VU Meter Buffer Circuit

[Mallette]

I've a pair of 40's vintage Weston VU meters (such as found in old radio boards) I'd like to show off the efficiency of the Klipschorn's with. My thought was to use a switched buffer circuit to set range to 1/10/100 watts full scale.

Anyone know where to find a schematic for such a resistive buffer? I know this is simple, but I've long forgotten what little I knew of such matters.

Dave

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David A. Mallett

Average system component age: 30 years.

Performance: 21rst Century

[Mallette]

Golly, I thought this would be an easy one. No takers at all. Hope I don't come up with a REAL poser!

Dave

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David A. Mallett

Average system component age: 30 years.

Performance: 21rst Century

[WMcD]

I don't know of any such circuit but it is not something I've kept up on. It is probably out there someplace.

The following is guesswork but no one else seems to be fielding the question. You probably know more than I do.

One thing I see is that 0 VU is set at 1 milliwatt into 600 ohms. That translates to about 0.74 volts RMS. So the meters are just registering voltage.

My understanding is that this goes back to putting a signal on a telephone line where the assumed impedance of the line is 600 ohms.

I don't know if the meters you have would read 0 VU if you put them across a 0.74 volt source. Probably not. As you mention, they might need an amp for that. An op amp circuit might do the job. There are also some chip based audio amps from Parts Express for 30 bucks or so, too.

If you find a schematic, for an amp to drive the meters for 1 milliwatt into 600 ohms, you're not quite where you want to be.

One issue is that it takes a sophisticated circuit to measure power (volts times amps), as the McIntosh meters do. The fall back is measure voltage into an assumed load.

In the world of speakers, the load is assumed to be 8 ohms. Power = Volt * Volts / Load. The load is 8 ohms. Therefore, the voltage for one watt is square root of 8, which is 2.828 volts. This is why you see the spec on speaker testing of "1 watt at one meter (2.828 volts)".

Therefore, if 0 VU is to be 1 watt, the meter amp gain would have to be calibrated to read 0 VU with a buffer amp input of 2.82 volts. This naturally assumes you're putting the hi fi amp output (same as speaker input) into the input of the meter buffer amp.

If you come across a circuit, please pass it along.

Gil

[Mallette]

Thanks for the response. You are right about the specs. However, a simple resistive circuit is all that is necessary to provide various scales. The circuit is inserted across the output and is sufficiently high to be invisible to the amp.

I may have to get out my old books and see if I can puzzle it out. It's been 25 years since I dabbled in such things.

Dave (Klipschorns to be deliver MONDAY!)

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David A. Mallett

Average system component age: 30 years.

Performance: 21rst Century

[Al Klappenberger]

Mallett,

I looked it up in a book I got a few days ago but decided not to answer because of the omission of some improtant details. VU meters are caliobrated to 0 VU = 1 mW (.001W) in a 600 ohm system. They requre an external 3600 Ohm resitor. To change the range, an additional "T" attenuator is needed. The detail they left out was the impedance of the attenuator! It's probably 600 Ohms, but I'm not sure.

Al K.

[SOUNDJUNKIE]

Not exactly what you're looking for or want to hear but... maybe a pair of Simpson wattmeters might work out better. It's a good read site.http://www.digido.com/integrated.html#anchor3646434

[Mallette]

Al, Soundjunkie.

Thanks. What is a "T" attenuator? I assumed that to achieve a 1/10/100 watt scale I'd need three resistive paths with a three-way switch. Isn't one simply both isolating the meters from the amp output and then insuring properly calibrated response from the meters?

As to using another set of meters, about half my goal is to put these beautiful Westons to work and on display. I spent many years in my youth staring at them on Gates boards in my radio days. I love the bakelite cases and the immediate response. They're also gorgeouse inside. They'd cost a fortune to build today...if anyone even attempted such quality.

Dave

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David A. Mallett

Average system component age: 30 years.

Performance: 21rst Century

[Al Klappenberger]

Dave,

A "T" attenuator circuit has 3 resistors connected in a "T" shape. two resistors in sereis with a third connected between the two to ground.

VU meters will fool you. They can't keep up with the peek power and you could blow a tweeter if you try to use them to monitor your power level. You need a peak-locking meter. It's not any easy job to build them. The require lots of electronics to detect, store and display peaks. My advice is not to do what you are suggesting.

Sorry to rain on your parade, but I have a power monitor that is peak locking and the difference between its readings and a VU meter or any other kind of simple "DB" meter is quite dramatic.

AL K.

This message has been edited by Al Klappenberger on 05-19-2002 at 06:54 PM

[Mallette]

Not raining on my parade at all. Peak meters were once known as "BBC meters" in the U.S. The FCC did allow their use for reasons I don't recall. I've spent so much time using both kinds that I became very adept at determining what the peak was on a true VU. For voice, VU is the better type as voice goes from nowhere to peak so fast that a peak type doesn't tell you what is really going on (which, I think, had something to do with why the FCC required true VU response).

Anyway, I've more interest in simply displaying these old beauties in a functional environment than actually using them. Your description of the T attenuator jibs with my memory of such circuit. Now, if I can just get the values necessary...

Dave

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David A. Mallett

Average system component age: 30 years.

Performance: 21rst Century

[Al Klappenberger]

Mallett,

The VU meter has a standardized motion and is optimized for speach. Peak locking is an extreme situation. As long as you are firmiliar with the difference there shouldn't be any danger. Just don't beleive that what a VU meter says is what's actually happening. It's just a sort-of "average".

I can easily calculate the values for a T pad, but I would have to assume that 600 Ohms and 10 dB is the right thing. I am not sure about that. The values would be two 311.7 Ohms in series with 421.6 Ohms between them to ground. You should use +-1% or +-2% resistors.

Al K.

This message has been edited by Al Klappenberger on 05-20-2002 at 06:02 PM

[WMcD]

If I may voice some opinions.

T networks are used as attenuators where it is important to maintain a specific input impedance to the network.

Here, I think we're just using the output of an audio amp to drive the network. The audio amp is not going to affected by a slight increase in the load, so I think it is not going to be an issue.

My main concern is what voltage will be necessary at the input to the meter to get a 0 VU reading. I'd guess that level on the meter is about 2/3rds of full scale.

My belief is that the VU meter is simply an dc volt meter fed by a rectifier. The problem is that germanium rectifiers don't turn on 'til 0.4 volts and selenium rectifiers don't turn on 'til 0.7 volts. Therefore I think you'll find that it takes levels higher than that to get the meter to start registering.

I believe the only think to do is to test the sensitivity of the meter that you have with some test equipment, improvised or otherwise. RS has some cheap digital volt meters. You should be able to find some test CDs to provide a tone.

It may be that you find it takes a few volts a.c. to get to 0 VU. Of course this is a bit of guess work.

Gil

[SOUNDJUNKIE]

The included URL in my previous post says quite a bit. Up front, it says this about a VU construction; "Summary of VU Inconsistencies and Errors

In General, the meter's ballistics, scale, and frequency response all contribute to an inaccurate indicator. The meter approximates momentary loudness changes in program material, but reports that moment-to-moment level differences are greater than the ear actually perceives.

Ballistics: The meter's ballistics were designed to "look good" with spoken word. Its 300 ms integration time gives it a syllabic response, which looks very "comfortable" with speech, but doesn't make it accurate. One time constant cannot sum up the complex multiple time constants required to model the loudness perception of the human listener. Skilled users soon learned that an occasional short "burst" from 0 to +3 VU would probably not cause distortion, and usually was meaningless as far as a loudness change.

Scale: In 1939, logarithmic amplifiers were large and cumbersome to construct, and it was desirable to use a simple passive circuit. The result is a meter where every decibel of change is not given equal merit. The top 50% of the physical scale is devoted to only the top 6 dB of dynamic range, and the meter's useable dynamic range is only about 13 dB. Not realizing this fundamental fact, inexperienced and experienced operators alike tend to push audio levels and/or compress them to stay within this visible range. With uncompressed material, the needle fluctuates far greater than the perceived loudness change and it is difficult to distinguish compressed from uncompressed material by the meter. Soft material may hardly move the meter, but be well within the acceptable limits for the medium and the intended listening environment.

Frequency response: The meter's relatively flat frequency response results in extreme meter deflections that are far greater than the perceived loudness change, since the ear's response is non-linear with respect to frequency. For instance, when mastering reggae music, which has a very heavy bass content, the VU meter may bounce several dB in response to the bass rhythm, but perceived loudness change is probably less than a dB.

Lack of conformance to standards: There are large numbers of improperly-terminated mechanical VU meters and inexpensively-constructed indicators which are labelled "VU" in current use. These disparate meters contribute to disagreements among program producers reading different instruments. A true VU meter is a rather expensive device. It's not a VU meter unless it meets the standard.

Over the past 60 years, psychoacousticians have learned how to measure perceived loudness much better than a VU. Despite all these facts, the VU meter is a very primitive loudness meter. In addition, current digital technology permits us to easily correct the non-linear scale, its dynamic range, ballistics, and frequency response." Therefore the VU is LOGARITHMIC, so a simple bridge will be insufficient.

It says this about VU=0;

"This is an incredible testament to the effectiveness of the 83 dB SPL reference standard proposed by Dolby's Ioan Allen in the mid-70's, originally calibrated to a level of 0 VU for use with analog magnetic film. The choice of 83 dB SPL has stood the test of time, as it permits wide dynamic range recordings with little or no perceived system noise when recording to magnetic film or 20-bit digital. Dialogue, music and effects fall into a natural perspective with an excellent signal-to-noise ratio and headroom. A good film mix engineer can work without a meter and do it all by the monitor, using the meter simply as a guide. In fact, working with a fixed monitor gain is liberating, not limiting. When digital technology reached the large theatre, the SMPTE attached the SPL calibration to a point below full scale digital. When we converted to digital technology, the VU meter was rapidly replaced by the peak program meter." It

would appear that VU=0 is calibrated to 83 dB SPL. Mallett, It would seem to me that in order to "show off" the efficiency of your Klipsch speakers(presumably versus another set(s) of speaker)with these agruably pretty meters, you would have to implement the VU meters by using them to monitor the SPL of each speaker set in conjunction with wattmeters to show power output(1W) with a test tone. The VU meter itself, sans electronics, is merely a current meter, with an associated mA or uA FS value.

[WMcD]

Thanks for the info.

So the raw meter itself is a d.c milliameter? Which makes sense.

My recall is that a typical voltmeter starts with a milliameter. Then a series resistance is added to register applied voltage.

Then some sort of rectifier is used to allow measurement of an a.c. signal.

Naturally we want to be careful with the meters Mallett has.

Gil

[SOUNDJUNKIE]

Mallett, knock your socks off! http://sound.westhost.com/project55.htm

This message has been edited by SOUNDJUNKIE on 05-22-2002 at 08:53 PM

[Mallette]

Jeez guys, I hadn't looked over here in a couple of days. Give me some time to digest this.

However, let me say that I spent the first 10 years of my career in radio, then in audio engineering, staring at true VU meters. I learned to determine what was REALLY happening as opposed to what I was seeing. As with any tool, one has to really spend time with something to get the most out of it. Most all location work I have done was done with VU, music as well as voice, and I am quite comfortable with them.

As you said, SJ, I am looking more to show off these gorgeous hunks than accurately read anything. My ears tell me all I really need to know about what is feeding my Khorns...

BTW, Gil McDermott of the list is going to visit Saturday afternoon to help me lay out the proper T attenuator circuits...as well as listen a bit.

Sunday the 10th will be the inaugural meeting of the Dallas Area Klipsch Forum here with 5 forum members in attendance.

Get Klipsch, and suddenlly you've got friends.

Dave

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David A. Mallett

Average system component age: 30 years.

Performance: 21rst Century

[WMcD]

We took a look at Dave's meters. Gems.

They do work as AC voltmeters. My old HP signal generator was noisy so an exact calibration was not possible. It does look like 0 vu is where one would expect.

I was able to find a commercial buffer circuit on the web and gave him a hard copy.

We did a short experiment running them off the speaker level output.

I hope he and I can get together again.

Dave, thanks for your hospitality.

Right now, I'm hoping that you investigate the commercial buffer. I was getting nervous about the possibility of running them off a speaker level. Too easy to peg. We really don't know if there is any protection circuit in there.

On the other hand, with the buffer circuit, you could run them off a line level. It would still be a lot of fun and wouldn't depend on the volume control.

More on the "imaging" post.

Dave, thanks for your Texas hospitality.

Regards,

Gil