Preamp output impedance question

[gwell1]

Design question regarding impedance
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?

[Leo_Gnardo]

Those are mighty low impedances to try and drive with a 12AX7. You could look at a final stage having a cathode driven output, drive a transformer with a very high primary impedance and secondary that will be happy driving loads in the 2K to 5K range. Plenty of gain in a sound board if the level comes up low. Or use your op amp plan.

From Mouser try a $3 Xicon TM019 transformer, 10K to 600 ohms. Don't mind the bandwidth specs, this series of transformers deliver surprisingly good bandwidth for the price as long as you don't run 'em to their power limits. If you're feeling spendy once you proved your circuit works well, then you can throw down for a fine quality Dean Jensen which should run you about 50X as much. You'd be surprised where similar little fingertip size Xicons turn up, preamps that retail for hundreds of $$$.

http://www.mouser.com/ProductDetail/...Dj7UeeLO0fc%3d

[gwell1]

Well my goal was to end up with an output impedance of about 600 ohms at the main output, the transformer. Cathode follower is not a bad idea. Thanks!

[Mike Sulzer]

Why not use the 10Mohm inputs on the Digi? The B also has high input impedance inputs, but I do not know how high or when they can be used.

[Chuck H]

You can look up any number of "tube effects loop" schematics with a *oogle search. You'd be interested in the "send" circuit only as your output. Most have an output impedance at or below 600R and provide a "line level" signal that can drive contemporary DSP's and other devices appropriately. Since that circuit only requires one triode and you're using two 12ax7's that leaves you with three triodes for input and signal processing (EQ?).

[Zozobra]

Lots of scope to use a MOSFET here. There are some solid designs using the irf820 as the send part of an effects loop. It's fairly low part count and runs from the b+ supply so easy to implement in pretty much any amp.

[Mike Sulzer]

Lots of scope to use a MOSFET here. There are some solid designs using the irf820 as the send part of an effects loop. It's fairly low part count and runs from the b+ supply so easy to implement in pretty much any amp.

But it would be easier to put a 100:1 voltage divider on the output, maybe 100K:1K. Then you can run into either impedance input. Presumably the purpose of this preamp is to get some tube sound, otherwise just use the preamp in the Digi. Using the attenuator forces you run the output level on the last tube stage up a bit so you get some coloration. Might have to fool around a bit with the values.

[gwell1]

You can look up any number of "tube effects loop" schematics with a *oogle search. You'd be interested in the "send" circuit only as your output. Most have an output impedance at or below 600R and provide a "line level" signal that can drive contemporary DSP's and other devices appropriately. Since that circuit only requires one triode and you're using two 12ax7's that leaves you with three triodes for input and signal processing (EQ?).

Correct. The other three triodes are used for tone shaping.

[gwell1]

But it would be easier to put a 100:1 voltage divider on the output, maybe 100K:1K. Then you can run into either impedance input. Presumably the purpose of this preamp is to get some tube sound, otherwise just use the preamp in the Digi. Using the attenuator forces you run the output level on the last tube stage up a bit so you get some coloration. Might have to fool around a bit with the values.

Could you elaborate a little? Do I put the voltage divider on the plate?

[Mike Sulzer]

Could you elaborate a little? Do I put the voltage divider on the plate?

Well, after the plate coupling capacitor. That is, on the low dc voltage side. Perhaps a 100K resistor for the series element and a 10K pot as the shunt element, with the wiper as the output. Then you can make adjustments similar to the way an electric guitarist uses the gain and master volume controls to adjust the distortion. Only yo do not want a lot of distortion, just good control over the coloration caused by the tubes.