Diy Phono stage

[fdegrove]

Hi,

Without the noise concellation circuit, the PS noise may already be considerably below the signal level, depending on the nature of the PS. Let's say it's 60db below for example.

Wouldn't you say this is fascinating enough to merit a thread of its'own?

Mind you, I do have great interest in this, anything low signal level is a bit like RF, it does have a story to tell...and the smallest of small signals matters.

It also shows you what could go on at higher signal levels...totally capturing if you'd ask me...

Like mainframes in computors, there's a lot of info and tips to be found in the pro world.

Cheers,

[runeight]

Would make a very interesting thread! If we could get Mr. Broskie to join the forum, he could run that thread with one hand tied behind his back.

As a poor to average tube circuit designer, unfortunately, the best I can do is to refer everyone to TCJ where John gives many examples of noise cancellation tricks.

In this particular phono preamp, the output stage is a common cathode diff amp. The second grid is normally grounded, but in this case Broskie feeds the PS noise into the second grid and this noise, in phase with the PS noise introduced normally into the rest of the circuit, cancels the normal PS noise (similar to the way NFB would be employed in this topology). This design is very clever (at least to me) and with the right values of capacitors should have an extremely low noise floor.

Or did you have something else in mind Frank?

[fdegrove]

Hi,

This design is very clever (at least to me) and with the right values of capacitors should have an extremely low noise floor.

It would indeed.

Better still would be to apply the same technique of noise cancellation in the PS in the first place.
Which is excactly what you do by using series regs.

That, in a nutshell, is my major quibble with TCJ; if your not savvy or an engineer you're just left second guessing.

Now, combining both appropriately could yield very high noise reduction indeed and hopefully a very good sounding phono preamp.

BTW, it's worthwhile to go through the pages following the original article.

FWIW, noise cancellation techniques are not new at all and can be found in Pro equipment from the Fifties or maybe even prior to that period.

Thanks to J.B. we now have most of it in one convenient place on the web.

Or did you have something else in mind Frank?

No, not really...I had my 2 cents added. TA.

Cheers,

P.S. You're much better than you give yourself credit for IMO.