Input Impedance Tollerances for the NE5532??

[Minion]

Hi, The Tubes have an output impedance of 470K and I am useing 2 tubes in series which should bring the Output impedance close to 1M.....

Even if the Output of one Tube doesn"t effect the output impedance of the other the Impedance is still at least 470K??.....

here is a Diagram of one of the Tubes curcuit:

The 100K resistor at pin 1 of the Tube can be replaced with a 100K Pot, the Less resistance the More Current and the More Gain, it runs at unity gain at 100K.....

I have it set up so the First tube is run at unity gain to add the Tube color to the sound and the Other Tube has the 100K pot on it which allows you to overdrive the second tube.....


ThanXX!!!

 

[Samuel Groner]

The OPA627 connected as voltage follower sounds like a good plan to me for this circuit. I'm not enough into tube design to seriously comment on the 2x 470 kOhm output impedance (I don't think you can simply add them up), but the output impedance will likely be high enough to justify the FET buffer anyway.

Samuel

 

[pstamler]

Some things need to be clarified. First, the output impedance of the tube stage. As drawn, the book spec of the tube's output impedance open-loop may be 470k, but there's also a 100k plate resistor which is effectively in parallel with the tube, so the output impedance, open-loop, would be something like 80k.

But that's open-loop, and the tube is not being operated open-loop. The 340k and 604k resistors connected to the grid turn the stage into an "anode follower", applying nearly 100% feedback to bring the gain down to near unity. Without knowing the open-loop gain of the stage, I can't say exactly what the output impedance would be under those circumstances, but I'd guess at somewhere between 1k and 2k. A 5532 would make an excellent outpul buffer stage under those circumstances. (And you will still need an output buffer to drive the next piece of gear; the tube wouldn't be happy supplying the current needed to drive 10k, let along 600 ohms.)

I question the appropriateness of this design for what you're trying to do, though. You're after some "tubey" sound, and the thing about anode followers is that they're pretty darned clean, as all that feedback wipes out the pretty distortion products you wanted in the first place. Why not. instead, replace these stages with a couple of stages of open-loop triodes, say the two halves of a 6SN7 or 6FQ7, with a drive-control pot before the first one and a level-control pot after the second one, feeding into a 5532? That'd give you lots of nice triodey distortion. You could install a bypass switch for when you didn't want the tube stuff in the line.

Peace,
Paul