my new mic pre-comments please

Hey Everybody,

I don't post here often and when I do it's ususally to ask some basic questions. I have learned so much from this forum, ever since it was on that "other" board.

I have been quietly building N e v e clones, A P I clones, and power supplys over tha last year or so. As my confidence grew I decided to design my own mic pre.

It started out as a transformer input and output using the Neutrik NTM4 & NTL1 respectively, with the OPA604 as the active gain stage. This has mutated somewhat to include pads for 3 different input transformers: NTM4, LL1587,or LL1583 and 5 different output options: NTL1, LL2811, LL1582, OLI3 (unbalanced), or electronically ballanced using a Burr Brown DRV134 on a daughter board that uses the LL2811 footprint.

I have also added a 2520 footprint if a DOA is desired.

It runs on a bipolar 18v supply that also supplies 48v phantom. All I/O, power, and ground connections are on a 15 pin .100 center card edge connector. Once the pre is plugged into the card connector the phantom power curcuit is complete. I put the 6k8 resistors and a cap on the connector pcb because the preamp pcb was getting crowded.

I'm still working on the enclosure but it is going to be a rackmount 2U high and I will be able to fit 12 of these pre's vertically.

Here is the schematic and pcb for the pre:

PCB
Schematic

Any and all comments are welcome.

thanks again to everybody on this board.

cheers,

James Lee
Big Ugly Van Audio

It is a long way from the ouput all the way around the board to the rear ... and past the input trafo.
... could be ok
I guess you saw my one that put the input at the front anf the output at the back. The Mic comesin on sheilded cable to a front switch board for phantom and pad and then drops to the PCB away from outputs and power.

Look to Joe's schems and you may find he has a resistor option from op-amp output to OP trafo. You have the Cap (a good thing) Some Op-amps don't come up cleanly from turn on and can self destruct.

I've only had a quick look at your output trafo config ...step up ??
Many people like to make use of the multiple outputs of the original.

You have dropped the diode protection ... being on a card I guess you could say it is redundant ... even though the original was also on a card. :roll:

The resistor inductor option on the output in place of the trafo (did I get that right) is very cool and interesting.
I didn't go that way cos I was thinking OFF board trafo (big)

Generally speaking ... neat and tidy ... but I'm sure your version two will be different.

We love our DIY
:thumb:

thanks Kev,

Yeah, I know it's a long way from the out trafo to the card edge. It couldn't be helped because the polarity switch is on the output at the front of the card. in re: the resistor on the output, I'll assume you mean in parallel with the output cap. This was mentioned to me also by Avedis @ B. Averill's place. What would be the benefit of it?

I have 2 prototypes running now that don't have the output cap. They're using OPA604 for gain and the BB DRV134 for balanced out. I figured I didn't need the cap because the 604's have such a low DC offset, was I wrong?

As for the output trafo, yes the schematic shows a step up of 1 to 2 but this only applies to the Lundahls. The Neutrik is strictly 1 to 1. Also if the electronically balanced daughterboard is used it gives an additional 6db of gain, the same as the Lundahls.

Thanks,

James

The ouptut cap and resistor may be more for the Descrete op-amps than the likes of the OPA604 and the BB DRV134.

Offers some protection to 2520, 990, 99v and 1731 ... etc
AC couple to the ouput trafo and to limit some current in the event of a short to ground sort of stuff.

If there is option on the board you can chose to use ... or to not.