RCA BA21A help needed.

[gary o]

http://www.waltzingbear.com/Schematics/RCA/BA-21A.htm

Does anyone know anything about these BA21A mic pre?s, I have a PDF of instruction manual, Im wondering what the transformer ratios are, im fairly green as a diyer & some time back I soldered one of these together from the schematic & im now tryin to retrace my steps & correct a few things as I learn more, I used a Sowter 9045 1:10 tran for input, question does it matter too much about the input ratio? I used a Sowter 9050 push pull output tran but this tran doesn?t have the terriary winding for the neg feedback so on mine its either off or to ground, the amp sounds fantastic to me IMHO especially thru my homemade BA6A don?t know if this is anything to do with being a push pull like the BA6A or because RCA design, but I haven?t heard an original & im wondering how it would sound with the right tranny? my main problem is it has too much level, theres instructions in the manual on how to fit a bridging gain control which seems to connect across the input tran anyone know anything about this, theres a kind of dual pot thing shown on the schematic for a BA1A pre wonder if the same if anyone has any thoughts or ideas I would love to hear em thanks

Gary O.

 

[NewYorkDave]

Gary, your post is a run-on sentence with a bunch of questions crammed together, which makes it hard to read. But it seems you're saying that you built the circuit with no negative feedback loop. Is that the case? That would explain the excessive gain.

If you like your pre as-is and simply want to control the level, use an attenuator on the output. I've posted several designs here. If you want an input pad, Winston O'Boogie posted a nice switchable one in the "RED" thread several weeks ago.

The "bridging input pot" you refer to was probably a dual pot across the primary of the input transformer, with series resistors going to the wipers, and signal injected at these series resistors. That was a pretty standard configuration for adding a variable bridging pad to a mic preamp input. Can't say for sure without seeing the same manual you're reading.

 

[gary o]

Sorry?. I do ramble on as Nat king cole would say,
Yeah I did make the circuit as you said, pin 5 on op tran goes straight to ground instead of thru that little winding on the op tran, the amp is quieter this way than when feedback loop is not connected, so I think there is a feed back loop of sorts.

I don?t think the amps level is any higher than it should be I just need it to be quieter for my purpose

Yeah I have your various pads on the PC waiting for resistors & switches so thanks for those NYD.

Its not that im dissatisfied with the sound of my pre amp, just wanted to learn about & hear the difference with the designed feed back loop, looks like I wont be able to do this with my current OP tran.

For the moment I have a 1K pot accross & 2 X 270R res on the output, tho its not quite enough reduction still.

I think your right about the bridging input pot, I might hook one up to try it out, theres one on this Ba1A schematic
http://www.waltzingbear.com/Schematics/RCA/BA-1A.htm
I also made a BA1A at same time as BA21A, it has much much less level & a clean sound,nice tho, no need to attenuate

Also made some other bits of kit that im gonna check over & possibly tweak
Altec 1566A, Pultec MB1,RCA BC2B, V72, some jenson cascode thingy & an LA2A should keep me bizzy so might have some more questions soon if no one minds

Thanks for your wisdom & time sorry to be a pain,I am learning slowly but surely

Cheers Gary O.

 

[CJ]

Gary, talk to Adrian. He has a whole rack of those things I believe.'

 

[gary o]

Cool thanxs CJ.

Gary O.