GE 4BA21A Germanium Mic Pres

[RSRecords]

So I pick up a couple of 4BA21A mic pres. Anybody familiar with these old Germanium pre's? Came out of a General Electric 4BC21A1 console circa 1961.

I found a schematic but I haven't checked how accurate they are. Seem to run off -25volts. I probably add an output transformer for a balanced output. Any suggestions looking at the schematic? 1st thing is a recap and cleaning of course. Some of the cap values seem low but I know cap size was an issue during this time. Anything you guys would look at changing?

EDIT: Schematic attached on post #4!


Thanks!

 

[RSRecords]

Here's a pic of the units themselves

 

[L´Andratté]

hi rsrecords i get
404 - Attachment Not Found
is it me only?

 

[RSRecords]

Hmm weird,  I'll try again. thanks for the heads up.

 

[mjrippe]

I had one of these cards without the frame and attenuator.  As I recall I wired it up with a pot and an output transformer and it made a decent little preamp.  Attached is the Rev C version of the schematic for posterity.  (Yours is Rev A)

I have also seen some with high and low EQ on the faceplate, but no docs for those. 

 

[abbey road d enfer]

I probably add an output transformer for a balanced output. Any suggestions looking at the schematic? !

Something's very wrong with this schemo, with the 5.1k emitter resistor loaded by 300R!
I see it's been corrected on the other schemo. Anyway, the ouput capability of this stage is not very good and would be at pain driving a xfmr, unless you want an FX box.
There's a lot to be said about the noise and distortion performance, due to the architecture; there are period products that take advantage of a much better design (think Langevin).

 

[RSRecords]

I had one of these cards without the frame and attenuator.  As I recall I wired it up with a pot and an output transformer and it made a decent little preamp.  Attached is the Rev C version of the schematic for posterity.  (Yours is Rev A)

I have also seen some with high and low EQ on the faceplate, but no docs for those. 

Awesome thanks!

Something's very wrong with this schemo, with the 5.1k emitter resistor loaded by 300R!
I see it's been corrected on the other schemo. Anyway, the ouput capability of this stage is not very good and would be at pain driving a xfmr, unless you want an FX box.
There's a lot to be said about the noise and distortion performance, due to the architecture; there are period products that take advantage of a much better design (think Langevin).

I actually haven't received the pre's yet so I can't say if the schemo I posted is accurate. I may make the modifications you referenced in Rev C.
I also have some THAT 1646 ic's left over from another project.  I was thinking I could use them since I have a +/-16 and 24v PSU around too. I kind of feels wrong though...other thought was since there seems to be 2 outputs I could choose between xfmr out and chip. Maybe that would make a better spot for a +48v switch. I don't know, I just thought they were cool looking and might be a fun color piece guitars or something.

 

[mjrippe]

They were likely just driving a mix bus in a broadcast console, so simply adding an output transformer is not ideal.  Like I said, I did it anyhow and it worked fine into modern (10k+) loads.  Probably lost some headroom but I was looking at it as a colored sounding preamp, not clean.

 

[emrr]

For the A version:
output load Z 300-1000Ω, nominal 600Ω
output level -10dBm with 1/2% distortion (50-15K)
+/-1dB 50-15K
40dB gain strap max in -25dBm
50dB gain strap max in -35dBm

I have a pair of the larger program amps available if anyone wants them.    The input transformer there (same as) is unloaded, so the mixing bus load is the Ω definition.  Should work fine as higher gain preamps, and +30dBm output rating. 

I also use the stand-alone GE BA-25 preamps which do have an output drive section and output transformer, very similar in sound to RCA BA-31/41/71/72. 

 

[RSRecords]

For the A version:
output load Z 300-1000Ω, nominal 600Ω
output level -10dBm with 1/2% distortion (50-15K)
+/-1dB 50-15K
40dB gain strap max in -25dBm
50dB gain strap max in -35dBm

I have a pair of the larger program amps available if anyone wants them.    The input transformer there (same as) is unloaded, so the mixing bus load is the Ω definition.  Should work fine as higher gain preamps, and +30dBm output rating. 

I also use the stand-alone GE BA-25 preamps which do have an output drive section and output transformer, very similar in sound to RCA BA-31/41/71/72.

Awesome thanks for the info. I found a manual for the whole 4bc21A1 console. It has some details about the preamp which are pretty handy.

Interestingly enough the preamps I received have a triad A57j already mounted on the chassis.. ;D

 

[RSRecords]

and another pic

 

[abbey road d enfer]

For the A version:
output load Z 300-1000Ω, nominal 600Ω
output level -10dBm with 1/2% distortion (50-15K)
+/-1dB 50-15K
40dB gain strap max in -25dBm
50dB gain strap max in -35dBm

These figures are inconsistent.
The last two lines suggest an output level of +15dBm, that is 13Vp-to-p into 600 ohms. There's no way the 2400 ohms (5100 in the 1st version) emitter resitor can supply that with 25V rail voltage. However it may be possible into 10kohms.
-10dBu nominal into the program bus resistor (probably about 10k) is plausible and compatible with about 20-25dB headroom.

 

[RSRecords]

I had one of these cards without the frame and attenuator.  As I recall I wired it up with a pot and an output transformer and it made a decent little preamp.  Attached is the Rev C version of the schematic for posterity.  (Yours is Rev A)

I have also seen some with high and low EQ on the faceplate, but no docs for those. 

What are the values of C5 and C6 on the rev C version? This might be a silly question but I haven't seen this prefix  before "uuF".  Is that nano-farads?

 

[abbey road d enfer]

What are the values of C5 and C6 on the rev C version? This might be a silly question but I haven't seen this prefix  before "uuF".  Is that nano-farads?

They are clearly labelled as "double-micro" (mu.mu) i.e. picofarad

 

[RSRecords]

They are clearly labelled as "double-micro" (mu.mu) i.e. picofarad

Ok thanks. I saw the label "mu mu" and thinking about it that makes sense. I just haven't come across that before.  Thanks for clarifying.

 

[emrr]

These figures are inconsistent.
The last two lines suggest an output level of +15dBm

These are the quoted specs.  You are ignoring the interstage gain control.  Remove C2 for 40dB gain.  The full manual shows a passive broadcast mix bus, differing physically  in that the resistances you see in the output switching within the pre are the defined bus resistances. 

 

[RSRecords]

I'm trying to figure out what's going on with the "+24v" on the schematic. Is this really a positive 24vdc rail?

 

[abbey road d enfer]

These are the quoted specs.  You are ignoring the interstage gain control.

If the gain control is used to attenuate, the gain is not the specified 40 or 50dB.
So OK, we may accept it's a convoluted way of assessing the input stage's headroom.
But still, quoting levels in dBm, which implies 600 ohm loading, is in contradiction with the intended usage and what the circuit can handle.

 

[emrr]

I'm trying to figure out what's going on with the "+24v" on the schematic. Is this really a positive 24vdc rail?

No it's common.  This is also a schematic standard of the era. 

 

[emrr]

If the gain control is used to attenuate, the gain is not the specified 40 or 50dB.
So OK, we may accept it's a convoluted way of assessing the input stage's headroom.
But still, quoting levels in dBm, which implies 600 ohm loading, is in contradiction with the intended usage and what the circuit can handle.

This is the way American equipment was frequently specified at the time.  Gain is always max, headroom spec may or may not be total headroom, may be input stage overload point, dependent on whether there's an interstage gain control.  Output level is quoted at -10dBm, that spec is always included.  Up to the user to run the gain math. 

We know the input transformer of the PGM amp is unloaded, and there's the output series resistor and the internal load resistor.  There don't appear to be any insert points in the console, so yes, dBm is an aberrant spec to include.