by cipher <nospam@[EMAIL PROTECTED]
>
Oct 9, 2008 at 03:10 PM
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
