My RIAA Phono Stage Project

[sorenj07]

here's the schematic:




hopefully i'll get to building soon.

edit: i should cite this site ( http://www2.famille.ne.jp/~teddy/pre/pre3b.htm ) as a source, and the folks at diytube for some excellent insight, as usual.

[Original Burnedfingers]

I think I will try it also. Looks simple enough.

[SY]

It's interesting to do some simulations for the power supply. A few minutes with PSUD will convince you that at these low currents, three cascaded RC sections will have two or three orders of magnitude lower ripple and noise. The Rs are necessarily large values because of the need to drop voltage and that makes ripple currents low. For fun, start with an RC input and two more RC sections with about 3k and 100uF in each section. I like to put small resistors (a few ohms) in series with the filter caps to swamp out any nonideality that they have; in PSUD, you can do that directly in the cap specification. Anyway, that will get you down to a couple of microvolts.

edit: why is that 3k resistor 10W?

[Tweeker]

RCs will remove HF noise better too, though I like choke input for 1st section.

[sorenj07]

OK, first off, this is an OK schematic right? It's getting a lot of flak from different people, mostly in that the output Z is high (I don't know how to work Pspice so I haven't simmed it), that 12AX7's are bad, that feedback is bad, etc. The guy who built it really seemed to love it, which caught my fancy.

Originally posted by SY
why is that 3k resistor 10W?

I'm not sure, I just figured 10W was safe. 280V * .003A = .84W so 2W would probably be fine. Thanks for pointing that out

I also hear I should parallel a 20K resistor to the first 22uF cap in order to get a good current draw?

guy who built it = here at
http://www.diytube.com/phpBB2/viewtopic.php?p=5379#5379

[SY]

The resistor is not dropping 280V, it's dropping about 10V. 3mA x 3k, more or less.

The preamp circuit itself... well..... errrrrrr.......

Doing something right in the Dynaco format is a very hard job. And that's the reason it hasn't been done after 5 decades and millions of experimenter hours.

[sorenj07]

Originally posted by SY
The resistor is not dropping 280V, it's dropping about 10V. 3mA x 3k, more or less.

The preamp circuit itself... well..... errrrrrr.......

Doing something right in the Dynaco format is a very hard job. And that's the reason it hasn't been done after 5 decades and millions of experimenter hours.

What do you mean, the "dynaco format"? Is that the whole idea of building up gain, then using the feedback to cut out the EQ? I'm shopping for a better schematic, so don't feel bad about not pulling any punches

[SY]

Two 12AX7 in cascade, no buffering of the output or the drive of the feedback loop. There are a couple of variations here, mostly detrimental to performance.

The problem is that an RIAA stage with low noise, accurate equalization, low distortion, and good overload characteristics is hard to do. Morgan Jones's designs in his book seem excellent. So do Allen Wright's in "The Preamp Cookbook." I think he publishes some of these on his website, www.vacuumstate.com . You'll notice cascodes, current sources, complex buffers, and active regulation. Like I said, hard.

edit:

http://www.vacuumstate.com/various/SP-15_Article.pdf

http://www.vacuumstate.com/schematics.htm

[Original Burnedfingers]

The preamp circuit itself... well..... errrrrrr.......

SY,

If you have a suggestion....


Granted the supply can use some tweaking and the circuit might not be the greatest but then again I really haven't seen anything great pop up here recently.

Its about in line with the average piece arriving from Hong Kong being marketed at the $250 price figure. It would be an inexpensive piece to try and maybe something that would lend itself to some cheap tricks. Just MHO.

Being phono preamp poor I personally would jump at about anything..... I have managed to purchase two turntables and have a stack of records but no RIAA preamp.

[Giaime]

The feedback circuit seems, at least to me, missing a capacitor, but I may be wrong.

For a simple phono stage: