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Old 21st March 2009, 12:31 AM   #1
surfstu is offline surfstu  United Kingdom
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Default vu meter

i've completed a choke input powersupply, with help, consisting of transformer 300-0-300, 2 large chokes and some large capacitors... the B+ is 320V unloaded which i am told will drop to around 280V once i've built my linestage for it. I'm a total newbie here but i wish to build franks simple 12b4a linesatge which should be easy enough now that the PSU is complete. For fun i'd like to place one or two vintage VU meters in the unit and i'm wandering how this is done, is it recommended and will it impair the sound quality?
any help from those in the know would be appreciated...
cheers stu
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Old 21st March 2009, 05:07 AM   #2
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One thing you want to do is place the meter sensing circuit in a relatively high impedance position with a precision rectifier and amp circuit.

A couple of years ago I started a GB which never went anywhere -- a wide dynamic range level meter -- GB -- "VU Meter Amplifier" PCB

There's also an NEC replacement meter chip which you can still find at Mouser -- I found it in my Yamaha A10. You can also use the LM3916 from National which is a dB reading chip for driving LED's
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Old 21st March 2009, 01:00 PM   #3
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Quote:
A couple of years ago I started a GB which never went anywhere -- a wide dynamic range level meter --
Maybe its time to try it again
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Old 21st March 2009, 01:29 PM   #4
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The replacement chip is the TA7318P -- a dual meter driver -- the replacement part number is NTE1498 and it's available from Mouser for $4.04 -- there's a link to the datasheet from Mouser's website.

There wasn't that much interest in the VU Meter Board -- I just had a couple of them made up for my own experimentation.
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Old 21st March 2009, 03:58 PM   #5
kuroguy is offline kuroguy  Germany
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I built an amp with analog VU meters. I placed a 10K pot across the secondary of the output transformer and connected the wiper to the meter with a series resistor. the other side of the meter runs to ground. Adjust the pot for the deflection you want.

Works great.
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Old 21st March 2009, 09:36 PM   #6
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Default The VU meter is basically an AC watt meter

....designed to read .773 ACvolts as O vu (I think). If you buy a standard VU meter then it's basically a 0-250 uA DC movement with a bridge diode that converts the AC to DC and a 3.9k series resistor where it can be read by a DC meter. It was also designed to be across a balanced line of 600 ohms, ie a transformer coupled line output. Watts have to be measured across a known resistance. Ideally it should be isolated from the rest of your output circuit with some sort of buffer amplifier with a calibration pot where you can set it to some specific value corresponding to a known output of your preamp If you don't obey what it was designed for then it's just there to wag with the music and as a conversation piece, I guess.

The British have their own meter called the PPM (peak program meter) which is a 0-1 DC MA meter again with a special buffer amplifier. But it reads peaks much better than a VU. But both of them are set to a standard; the Bell or decibel in the case of the VU and an arbitrary 0-8 scale on the PPM.
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Old 22nd March 2009, 03:39 PM   #7
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Default CORRECTIONS!!!!!! .775V ACROSS 600 OHMS=0dBu= 1mwatt

-10dBu is 0.3162v

Yes, just putting it across an unbalanced line will probably affect the signal audibly as it is a coil( ie a filter) so I would recommend an isolation buffer before the meter; small amplifier (two transistor) with a gain of about 10 with an input potentiometer. If you do this then it opens up much more possibility. Some meters you run into are 0-1dc ma which will indeed require more gain in the buffer. There are some on ebone right now made by SIFAM which are 0-1 dc ma. You need to know 1. does the meter have an internal bridge rectifier and 2. whether it has an internal 3.6k resistor and 3. the sensitivity of the meter coil; 0-250 dc ua or 0-1 dc ma. Good luck! Ray
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Old 22nd March 2009, 04:09 PM   #8
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Default Meter on ebone

This is a Sifam 0-1 DC ma meter on ebay right now.
http://cgi.ebay.com/ws/eBayISAPI.dll...m=120300676102
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Old 22nd March 2009, 07:27 PM   #9
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you need a nice precision rectifier up front -- with a pair of rail-to-rail opamps and Schottkey diodes.

when I was researching the topic I found that some of the old VU meters used diodes which imparted some unpleasant characteristics to the sound -- seems that engineers got used to it.
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Old 23rd March 2009, 01:30 AM   #10
surfstu is offline surfstu  United Kingdom
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thanks very much for all the replies, am i right in thinking that the dual opamp circuit will act as a buffer, thus not putting any strain on the output of the linestage? I do not wish to implement this if it affects the sound of the linestage, it is purley for fun afterall and if i believe its detremental to the sound quality i will stop enjoying it!! I have posted a schemtaic of the simple linestage, do i simply run this circuit in parallel from the tube output? or after the final .33uf capacitor? or after the final 470K resistor?
thanks in advance... stuart
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