plumsolly
Well-known member
Ok, so Im building what is essentially a 312 type preamp. I want to put attenuators after the output transformer so I can drive it and still get good levels to my converters. I've been reading about pads at http://www.uneeda-audio.com/pads/ and this thread: http://groupdiy.com/index.php?topic...tenuator&sid=3808b232696eaaa1a8c96655ff338487
and looking at these schematics:
http://twin-x.com/groupdiy/displayimage.php?album=random&cat=10184&pos=-1683
and http://www.jlmaudio.com/PAD.gif
Mine will be at line level and impedance so it will be slightly different than these. I plan on using a U-pad. On the Uneeda-Audio page it talks about trying to mimic the impedances present before the pad:
my first question is: Should I consider the output impedance of the preamp 600 ohms because it comes off of a 600:600 transformer? And what should I consider the input impedance of the next device to be? Obviously it won't always be connected to the same device, but what might be a good average. The input impedance of my converters is 10K.
second question: I cannot reconcile the JLM U-pad schematic with the formula from Uneeda-Audio: for -10db on the JLM schematic he's got 270R for the shunt resistor and 470R for the series resistors. according to the Uneeda page, for a 270R shunt resistor, the formula should be:
Rseries = 270* (3.16 - 1)
or 583.2R total (291.6R for each resistor)
What am I doing wrong?
Many thanks in advance, Ben
and looking at these schematics:
http://twin-x.com/groupdiy/displayimage.php?album=random&cat=10184&pos=-1683
and http://www.jlmaudio.com/PAD.gif
Mine will be at line level and impedance so it will be slightly different than these. I plan on using a U-pad. On the Uneeda-Audio page it talks about trying to mimic the impedances present before the pad:
my first question is: Should I consider the output impedance of the preamp 600 ohms because it comes off of a 600:600 transformer? And what should I consider the input impedance of the next device to be? Obviously it won't always be connected to the same device, but what might be a good average. The input impedance of my converters is 10K.
second question: I cannot reconcile the JLM U-pad schematic with the formula from Uneeda-Audio: for -10db on the JLM schematic he's got 270R for the shunt resistor and 470R for the series resistors. according to the Uneeda page, for a 270R shunt resistor, the formula should be:
Rseries = 270* (3.16 - 1)
or 583.2R total (291.6R for each resistor)
What am I doing wrong?
Many thanks in advance, Ben
