audio transformer for DI box

[9 volts]

Hey there, I have a transformer from a microphone input (primary 5.5k secondary 55ohm) which I'm wondering if I could use as part of a DI output on my valve amp. Eg after preamp? Does anyone know or have a link about this stuff. first port of call was GEO but there wasn't anything there about it.....this was a first!

[d95err]

You can find lots of transformer based DI schematics here:

http://www.jensentransformers.com/apps_sc.html

[BrianJ]

You probably could, but the primary impedance is so low that it would be hard to interface with a tube circuit, you would need a high impedance buffer in front of it.  No guarantee it would sound very nice though.  The jensen DI transformers is phenominal, I've made a couple of boxes with them.  But $75 is steep for DIY.

I say try it.

[9 volts]

Thanks I'll press on and give it a try, seems a waste not to do something with it.

[RaceDriver205]

Why not an active design?

[d95err]

Why not an active design?

(Just guessing) an active design would mean adding another tube or adding an extra powersupply for solid state voltages. Neither may be particularly attractive to do in an existing tube amp design.

[d95err]

You could try adding a high voltage MOSFET source follower before the transformer to get better impedance matching. See "MOSFET Follies" at www.geofex.com.

[BrianJ]

If your tube amp has a negative voltage supply for Power tube bias you can tap off of that for a small low current circuit.  Surely a follower or two wont hurt, and you may be able to use it without regulation.  Since your signals will be so small in the DI circuit you may not need to regulate the B-.  Anything between 10 and 50 Volts will probably work depending on the maximum voltage of the devices used. 

If you don't have a B- supply in you amp try a Diode capacitor voltage doubler connected to your Heater supply.  You should get 8 - 15v DC from that.