NFB and presence control on a standalone preamp?

[jarrodthebobo]

Been screwing around with my preamp build which has undergone a boatload of changes... Was just wondering if anyone had any sort of idea to how I can implement this?

In my SE builds Ive added,nfb via tapping off the OT and sending that to the cathode of my last triode which I use as a "fake pi"... But with no OT here, is this possible?

Thanks.

[roberto]

Do you want to have negative feedback on which stages of the preamp and to obtain what?
It is just to have a presence-like equalisation?

[jarrodthebobo]

Do you want to have negative feedback on which stages of the preamp and to obtain what?
It is just to have a presence-like equalisation?

Correct. I'd like NFB on the "PI" stage in order to simulate what you'd get in a full amp.

[roberto]

If you mean the frequency part of "what you'd get in a full amp", you can use a rheostat (and a resistor in series) to partially bypass a cap on the cathode of the last stage.
If you mean the overall sound (harmonic complexity, dynamics, etc...) of a power amp, it won't work.

[jarrodthebobo]

If you mean the frequency part of "what you'd get in a full amp", you can use a rheostat (and a resistor in series) to partially bypass a cap on the cathode of the last stage.
If you mean the overall sound (harmonic complexity, dynamics, etc...) of a power amp, it won't work.

So, without implementing NFB? Would this cap and rheostat combo be bypassing the last stage such as a normal cathode bypass cap?

[roberto]

This is one option, otherwise you can have a voltage divider with a cap and the rheostats to have the same shelving effect.
I've posted a thread with something similar but for a different purpose, months ago.

[jarrodthebobo]

Gotcha! I dig some digging around here and the sloclone forums and found something that ended up working pretty well for what I wanted. I'm currently rendering a video including some audio of the finished product along with some info about the topology. I keep promising to make a schematic of this thing for others to screw around with, but always end up changing the preamp; hell, just today I finally decided to add a 3rd 12ax7 for some cathode follower action

[jarrodthebobo]

For anyone interested, here's what I needed this idea for. Although I didn't end up implementing NFB in the way that I thought I'd be able too, I think the result was pretty good.

Thanks for the help, Roberto!

[roberto]

Now I've got what you are looking for.
For that sound, just use a standard plexi preamp, then add after the eq two triodes in parallel, 27 to 47k on the plates, 3k3 to 8k2 on the cathodes.
Snubber some highs if needed. You'll find two threads on it on sloclone.

[jarrodthebobo]

Now I've got what you are looking for.
For that sound, just use a standard plexi preamp, then add after the eq two triodes in parallel, 27 to 47k on the plates, 3k3 to 8k2 on the cathodes.
Snubber some highs if needed. You'll find two threads on it on sloclone.

That sounds interesting... I'd never thought about running two triodes in parallel after the tone stack... What should I search for on the sloclone forum? I wanna see some more of this haha

[jarrodthebobo]

Been messing around a bit more and swiped the last two stages of the Bogner Fish in order to try to implement presencr how it was done in in that circuit; and I must say its much more effective than I expected. The only oddity I've noticed is that when the presence is maxed, it seems more like a mid boost and also increases gain quite a bit vs. When its backed off. Its real nice when I'm going for a soaring lead tone, but If im trying out some " chigga chiggas" having the presence up high adds too much mids and I loose the "chug". Turning it down brings it back back some gain is lost.

How exactly does this presence circuit work? Is it utilizing local feedback?