Single Tube RIAA preamp too good to be true?

[alexmoose]

Hello everybody!

since I built my tube power amp, I have wanted to build a Phono preamp to obtain a "Transistorless" stereo. I found a design that uses a Single 12AX7 (1/2 per channel), and I wanted to know what everyone thought of it.

I also wanted to know if it was possible to lower the B+ on it, I know that then I would have to play with the Bias in some way, any suggestions?


http://www.bonavolta.ch/hobby/en/audio/1_ecc83.htm

-Moose

[Eli Duttman]

Moose,

Notice the 270 KOhm load resistor on the 'X7 section. The large load resistance is necessary to get sufficient gain out of a single stage. If Ib is 1 mA., 270 V. are dropped in the resistor. That 400 V. B+ rail is the price that must be paid to use a high value load. TANSTAAFL! Also, that circuit has a HIGH O/P impedance. IMO, it must be followed immediately by a buffering voltage follower.

[lineup]

Originally posted by alexmoose

Since I built my tube power amp, I have wanted to build a Phono preamp to obtain a "Transistorless" stereo.
I found a design that uses a Single 12AX7 (1/2 per channel),
and I wanted to know what everyone thought of it.

http://www.bonavolta.ch/hobby/en/audio/1_ecc83.htm

Hello.

You have MM cartridge or you have MC, which requires a level more of gain?

If you like most people have MM
you should be able to find quite simple RIAA, using 2 Triode Halves per channel (=1 tube/ch).
Do not expect to find a really good RIAA using only 1 triode.
Because if they do, you most surely will have some else preamplifier to feed your power amp input.

RIAA, even Moving Magnet pickups, needs plenty of voltage gain.
This is why we need at least 2-3 triode stages per channel.
------------------------------


I went to have a look at website in your link.
There are many interesting and old tubes amplifiers of all sorts.
From that Tube RIAA collection
I judge this one as the most serious and possible best performer:

Hiraga RIAA

That's my preamp. I love it ...
I named it "First", because it was my first project.

Actually, I'll probably modify it by splitting the control buttons (source selector and volume) in a separate box, the preamplifier will be only an in-out box.

Schematic by Jean Hiraga and published in L'Audiophile n° 21 (Sept. 81).
.

Hiraga died not long ago and is one of the Great Legends of early audio amplifier designs.
His published amplifiers are still built by many, and after many years,
yet, they can be regarded very high quality.


He uses 3 stages and Two ECC83/12AX7 Tubes per channel!
The output stage, using cathode follower into a constant current source,
is a clever idea and should be able to drive normal loads.
In schematic one 100 kohm potentiometer used for output.

Included is also Power Supply schematic, with many details.
Enjoy!

lineup





HIRAGA ECC83 - 12AX7 RIAA ... Project page

[kevinkr]

Hi Alex,
IF you check out my site you will find some very simple phono stages that work well and have been built by lots of people over the years.
The simplest requires a single 12AX7 per channel, but does have pretty high output impedance.

Here is a link to the page:
http://kta-hifi.net/projects/pre_pag...amp/ots11.html
It was originally published in Positive Feedback longer ago than I would like to admit..

There are several circuits shown, one should suit your needs quite well. I'd recommend the two tube per channel version, it's much quieter than the single tube implementation, more transparent and a lot more dynamic. It'll drive lower impedance loads better as well.

I recommend the 5751 as being somewhat quieter than the typical 12AX7. Power supply could be a well filtered unregulated 300V. Filaments should be well regulated 12.6Vdc particularly as the cathodes are unbypassed. (A single 7812 regulator with a diode in series with the ground lead, and properly heat sunk is all that is required to get the 12.6V @ 600mA this design requires.)

Note that the single tube version has an output impedance around 60K and the two tube version around 10K. Use low capacitance interconnects!

[alexmoose]

Originally posted by kevinkr


I recommend the 5751 as being somewhat quieter than the typical 12AX7. Power supply could be a well filtered unregulated 300V. Filaments should be well regulated 12.6Vdc particularly as the cathodes are unbypassed. (A single 7812 regulator with a diode in series with the ground lead, and properly heat sunk is all that is required to get the 12.6V @ 600mA this design requires.)

I really like the design, as most of the capacitors I already own, and the resistors are not hard to track down. However why do the Heaters on the 'X7s need to be so well regulated?

-Moose

[kevinkr]

The signal level at the input of a phono amplifier is only a couple of mV so hum from the filaments will be quite audible unless a lot of precautionary measures are taken, that and the fact that I eschewed the use of electrolytic cathode bypass caps in this designs means that hum and filament circuit borne noise can couple through the filament to cathode capacitance. (Electrostatic and magnetic coupling from the filament wiring could be an even bigger issue frankly.) It is a lot easier just to use regulated dc on the filaments to avoid this issue, also adequate filtering with CRC may actually cost more than the regulator circuit itself. The filament circuit in any case should be elevated about 80V above ground using a resistive divider and 10uF bypass cap. (Say 220K and 82K) I can provide additional help and details if you are interested - as my schedule permits..

[astouffer]

Try these if you are looking for something simple:

http://margo.student.utwente.nl/el/t...p/fb-e180f.gif

http://www.geocities.com/rjm003.geo/.../diy_pho3.html

I just built the 6DJ8 preamp and it works very well. Nobody seems to have tried the first schematic but I plan to sometime soon.

[alexmoose]

okay, I am convinced that I have to make a phono stage with two+
tubes. how much smoothing do I need in the power supply of this new preamp? I like the designs that run at about 250-300v as I already have a power supply built for such voltages. The problem with this is the power supply may not be smooth enough.

----340uf---4ohm----340uf---100ohm----110uf--B+

is this smooth enough? or should I add something eles in there?

[SY]

Power supply requirements are completely determined by the circuit design. Some circuits (e.g., CCS-loaded common cathode) have very good power supply rejection. Others (e.g., cascode) have nearly none. What you need to do is start with the signal-to-noise you would like to achieve, then working backward, determine how good the supply has to be to hit the target.

If you don't want to go through that exercise, just build a total overkill supply. You can't go wrong that way.

[astouffer]

Those resistor values are too low to do much filtering. Maybe change that 4 ohm to 400 and 100 to 1k. Adding a choke or two won't hurt either