Custom Active Studio DI...

[pucho812]

Well in going through the Studio storage. I found a DI box that the owner is swearing is the best DI box ever.  That people in the past booked the studio at the old location and raved about this DI box.  While I can't confirm that, I can confirm I have it working and it sounds great and is a completely different design then the passive Westlake boxes we have.

I have not traced a schematic yet but here are some basic design notes from a first look.  it runs off a single 12VDC supply that splits it to +/-6VDC. It outputs  balanced level, not mic level but not really line level  it is amplifying the signal so the mic pre can be lower in level.  It's centered around a single 5534 and has a 6 position gain switch on it.  It's a rats nest inside but will get cleaned up and traced out  down the road.  I'll have to make some recordings, asap...

 

[JohnRoberts]

I've used the tl07x bifet op amps for active DI, with good results.

The bipolar 5534 is not as happy at very high impedance. Nor does the 5534 do unity gain unless using additional external compensation cap. With only 12v rail, and gain (+10dB min for stability without external compensation) could be marginal for headroom from hot guitar pickups.

DI generally need to scrub off a bunch of gain not add it to interface with mic inputs from MI sources.

Of course if it sounds good, it is good, just not optimal IMO.  8)

JR 

 

[pucho812]

Oh I would agree, especially on the Ω part of the show. However I can attest it sounds pleasing to our ear. 

Perhaps in the future I can make a version that runs off phantom power and a DC-DC converter?

 

[Gus]

You should be able  to run it from phantom without a DC to DC

You can often adjust circuits to run from one supply

 

[pucho812]

You should be able  to run it from phantom without a DC to DC

You can often adjust circuits to run from one supply

true  it should be possible.  let me trace the schematic as is and go from there....

 

[JohnRoberts]

true  it should be possible.  let me trace the schematic as is and go from there....

Yes, more than enough voltage/current  from phantom to power a TL07x...(thats what I did).  553x uses a little more current but no need for a dedicated PS or batteries if you have phantom available.

JR

 

[pucho812]

Yes, more than enough voltage/current  from phantom to power a TL07x...(thats what I did).  553x uses a little more current but no need for a dedicated PS or batteries if you have phantom available.

JR

well as it was built V1 ran off a 12V wall wort.  I just found it and got it back to running again. 

 

[abbey road d enfer]

Yes, more than enough voltage/current  from phantom to power a TL07x...(thats what I did).  553x uses a little more current but no need for a dedicated PS or batteries if you have phantom available.

JR

My first DI box would work only with TL061 or LM308. TL071 wouldn't do, not to mention 5534.
After some headscratching, I found out the API mixer had 10k resistors before the 6.81k, instead of the specified 100r's. Factory mistake. Worked on the Neve, though.
For production I used TL071 then TL072.
Today I use OPA2132, about 10dB quieter than TL07x.

 

[RuudNL]

Here is a design of a very simple phantom powered active DI box.

 

[Bo Deadly]

Here is a design of a very simple phantom powered active DI box.

Bad circuit. Tascam problem and hot and cold appear to be reversed. Phantom power looks sketchy.

I would think a little class A discrete with OT would work well. A little 4:1 or 8:1 step down so that limited phantom power can drive the line properly.

 

[abbey road d enfer]

Bad circuit. Tascam problem

This is debatable. I agree that faced with an unbalanced input, one of the opamps would distort heavily because seeing only 100r.
I built and sold about 20 000 DI boxes that used a very similar circuit, although the output res were 680r, which made a difference. However, the probability of someone connecting a DI box to an unbalanced input is very small.
The most unwashed user knows that a DI box is to be connected to a balanced mic input.

and hot and cold appear to be reversed.

Check te date, 1985. Pin 3 hot was era-correct.

Phantom power looks sketchy.

It works, though. The only issue is that the output caps are reverse-biased when there is no phantom. In practice, aluminium electrolytics are quite tolerant to this situation.

I would think a little class A discrete with OT would work well. A little 4:1 or 8:1 step down so that limited phantom power can drive the line properly.

This DI's BOM is about $10. Including a half-decent xfmr would push it north of $20. Not an issue for DIY, but in manudfacturing, where every dime spent on the BOM costs a $ to the customer, it is.