What do you think about this tube preamp?

[diegot]

It's from a magazine published in 2000.

I'm thinking to built it, but I need your opinion about it.
It's my first DIY tube project, so I need help.

[Giaime]

High gain and negative feedback, high output impedance lowered by NFB.

I'm sure you don't need that

With modern sources, that can drive a power amp directly, you need a preamp that's as much transparent as possible. If not, better to use a passive attenuator and connect the CD player to the power amp.

[SY]

I wonder whether the author ever actually tried his shown pentode option. It's got a major flaw, I think.

[Giaime]

Originally posted by SY
I wonder whether the author ever actually tried his shown pentode option. It's got a major flaw, I think.

Are you referring to the fact that if you connect that dotted connection you'll short the output to ground?

[diegot]

I want a tube-preamp to "warm up" de sound.

I'm still looking for the audioxpress june/july 2002 tube preamp project.
If anyone have this article, please send to me.

[Joel]

#1 pet peeve:

I want a tube-preamp to "warm up" the sound.

Then why don't you just build a tone control?

[Giaime]

Originally posted by diegot
I want a tube-preamp to "warm up" de sound.

Then take it simple I think you'll be more pleased with the sound of a simpler preamp. Take a look at SY's Heretical (do a search in the forum), or even take a look to the various preamp schematics on my website.

http://www.tnt-audio.com/clinica/ta/mwprmk2_e.html
this sounds very good.

[SY]

Originally posted by Giaime


Are you referring to the fact that if you connect that dotted connection you'll short the output to ground?

Yup. And if you don't, the screen isn't bypassed properly to the cathode.

BTW, the Heretical will not warm up the sound.

[billr]

I think that you may be missing the point with the dotted connection.

It is an alternative to connecting the screen grid to the Ht.

If the grid is connected via the dotted line, and not to the HT, you have a triode.

Hope that this helps.

by the way the diagram is from Rainier Zur Linde's book, which I have lying around here somewhere.

kind regards

bill

[Johan Potgieter]

Nobody seems worried about the exorbitantly high interstage and output coupling capacitors, forcing the use of electrolytics, which in the case of the interstage will probably have enough leakage current to influence the pentode working point.

2.2uF for the interstage and 44uF for the output coupling if I read correctly?!!

To take you to a -3dB cut-off at a low 5Hz interstage would need only 33NF, and likewise for the output, of 1uF. And that is without taking the further extending action of the feedback into account (I have just done a hurried calculation without determining how much feedback is left at say <20Hz.)

One does not always want to sound "wiser-than-thou", but I always wonder why when using feedback to get the RIAA characteristic, the designer does not also make use of an excellent opportunity to build in a low-cut attenuating characteristic below say 20 Hz. It calls for just about no extra components, and a cut-off of 16 - 20 dB/octave can easily be obtained. No turntable is perfectly rumble-free, there are also possibility of bumps that can dislodge the speaker cone, etc. (Rumble-wise, it makes no sense to me to pay several $100 extra for what can be achieved by $5 components.)

I have used 3 such designs in the past with great acceptance (one designed to replace the 1st stage in the Quad II control unit), but must hang my head in shame for not being able to post the circuits right now - PC problems. There have been a host of excellent circuits here in the past so I hesitate, but should there be any interest I will be happy to post them as soon as possible (in about a week). They respectively use an ECC83, ECC81 and ECF80 each, with different gains.

So being unable to "deliver" now, I would at least suggest that you cut down on those unnecessarily high capacitors if you want to use this circuit. Get within the region of available polyesters for several reasons.

Regards