John Byrns wrote:
> 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?
>
I have put one here:
http://www.ianbell.ukfsn.org/data/RCA-76-B2-B4-Console.pdf
Cheers
Ian
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