In article <gcn0bv$sae$1@[EMAIL PROTECTED]
>,
Ian Bell <ruffrecords@[EMAIL PROTECTED]
> wrote:
> John Byrns wrote:
> > In article <gclmeo$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"?
>
> Thinking about it I neither am I - I'll check.
I'll be interested in hearing what you find.
> > 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.
>
> Yes, the two secondary transformer is a good idea. I don't have any
> examples of stereo Consoles that used that technique - do you??
No, I don't either, however that doesn't mean much as I have never
followed
mixer technology, except to a limited extent as related to microphone
preamplifiers. One monophonic mixer that used the hybrid transformer
approach
was the BBC Type A mixing desk. In the BBC mixer a hybrid was used on the
output of each microphone amplifier to prevent "howl around" related to
the echo
send and return functions.
One complication of using the hybrid approach is that the amplifier must
be
designed to have the output source impedance needed to balance the hybrid,
the
amplifier can't simply be designed for as low an output source impedance
as
possible. I suppose an alternate approach, if you are willing to accept
an
additional 3 dB loss, would be to design the amplifier to have a low
source
impedance and then use a build out resistor to set the impedance seen by
the
hybrid to the required value.
> >> 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.
>
> I have posted it here:
>
> http://www.ianbell.ukfsn.org/data/RCA-BC-6B-Console.pdf
You also mentioned consoles where "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." Could you post
the
"pdf" of an RCA mixer that used this approach for comparison?
--
Regards,
John Byrns
Surf my web pages at, http://fmamradios.com/
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