Q about feedback in the ZV9

[lordvader]

Hey all.

I have a question about the feedback loop in the ZenV9. The feedback basically lowers the output impedence and decreases (increases ?) the damping factor.

If it is known that the amp will be driving a nice load (the sort of load you may present a tube amp), can the feedback be ommitted (ie, leave out r17 and c8, leaving an open circuit)?

Being a very simple design, I would like to A/B the circuit with and without the feedback, but really just want to make sure it's just a matter or removing those components (eg, do those componets do anything else in the circuit ?)

Thanks

[flg]

Lordvader, there are 2 things you need to consider if you'ld like to remove the F.B.
1) The JFET needs a path for a very small gate current to GND. It gets this path currently through the R17 and R7 components. You will want to provide a high value resistor to GND if you remove R17.
2) Leaving R16, the 9.09k, in place will needlessly limit high frequecy response. Lowering it's value substantially will improve bandwidth.
Both of these Ideas are dealt with and illistrated in the ZV9 paper, in figures 2 and 4. You will also experience slightly more gain
I can imagine the "possibility" of instability due to these changes. In my inductor loaded test cicuit however, I have run it like this several times and never experienced any instability

[lordvader]

I was about to ask where would this large gate to ground resistor goes, and why wasn't it in the non-feedback circuit (fig4), till I realised that THAT circuit had a 47.5k resistor at the input.

Would that be what you're referring to ?

If that's the case, switching between feedback/non-feedback is REALLY straightforward !

Can't wait to try it out (waiting on another zv9 pcb groupbuy at the moment ...)

[flg]

That would be the one
My circuit actually has 100k on it's input...
PCB ??? There are not enough parts to worry about a PCB???

[Vix]

Originally posted by lordvader
If that's the case, switching between feedback/non-feedback is REALLY straightforward !

You can easiy make a switch to feedback/non feedback. Make it with small relays. In a feedback mode, it will connect the feedback resistor.

In a non-feedback mode, feedback resistor (47k)will be disconnected. Another relay could short the input (10K) resistor. Another one will disconnect the aleph CS. Current sensing resistor could be shorted as well. Lastly, if you want if fully "automatic", you will put one more relay to connect a 22 ohm resistor across the output terminals. Seems complicated, but it is not. You just need a bunch of small relays. Make a small switch with twin colour Led. No feedback red, feedback green. 2 in One. Cool.

I was running my ZEN V8 without feedback quite sucessfully. It sounded airy and relaxed.