EZ-DO ---> EZ-Direct out PCB

A little PCB I did for all those times someone wants a direct out on an old mixer. Did one old Yamaha for a friend and used this same circuit on perfboard, which is OK for a couple of channels, but after a dozen I decided to make the PCB and simplify my life.  Posted on the Black Market to sell some extras, and told buyers I would post the schematic here.  Anyway, pictures worth a thousand words.  Really simple circuit actually, non-inverting opamp, can be set up as a unity buffer or with some gain. Impedance balanced output, and little SMD pads if you want to connect TRS sleeve to circuit ground. I usually don't. 

Schematic (sorry it's kinda blurry, I can never get eagle to export decent pics without trying ten different settings.. There are no values in anyway, so I think it's self explanatory anyway).



I usually jumper the series input resistor, but you could use those pads to put in a load R if the circuit needs one (as a makeup amp for example)

I've made a few other "utility" PCBs over time, and I'll be posting extras in the BM thread as I find them.

http://groupdiy.com/index.php?topic=59028.msg749994#msg749994

Pic of PCB


Pic of assembled PCB

Someone PM'd me for a BOM.  Since everything is case specific, you'll have to figure out what you want to put in, but here is what I generally use for direct out use.

TRS Jack is REAN/Neutrik NYS216
One DIP8 socket
One DIP 8 single opamp (pick your favorite, last one I used was TL071, worked fine even though people "hate" them).  LME49710 should be "better."
2x 0.1uF ceramic caps, X7R type (C4, C5)
1x 10-22uF electrolytic input cap (C1)
1x 100uf or higher electrolytic cap (C3)
2x MATCHED 1/4W 47-100 Ohm resistors (R3, R4)
Gainset resistors, 1/4W (R2, R5)
1x 68-100pf ceramic C0G capacitor (C2)

J2 is a pair of SMD pads towards the front of the PCB. A solder blob across these will connect PCB ground and TRS sleeve. I normally don't do that but it's there if you want to.

R1 is usually a jumper

R2, R5 set the gain, C2 limits the bandwidth on the top end.

For unity: R2 = 0 Ohms (jumper) other two are not installed
For higher than unity gain, I use 10K for R2, and vary R5 for the gain I want. C2 creates a LPF is 68-100pF

You may notice in the picture C2 is not installed, it's not because I want super ultra mega bandwidth out of my TL071, but because I forgot to order some. 

In your schematic, The +ve input of the op amp is floating. It needs some means of getting its dc bias.

Cheers

Ian

You are right! Crap, that slipped by me.  Thanks for pointing that out. 

I used to build these on perfboard, and there was something like a 100K there.  I wonder, since I got the PCBs I've only used FET input opamps... maybe that's why I have not had any problems?  I will try to swap a BJT into one to see.

The good news is, There's an easy fix.  I will take a look tomorrow and post a pic.

I was about to point the same thing... you could just make the advice "use a leaky cap here!". Or just shorting the cap out and getting that DC from the source.

JS

I have some old leaky caps, anyone want some?  Anyway, I really should have seen this, I ordered 3 different PCBs, and exported all the gerbers the same night to get them all shipped together.  :

But yes, good news is that removing the input cap and installing a jumper, is one way to do it,  and for cap coupling you can use the pad from R1, which is normally a jumper. 

Everyone who ordered gets some extra PCBs for this. I knew there was a reason I didn't make it to the post office Friday.

I promised I'd post a pic of how to install the bias resistor when needed.  It probably should always be installed, but not installing may not cause a problem in some cases.    Put a 100K resistor between the ground pin of the inside set of power pins and R1, going into the pad closest to the power pins.  Thread that lead to the other pad of R1.  See attached pic. 

I should repeat that I gave up on daisy-chaining power from one Ez-DO to the other, because I couldn't handle the Molex crimping.  If you still want to daisy-chain power, then I would install this resistor under the PCB.