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There are dB and then there are dB - Amertran data sheet curiosity




 
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Author Topic: There are dB and then there are dB - Amertran data sheet curiosity  (Read 354 times)
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Patrick J. / KD5OEI
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« on: January 20, 2018, 01:36:27 AM »

The attached data is from an old Amertran catalog. Bulletin 1002B, 1935.

In the left column, wattages are shown. In the center DB are shown as '+24', +'37'. etc..
These are basic 500 Ohm line to various secondary speaker or other line impedances.
I take it that the Wattages are the actual handling ability.
Because they specify Ohms-to- Ohms, I do not believe these are for 'constant voltage' line to speaker' service. Did those exist in 1935?

in my table below, the leftmost two columns are from the data sheet. The rightmost two are what the watts and dBm should be, if translated from the data sheet values. It does not match. I'm sure I am missing some very simple thing.
- application Wattage - - Max. DB -but..       - the Wattage is dBm - - The DB are Watts -
4.5+2836.50.63
15+2441.80.25
30+3744.85.0
30+3744.85.0
15+2441.80.25

What is to be made of this? How has a 4.5W transformer more DB than a 15W transformer?


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« Reply #1 on: January 20, 2018, 08:50:00 AM »

There was different dB level-to-wattage data that was in use when that xfmr was made.  I don't have the data handy right now but research it.  IIRC it has something to do with the way dB is stated.  Just "dB or "dBm" on the xfmr rating plate.

Fred

OK I think I missed what you were asking.  I see what you were questioning.  The first two xfmrs on the sheet.  First one is +28dB and only rated at 4.5watts.  Second one is +24dB and rated at 15watts.

Good question,  don't know except maybe it has something to do with frequency range.

Looking at the specs again,  seems the only difference would be the possibility of DC current on the primary.
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« Reply #2 on: January 20, 2018, 02:26:30 PM »

  The "Radio" Handbook, Sixth Edition (1939), on page 233 (the section on microphones and speech amps) states that for audio the 0 dB reference level is 6 mW.  Working backwards on the transformer numbers that level indeed appears to be what they used to convert watts to dB.

  I have never seen anything written about when that reference level was used or when it changed to today's common 1 mW.

  I thought you might find this tidbit interesting.

  Dean
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« Reply #3 on: January 20, 2018, 02:34:37 PM »

 The "Radio" Handbook, Sixth Edition (1939), on page 233 (the section on microphones and speech amps) states that for audio the 0 dB reference level is 6 mW.  Working backwards on the transformer numbers that level indeed appears to be what they used to convert watts to dB.

  I have never seen anything written about when that reference level was used or when it changed to today's common 1 mW.

  I thought you might find this tidbit interesting.

  Dean

Dean,  You can tell what dB scales were used by the way the rating is stated.  Just "dB" (500ohms) indicates the old reference,  "dBm" (600ohms) indicates the new reference.

Fred.
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« Reply #4 on: January 21, 2018, 06:58:39 PM »

Wow how about that! Thanks!  This might be handy for working with very old mikes that would have used that system. There's an ancient ribbon mike buried in a building, been looking for it.
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