emrr

RCA BA-78A rack-up
« on: July 09, 2008, 12:48:09 PM »
Early 1960's RCA BA-78A Cue / Intercom amplifier.  Set up with variable input pad and output attenuator on a General Electric rack panel.   Dig that 'screwless' GE front face.

The BA-78 'A' version is a real era-bridging oddity that you usually only see as a sample circuit in an old transistor manual.  Fully push-pull, with a transformer between every stage; 5 transformers!  90ish dB gain, several watts max output.  Found in BC-7 / BC-8 type solid state consoles, meant for talkback purposes using relay switched 8 ohm speakers, limiter circuit built in which works at a very low threshold.  Limiting achieved by variable shorting of a tertiary winding on the input transformer, which in turn varies the impedance reflected to the mic/speaker, and causes level losses.  How's that for weird?  There is a BA-78B version which is much more common.  It uses the same method and has the same gain max, but a higher max output and it ditches the PP circuit in favor of a more traditional type with only input and output transformers.  

The variable input pad is a very large string of series resistance maxing out in the 100K range, for line level inputs.   The output is either 8 or 50 ohms, and this has a 50 ohm Mallory ladder which is almost always nearly all the way down.   The limiting has an 'ideal' threshold, and RCA provided a chart for establishing ideal input pads.   In the consoles, the output attenuator was on the faceplate.  

What do you use it for?  I put a mic behind a drum kit, turn the input pad to the lowest attenuation setting, and let it smash up a room sound.  It mashes up a flat-lined wave form, and if you look at RTA, if also mashes all frequencies up to the same level.  Somewhat upper mid-heavy, it is better with dark mics.  I've used it as a talkback mic too; gee who would have thought.    Harder to use with line level stuff; definitely a one trick pony with surprisingly clear sound, given what it's doing.  







Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounded g


MartyMart

RCA BA-78A rack-up
« Reply #1 on: July 09, 2008, 12:59:08 PM »
That's just GREAT Doug - I love that "hamerite" finish !!
Very "Retro" in a cool way .

MM.
"Success is the ability to go from one failure to another with no loss of enthusiasm"

emrr

RCA BA-78A rack-up
« Reply #2 on: July 09, 2008, 01:04:21 PM »
I'm a sucker for the hamertone / hamerite finish.  Hides dirt well, don't have to wash it so often, only when it smells a bit.....
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounded g

MartyMart

RCA BA-78A rack-up
« Reply #3 on: July 09, 2008, 01:11:04 PM »
Quote from: "emrr"
I'm a sucker for the hamertone / hamerite finish.  Hides dirt well, don't have to wash it so often, only when it smells a bit.....


Sounds like the way I treat my socks  !!!! :grin:
"Success is the ability to go from one failure to another with no loss of enthusiasm"

Viitalahde

RCA BA-78A rack-up
« Reply #4 on: July 09, 2008, 01:32:08 PM »
Now how cool is that!  :grin:

There must be hope in here, afterall!

rafafredd

RCA BA-78A rack-up
« Reply #5 on: July 10, 2008, 09:56:03 PM »
man, that´s wierd unit! Five transformer!!! WOW. I would love to hear what it really does.

emrr

RCA BA-78A rack-up
« Reply #6 on: July 10, 2008, 10:13:32 PM »
Official response curve is more voice oriented; down a good bit by 100 and 12kHz I think, but seems to 'mash in' more than promised.  I tried doing a response plot once, and it wouldn't chart anything static since the threshold is so low(!).  Just looked like it was squirming to get away like a worm on a hot sidewalk.   In contrast, other limiters will give you a stable plot with GR.  Will have to revisit a plot with 80 dB of attenuation on the front end sometime.
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounded g

Re: RCA BA-78A rack-up
« Reply #7 on: May 01, 2017, 11:49:05 AM »
Quote
Will have to revisit a plot with 80 dB of attenuation on the front end sometime.

That's about what I'm getting.  35 db external pad plus -40db line signal coming out of DAW gives essentially a clean uncompressed signal.   Even just a 57 or ribbon ran straight in as a mic pre with no pad runs into clipping pretty quickly on vocals.  You can run the output directly into a speaker cab and use it for imitating the molten sound of an ancient PA too.


mjrippe

Re: RCA BA-78A rack-up
« Reply #8 on: May 01, 2017, 01:09:01 PM »
I have one of these in my project drawer and was considering adding a switch to bypass third gain stage.  As in, take the collectors of Q3 and Q4 and connect them to the primary of T4 instead of T3.  Has anyone tried that?