"John Byrns" <byrnsj@[EMAIL PROTECTED]
> wrote in message
news:byrnsj-095BA6.11550714052008@[EMAIL PROTECTED]
> 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.
Hello John
In prof recording and broadcast, with analogue tape at 2% THD
the programme material was plenty warm enough:-) And besides I
don't think anyone would want a meter which changed the sound of their
system when it was connected. But, hey, wait a minute, this could be
the ultimate in high-end audible improvements. "Connect a VU meter
and both see and hear the improvement!" :-)
> 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?
I made a quick comparison with a Radford LDO4 low distortion
oscillator 600 Ohms at +4dB (1.22 V) I then fed this to a preamp
with attenuation and thence to a power amp. The distortion added
by hanging the meter across the line was clearly audible. I seem to
have discarded the scrap of paper on which I noted the figs.
> 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.
Yes I also have some articles somewhere. The whole story
of the VU and the ANSI specification of "Volume Measurements
of Electrical Speech and Program waves" C16.5-1942 (which is
now incor****ated into IEC 60268-17) is a fascinating one.
> 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?
I have no idea what came before the VU, but I do remember
seeing tube driven VU meters at Decca. There was a shelf full
of them in what we called the Redundancy Store. In retrospect,
Aladin's Cave might have been a better name:-) The buffer amp
was just a simple cathode follower 12AU7. I used to own an
Ampex 351-2 tape machine with VUs but I cannot recall how
the metering was done. But I do know that the meter units for
European tape machines like the Studer C37 and J37, had a
tube buffer amp just like the one I describe.
>
>> 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.
That's an interesting meter. What would you use it for?
I am keeping an eye open for PPM's The best ones in Europe were
made by Ernest Turner (a company which ceased trading in the 1980s
after a takeover)
>
> 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.
John. Such a project would make a fascinating thread on RAT. I
hope that you will persevere with it.
Regards
Iain
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