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

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.

The mic pre was interesting also because it used a pair of 12AY7 tubes 
on a PCB. The first 12AY7 is wired with global NFB giving 26dB of gain 
and incidentally they used selected 12AY7 tubes for this stage for 
lowest noise. The second 12AY7 was wired as a triode stage with local 
NFB followed by a  cathode follower. The two tubes together give a total 
gain of 48dB which with a 10:1 transformer gives a maximum channel gain 
of 68dB. With a 20dB pad on the transformer the first stage has a gain 
of 26dB and can produce +12dBV at negligible distortion (simulated) 
which means a max mic input level of -14dBV can be handled.

Since the 12AY7 is hardly common today so I re-jigged the circuit to 
work with 6N1P tubes which have a similar mu but higher gm and lower ra 
and this gave even less distortion. The pair of tubes is capable of 
delivering +12dBV into 22K at less than 0.1% harmonic distortion most of 
which is 2nd harmonic.

So simulation seems to suggest this is quite a good design so I am now 
off to build one and put it through its paces. I'll let you know the 
results.

Cheers

Ian