Why did they tie the + rail to the mic input xfmr?

[MediaWorksOKC]

Hello,

I’m working on an old Rauland mic mixer, never found a schematic. I got extra curious so I started tracing and see that the + of the +/-12v rails is tied to pin 5 of the mic input transformer. They tied all 8 pin 5s together to the +12. Any idea why? Doesn’t seem a convenient tie point as the routing is complex to do so, it seems to serve a different purpose. Nothing else is connected to pin 5 and the +12 is always live. What were they doing?

Phantom power was optional and is handled separately.

Thanks,
Chris

 

[JohnRoberts]

if it works the preamp must be happy with that, maybe trace out more of the circuit to reveal why.

JR

 

[Bo Deadly]

It's part of the circuit. It bias for the first transistor probably. A little strange but early discrete designs did some funky stuff.

 

[Brian Roth]

Is that a plug-in device/accessory socket? If so, some sort of gizmo option might have required powering.

In that context, many Ampex recorders had an option socket on the rear panel. One of the (eight) pins supplied DC power for an optional mic preamp plug in module. Two other Ampex options were a couple of different input transformers, or a dummy plug, both of which ignored the power applied to one of the pins.

If Rauland did NOT have plug in options I have no idea.

Bri

 

[abbey road d enfer]

They tied all 8 pin 5s together to the +12. Any idea why? Doesn’t seem a convenient tie point as the routing is complex to do so, it seems to serve a different purpose. Nothing else is connected to pin 5 and the +12 is always live. What were they doing?

You sure it's not a center-tapped primary? Looks like a convenient way of feeding P12...

 

[JohnRoberts]

the OP said Phantom power was separate.

I like the idea it was an optional power supply output to power active accessories. I think I've seen things like that for install (background music) market.

JR

 

[untune]

Is that a plug-in device/accessory socket? If so, some sort of gizmo option might have required powering.

In that context, many Ampex recorders had an option socket on the rear panel. One of the (eight) pins supplied DC power for an optional mic preamp plug in module. Two other Ampex options were a couple of different input transformers, or a dummy plug, both of which ignored the power applied to one of the pins.

If Rauland did NOT have plug in options I have no idea.

Bri

I came across this recently while studying Ampex schematics as I have a few of the Ampex octal line bridging transformers. I was curious why pin 5 had B+ going to it, until I realised that it was to cater for the plug-in mic preamps, the line transformer pin 5 is (presumably) disconnected internally and therefore it makes no difference

 

[abbey road d enfer]

I came across this recently while studying Ampex schematics as I have a few of the Ampex octal line bridging transformers. I was curious why pin 5 had B+ going to it, until I realised that it was to cater for the plug-in mic preamps, the line transformer pin 5 is (presumably) disconnected internally and therefore it makes no difference

Of course! I didn't realise it was a socket.

 

[mjrippe]

I have a couple Rauland SS mixer schematics, but neither has transformer inputs. I've attached one that might be similar, at least in the output section. I agree with the folks who say there may have been active modules that needed voltage, like the Altec, Ampex, Dukane, etc. preamps.

 

[MediaWorksOKC]

Is that a plug-in device/accessory socket? If so, some sort of gizmo option might have required powering.

In that context, many Ampex recorders had an option socket on the rear panel. One of the (eight) pins supplied DC power for an optional mic preamp plug in module. Two other Ampex options were a couple of different input transformers, or a dummy plug, both of which ignored the power applied to one of the pins.

If Rauland did NOT have plug in options I have no idea.

Bri

That's true, and makes the most sense, that didn't occur to me as this is a standard 7-pin tube-type socket for the transformer so it's rather small to have had an active option IMO considering the age/era but anything is possible, all the active plug-ins I've seen were 8-pin octal and maybe a few 9-pin types over the years.

Thanks!

Chris

 

[MediaWorksOKC]

"Mystery" solved, a kind gentleman sent these images which I compiled into a single image. It shows another mixer's input but the transformer pin out and socket diagram tell the tale relative to what I found. They tied the +12v to pin 3 which is the transformer's case pin when installed, waiting around for an optional active plugin module as surmised. My original drawing numbering is reversed from Rauland's.