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converting 70v line output to line level 600 ohm
Old 28th January 2013
  #1
Gear Maniac
 
JOliver's Avatar
 
🎧 10 years
converting 70v line output to line level 600 ohm

I just bought an old Altec mixer/amplifier which has a couple 1410 preamps mounted in it. I noticed that the output transformer for the unit has only 70v line, 16, 8, 4, and 0 ohm outputs and ground. What would be the best way to get a 600ohm line level output from this thing to my converters? I noticed there are some stepdown units by jensen, grommes etc. Is this the only way to do it? Can I take the straight 70v line out? thanks
-J

Here is the grommes I was looking into:
Accessories
Old 28th January 2013
  #2
Lives for gear
 
brianroth's Avatar
 
🎧 15 years
Out of curiosity, which model Altec mixer/amp?

As for "matching" the output of the amp, all you will need is a pad built from some resistors, along with a "dummy load" resistor across the amp's speaker output so that the amp "sees" a proper load resistance.

Bri
Old 28th January 2013
  #3
Gear Maniac
 
JOliver's Avatar
 
🎧 10 years
Thanks for the quick reply Brian. The altec mixer amps are the 1430s shown here http://www.vintageaudio.net/members/view.php?item=5
With the 1410 dual preamps installed.
Old 28th January 2013
  #4
Lives for gear
 
🎧 15 years
Hi
You need a 'load' of some sort so that should be something like 150 Ohms 100 watt resistor across the 70 Volt tapping to ground. (you don't need to load it to the maximum rated output, just a decent proportion).
To get a 'line level' output you can use the 0-4 or 0-8 volt tappings which will be about the right voltage. You may wish to 'pad' it out to 600 Ohms in which case a pair of 270 Ohms, one on each 'leg' will be close enough.
The use of the 6AL5 looks interesting, a form of limiting (soft voltage clipping) I presume. I have not examined the circuit properly as it is a bit small and dark.
Matt S
Old 28th January 2013
  #5
Gear Maniac
 
JOliver's Avatar
 
🎧 10 years
Thanks Matt, but I am still a bit confused... so you are saying as long as I have a dummy load on the amps 70v line I can use the regular 8 or 4 ohm speaker outs to a line level input?
Old 28th January 2013
  #6
Lives for gear
 
Richard Crowley's Avatar
 
🎧 10 years
Yes. The load allows the power amplifier to operate within its design specifications. The 4 or 8 ohm speaker output voltage is approximately equivalent to "line level". If you also really need "600 ohms" (not much modern equipment cares about source impedance), then you can put in the 270 or 300 ohm resistors on either side to create 600 ohm source impedance as Mr. Syson suggested.

The 70V output is around 100x to high for "line level" and would NEVER be appropriate to connect to anything but a 70V speaker transformer. Using a step-down transformer may be one solution, but finding one that will handle that particular source voltage will be difficult to find and likely expensive. Using the 4 or 8 ohm output is that rare triple-play of easy, cheap and fast.
Old 28th January 2013
  #7
Lives for gear
 
🎧 15 years
Hi
A dummy load SHOULD be applied for 'safety' (avoiding killing the output transformer if overdriven with no load) but it only needs to be about half of the nominal output power. If easier to find you could load any of the tappings.
Matt S
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