Beginner's very simple line pre-amp

[Giaime]

Hello everybody!

I've had some interest in the linestage portion of the preamp I recently built, mainly with some italian friends.

So I decided to make things a little clearer, and to give a response to everyone is seeking for a SIMPLE linestage, with gain and reasonable performance (althought it seems that's more than reasonable...)

here it is:
http://www.giaime.altervista.org/plate_follower.html

I know there are many many people on this forum that eat tubes for breakfast so comments / suggestions / corrections are welcome!

Maybe following will be a simple regulated power supply and the developement of the Plate Follower Stage II: The Constant Current Loaded One

[EC8010]

I'm afraid you need to think again on your Super Plate Follower. What you start with is a mu-follower. That's good. But then you've used the lower output. That's bad. The idea of the cathode follower up top is to provide a high impedance load to the lower valve so as to reduce distortion. Take the output and feedback from the cathode of the upper valve. A well-designed mu-follower with carefully chosen valves gives very low distortion and excellent rejection of power supply noise. In theory, wrapping feedback around it should improve things further. I think you'll have to try it to see whether the feedback is a good idea.

Oh, and input impedance is still tending towards 47k.

[Giaime]

Hello EC8010, and thanks for your response.

In fact, the super plate follower (as I like to call it) is still an idea, the target was the original simple plate follower. In fact, I'm thinking of developing a solid state CC source, to provide the high impedance load.

Can you please explain me the part

Take the output and feedback from the cathode of the upper valve

are you suggesting to move both the output of the circuit and the feedback signal from the lower anode to the upper cathode?

And yes, about the input impedance, that was math error I will correct in the webpage.

[kevinkr]

Hi Giaime,
The plate follower is old hat (sometimes a good thing) and not related to a mu follower at all. (Not quite sure what EC8010 is on about here.)

You don't need R2 in the schematic as shown due the the feedback network and R7. This will net you a bona fide 100K input impedance. (There is still a grid bias path should the pot wiper be intermittent.)

I used a simular output circuit with 12AX7 in some of my early pre-amplifier designs, both with and without a cathode follower.

Incidentally I did not like the sound of the 12AX7 mu follower with feedback wrapped around it, perhaps because of the relatively high level of global feedback extant as I used this as a unity gain output buffer after a LTP configured to allow output polarity reversal.

This circuit is an excellent match with some tweaking to the 5687, with NFB applied for a modest amount of gain you can get source impedances of well under 1Kohm. (Unity gain <150 ohms)

You can also use a T type attenuator network in place of R6 if you need gain and are concerned about the quality of higher value resistors.

[Giaime]

Thank you Kevin, your suggestions are priceless, I've taken the freedom of linking this thread in the webpage

[EC8010]

Originally posted by Giaime
Can you please explain me the part; are you suggesting to move both the output of the circuit and the feedback signal from the lower anode to the upper cathode?

Hello Giame, yes; exactly.

[kevinkr]

Sorry EC8010, for some reason I did not see the mu-follower on that page the first time I looked at.

Some people do connect mu-followers this way, without feedback and into a following high impedance stage. The argument for doing this is supposedly improved linearity, the follower on top being used solely as a current source.

My comments about 12AX7A based mu-follower still stands, lifeless sound when global feedback is applied. Not sure about other tube types though. I have used this topology in phono stages (no global fdbk) where the gain is useful, but thought the sound a little on the euphonic side of things.

[EC8010]

kevinkr, no apology necessary. I can see the arguments for taking the lower output of a mu-follower into a high impedance stage such as a cathode follower although I've never needed to do it myself.

Bearing in mind that a mu-follower is a series amplifier, the HT needs to account for the Va of upper valve plus lower valve plus the drop across the load resistor, so choosing a high-mu valve (which invariably requires more Va than a low-mu valve) seems doomed to failure unless you have an extraordinarily high HT. Perhaps being forced to operate with a lower-than-ideal Va is why you didn't like the results from your experiment?

By the way, I edited the title of the thread as I'm sure the threadstarter wanted to infer that the line pre-amp was very simple, rather than the beginner building it...

[Giaime]

Thank you EC8010, that was what I intended. English is not my primary language... sorry

[EC8010]

You will only need to apologise when I can write Italian as well as you can write English.