Masuto

RCA BA-73 as mic preamp - help
« on: February 17, 2019, 09:25:38 AM »
Hello. My plan is to use this BA-73d as mic pre. I read somewhere this advice: “tap in at C14”.. err what does that mean? Should i just input audio from there and just use the last two transistors+power trannies and output transformer?! i am hoping a kind soul will chime in..
Thanks
« Last Edit: February 17, 2019, 10:07:34 AM by Masuto »


Re: RCA BA-73 as mic preamp - help
« Reply #1 on: February 17, 2019, 03:50:10 PM »
I read somewhere this advice: “tap in at C14”.. err what does that mean?
1.  BA-73 family has arguably excess gain,  practical to bypass center gain stage
2.  remove C13 and R27.  jumper 5 to 7 at connector.
3.  one can access the output stage at C14; however suggest leaving R25 and R26 in place for enhanced stability.
4.  attached schematic is for the "c". 
5.  polarity notations  reversed.


Masuto

Re: RCA BA-73 as mic preamp - help
« Reply #2 on: February 17, 2019, 08:39:44 PM »
1.  BA-73 family has arguably excess gain,  practical to bypass center gain stage
2.  remove C13 and R27.  jumper 5 to 7 at connector.
3.  one can access the output stage at C14; however suggest leaving R25 and R26 in place for enhanced stability.
4.  attached schematic is for the "c". 
5.  polarity notations  reversed.

Thanks!
Just a couple of questions: modifying the board as per your suggestions will leave my terminals as per schematic-alas no other mod is required?

By “polarity notations reversed” you mean the D is a negative ground circuit?

Re: RCA BA-73 as mic preamp - help
« Reply #3 on: February 17, 2019, 09:06:21 PM »
modifying the board as per your suggestions will leave my terminals as per schematic-alas no other mod is required?
no other changes. 
By “polarity notations reversed” you mean the D is a negative ground circuit?
audio polarity.

Masuto

Re: RCA BA-73 as mic preamp - help
« Reply #4 on: February 17, 2019, 11:39:01 PM »
Terrific, you’ve been of great help. Cheers.

Masuto

Re: RCA BA-73 as mic preamp - help
« Reply #5 on: February 28, 2019, 11:26:42 AM »
Ok sorry guys but i seem to be struggling with ponytailing this pre..
What am i doing wrong?
I carried out the mod described above and now i have these connections: terminal pins 5 & 7 are jumpered.
I am using an external psu: -30v to terminal PIN4, +30v to terminal PIN 16;
Input: terminal PIN 13 to female XLR 1, PIN 14 to XLR 2 - pin 3 is unused
Output: PIN 12 to male XLR 1, PIN 24 to XLR 2 - pin 3 is unused
Attenuator: 5K A pot, PIN 6 to pot’s PIN 1, PIN 7 to pot’s PIN 3, PIN 19 to pot’s PIN 2.
Awfully sorry to bother you pros with this, but i really feel a bit confused.. maybe it’s something easy that i am missing..
HELP ME PLEASE!

dmp

Re: RCA BA-73 as mic preamp - help
« Reply #6 on: February 28, 2019, 02:44:36 PM »
I am using an external psu: -30v to terminal PIN4, +30v to terminal PIN 16;

That sounds like you are giving it 60 V
You want to supply 30v.
The schematic says common (0v) at pin16 and -30 at pin4

Was it working before you did the mod to lower the gain?

Masuto

Re: RCA BA-73 as mic preamp - help
« Reply #7 on: February 28, 2019, 04:43:10 PM »
Hi erm i dont know-i recapped and checked all the transistors, wiring. It should be working unless there’s a transformer failure (unlikely!). Being a positive ground circuit, i thought +30 had to be on pin 16. Am i wrong?
Cheers

dmp

Re: RCA BA-73 as mic preamp - help
« Reply #8 on: February 28, 2019, 04:46:23 PM »
Did you supply ground (0v) to pin4 or -30v?


Masuto

Re: RCA BA-73 as mic preamp - help
« Reply #9 on: February 28, 2019, 07:30:55 PM »
Pin 4 is currently fed with -30v.

 :-[
« Last Edit: February 28, 2019, 08:10:42 PM by Masuto »


Masuto

Re: RCA BA-73 as mic preamp - help
« Reply #10 on: February 28, 2019, 08:13:24 PM »
I am attaching another diagram of the module connections from an RCA brochure

Masuto

Re: RCA BA-73 as mic preamp - help
« Reply #11 on: March 01, 2019, 09:40:30 AM »
Anyone...please?

dmp

Re: RCA BA-73 as mic preamp - help
« Reply #12 on: March 01, 2019, 09:48:34 AM »
You need to provide the correct power to it and then start measuring voltages / tracing the signal.

I don't know if it is miscommunication or a mistake but if you are putting +30v on pin 16 and -30v on pin 4, then the amp is seeing 60v. That is wrong. It needs 30v between pin 16 and pin 4

EmRR

Re: RCA BA-73 as mic preamp - help
« Reply #13 on: March 01, 2019, 10:31:30 AM »
30VDC PSU
+ to 16
- to 4
16 and 18 tied together at the PSU end.    If + isn't grounded, the whole thing may oscillate.  If you miss that part on some SS Altec amps of the same era, they will immediately self destruct. 

Your XLR wiring sounds wrong. 
You also have to connect the transformer strapping as per the notes.

You've connected the attenuator as if you are using the 2nd stage, which you've bypassed as per gridcurrent's notes.  Can't possibly work.   Think about it.    Gridcurrent plan also leaves out the volume control completely.  Also think about that. 

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 sounde

Masuto

Re: RCA BA-73 as mic preamp - help
« Reply #14 on: March 01, 2019, 10:43:06 AM »
Thanks!! Wow Doug you are a legend, so grateful you chimed in.. embarassing yes, i am a complete noob at this. I will ponder over your post. Thanks again. Very helpful.
My only experience with altec ss has been recapping and powering a 1592b using the same psu i will use with this..


 

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