buffered direct outs in old mixer

[mitsos]

I've modded a yamaha m512 for direct outs, similar to a 508 in this thread:

http://www.groupdiy.com/index.php?topic=50521.0

The output is low (subtly veiled) and I'm afraid it probably won't drive loads too well. 

I want to buffer it and add some gain, and while I'm at it, might as well balance it. 

First I thought of doing a single opamp for gain, but as I have a ton of TL07x's I know the current drive wouldn't be sufficient in 600 Ohm situations (most likely this will go direct to an interface, but you never know, so just thinking ahead, opamps are cheap). 

So I thought TL072, one for gain, one for added current? 

Finally, I thought with the TL074 I could use the other 2 to balance the signal...

Anyone see any issues with this? Or an easier way that doesn't require any opamps I can't get locally (most well loved opamps are impossible to get here, even 5532/5534 I sometimes have trouble finding, hence the preference for TL07x)

I'll sketch up a schematic later tonight, but it's pretty basic stuff. 

thanks for the help.
cheers!

 

[JohnRoberts]

Driving 600 ohms seem an archaic requirement unless you have a bunch of legacy gear that requires that.

The larger issue IMO when grabbing a single ended feed from inside a console is getting a good low reference too.

I would suggest using at least a differential connected (4 resistors) opamp between the internal circuit node, and it's clean low reference, and the output jack referenced to chassis ground at the jack. An XLR with appropriate resistor to chassis from pin 3 would impedance balance the output. 

The tl07x will drive 2k total which should be enough for most modern gear input Z over modest cable distance. Adding a bunch of high current output drivers could consume PS budget not in the original plan.

JR

 

[outoftune]

not familiar with this circuit but if it has a bipolar PSU...have you considered the THAT 1646 balanced output driver?

 

[mitsos]

not familiar with this circuit but if it has a bipolar PSU...have you considered the THAT 1646 balanced output driver?

The psu is +\-25Vdc, the plan is/was to either further regulate to +\-16, or pull directly from the power trafo, rectify then regulate. TL07x only use a couple mA's per amp so so I dont think it will be a problem?

THAT stuff is nice but not available here. Would require importing and taxes are 100% when alls said and done. Plus 1-2 months waiting. So as much as i like them, they are off limits for now.

John, you're right, so forgetting about driving 600 ohms, would you say the diff amp would be better than a dual opamp balancer with one opamp driving the hot output and the negative in of the other opamp? I'd like to get at least 10dB of gain out of this as well.

Thanks for chiming in guys!

 

[JohnRoberts]

John, you're right, so forgetting about driving 600 ohms, would you say the diff amp would be better than a dual opamp balancer with one opamp driving the hot output and the negative in of the other opamp? I'd like to get at least 10dB of gain out of this as well.

Thanks for chiming in guys!

If you want some easy gain using two op amps with one a simple inverted version of the first,  will give you 2x or +6dB of voltage gain. Of course you can put gain in the differential amp too.

If you have +/-25v rails how hot are the internal audio signals that you will be buffering? You don't want to clip the buffer, before the internal path clips.

The dual opamp buys you some easy extra voltage gain/swing but will generally not outperform the one opamp differential for short length insert I/O. You also need to be careful not to short one side of the simple buffer with single ended wiring.

Note: If you can trust the balanced differential input of the external product you are sending signal to, the output buffer could be two inverting opamps grounded to the chassis at the output jack. The second inversion brings you back into correct polarity, and the ground errors will be common mode to the balanced output. i personally prefer to do the differential inside the console so even the single ended output signal is clean but YMMV. 

JR

 

[mitsos]

Hey John, thanks for this. haven't been online in a few days, but I'm going to give the diff amp a try.. For 4x gain, something like 5K for the series Rs and 20K for the others?  Something like the attached schemo?

 

[mitsos]

OK, I was about to put this thing in, and I'm changing my mind on one thing. I have the direct outputs wired with switching jacks, so that the bus works normally when there is nothing plugged in (idea came from thread linked above)... but now I realize I'd prefer to not have this, IOW, I'm going to try a simple non-inverting buffer tapped off the post-EQ and hope for the best.  This way, I get the direct out and bus simultaneously.

Any warnings/words of wisdom are welcome..

thanks!

 

[mitsos]

Just to post back, this works just fine.  I used a TL071 wired non-inverting with 2x gain.  Tapped the signal at the top of the FB pot, and I have a freaking hot ass signal at the direct out, plus FB and echo buses working normally.  Only change I'm going to make now is go to a unity gain buffer, (even less parts! ) because there is a bit too much gain as is.

thanks JR for the tips.

cheers.