I'm not sure I understand the details, though. I guess you're buffering but then attenuating the signal to get it back down to the voltage range (plus a bit extra) that a U47-type circuit expects, and the 60pF cap is to match impedance too. Is that right? (My understanding of analog circuits and impedance is very weak.)
There must be a ton of ways to do this, it was just most logical to my non-electrical engineer way of thinking.
First balanced unity gain opamp is to get rid of any noise the cable picks up, it comes from my audio interface, line level. But it could from any outboard gear. If i want to attenuate signal, i do it from my audio interface. k67 (probably the same as k47) puts out 390mV RMS polarized at 60v, at 117db of spl.
Line output puts out more than this, so there is no boost really needed if you want to drive the tube as a capsule would. 60pf can really be omitted, but this would eliminate capsule - grid resistor interaction which forms hpf filter. So i keep it in. k47 has about 80pF capacitance, so that cap should in reality be a bit higher. 80pF and 60Meg grid resistor form hpf at 31.9 hz. This is important to have in place as low end content affects tremendously how things sound when saturated. 60Meg sets tube input impedance, and since you have that pF range cap emulating capsule in place, the tube doesn't really care what came before it, is it an op-amp, or a real capsule generating the signal.
I added the second stage op-amp after first buffer stage just in case i want to go crazy, and if the signal i'm sending from the daw is low for whatever reason.
That brings me back to my earlier question: is there tube-head-amp goodness that you can only get with something very like a head amp, operating on the kinds of signals you get from a capsule, rather than mic-level signals into a preamp?
I would say most definitely yes, because pre-amps, tube or not, have quite different topology, probably multiple stages, before if any transformer. There's also a question of existence of input transformer. There's that interaction between EF14 (VF14) through coupling cap to BV8 style transformer, to whatever preamp you use. And i do go back into a mic pre, just like i would with a mic!
Maybe you would presume my color box sounds always great, and i can promise it does not, just like regular mic it sounds best at moderate levels, just like a real capsule would create. Overdriven heavily sounds like typical one stage triode. Can be fun, but often not.
As far as I understand it, or misunderstand it, head amps are typically low gain (even unity gain) and mainly used for increasing current, leaving it to the preamp to actually increase the voltage.
Yes, at the end of the mic circuit, but in a typical c12, u47... type circuit there is quite a bit of gain from the tube, the voltage swing is determined by plate voltage, so it can go quite high in c800, c12, less in u47... That voltage goes back down to output level, and is lowered by the output transformer. So there's that interaction when tube pushes the OT transformer at those voltages, and how it can saturate it's core. And then that OT transformer interacts with pre that follows. BV8, T14, and other transformers are quite different than your preamp transformers.
Do low-level low-gain tube amps distort characteristically differently from higher-level higher-gain tube amps? Does head amp distortion therefore have a different "tube sound" than preamp distortion?
I think i already touched upon this, it will depend on the topology, but in general yes.
To add further to the madness, I made an IR from VF14 shootout files posted here, which generates characteristic VF14 microphonic ''reverb'' effect, which comes from the sound hitting the tube, and has nothing to do with signal going through the circuit....
What a marvelous idea for a commercial product i just gave away for free
![Rolling on the floor laughing :rofl: 🤣](https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f923.png)
I hope i, at least, get credited if it ever comes to fruition. On the other hand, the concept is so logical to me, and I thought of it the second i injected the signal for measurement of the first mic circuit i measured. So it could be there are people who have already done this.