Dumble hybrid PI

[greiswig]

Hi, all,

Posted this on the D-Discussion list, too, but didn't get many nibbles.

I'm having what is probably a bad idea, but I hope to learn something from it. I have tried two phase inverters in my 50-watt non-HRM D'Lite: the standard PI and the Bluesmaster. The standard one has more volume and the notes have a harder attack, and the Bluesmaster is more compressed and has more bloom and harmonics. I actually find myself wondering if there is a good middle ground that gets some of the benefits of both...a bit more compression than the standard PI, but some of the bloom and harmonics of the Bluesmaster.

I attach a pic of what I had in mind: in black text are the standard PI values. In blue are the Bluesmaster values. In red are the hybrid values I was thinking of.

To that end, my idea was to split the difference between the two. My problem is that I don't know enough technically to know when I am getting myself in trouble. It doesn't hurt to try, but if there is a good reason not to, I'd love to hear it.

For example: is there something magic about the coupling caps and their relationship to the long tail pair that I need to be aware of that would make my hybrid design cut off too high or something? Or maybe it's the case that the compression comes from one section of the PI, and the harmonic bloom from another, so I should actually be not averaging the component values but changing things up in a different way? Any other reasons why this might be a bad idea?

[Firestorm]

I'd suggest just changing one or two things at a time so you can gauge the effect and actually know what caused it. Leave the feedback loop alone at first (your proposed loop values have almost the exact same ratio as the Bluesmaster values anyway). Besides if you can find a 13K pot somehwere, you're a better man than I am.

Iit looks like most of the difference in the amps comes from manipulating the resistor values in the PI -- especially the tail resistor. The smaller the tail gets the more out of balance the two sides become and the more "interesting" the harmonic content gets. Start there. Then, maybe change the coupling cap values if you want (but I'll bet you won't hear much difference between .1 and .05 or between .02 and .05).

[greiswig]

...Leave the feedback loop alone at first (your proposed loop values have almost the exact same ratio as the Bluesmaster values anyway). Besides if you can find a 13K pot somehwere, you're a better man than I am.

Thanks for the input. I wasn't sure whether the whole PI (including the presence circuit) needs to be treated as one cohesive whole or not.

When you say "same ratio" for the loop in the feedback, are you talking about the ratio of, say, 4700/390? If so, aren't the Bluesmaster and standard PI's about the same in that regard as well?

As for the pot, I was planning on a 25kL pot with a 25k resistor in parallel. Not perfect, but it would work.

[Firestorm]

There are interactions, but it's hard to know what change produced what result if you change too many things. All of the loops are pretty close, but the stock loop is about 12 to 1 and the Bluesmaster and your new one are around 14.5 to 1.