Re: Wot? No tube mixer??(YES IM STILL HERE< TY)

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