[ProAudio] VU meter permitted overdrive with a sine wave...

Bill Whitlock engineer_bill at verizon.net
Thu Jan 28 12:42:04 PST 2021

I tried to post this this morning, but hitting reply only sent it to Bob - apparently the server for this list works differently from the SAC list, which takes care of the "reply to" field automatically. Anyway, here it is ... I hope!

-----Original Message-----
From: Bill Whitlock <engineer_bill at verizon.net>
To: bobkatz at digido.com <bobkatz at digido.com>
Sent: Thu, Jan 28, 2021 10:12 am
Subject: Re: [ProAudio] VU meter permitted overdrive with a sine wave...

I'll tell you what I know about vintage VU meters: internally, there's a series resistor, 3.9 kΩ usually, going to a copper-oxide rectifier and then to the meter "movement" coil, which I believe has a full-scale sensitivity of about 1 mA.  Electrically, even if the resistor is only a 1/4 watt type, an input of 31.2 Vrms or +32 dBu would do no damage to the electrical parts. I doubt that any mechanical damage would occur if the level slowly rose to that level and remained indefinitely. But if suddenly or intermittently applied, I'd expect that the "slamming" of the needle against the stops might bend the needle!
Bill WhitlockWhitlock ConsultingVentura, CAOffice (805) 755-5018engineer_bill at verizon.net

-----Original Message-----
From: Bob Katz via ProAudio <proaudio at bach.pgm.com>
To: proaudio at bach.pgm.com <proaudio at bach.pgm.com>
Sent: Thu, Jan 28, 2021 9:26 am
Subject: [ProAudio] VU meter permitted overdrive with a sine wave...

  This list has been remarkably silent, but I am very grateful that it's around when I have a question to ask AUDIO EXPERTS. 
  Who here knows the acceptable rating of a standard mechanical VU meter above 0 VU with a sine wave?  If I recall correctly, they are designed to accept as high as +6 VU or even higher. For example, when doing testing of a tape recorder for headroom. It's still ideal to shut off the meter, but in some cases that's not possible. 
  Thanks and happy New Year! 

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