THAT 1646 unbalanced to balanced line driver

Some time ago I started a thread here to design a power supply. I will order the pcb's somwhere next week and do some prototyping. However a power supply needs something to power. I made it primarily to be used with an unbalanced to balanced line driver board. I have also made a design of that board and I'd like to hear your thoughts on it. It isn't exactly headphone related as it will be used to send an unbalanced signal out of a mixing console to some powered dynacord monitors. The length of the cable is about 20m. Right now it is used with an unbalanced cable but there is quite a bit of hum, hiss and cellphone noise.

The circuit consists of an OPA227 at unity gain driving a THAT 1646. The OPA227 can be replaced with the OPA627, but only for small channel counts or by those with slightly to much money. the connector is an amphenol ACJS-MHD double trs. I couldn't find an eagle library for it so I made one myself. 

It's a bit hard to see but the trace to pin 12 on the connector isn't connected to the ground plane

The THAT1510 is a preamplifier. It is generaly used as a mic preamp. It takes a balanced signal and amplifies it up to 60dB. So I think it isn't really usable in this situation.

edit:

And it's exactly the wrong way round. it has balanced inputs and unbalanced out. If that were the aim, I would use a THAT1246. Which is a balanced line reciever.

No problem, just check the datasheet and you'll see 

U1 is there because it is stated in the data sheet that it should preferably be driven directly from an opamp's output to maintain specified performance (page six in bold). Several articles I have read indicate that they actually mean it and that when it isn't used in this manner it tends to be very noisy. Will add the resistor tonight, a large dc offset would be bad .

An opamp, but with at least a meter of cable between them. I think the key word is directly ie not more than a few cm. 

You are never a pain in the *ss , and you are right(as always). I've done a bit more reading and U1 isn't neccesary and could be ommited. But I like the piece of mind it gives. You can connect any piece of gear no matter what output impedance it has. So I think I'm going to leave it in anyway. 

Quote:

Originally Posted by headphoned 

 

I'd already done that much before I saw this thread. ;) You're such a modest guy, are you.
 

Always 

Adding a gain option is possible but it would add some resistors and jumpers/switches. The unbalanced to balanced conversion in the that1646 already gives you 6dB of gain and I don't need any more since it is just a line driver. I could add some test points/pads so that gain resistors can be added in. 

edit:

How does one put components, or at least silk screen, on the bottom of the board? Or just remove the silkscreen of a single component? I could put 2 resistors on the bottom of the board and you would leave them both out for unity gain or cut one trace and put them in for gain. 

I changed the board layout to include a 100k resistor from input to ground and 2 optional resistors to set gain. The gain resistors are on the bottom of the pcb.

If you don't want any gain the resistors can be left out and if you do want gain you need to cut one trace: