen and other "heavyweights" were all participating in the discussion. I was just observing and taking notes <g>.

3600 Ohms is the correct series resistor ****IF**** the meter is installed into a "true" 600 ohms impedance system. In that case, the signal's source impedance is actually 600 Ohms, and the termination is also 600 ohms. Thus, there is an effective 300 Ohms series impedance into the VU meter.

However, when driving the VU meter from an opamp with essentially zero ohms output impedance, 3900 Ohms becomes the correct series resistor.

I tried to find other supporting info for the 3900 Ohms value and the best I could find was from an article which Jay Mcknight posted regarding the test procedures to ensure that a so-called VU meter actually conformed to the Olde Specs. In that paper, Jay states:

The overshoot must also be measured. For this measurement, the meter must be operated from the proper impedance (usually 3600 Ohms
in series with a 300 Ohm source resistance, for 3900 Ohms total).

I can dig back through other references/citations to further back-up the 3900 Ohm value for the common situations when the meter is driven from a low source Z.

The issue becomes whether or not the VU meter (Jay refers to the meter as a "Standard Volume Indicator", which likely harkens back to the ancient specs) has the proper "ballistics" as defined in Ye Olde Specs. If the source resistance into the meter is wildly off, then the attack and release times of the meter will also suffer, at least according to the Olde Specs.

Bri

 

[deveng]

I finally got back to this item (since I need to finish my vacuum tube mic pre) and have an update on it. The circuit as described by PRR and Mediatech works. No surprise huh. There's no meter slamming on either power up or power down. Just to be clear though those of you using meters without a rectifier will need to add one and it needs to be a full wave to work. I do have one issue. One opamp has a trimmer in the feedback loop to fine adjust the meter. At this point I can only get about 1 1/2 VU adjustment. But aside from that it works quite well. I'll get this worked out and give another update.

Jeff

 

[PRR]

> those of you using meters without a rectifier will need to add one and it needs to be a full wave to work.

It really wants to be the specified Copper Oxide rectifier; indeed if there isn't one in or with the meter it is highly unlikely to be True VU.

If you got an old fake-vu off a consumer tape deck: Silicon rectifier will give very lazy response below -10"VU" due to Si's high voltage. A cheap 10A Bridge may be a bit better than four 1N914s. Ge is better but becoming collectable. Schotkey rectifiers have lower drop than common Si.

> One opamp has a trimmer in the feedback loop to fine adjust the meter. At this point I can only get about 1 1/2 VU adjustment.

My suggestion was a pair of simple followers. Not sure how you got a gain trim.

If the meter is perfect, and you want +4dBu on the line, unity gain with 3,890 ohms should be "exact". (3K9 minus a few ohms buffer Z.) A +/-500 ohm trim on the "3K9" should give +/-1dB of zero-VU trim, with not enough ballistic error to see unless you been working with too many True-VU meters far too long.

If you got an old fake-vu off a consumer tape deck: the resistor value must be found by experiment against a steady +4dBu source, and let the ballistics fall where they may. (It would be wiser to salvage the meter driver from the deck: it probably has un-True but usable ballistics, and its sensitivity would be easy to trim.)

 

[deveng]

I first tried the Schotky's and the one's I found dropped 0.330 volts. I ended up with Germaniiums which drop about 0.23v. These can still be found and NTE has a standard part. I coulnd't find copper oxide anywhere so the Germaniums will have to do for now.

The gain trimmer in the feedback loop was leftover from my original single supply design. Rather than take it out I just initially matched the feedback resistance (with the trimmer) for both opamps. Then once the circuit worked tried to adjust. With them both at unity, I was down 1/2 VU. So the trimmer in the feedback loop does allow enough jiggle to get calibrated.

Jeff

 

[deveng]

First its PRR and Mediatech, not Waveborne as I previously stated. Sorry Mediatech. It was a quick post.

Yes, your circuit does show followers! Since I adapted my circuit to yours, mine are actually unity gain amps and my original circuit had a trimmer in the feedback loop.

Regards,
Jeff

 

[deveng]

Update on this meter circuit (or as in my case, very similar circuit). I finally installed the circuit and tested in my new vacuum tube mic pre with the Jensen JT10k61 output transformer. It works generally well. Also, in circuit the trimmer actually will allow a wide range of adjustment (not just the 1 1/2 dB I had previously mentioned). It still does push the meter to full deflection upon turn on and turn off. But, it's not slammed like before, its more of a gentle rise and then gradually drops as the output cap discharges.
This concerns me more about the DC through the primary of the transformer than with the meter issue at this point. which is discussed in the "transformer" thread.

Regards,
Jeff