What do you think bout that???

[costiss]

I had a look into the 6N1P datasheet and thought it could make a nice small preaplifier... I came up with this shematic.. simulation results seem quite good - gain about 7 ...

[costiss]

the simulation.......

[Positron]

Hi Costiss.
Not too bad but as a suggestion I would isolate the two stages more. Frequency dependent feedback thru the power supply (PS) will occur as the frequency goes down. The feedback will be phase shifted more and more until it approaches 90 degrees.

The amplitude of the feedback reaching the output may only be some 50db down or so at 20hz.

Hope this helps.

[costiss]

you mean there would be feedback from the common power supply? using seperate regulation for each section could isolate the two sections, right?

[Brett]

Originally posted by costiss
I had a look into the 6N1P datasheet and thought it could make a nice small preaplifier... I came up with this shematic.. simulation results seem quite good - gain about 7 ...

Simulation results have no real bearing on how it will actually sound.

The 6N1P is a nice little tube but loading it with 10k when it has a plate resistance near 8k is a bit low. I also have a particular dislike of cathode followers because of their consistently horrible sonics.

Steve (Positron) makes some good points about the power supply.

[fdegrove]

Hi,

I also have a particular dislike of cathode followers because of their consistently horrible sonics.

A bit of a blanket statement but yes, if you MUST use a CF, then better use a good one.

A good one is the White CF to my ears, sounds almost like nothing at all but it costs you an extra triode AND a good PS.

And that does mean $$.

Question is, do you need such circuit?

Don't think so...
Unless you need to interface atypical impedances, you don't.

Cheers and keep it as simple as need be,

[Positron]

Brett and Fd make good points about Rl, Ra, and removing the CF. Keep amp input Z up and ICs' capacitance low so as not to "load down" the preamp tube.

I recently measured the capacitance of an aquaintances 25 foot interconnect, I think it was home made, at 15,500pf. That is .015uf for those who may be unfamiliar with capacitance terms.

If you must use a cathode follower, yes, separate filters and/or regulators for each stage should help isolate. But they must be good as what feeds back could be up to 180 degrees or more out of phase, depending on the design.

Hope this helps.
Steve

[costiss]

so.... what?... parallel the sections and make just an anode follower? ...with a Zout equal to 1/2 (because of the two parallel sections) of the pl.res. = 1/2*4400= 2.2k? when the cf would have sth like ~1/gm=113 ohm... I dont know... maybe im kind of predisposed.. have been using opamps (!!!!!!) ... for some time... howcould i get a better cf, apart from psu?

[Brett]

From your last post, I see you're referring to the plate resistance as 4400R. It's more like 8k. Svetlana transposed the transconductance and Rp numbers and have never corrected them.

[costiss]

ahem... hmmmm.. upssss... i didnt know bout that........plans change, then... what about 5687 with parallel sections , 300V, 2 x 12k anode + 132 R cathode resistor(4V bias), unbypassed (drew this on the curves)... zout would be (???) ~1.8k