Voltage levels and VU meters...

[Jaroslav Lukesh]

Hi, for this application is LM 324 good choice (I not love to use twin
supply, I say that thing must be able to operate from cheap 12-15V
adapter). But output res can be true omit, because output of rectifier
stage has drive diode, which have some resistance too. But I use as R1 (100
ohm) and cap 10uF, R2 47kohm). But I think that I will change R1 with
"short wire" and decrease cap. (I really need fast rise time). But if you
use different design of rectifier, you can need to use R1 - its true.

You have true, that VU originally mean Volume Level and has slow rise and
fall time. Note, that original VU meter has non-linear frequency
characteristic too. But in digital age the VU meter would be need little
changed - linear freq. char., ultra fast rise and long fall time. All of
these is your own options for your own specific design. If you will use my
component values of this level indicator for digital reverb unit, it will
good. But for passing signal through analogue vocoder or VCA, it is not
good.

Regards

Jerry LSH


> DANGER Will Robinson....
> watch out, don't call an LM324 a "quad 741". They are very different. The
LM324
> is specialized for single supply operation. And its current source/sink
> capability are not symmetric... which could be a problem here. It also
has a
> nasty crossover notch if used on a bipolar supply as the input swings
through
> zero. Someone suggested a load resistor to the negative rail might cure
this,
> but I never tried..
> 
> If you use an active rectifier and a very small cap, followed by a buffer
you
> don't have to worry about how much current is available to charge the
hold
> cap... Is peak hold really a VU response, or do you really want some
rolloff on
> the leading edge (i.e. you really need R1.)
> The Volume Unit was supposed to give a quasi-average reading... it would
allow
> peaks that were too short to be obvious as distortion when recording to
analog
> tape. A good VU meter lets you drive a good hot signal to tape without
> (audible) clipping.
> 
> :^) Harry
> 
> Jaroslav Lukesh wrote:
> 
> > > On Tue, 27 Jul 1999, Chris Crosskey wrote:
> > >
> > > > If it needs DC then what do I do, if I rectify it with signal
diodes,
> > how do
> > > > I go about smoothing it when the signal might be anywhere between
20Hz
> > and
> > > > 20kHz?....I know that I'm unlikely to need the bottom octave of
that or
> > the
> > > > top two but I want it to be capable of the full range....
> > >
> > > You might want to use active rectifier to get rid of the 0.6V diode
drop.
> > > For smoothing, make an RC lowpass filter with cutoff set low enough
(say,
> >
> > > 20Hz).
> > > Disclaimer: I don't have any experience using VU meters so this
advice
> > > might be wrong.
> > >
> > > Antti
> >
> > Hi,
> >
> > Vu meter shoud have following charecteristic:
> >
> > 1. FAST response (<1 msec)
> >
> > 2. SLOW decay (0.05 to 0.2 sec)
> >
> > How to do it:
> >
> > in o--R1---+------+--------o out
> >            |      |
> >            C      R2
> >            |      |
> >           gnd    gnd
> >
> > decay: t=R2*C
> > response (rise time): t=R1*C
> >
> > R1 may be omit, but output of prev. stage must have good current
limiter
> > (should be used 741 type opamp or LM324 - quadruple 741, max. output
> > current is 25mA)
> >
> > BUT Dont forget for equation
> >
> >                 C*U=I*t
> >
> > if max detected voltage is for example 5 volts ie., C*5=0.025*0.001 ;
> > C=0.025*0.001/5=0.0052
> > ie., C must be less than 0.0052F, if we use for example 10uF, condition
is
> > OK, rise time is OK too.
> >
> > Best Regards
> >
> > Jerry LSH
>