RCA BA-78A rack-up

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emrr

Well-known member
Joined
Apr 12, 2006
Messages
8,525
Location
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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.

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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.....
 
[quote author="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.....[/quote]

Sounds like the way I treat my socks !!!! :grin:
 
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.
 
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.

 
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?
 

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