Phono pre-amp Question

[athos56]

So I'm looking at this schematic and I realize there is a learning opportunity. Here is the question, How do I pick the input and output transformers? I think I understand the input transformer. You pick it on the basis of the MC cart? So if the cart wants a certain Z load you pick the Input transformer to match? Now the output transformer, here is where I'm confused. The Tube-Cad blog says that the output transformer needs a 1:14 step up, so would that be another input transformer? I ask because most of the ones I see step down to drive speakers. Also what about the z presented to the amp? I would want to keep the Z as low as possible? Thanks for helping me work this out

[athos56]

Two Sowter 9580 perhaps?

[analog_sa]

Originally posted by athos56
So if the cart wants a certain Z load you pick the Input transformer to match?

Not exactly. The impedance is of secondary importance. First pick up the step-up ratio based on what the cart outputs. Then adjust the 10k to match the desired loading.

The output impedance with the 1:14 seems too high to make much sense. It is probably just a proof of concept rather than a real phono stage but some sort of output buffering would be very desirable.

I don't remember seeing this article, so there may be more than meets the eye here. Link?

[athos56]

Originally posted by analog_sa



Not exactly. The impedance is of secondary importance. First pick up the step-up ratio based on what the cart outputs. Then adjust the 10k to match the desired loading.

I see, that is very helpful.

It is probably just a proof of concept rather than a real phono stage but some sort of output buffering would be very desirable.

Yeah, your probably right.

I don't remember seeing this article, so there may be more than meets the eye here. Link?

Here it is. http://www.tubecad.com/2007/09/blog0120.htm

Thanks for the help, I saw the low part count and went hmmm.

[kevinkr]

Deleted my post after reading the article.

Yeah, the output impedance will not be particularly low with the proposed step up, which is undoubtedly available somewhere.

MC input transformers with the required gains are easy to source, Sowter makes several suitable models, but they are very expensive.

Given the expensive iron employed I would not use the 12AX7A, its modest gm makes for higher than necessary noise levels although the gain provided by the input transformer will probably swamp this. Still I would use something like a pair of D3A, and that might allow the ditching of the 12AU7A CF or replace with 5687?

Search for some of the Thorsten Loesch phono designs here. (SP?) They're very good and carefully calculated designs.

Some cheap, simple phono stages are on my site as well. OK for MM and HO MC cartridges. These are very old designs I did in the late 1980's, but perform well with good parts, and can be built very cheaply.

[athos56]

Thanks kevin I'll look into your suggestions!

[Jeb-D.]

12AX7A, its modest gm makes for higher than necessary noise levels

I wouldn't choose a 12AX7 either, but do you think noise would be an issue with CMRR on his side?

[kevinkr]

Broadband tube noise is generally gaussian in nature and not correlated so I would not expect any improvement from the differential topology, the reverse in fact, perhaps ~3dB worse.

[athos56]

So was this something that could actually be built and used or a proof of concept? kevinkr, I looked at the posts you linked and have added them to my list of things to ponder. There was also this other phono-preamp on the site which used an opamp, which looked interesting.

[athos56]

And supply, there would be regulators between the +/- 22v to make the +/- 15 needed for the opamp