Phono Preamp

[bayermar]

Hi everyone,

I unearthed my old record collection from boxes after moving back to Europe recently. Now all I need to play them, is a phono preamp to connect the old technics player with my tube amp (Gingertube's Baby Huey).
The MM cartridge (technics P30S) is mediocre but I will have to go with it for now.
All I know about it is that it needs 47-100k (no words on capacitance, though - I will try 47p + tube +cable).

For the moment I am thinking about building a circuit published by R. zur Linde in a book called 'Audio and Guitar Circuits' (see attached schematic).
The power supply has regulated DC on HT and filament (not shown).
I just got a beautiful transformer from 'AskJanFirst' that would fit the suggested power supply nicely. I have built other circuits from that book and they all seemed to work ok.

Now my question: What do the more experienced members here think about a circuit like that?
Would it make sense to replace the resistors and caps of the 2nd tube with a 1.5V LED? As I understand the circuit the cathode resistor of the first tube is part of the RC network in the feedback loop and has to stay.
Or is there a similar simple circuit that would me much "better"?

I love the sound of my EL84 Baby Huey and would like to have a phono preamp that can meet it on eye-level.

I appreciate any comment.
Thanks,

Martin

[kmtang]

Hi Bayermar,

The circuit seems very classic like the Marantz 7C one. I would say built one and tweak it with other later. You may add the B+ RC filtering for each stage and separate RC filter for the L and R channels.

In order to get the good sound out from it, it is very important to build a very good power supply with good coupling and bypass capcitors.


Johnny

[bayermar]

Hi Johnny,

Thanks for your comment.
I realize I should have posted the power supply, too.
It uses 2 stages of CRC filter followed by a 3-pin regulator which is followed by another RC filter.
As you suggested it might make sense to have a regulator with RC filter for each channel separately.

Thanks,
Martin

[bayermar]

OK, this is the suggested power supply.
The voltages are slightly different as needed.

I will probably use a LT 1085 for the filament supply since my transformer has a 6.3V winding.

Cheers,
Martin

[Eli Duttman]

Martin,

IMO, the 12AX7 is a POOR choice for a cathode follower. Consider its low gm.

Tubes are not particularly good candidates for driving active EQ networks. Also, there is the issue of overload headroom. Passive EQ phono preamps are in vogue for good reason.

May I immodestly suggest you check this thread out. The thread and the places it sends you "surfing" describe a tweaked version of the "classic" passive EQ RCA design.

Building a nice regulated 12 VDC heater supply energized by a 6.3 VAC filament winding is very easy. Greinacher ("full wave") voltage double using 2X Schottky diodes and follow with a 7812 3 terminal regulator.

[kevinkr]

Have to agree with Eli - certainly not what I would recommend. There are so many tubes that are so much better suited to the task, and I am a fan of passive eq and two stage designs where possible.

There are a lot of really good designs lurking on this forum, and this is not one of them IMO.

Consider designs using the D3A, 5842 and other really low noise, high transconductance types. The first is very readily available in Germany - it's what I use and as a phono stage input tube it is really hard to beat.

[dsavitsk]

Also look at the 6GK5 -- cheap, plentiful, and a good performer. Easier to work with that the D3a or 5842, too. Here's one example using it, though there are plenty of others: All 7-pin Tube Based Phonostage

[Eli Duttman]

SY has pointed out that the high Miller capacitance of a 'X7 section can interact badly with some carts. and SUTs. A 6GK5, instead of an 'X7 section, in the 1st gain block of the RCA topology neatly disposes of the issue. Also, thanks to a much lower R_P, stage gain will be greater out of a 6GK5.

Frankly, I foul RIAA EQ networks up, with distressing regularity. Would some member please provide the proper values for a 6GK5 1st gain block working into a grid leak biased 'X7 section 2nd gain block. Feedback from the field has confirmed my selection of 20 MOhms as that grid leak resistor value.

[kevinkr]

Originally Posted by Eli Duttman

SY has pointed out that the high Miller capacitance of a 'X7 section can interact badly with some carts. and SUTs. A 6GK5, instead of an 'X7 section, in the 1st gain block of the RCA topology neatly disposes of the issue. Also, thanks to a much lower R_P, stage gain will be greater out of a 6GK5.

Frankly, I foul RIAA EQ networks up, with distressing regularity. Would some member please provide the proper values for a 6GK5 1st gain block working into a grid leak biased 'X7 section 2nd gain block. Feedback from the field has confirmed my selection of 20 MOhms as that grid leak resistor value.

It's easy enough to do.. One thing we do need to know is the approximate rp at your chosen operating point, assuming a ccs is used as the plate load. I have a qbasic based calculator program I can run with your numbers (I wrote eons ago...) that gets really close provided you have a Lipshitz based single stage EQ - if not Doug can probably help out.

[dsavitsk]

Unfortunately, I don't have any great insights. My calculations were done using Jim Hagerman's calculator. I accounted for rp, but not the next stage, so my design is just an approximation -- I figured rp would differ from the datasheet and change over the life of the tubes enough that getting close was good enough here. Thus, tuning was done by ear. All I can say is that people who have heard it like it, and that approximations are OK for listening to old Misfits records