tube phono stage

[dsavitsk]

I am looking around to upgrade an old opamp based phono stage. Looking through my files I stumbled across a schematic I downloaded from here (I think) some time ago. It looks simple enough to put together, but I am initially stumped by one thing. The cathode of the ecc88 is connected directly to ground as, seemingly, is the grid. Is this correct? Can anyone explain this.

Also, any comments on whether this is a worthy project, or, if not, what might be? Also, I was thinking that I might change the second tube into a mosfet to give a little lower output impedance. Thoughts?

[barretter]

The ECC88 grid is not connected directly to ground but through a ten mega-ohm resistor : this is known as grid-leak biasing and is permissible, but I wouldn't use it. Nor would I use an ECC83 as the input valve for a phono stage ; or use all-in-one passive RIAA filtering. It will work but the output will probably be inaccurate vis-a-vis the RIAA curves.

[analog_sa]

Originally posted by dsavitsk
The cathode of the ecc88 is connected directly to ground as, seemingly, is the grid. Is this correct? Can anyone explain this.

I fail to see how the grid is connected to ground. The ecc88 works in grid-leak mode - nothing strange or unusual here. The Counterpoint SA-7, to mention one commercial example, does the same. I may have posted a circuit previously.

Don't do the MOSFET thing. Unless designed by NP MOSFETs sound atrocious

Seriously, what's wrong with 2k output impedance? If you really must cut it consider paralleling triode sections.

Good luck building it.

[Eli Duttman]

The 2nd gain block is set up for grid leak, AKA contact, bias. Notice the large (10 MOhm) grid leak resistor. Electrons striking the grid bleed off slowly. An "equilibrium" is reached between electrons striking and bleeding off, with the grid at a small negative potential. The technique works only at low signal levels, like a phono preamp. There is good reason for using grid leak bias in the 2nd gain block of a phono preamp. Bass extension is improved. I believe that design is by Thorsten Loesch.

If you want to lower the O/P impedance, DC couple a ZVN0545A MOSFET source follower to the 2nd gain block, as described in MOSFET Follies.



Edit: fixed typo

[dsavitsk]

Originally posted by Eli Duttman
The 2nd gain block is set up for grid leak, AKA contact, bias. Notice the large (10 MOhm) grid leak resistor. Electrons striking the grid bleed off slowly. An "equilibrium" is reached between electrons striking and bleeding off, with the grid at a small negative potential.

Interesting, I'd not seen that before. I think you are right that it is a Thorsten design.

This seems like such an easy build, and really, I only use my (vintage 1974 Linn, sn around 2000!) turntable to listen to old punk rock and blues records of mine, so the pinnacle of performance is not necessary. I think I'll try to get one together one of these days. Anything would be better than the opamps in use now.

Thanks

[Giaime]

I would suggest this design from Thoersten, the one in the first post bah... I didn't like it, too little gain and bah, grid leak biasing.

This one sounds much better
http://www.diyaudio.com/forums/showt...687#post959687

I'm developing a PCB for it.

[Shoog]

I built this phono preamp, and indeed it is by Theorsten. Sounds very good and I think all the design decisions were carefully made.
I can only really compare it to the VSPS with OPA627, and it is superior to that one.
If you look at the circuit you will see it is not quite as easy to implement as it first appears. There are some big high voltage smoothing caps in there. I cheated and used bypassed computer grade electro's.
Be careful using chokes in the power supply, as I did initially. I managed to introduce a resonance at 50hz or so. Used a bigger choke and this went out of audable range.

Shoog

[Eli Duttman]

Geek recently linked a 250 V. regulated B+ PSU in another thread. I'm linking the regulator below. If ever there's a time to use regulated B+, it's in a phono stage. CRC filtration in front of this "baby" should yield a result that make Church Mice seem noisy.

Geek's Regulator

[Idefixes]

I will use this regulated power supply for this job



The riaa is waiting for enclosure before power on

the power supply board





Thorsen RIAA based on the first schéma ECC83+ECC88, I made a board since i am not really comfortable without. HT is localised in the middle of the board.





I use a isolating power trafo to provide HT. It commes from rubbish



Marc

[dsavitsk]

Here's my version of a board, or a first version of one anyway. I used bypassed electrolytics as well as there was no room otherwise. One could, I suppose, just connect nicer caps via wires, which may be worth the experiment.

I left space for Relcap RT polystyrenes in the filter which may be a silly thing to do. They're pretty cheap in these sizes so hopefully this is okay. I think the output caps are large enough for Auricaps or something similar.

I do have a high voltage regulator I can use which I think is a good idea.