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
any help from those in the know would be appreciated...
cheers stu
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 -- http://www.diyaudio.com/forums/showthread.php?s=&threadid=84003&highlight=
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
A couple of years ago I started a GB which never went anywhere -- a wide dynamic range level meter -- http://www.diyaudio.com/forums/showthread.php?s=&threadid=84003&highlight=
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
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.
There wasn't that much interest in the VU Meter Board -- I just had a couple of them made up for my own experimentation.
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.
....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.
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
-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
Meter on ebone
This is a Sifam 0-1 DC ma meter on ebay right now.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAX:IT&item=120300676102
This is a Sifam 0-1 DC ma meter on ebay right now.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWAX:IT&item=120300676102
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
thanks in advance... stuart
Attachments
Rod Elliott to the rescue.
VU and Peak Program Metering Circuit and description.
http://sound.westhost.com/project55.htm
Cheers,
Ian
VU and Peak Program Metering Circuit and description.
http://sound.westhost.com/project55.htm
Cheers,
Ian
You need to decide what you want...
...whether you want to measure the signal accurately ie. a decibel -wattmeter in the case of the VU meter or peaks in the case of the British PPM. I have copies of both circuits using ASA standard VU meters by Simpson or Hoyt or API or Weston and the British Earnest Turner meters--- a circuit designed by D. Robinson of Dolby that was published in STUDIO SOUND MAGAZINE but I can't post them without permission from the authors and publishers. They are out of copyrighted books. These meters are not cheap as they have mechanical ballistics that obey the standards. Ray Hughes
...whether you want to measure the signal accurately ie. a decibel -wattmeter in the case of the VU meter or peaks in the case of the British PPM. I have copies of both circuits using ASA standard VU meters by Simpson or Hoyt or API or Weston and the British Earnest Turner meters--- a circuit designed by D. Robinson of Dolby that was published in STUDIO SOUND MAGAZINE but I can't post them without permission from the authors and publishers. They are out of copyrighted books. These meters are not cheap as they have mechanical ballistics that obey the standards. Ray Hughes
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