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

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.

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??

>>> 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.
> 
> Sounds about right.
> 
>> 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

Cheers

IaN