turning a tube DI into a mic preamp (?)

[kato]

I have an Alembic FX-1 tube DI designed for bass. It's modeled after some old Fender circuit - maybe the bassman (?) I can't remember.

Anyway, years ago I had to multitrack with a paucity of gear and used it as a mic preamp by plugging in a mic with an Archer inline transformer (from Radio Shack.) Listening to those tracks today, I'm impressed with the sound.

Should I modify it for mic level input?
What transformer would be best? Any ideas?

Kato

 

[NewYorkDave]

Well, I think you already answered your first question. You liked the way it sounded as a mic preamp, so why not go for it?

It's definitely a "color" box insofar as the Fender tone stack will not give you a flat response, but that built-in mid dip sounds pleasant on a lot of material, as you've found.

Any decent mic input transformer should work well. Look for something with about a 1:10 turns ratio. That's probably about what your inline transformer used. If you liked the sound with the inline transformer, you could even take that same transformer, remove it from its cylindrical case, and house it in a small steel box mounted on the chassis of the preamp.

The first stage only has about a volt of negative bias, though, so hot signals will overload it. I would recommend including switchable pad(s).

I haven't seen a schematic of the FX-1, but if it's nearly the same as this pre...
http://www.geofex.com/FX_images/alembpre.gif
...then the output is at a rather high impedance and will need to be buffered if you want to use it as a mic preamp. There are many options for buffering.

 

[kato]

Thanks NYDave!
My hunch was there'd be no room on the front panel for an XLR jack. Then after I posted, I realized, "Hey I've owned this thing for 10 years and I've never even opened it?" I had a mental image of what the inside looked like and I was way off...

It's a very heavy, thick enclosure. A lot less inside than I imagined...

Pleasantly surprised by the clean layout.

The red connector at the left is from the input jacks. I'm thinking I can hook the input transformer up right there but I'm not sure if it needs to interface with the circuit anywhere else. Can I just give it the +/- from the transformer?

Here's the tube. It looks socketed as far as I can tell. I was afraid to pull too hard and find out the hard way...

Looks like some sort of output transformer (at the right).

The power supply and crossover board.

 

[Test Point]

I have been looking for years for a schematic for the FX-1 and have drawn a blank...I have built several F2-B's for friends and one for myself (they are basically a Fender Bandmaster front end) but there are some op amps in this pre amp that aren't in the F2. If anyone has a schematic of the FX-1, I would love to get a copy of it! BTW, I just got a Peavey Max Bass preamp and so far, it sounds pretty nice. It's dual channel, one tube and one solid state and the channels are blendable as well...
Cheers,
TP

 

[clintrubber]

from NYD:

It's definitely a "color" box insofar as the Fender tone stack will not give you a flat response, but that built-in mid dip sounds pleasant on a lot of material, as you've found.

FWIW, I've seen an overview of tonestack-settings in some book and according to that book (I haven't verified it) 'the' stack is more or less flat at L-,M+,H- settings (- & + indicating fully CCW & CW settings). There's of course a lot of attenuation.

Bye,
Peter

 

[kato]

[quote author="clintrubber"]from NYD:
FWIW, I've seen an overview of tonestack-settings in some book and according to that book (I haven't verified it) 'the' stack is more or less flat at L-,M+,H- settings (- & + indicating fully CCW & CW settings). There's of course a lot of attenuation.
[/quote]

That's correct; the midrange control is "cut-only." It cannot boost.
The kind folks at Alembic say the nearest to "flat" this unit is capable of is with the tone controls set at Bass: 2, Mid: 10, Treble: 2.

TP - I am not skilled enough to make a schematic for a piece of gear this complex! I would otherwise.

What's a cheap 1:10 mic input transformer?
I was originally going to use a Jensen but I'm thinking that may be too expensive for this project. Perhaps a trashier sounding transformer would be in order. Something under $50 hopefully...

 

[clintrubber]

Nice thread, triggered by the info here I'll be trying a bit sadder version of it: a somethinglow:50k mic-transformer into a Peavey TBRaxx I still have around.

Bye,

Peter

 

[Test Point]

"TP - I am not skilled enough to make a schematic for a piece of gear this complex! I would otherwise. "

Kato...I appreciate the thought :grin: I was hoping that someone here might have one already that was stashed away somewhere....I do like the F2-b as I have a home made one myself but the internals of the the FX-1 intrigue me...I'm wondering if they are buffering the input with a FET input opamp and then buffering the output with a follower type opamp setup. I'm glad to see the pictures that you have though....


Cheers and bracing for another winter storm :evil:
TP