Design question regarding impedance

[gwell1]

So I'm designing a preamp for an acoustic guitar all tube. Two 12AX7's to be exact. I'd like to take the output from the plate of the last 12AX7. Then I'd like to plug the output into something a recording interface or a sound board. The input impedance of a Digi 002 is 2k. The input of a Behringer xr18 is 5k. How can I choose transformer that will allow me to plug this pre amp into virtually anything? I was thinking, worst case, run the plate output into an opamp to make it easier.

Any thoughts?

[nhbassguitar]

Four 12AX7-based gain stages is a hell of a lot of gain. Can you change the design? Make V1 a 12AX7 and V2 a 12AU7. Make V1A, V1B, and V2A common cathode for gain, and make V2B a cathode follower, which should be able to drive anything.

As a point of reference, my everyday bass preamp, which includes some passive bandpass filtering but no variable tone controls, uses only half of a 12AX7 and half of a 12AU7. V1B and V2A are properly biased but aren't in the circuit. It's got so much gain, I can run it into a Radial JDI (about 130 Kohm input Z, 12-14 dB attenuation) and from there directly into a power amp, and let me tell you, that sucker can get loud. At some places I play, I have to engage the JDI's pad; otherwise I'm forced to run the preamp's gain control barely open, like 8:00 or 8:30.

[doctor]

The best solution for your purposes is a cathode follower. You obtain low output impedance, which will be suitable for all types of inputs (amps, recording interfaces, etc.).
Conditions of properly work: output impedance of a source (preamp in your case) should be much smaller than the input resistance of the device, to which the source is connected.

[R.G.]

Cheat. Use a MOSFET as a final source follower. Hide it under some of the other circuit stuff.

You will not be able to hear any tone pollution from this.

Of course, if this preamp is for sale, it precludes advertising it as "all tube".

[Chuck H]

It also adds a device where one may not be necessary. I would have thought that two 12ax7 tubes with the last stage as a cathode follower is plenty for an acoustic guitar preamp. Even if you add a passive EQ.