Lab series L5 with DI

[omnia:arts]

Greetings
First time poster, long time lurker. Okay so I've built several of the Aion FX recreations of the Lab Series in 2u racks with SS power amps as my colleagues and I really liked them. I am currently putting one in a pedal and want to incorporate an active DI.
I would love some suggestions on this.  I am going to make it switchable and would like to use op amps.
Would I need to be worrying about gain staging? I know the Bo Hansen active DI is a big favorite but would like to keep it more compact as well as save on some costs. Is there a schematic anyone knows of?
Could I just use a buffered unbalanced to balanced circuit?
Thanks in advance
Dylan

[PRR]

Welcome!

... ...recreations of the Lab Series  ... want to incorporate an active DI. ....

Why?

A DI is designed to do minimal loading on an electric guitar. Your Lab Series Preamp is much stranger than e-guitar, loading is not an issue.

[omnia:arts]

Thank you for replying sir,
I was planning on running an acoustic threw it, with one unbalanced going to an amp and another to the Venue's system/mixer.  Possibly adding it to a mic'ed signal while in the studio. I should have said so, my apologies.

[MikeA]

You can just add an unbalanced-to-balanced circuit at the output of the L5 Pre.  One unity gain opamp buffer to pin 2 of your XLR out, a unity gain inverter opamp to pin 3, and pin 1 to ground.  You will probably want to power them with +/-15V to provide sufficient headroom since that's what's going on upstream. 

I think Paul's point was that a DI, used alone, reduces the load on a guitar pickup, which is not needed here since the input buffer of the L5 Pre has a sufficient input impedance to do that for you.

Speaking of loading, an acoustic guitar with a magnetic pickup should be fine into the L5 Pre, but you may want a higher input Z if it's a piezo pickup.

[omnia:arts]

Thank you Mike and Paul,
That is what I thought but doubted myself. So if I were to use a transformer, albeit doubtful but I am curious) do you think I should buffer it prior to the xfmr or just run it from the L5 output.
And seriously guys,
Thanks

[omnia:arts]

And to add the L5 runs on an internal +/-15v so I think I'll bypass that and build a separate psu to use for both circuits.

[MikeA]

I think you can just tap off the output of the L5, since Q1 (2N5457) is already in place as a buffer.  You can see a similar layout in the Intersound IVP which I've been working on recently.  A closeup of the unbalanced and balanced output section is below.  Top jack (Main) is normal unbalanced out, middle 3-pin is balanced out, bottom jack is low-level unbalanced out (-10 dB I believe.)  You can see the transformer is just tapped onto the main output through voltage divider.   

[omnia:arts]

Mike,

That's awesome. Curious do you know what xfmr was used for the IVP? A google search didn't give me much of anything.

[MikeA]

<EDIT: this is the info for the Tube Voice xformer, not the output xformer that the OP was asking about!  Mike>

The original was marked ""80TM018 HONGKONG" which no one can quite identify.  However, there's a current model that may be very close, it's what I use, 42TM018-RC by Xicon, 10k to 10k with a center tap, 550 ohm and 500 ohm.  Available at Mouser. 

There are related threads here:

https://www.diystompboxes.com/smfforum/index.php?topic=119131
https://www.diystompboxes.com/smfforum/index.php?topic=119132.0
https://www.diystompboxes.com/smfforum/index.php?topic=125723.0
https://www.diystompboxes.com/smfforum/index.php?topic=78881.0
https://www.diystompboxes.com/smfforum/index.php?topic=38033.0

Resources:  https://projectivp.wordpress.com/
PCB for the Tube Voice side, does not include Clean or 4-band EQ:  https://aionfx.com/project/isotope-amp-overdrive/

[PRR]

> "80TM018 HONGKONG" .... 42TM018-RC by Xicon

These pocket-radio transformers were standardized long long ago (before Xicon). The "80" is the core size. The "018" is the winding impedance. The 42TM018 is rated 300mW at 300Hz, 45V.... transposed to 80Hz that would be 12V which is much more than the PA console needs.

Feed it from a low impedance, <<10k. Problem: the LAB ends with a 2.7k resistor because it wasn't expected to run across the stage, much less to the goalposts. I think it wants a buffer; or because the soundperson is kinda expecting a MIC level, a heavy attenuator (22k:470) and a 600:600 (42TM009).

[MikeA]

My apologies, I was describing the Tube Voice transformer in the IVP, not the output transformer you were asking about.  The output transformer is marked "T-1562" and is described as a balancing transformer, rated for 8V into 600 ohms.  I'd go with Paul's analysis above on this.

[omnia:arts]

Thank you both. Really great information here.