Mallette Posted December 12, 2011 Share Posted December 12, 2011 Can't recall if this is third or fourth try over the years, but I know there are plenty here who can help with this if I can just describe it clearly. I've a couple of marverlous Weston classic VU meters from the 40's I've always wanted to use "Just for grins" in my system. Great way to show how efficient them big ol'horns are. What I need is the resistor values and configuration to do this with a three way switch that, when inserted in an 8 ohm speaker output line, will read 1w, 10w, or 100w full scale depending on the switch position. This diagram should be construed as a communications tool, not a real circuit diagram, as it's only my best guess at how this might work. I've forgotten what little I knew about resistor circuits. I hope this at least can give you tech heads a clear image of what I need. Hope it is, in fact, the third time...and a charm. Dave Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted December 12, 2011 Share Posted December 12, 2011 Here's some info which will help you get started on your "quest!" http://www.circuitstoday.com/audio-wattmeter-circuit http://sound.westhost.com/articles/meters.htm Quote Link to comment Share on other sites More sharing options...
Mallette Posted December 12, 2011 Author Share Posted December 12, 2011 Thanks. The below definition describes what I am looking for in values. Assuming the amps RMS output is at a 1w level, I want the meter to read 0db. When switched to the second position, I want (I think...) 10mw to read as 0db at 10 watts. In the third position, I want the meter to read as 0db at 100 watts. Seems like this should be obtainable with resistors alone, though I can see one of them being variable to fine tune it for accuracy. OTOH, if I REALLY knew anything I wouldn't be asking such question. The reference voltage for the decibel volt (0 dBV) is 1 VRMS, which is the voltage required to produce 1 milliwatt [mW] of power across a 1 kiloohm [kΩ] load. The reference voltage for the decibel unloaded (0 dBu) is the voltage required to produce 1 mW of power across a 600 Ω load (approximately 0.7746 VRMS). Dave Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted December 12, 2011 Share Posted December 12, 2011 Calibrating the meter is the least of the problem. Keep in mind that VU meters usually have a damped movement which will present kind of an averaged reading as opposed to showing the peaks. Ideally, you want to use a driver circuit for the meter which will put a very high impedance across the amp's output so as to have no effect on the sound. Are you sure that you really want to bother doing this??? Quote Link to comment Share on other sites More sharing options...
Mallette Posted December 12, 2011 Author Share Posted December 12, 2011 Keep in mind that VU meters usually have a damped movement which will present kind of an averaged reading as opposed to showing the peaks. I was told by engineers back in my radio days that good VU meters like the Westons have needle tips that exceed the speed of sound. I have always prefered analog to digital meters because I can tell more about what is REALLY going on from them. Most meter buffer circuits I've seen appear quite simple. It's been surprising to me that locating plans or specs for them is such a trial. Maybe I should approach Ramsey about it. Dave Quote Link to comment Share on other sites More sharing options...
Mallette Posted December 12, 2011 Author Share Posted December 12, 2011 A bit more complex than I would like, but this looks doable. Dave Quote Link to comment Share on other sites More sharing options...
Mallette Posted December 12, 2011 Author Share Posted December 12, 2011 One of the problems is that I keep forgetting I want 1w, 10w, and 100w Odb levels, switchable. This was fairly common a couple of decades ago, and, in fact, PWK had meters set up that way in the old Hope listening lab. The circuits I've found are single purpose. Dave Quote Link to comment Share on other sites More sharing options...
Guest " " Posted December 12, 2011 Share Posted December 12, 2011 I think some diodes are needed in the first picture with the resisters. most of the meter's I've seen use both diodes and a small capacitor. There's some calibration needed as well. Maybe some pots in place of the 1K resister for the calibration. Not sure if you will need calibration pots at each step selection circut or one for the whole circut. Kinda think you will need one calibration circut for each step selection circut. Anyway, fast recovery singnal diodes and maybe even a small capacitor to hold the meter in swing position...I think 10uf is the most common size. with out the cap...the needle is going to do wiggly woo on you. Quote Link to comment Share on other sites More sharing options...
JJkizak Posted December 12, 2011 Share Posted December 12, 2011 Weston made a lot of meters. How do you know what the impedance and load factors are? Their AC meters for measuring ljne voltage were low impedance and required a bunch of power to run them. They were also highly damped. (they would load the circuit down but measuring AC line voltage it was not noticeable) They probably manufactured other types. In the old days we switched to VTVMs to keep from loading the circuits down. JJK Quote Link to comment Share on other sites More sharing options...
Mallette Posted December 12, 2011 Author Share Posted December 12, 2011 There was ONE classic Weston meter I used across numerous boards, starting with the classic "Yard" board by Gates. This is it. Not sure how it is damped or anything else other than it would have been "standard." Sorry about the pix, but the closet was dark and the "auto softness" feature of the camera couldn't be overridden. But if you know these meters you will recognize them. Yes, the built a number of meters, but these are the ones that set the standard and once you'd used them you could read them and get max level without clipping....the Klipschorn of VU meters. I suspect pretty much a lost art... Dave Quote Link to comment Share on other sites More sharing options...
Greg Oshiro Posted December 12, 2011 Share Posted December 12, 2011 There was ONE classic Weston meter I used across numerous boards, starting with the classic "Yard" board by Gates. This is it. Not sure how it is damped or anything else other than it would have been "standard." <snip> Dave Sometimes equipment manufacturers buy non-VU meter movements with VU scales. If you have an oscillator, voltmeter and a resistor in the 1K to 10K range (3600 ohms would be *really* helpful), we can sort this out. The schematic in your original post will work, but the meter ballistics will be non-standard. The ballistics can be made standard by adding a few resistors. We'll need to know the output impedance of the oscillator. You'll need a sine wave source and an AC voltmeter with known specifications to verify the calibration. If you don't have the test gear, Let us know and I'll *ASSUME* it's a real VU meter and post a circuit in a few days... Quote Link to comment Share on other sites More sharing options...
Mallette Posted December 13, 2011 Author Share Posted December 13, 2011 We'll need to know the output impedance of the oscillator. I have an AC voltmeter, but the only sine wave source I have would be a digitized 1khz tone. What oscillator are you refering to? Anyway, if there was EVER a real VU meter these are it. They were standard issue throughout the radio industry from the 40s through at least the 50s. I gotta get some better pix and show the insides. It's been a few years since I had one open, but they are dated 1940 something inside and the guts are gorgeous "made in America to last a lifetime." Dave Quote Link to comment Share on other sites More sharing options...
Don Richard Posted December 13, 2011 Share Posted December 13, 2011 Going waaaay back in the memory banks... there are 2 types of VU meters, the ones designed for radio calibrated in % modulation and audio VU meters. It appears the ones you have are the audio meters from the pics posted. I am not sure what, if any, electrical differences there are. Here is an article from Wikipedia that gives a calibration procedure: http://en.wikipedia.org/wiki/VU_meter Quote Link to comment Share on other sites More sharing options...
Mallette Posted December 13, 2011 Author Share Posted December 13, 2011 Going waaaay back in the memory banks... there are 2 types of VU meters, the ones designed for radio calibrated in % modulation and audio VU meters. US radio broadcasting used VU meters at the board. Modulation meters were used at the transmitter to ensure one stayed within FCC regs. They were usually set to ensure no more than a 100% modulation at +6db. Anyway, that's how I recall it. It's been 40 years since I was in radio so I may be a bit fuzzy. Heck, I am a bit fuzzy about last night... Dave Quote Link to comment Share on other sites More sharing options...
Greg Oshiro Posted December 13, 2011 Share Posted December 13, 2011 We'll need to know the output impedance of the oscillator. I have an AC voltmeter, but the only sine wave source I have would be a digitized 1khz tone. What oscillator are you refering to? <snip> Dave I was hoping you had a piece of test gear like this http://www.agilent.com/about/companyinfo/history/A03.gif Do you have any resistors in the 1K to 10K range lying around? Across what frequency range is the AC voltmeter accurate? I gave the circuit some thought: You may want a 100 milli-Watt range. I should measure what I'm pushing my K-horns with one of these days... Does the circuit need to protect the meter against 100 Watt input while set to the 1 Watt (or even worse, 100 milli-Watt) range? Quote Link to comment Share on other sites More sharing options...
Mallette Posted December 14, 2011 Author Share Posted December 14, 2011 They'll be a break in this before I attempt anything as my family and I will be traveling starting Saturday thru the first or so. Well, intermittent, anyway. As to protection, wouldn't the resistance itself be adequate? Never gave it any thought... Certainly wouldn't want to fry these classic beauties. I DO intend to get some hi def images inside and out. They are things of beauty. Not so sure about the milliwatt range. I don't blow out the walls every session but if I am using my 'horns, it's because I am LISTENING and not background music. I am a "realistic" level guy and wnilst I may turn it down just enough to allow commentary I just shut it off if the conversation moves to subject matter other than music. A house divided against itself cannot stand... Used a LOT of fine old audio oscillators in my day, but haven't owned one in decades. I suspect the digitized tone will be adequate as I am not looking for lab specs here, just a reasonable approximation. Not against the 1mw, though, unless it really requires some extra hassles. I'd prefer a simple resistive network as possible. Dave Quote Link to comment Share on other sites More sharing options...
Greg Oshiro Posted December 14, 2011 Share Posted December 14, 2011 See schematic attached. Please read the notes. I recommend lashing this together to see if it will really work before committing to "real" fabrication. Any parts you need should be available from http://www.mouser.com <snip>As to protection, wouldn't the resistance itself be adequate? Never gave it any thought...<snip> <snip>Not so sure about the [100] milliwatt range.<snip> Dave Well, If the range is set to 1W and the amp is putting out 100W, that's 10:1 in voltage. Simpson meter movements (your meters are likely different) are rated 10:1 overload for 1 Second. K-horns are 104 dB 1W/1M. At a distance of 10 ft [~3M] that's 96 dB. Two channels (power addition) makes 99 dB. 100mW would be 89 dB. Have a good holiday & look forward to hearing from you next year. Klipsch_forum_SK20111214A_spkr_meter_atten.pdf Quote Link to comment Share on other sites More sharing options...
Mallette Posted December 15, 2011 Author Share Posted December 15, 2011 Got this from Rod Elliot, an Aussy who sells buffer circuit boards and such: Dave,If you have the meters, the process is actually quite easy. Provided theyalready have internal rectifiers, you only need to provide a 3-position switchand attenuators for each range. The latter consist of nothing more than asuitably rated resistor (Ohm's law is all you need to work out the resistance)and a trimpot for calibration.See the article on 'Voltage Dividers & Attenuators' in the articlessection.Cheers, Rod If I have time, I am going to open one tonight and see if I see anything other than the large coil horseshoe and meter movement I recall. I'll post these as well and perhaps someone here can tell. Certainly worth looking at as they are very pretty inside. I haven't looked at the article on his site he referenced yet. Dave Quote Link to comment Share on other sites More sharing options...
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