In article <gclmeo$fs9$1@[EMAIL PROTECTED]
>,
Ian Bell <ruffrecords@[EMAIL PROTECTED]
> wrote:
> John Byrns wrote:
> > In article <gcl4de$rpc$1@[EMAIL PROTECTED]
>,
> > Ian Thompson-Bell <ruffrecords@[EMAIL PROTECTED]
> wrote:
> >
> >> cipher wrote:
> >>> Please continue the discussion and correspondance with me..
> >>>
> >>> This subject is still relevant as I still record quite frequently
and
> >>> have found that the "commercially available" products out there just
> >>> dont tickle my fancy
> >>>
> >>> I am not a troll.
> >>
> >> Hi Tynan,
> >>
> >> I recently purchased a DVD containing the manuals for a range old RCA
> >> and other tube based broadcast mixing consoles. One interesting
aspect
> >> was that the earlier ones all had mic pres which were transformer
> >> balanced *out* as well as in. Mixing took place using 200 ohm bridged
T
> >> stepped rotary channel faders connected to these transformers and the
> >> resultant mix fed into another transformer and amplifier. This meant
a
> >> lot of expensive transformers and faders.
> >>
> >> A later model in the same series (RCA BC-6 series) dispensed with the
> >> output transformer on each mic pre, included a 100K (stepped) pot
half
> >> way down the mic pre as a channel fader and a cathode follower output
> >> feeding 22K mixing resistors. The mix was amplified by the same basic
> >> mic pre topology with the group fader replacing the channel one.
> >
> > This scheme to eliminate the output transformers from the microphone
> > preamplifiers starts to get complex when you consider stereo mixers
with
> > the
> > need for pan pots.
>
> This scheme was designed simply for mono mixers although they were later
> adapted for stereo I believe where a switch selected L, R or centre - no
> real pan pot (the mono ones already had this per channel switch to
> select programme channel, monitor channel or off). I do not believe it
> is any easier with the transformers as the insertion loss with constant
> impedance faders is high and the pan pots would just add to this making
> the mix bus signal level very low. With the non-transformere scheme the
> channel fader is already buffered so that is no longer an issue. It
> would be relatively straightforward in a stereo mixer to replace the
> single CF of the original design by a pair fed from a pan pot and the
> insertion loss would be only 4 to 6dB depending on what you decide the
> centre position loss should be.
I'm not sure why you say "the insertion loss with constant impedance
faders is
high"? The fader loss is essentially 0 dB excluding the loss introduced
by any
"fading" introduced by setting the control to less than full up, in any
case the
loss in the fader is no different than in a similar monophonic mixer.
Making it
a stereo mixer does introduce two losses the monophonic mixer doesn't
suffer.
First there is a 3 dB loss in each channel that is the result of dividing
the
output power of the microphone amplifier between the two mixer channels,
the
left and right stereo channels. The power from each microphone amplifier
is
split by connecting the output transformer in a hybrid configuration to
eliminate crosstalk between the left and right channels of the mixer.
Second
there is a 3 dB to 6 dB loss, the exact amount depends on the designers
taste,
when the pan pot is centered. The added loss on either channel is only 3
dB
greater than with the non-transformer scheme.
> > Another advantage of retaining the output transformer is
> > that the same amplifier module design can be used for both the
microphone
> > preamplifier function and the mixer output function.
>
> I agree assuming you want balanced floating outputs. For a simple mixer
> directly feeding a recorder this may not be necessary. The RCA design
> added a further line amplifier that took the unbalanced mix outputs and
> boosted them to balanced line level which was +18dBm in those days
> apparently.
Sounds about right.
> I'll see if I can find a place to post the pdf of this mixer as it is an
> interesting read - good diagrams for instance showing actual bus bars
> for mixing.
If you can't find another place for it email it to me and I will happily
post it
on my web site.
--
Regards,
John Byrns
Surf my web pages at, http://fmamradios.com/
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