Help identifying gear please

[sonolink]

Can someone help me identifying this unit please?

Cheers
Sono

 

[JohnRoberts]

the OA20 may be a DOA, with input and output transformers. Could be a mic preamp from a mic company?Just my guess, but I am not the mic guy around here.

JR

 

[mad.ax]

OA20 is indeed a DOA, and its famous maker is written next to the OA20 ref...
As for the board it's a Neumann V472...

 

[TwentyTrees]

Mad.ax is 100% correct, it's a Neumann V472/2. Two channel line amp with Haufe transformers, max gain 46dB if the input transformer is strapped 1:2, though can be modified for additional gain at cost of increased distortion and decreased bandwidth. Makes a lovely medium-gain mic preamp, similar sort of topology to API but (to my ears) a clearer, faster sound. Datasheet and schematic are on Kubarth's site:

http://audio.kubarth.com/rundfunk/index.cgi

 

[sonolink]

Thanks a lot for your help!!
Cheers
Sono

 

[sonolink]

I apologize for my ignorance but what does RGK stand for? "Resistor Gain Knob"?

 

[TwentyTrees]

RGk is the gain set resistor - not sure what it actually stands for as it's in German (which I don't speak, beyond a few useful terms like "klirrfaktor" and "eingangsscheinwiderstand"), but the values to use are set out on the datasheet, with the "46" column being for 1:2 input transformer connection and the "40" column being for 1:1. Choose the values you want, calculate the differences between the values, get the appropriate value resistors, plonk them on a rotary switch in a resistor ladder config, and boom = gain switch!

 

[rock soderstrom]

RGk is the gain set resistor - not sure what it actually stands for as it's in German

It stands for Resistor Gegenkopplung (=negative Feedback), so gain set resistor is correct. +1 for your useful german terms list.

 

[TwentyTrees]

It stands for Resistor Gegenkopplung

Excellent, thanks! That's one more for the list.

 

[rock soderstrom]

To be precise, we would call the resistor Widerstand, although we use the international abbreviation R.
Gegenkopplungswiderstand it would then be called in its full glory

 

[sonolink]

Hey thanks a lot for the explanation (and the vocabulary ). So I guess I can use something like a Grayhill, and decide what gain setting I get at each position and apply the correct resistors according to the sheet, right,. something like the pic?

Cheers
Sono

 

[TwentyTrees]

So I guess I can use something like a Grayhill, and decide what gain setting I get at each position and apply the correct resistors according to the sheet

Pretty much. You want the total resistance at your given switch position to be the value indicated in the table, so you can either wire those resistors in place connected to a common output node, or do the switch as a resistor ladder and calculate the values so that each step had a cumulative resistance value as per the table. Hope that makes sense!

I find the latter easier to actually solder up, though it takes a bit more calculation upfront.

Alternatively, a 10k pot would give a gain range of +6 to +40dB / +12 to +46dB. You'd probably want to experiment with taper to get a good range of control though.

 

[sonolink]

Mmmm ok I'll look into all that
Thanks a lot for your help!
Cheers
Sono

 

[TwentyTrees]

Mmmm ok I'll look into all that
Thanks a lot for your help!
Cheers
Sono

You're welcome! I'm a big fan of the V472, in case that didn't come across.

 

[sonolink]

Hello again guys,

I first would probably want to test this thing and check it works and sounds ok.

So for testing purposes, feeding it with 24vdc, -24vdc, using a 10k pot and in and out xlrs would be ok, right?

Cheers
Sono

 

[mad.ax]

Mmm... Looking at the V472/2 schemo, I wonder where you would put the -24vdc?

 

[TwentyTrees]

No negative rail, +24V and 0V only for power.

 

[sonolink]

Ooops sorry I was looking at the wrong schematic (Tab v372)

So +24, 0v and a 10k pot.
Excellent!!
Thanks!
Cheers
Sono

 

[sonolink]

Does anybody know the aprox current draw of this thing? 60mA? 100?
Would a 240v/24v, 10VA trafo do?

Cheers
Sono

 

[TwentyTrees]

From the datasheet, 100mA max across the unit (ie both channels putting out +22dB into 300 Ohms). In my builds I tend to use 2x15V / 30VA toroids paired with a Silent Arts FP48PSU board, if that helps, though that's with relays and LEDs and such as well.

On a side note, I was intrigued by your note about a bipolar V372 above. Having worked on a few TAB V372 modules, I've never seen one with a negative power rail - as with most (all?) German broadcast modules I've seen, they were all +24V. Can you share a link? Thanks!