In article <LnvWj.578$_03.380@[EMAIL PROTECTED]
>,
"Iain Churches" <IainNG@[EMAIL PROTECTED]
> wrote:
> "John Byrns" <byrnsj@[EMAIL PROTECTED]
> wrote in message
> news:byrnsj-68097B.13303913052008@[EMAIL PROTECTED]
> > In article <XgiWj.249$_03.86@[EMAIL PROTECTED]
>,
> > "Iain Churches" <IainNG@[EMAIL PROTECTED]
> wrote:
> >
> >> We are all probably fettling away on tube projects:-)
> >> I have a tube driven VU level indicator unit just complete
> >> and ready for calibration. Pics available if anyone is
> >> interested.
> >
> > "Tube driven VU level indicator unit", what is that? Why are tubes,
> > transistors, or any active devices needed in a VU meter unit, the ones
I
> > have are designed to connect directly across a 600 Ohm audio line?
>
> Hang a VU across the line and measure the increase in
> distortion. You can "hear" the difference!
Sure, but the distortion is mainly 2 nd harmonic and adds to the nice
warm tube sound many are seeking. IIRC when the VU meter was originally
designed the amount of distortion produced, when the standard build out
resistor is used for +4 dBm at 0 VU, was considered insignificant. The
amount of added distortion is also strongly dependent on the source
impedance of the amplifier driving the line/meter/pad(s). How audible
is the distortion when a modern high feedback low output impedance
amplifier is used? Somewhere I have an article from the late 1930s when
the VU meter was designed, which discuses various factors in the design,
including the distortion you mention. I wonder what the nature was of
the meters used in American practice before that time, and also what was
used in Europe before the advent of the PPM?
> At least in the UK a buffer amp was commonly used.
Makes sense, also a pad was often used between the amplifier output and
the VU meter further reducing the distortion contribution of the meter,
although the amplifier will probably then be producing more distortion
since it will be operating at a higher level. For example the Ampex 351
recorder/reproducer had a 4 dB pad in the meter circuit which could be
strapped out of the circuit when a +4 dBm output level was desired
rather than the standard +8 dBm output. Pads were used when driving a
transmitter which commonly required +10 dBm in the US.
> > I would like to build a tube driven facsimile of a PPM, using a meter
> > movement I salvaged from a scrapped transistorized AM modulation
> > monitor. Any thoughts on a suitable circuit for a project like this?
>
> Which type of PPM are you referring to John.
> Does it have a scale to 12 with a red line at 8 ?
As soon as I posted I realized that my post was not sufficiently
detailed, and my requirements were not well spelled out.
I do not have a PPM meter of any sort. The meter I have is salvaged
from a solid state US style AM modulation monitor, an instrument used by
AM broadcast stations in the US to insure they aren't over modulating
their transmitters. The particular meter I have has three scales sort
of like a VU meter. The main scale is linear and calibrated in percent
modulation with a full scale reading of 140%. There is also a second
percent modulation scale whose readings appear to be 10 dB lower, or
more sensitive, than the main scale, presumably 10 dB of extra gain was
switched in to read lower modulation levels more accurately. The third
scale is a secondary scale calibrated in dB running from -20 dB through
+3 dB. I measured the full scale sensitivity of this meter once, but I
am not sure where my notes on it are, but IIRC it was 1 mA full scale.
What I remembered after posting is that PPM meters use a log amplifier
in their circuitry and I certainly wouldn't want that with the linear
scale on my meter. I was thinking of an all vacuum tube circuit,
including signal "rectifiers", which would include switching to indicate
either positive or negative peaks individually, or the largest peaks of
either polarity. So far all I have decided is to use a differential
amplifier, similar to those used in US style VTVMs, to drive the meter.
I still need to settle on a buffer circuit for the audio, and the peak
detection circuits with provision for selecting the polarity of the
peaks to be measured. It is this latter part of the circuit I need some
inspiration on. An added requirement is relative simplicity. Of course
the ultimate in simplicity was seen in the old tube type AM modulation
monitors which used a more sensitive meter and were driven directly from
the transmitter, requiring a drive of somewhere between 1 and 5 Watts
depending on the maker. The entire circuit consisted of only two
rectifiers, one to demodulate the transmitter output and provide an
audio signal, and a second diode to detect the audio peak of the desired
polarity and drive the meter which was shunted by a peak hold capacitor,
can't get much simpler than that.
Regards,
John Byrns
--
Surf my web pages at, http://fmamradios.com/
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