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.
>
I just checked out another old broadcast console manual, that of the
Collins 212G and it turns out I was wrong.
This console mixes nine inputs together with an insertion loss of 15dB
which is only a little more than the 12dB you would expect with passive
mixing. It uses constant impedance (stepped) rotary faders (they call
them mixers) which have a 600 ohm input impedance and 1200 ohm output
impedance and use padding resistors to maintain the impedance of
channels that are muted.
Their gain structure is a little odd. Their 40dB mic pre raises a -60dBm
signal to -20dBm, which then is assumed to lose 15dB in the channel
fader and 15dB more through mixing giving a mix bus level of -50dBm.
Another 40dB amp raised this to -10dBm, it loses 6dB in a master fader
to -16dBm, then goes through another 40dB amp to +24dBm and lastly a 6dB
pad to reach +18dBm at the output.
Cheers
Ian
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