GE 4BA21A Germanium Mic Pres

[abbey road d enfer]

Output level is quoted clearly at -10dBm, that spec is always included. 

That's what my gripe is about; it's dBu, not dBm.

 

[emrr]

There's a 'U' (universal) version with a multi-position input pad. 

What's the value of the attenuator?  I don't have that documented anywhere. 

 

[abbey road d enfer]

There's a 'U' (universal) version with a multi-position input pad. 

What's the value of the attenuator?  I don't have that documented anywhere.

It seems the OP has a fixed-gain version; that's probably why he needs external input attenuators.
EDIT: I forgot to untangle my brain's pipes and mixed up two different threads.

 

[abbey road d enfer]

Sorry,  I really have to wonder if you've even looked at his posts.  It quite clearly, in pictures and documentation, has an interstage gain control. 

Sorry for the confusion; I have mixed in my head two similar threads.

You clearly don't believe a word I've quoted from the manual for the unit he actually has.  You are successfully gas-lighting the situation enough to make me look at the documentation for the dozenth time.

It's not a matter of believing. The datasheet is confusing enough. And the fact that the pictures show a "-30dbm/-50dbm" switch that is not shown in either schematic does not help either.

 

[ruffrecords]

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.

In those days dBu had yet to be invented so levels were often (incorrectly) quoted in dBm irrespective of the load impedance (Even Neve were guilty of this).

Cheers

Ian

 

[emrr]

Ok, confusion makes sense, sorry. 

'Universal Input Module' version 4BA21U1.  No data available.    Assume from all surrounding data and images to be a 4 position input pad switch.

I don't have any pics or data for the version with EQ, nor do I know a part #. 

 

[mjrippe]

Ok, confusion makes sense, sorry. 

'Universal Input Module' version 4BA21U1.  No data available.    Assume from all surrounding data and images to be a 4 position input pad switch.

I don't have any pics or data for the version with EQ, nor do I know a part #.

Looked in my files again, the one with EQ is 4BA21E1.  I saw them on ebay and copied the following description.  Unfortunately I did not download the photos!

WARNING - This is information found on the internet.
"Here is a nice old GE Microphone Equalizer Module 4BA21E1 from an early Broadcast Console.

It uses 4 germanium transistors and needs about 36 VDC which is configured positive ground as are most germanium circuits.

Using 36vdc, a minus 30 db balanced input siginal I measured plus 2 db output into a 1Kohm load at the start of clipping. A higher DC supply voltage will raise this point. An external matching transformer should be used to match to any load below 1k ohm.

There are two outputs selected by the top buss switch with center position off.

Frequency response is a good 20cy to 20kc flat. The hi eq works nice cut and boost but the low eq is working in cut only with no boost. There may be a bad capacitor, bad connection, or miswire. The circuit is simple so the problem should be easy to fix.

This pre-amp module is in very good condition with a pair of connectors pig tailed and the power wires brought out. Red is plus which is also chassis ground and black is negative.

The input transformer is an ADC configurable for 600 or 150 ohms and the Daven attenuator is nice and smooth working."

 

[abbey road d enfer]

In those days dBu had yet to be invented so levels were often (incorrectly) quoted in dBm irrespective of the load impedance (Even Neve were guilty of this).

Cheers

Ian

You're right; although the "dbm" dates from 1929, it seems only about 40 years later that some took exception to its usage for voltage.

 

[RSRecords]

Well at least I don't feel as bad when I get dbm and dbu confused

I'm going to start with a recap and the power supply.    Each pre pulls 8ma according to the documentation.  So anybody got a line on an adequate ps pcb with a 25v rail and 48 v for phantom?

 

[Whoops]

Well, even if those units didn't sound any good the looks are beautiful anyway

 

[RSRecords]

Well, even if those units didn't sound any good the looks are beautiful anyway

Right? If they sound garbage I'll make something out of them.

So I was looking for power supply options. I found the MNATS http://mnats.net/adjustable+phantom_power_supply.html
It has a+24v rail and a +48v rail.  Would it be possible to use the +24v rail as the -25vdc I need? Has anybody tried this?

Thanks!

 

[Whoops]

Right? If they sound garbage I'll make something out of them.

So I was looking for power supply options. I found the MNATS http://mnats.net/adjustable+phantom_power_supply.html
It has a+24v rail and a +48v rail.  Would it be possible to use the +24v rail as the -25vdc I need? Has anybody tried this?

Thanks!

Yes you can use that, there's an LM317 regulator for that rail, the LM317 is an adjustable regulator you can adjust it to the voltage you want (between the limits of the regulator itself and the input voltage), you use the trimmer to set the voltage to 25VDC

 

[RSRecords]

Yes you can use that, there's an LM317 regulator for that rail, the LM317 is an adjustable regulator you can adjust it to the voltage you want (between the limits of the regulator itself and the input voltage), you use the trimmer to set the voltage to 25VDC

Ok cool. That's what I was thinking. My concern was with the 48vs common being the negative 25v rail.

 

[abbey road d enfer]

Ok cool. That's what I was thinking. My concern was with the 48vs common being the negative 25v rail.

So the actual layout does not match the schemo. My first thought was that there was an incompatibility between a +24/+48 PSU and the need for -24/+48 rails, and my fears were justified.
You'll need some serious reworking of the PCB in order to isolate the two different sections.

 

[Whoops]

Oh I'm really sorry for bringing some confusion

I missed the it was a negative 25V feed

I though you needed +25V

If you need -25V then better/easier to use a PSU pcb that is designed for adjustable positive and negative rails with an LM337 for the negative rail.
You just use that rail and dont connect the positive rail, you can check the schematic and leave the positive rail components out, we can help you on that

You can use any of the JLM audio PSUs for that,
Im sure theres also some offerings for similar psu's in the white market

 

[emrr]

You can also buy many off the shelf supplies.  The International Power modules are good in linear types. 

 

[RSRecords]

Oh I'm really sorry for bringing some confusion

I missed the it was a negative 25V feed

I though you needed +25V

If you need -25V then better/easier to use a PSU pcb that is design for adjustable positive and negative rails with an LM337 for the negative rail.
You just use that rail and dont connect the the positive rail, you can check the schematic and leave the positive rail components out, we can help you on that

You can use any of the JLM audio PSUs for that,
Im sure theres also some oferings for similar psu's in the white market

Right, that was kind of my back up plan. I've used both Five-fish and JLM with great results. I'll probably just go that route.

You can also buy many off the shelf supplies.  The International Power modules are good in linear types. 

The ones I've used (condor, power one) are probably overkill for my needs. Though they would probably fit, but I'd have to get a 24 and run it at max. 25 and a another 48v supply with chassis and ac that goes with it.

 

[Whoops]

you can use also this one from Gustav, get the +Version with the doubler for the 48V rail:

https://pcbgrinder.com/POWER-SUPPLY-PSU-LM317-LM337

 

[RSRecords]

I went ahead and picked up the Fivefish audio 2448. Little more expensive, but I've used him before, he's local and has really great customer service.

 

[RSRecords]

I'm ordering parts for a recap . Should I replace the carbon comp resistors too?