by Ian Bell <ruffrecords@[EMAIL PROTECTED]
>
Oct 10, 2008 at 08:26 AM
cipher wrote:
> Ian Bell <ruffrecords@[EMAIL PROTECTED]
> wrote in
> news:gclmeo$fs9$1@[EMAIL PROTECTED]
>
>> John Byrns wrote:
>>> In article <gcl4de$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.
>>>>>
>>>>> -Cipher /Tynan
>>>>>
>>>> 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.
>>
>> 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.
>>
>> 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.
>>
>> Cheers
>>
>> Ian
>>
>
> I Want to thank you again, Ian
>
> the recorder(s) in question are a sound devices 722 and a stellavox sp-
> 8
For anyone that is interested here is a link to the RCA broadcast
console manual
http://www.ianbell.ukfsn.org/data/RCA-BC-6B-Console.pdf
Cheers
Ian
