Reducing Hiss

[mooreamps]

Heh Heh, good point. I don't know what I was thinking when I wrote that last thread. Long day at work...

I'm sure there are other members who are still trying to learn how all this stuff works. As for me, since we can bias a FET gain stage, in the same respect as we can bias the gain stage of a vacuum tube, discussing the ability to lower the noise floor of an LNA RF amplifier using this techique, will be the topic of my next thesis paper. I've read most all the other papers written by others on this topic, but no one to data has captured the thermal noise content genereated by using the ultra-bright led ; or even any led. The others have only gone as far as capturing the noise content as generated by using a silicon pn junction, with respect to using carbon.

-g

[Bruce / Mission Amps]

It seems that if one were to replace the cathode resistor (R33 in the schematic) with a fixed 1.5 v bias supply, then the bias supply must be built robust enough to supply all the cathode current that that stage will draw. I forget that the cathode current for a 12ax7 is, but a quick look at a spec sheet will tell how much currrent needs to be supplied.
Anson

Here's a quick sketch of a preamp stage I messed with a couple years ago using a 2v @ 1 watt zener diode and 22uF@25v cap.
I can't remember if it sounded any different then using a larger (250uF@25v) cathode bypass cap...

*****************
Another thing I tried off and on was using an infrared diode as the cathode biasing resistor.
I think it's "turn on" voltage was around +1.2vdc.

[mooreamps]

I use the red ultra-brights for two reasons. The forward voltage drop is right at 1.7 volts. If i need a few 1/10's of volts more, then I'll add a shcokky or some such in series. Second, it provides the gain stage bite function that I'll looking for. Now, for the power tubes, the regular red led's work just fine since there is more current in play. But, I'd dropped the value of the plate coupling cap to properly attenuate the increased bass responce. 20 nF is just too large a value for the circuit as drawn.

Now that value for a triode input stage. I've not yet calculated the correct plate coupling for my cascode ef-86 input stage as yet.

-g

[Gregg]

To reduce hiss in high gain amps/preamps I'm using a modified EPFM Noise gate. The sound passes only through a Vactrol or similar optocoupler but the sound level must be dropped to line levels in order to have full and sharp NG effect and then reamplified to drive the power section.

[mooreamps]

To reduce hiss in high gain amps/preamps I'm using a modified EPFM Noise gate. The sound passes only through a Vactrol or similar optocoupler but the sound level must be dropped to line levels in order to have full and sharp NG effect and then reamplified to drive the power section.

I can source led's. How does one source a "vactrol" ?

-g