Re: 1646 in a Distribution Amp
Posted: Fri May 30, 2008 1:40 pm
Thanks for joining us.
I may have to give you some short answers since I'm under a bit of a deadline.
1) I would always feed a THAT1646 in that configuration directly from an op amp output. Any source impedance >100 ohm (and ideally <<100 ohm) will increase noise an decrease output balance.
2) I think keeping it unity gain bal in to bal out is probably the best option. If you were a radio station or TV plant you'd need trims. But you're "rebalancing" and for simplicity's sake just have the 1246/2143 feed the 1646s. But, realize that the input impedance of the 1646 is pretty low around 4K minimum. You really need a 5534-ish buffer between the 1646 and the 1246. So you do have an opportunity for an overall trim. You might want to make it inverting and then redefine the input polarity to keep it non-inverting.
3) The RCA ouputs I need to think about. If you take an unbalanced output off the 1646 you have to tie the opposing output to ground. One to get the +6 dB gain and also - this is very important - to keep the output noise down. An SSM, DRV or THAT line driver will all increase noise if an unbalanced load is not connected this way. The differential output noise on these parts is a lot lower than the common mode noise. This is due to CCOS SSM/DRV and OutSmarts. If the opposing output is left to float, the common mode noise will be converted to differential.
You might want to use a second 5534 or 1/2 5532 for the RCA output and give that one it's own level trim.
I guess they weren't so short after all. But it may be awhile before I can get back to you.
I may have to give you some short answers since I'm under a bit of a deadline.
1) I would always feed a THAT1646 in that configuration directly from an op amp output. Any source impedance >100 ohm (and ideally <<100 ohm) will increase noise an decrease output balance.
2) I think keeping it unity gain bal in to bal out is probably the best option. If you were a radio station or TV plant you'd need trims. But you're "rebalancing" and for simplicity's sake just have the 1246/2143 feed the 1646s. But, realize that the input impedance of the 1646 is pretty low around 4K minimum. You really need a 5534-ish buffer between the 1646 and the 1246. So you do have an opportunity for an overall trim. You might want to make it inverting and then redefine the input polarity to keep it non-inverting.
3) The RCA ouputs I need to think about. If you take an unbalanced output off the 1646 you have to tie the opposing output to ground. One to get the +6 dB gain and also - this is very important - to keep the output noise down. An SSM, DRV or THAT line driver will all increase noise if an unbalanced load is not connected this way. The differential output noise on these parts is a lot lower than the common mode noise. This is due to CCOS SSM/DRV and OutSmarts. If the opposing output is left to float, the common mode noise will be converted to differential.
You might want to use a second 5534 or 1/2 5532 for the RCA output and give that one it's own level trim.
I guess they weren't so short after all. But it may be awhile before I can get back to you.