Fully balanced from cartridge to loudspeaker - a little help needed!

[epistaxsis]

This years hi-fi mission is to build these power amps (6 monoblocks in 2 chassis) :-D

However I have a small problem - what about the phonostage / preamp?

The completely balanced nature of these amplifiers got me thinking along these lines:

"A cartridge is a loudspeaker in reverse so if it is a good idea to be balanced for a loudspeaker why not do the same for the cartridge?"

So can someone help me and point me in the direction of decent fully balanced phono stage / preamp circuits?

I can build from a circuit diagram but I cannot design

I have had a trawl through these froums but haven't had much luck in finding what I am looking for.

[zigzagflux]

How about DIYAudio's own SY :


His Master's Noise: A Thoroughly Modern Tube Phono Preamp

[MRupp]

That one's not balanced. If it has to be tubes, check out the RTP3C balanced phono stage on Allens site: Schematics.

If it can also be solid state phono, there are examples in the MPP thread. For balanced line and power amps there are lots of schematics out there.

[atmasphere]

It is possible to use a Circlotron circuit at the output of the preamp so you can have a direct-coupled balanced output. I have found that you can operate the whole thing with zero feedback. If properly designed, you only need three stages of gain, 2 in the phono and 1 in the line stage.

[DF96]

Why is it a good idea for a speaker output to be balanced? Does this require the crossover network to be balanced too? Very few are!

Actually, in a typical domestic audio setup, the cartridge input is about the only place where balance is helpful (except for the likely 3dB noise penalty). Fortunately, pseudo balance is good enough here: balanced cable to an unbalanced phono input.

[leadbelly]

Originally Posted by DF96

Fortunately, pseudo balance is good enough here: balanced cable to an unbalanced phono input.

This doesn't make sense the way you've written it; if you indeed replaced an unbalanced cable with a balanced one and then terminated it to a truly unbalanced input your noise would go up as your balanced cable would pick up more CM noise and the unbalanced input has no CMRR to speak of. I assume you mean use a balanced cable and terminate it to something that has good CMRR such as a transformer or in amp and then go unbalanced from that point on.

[ErikdeBest]

Morgan Jones explains a balanced RIAA design in the 3rd edition of his book (Valve amplifiers). Recommended anyway if you want to move from building to designing your own circuits.

[DF96]

I assume you mean use a balanced cable and terminate it to something that has good CMRR such as a transformer or in amp and then go unbalanced from that point on.

No, I really meant what I said. A normal phono cable has a signal pair (presumably twisted) which have an overall screen. The signal pair are subject to the same magnetic induction so provided the cartridge itself is electrically floating the differential-mode signal is clean. The screen stops capacitive pickup. The result is that you can run a balanced cable into an unbalanced input and still get low noise.

The weakness in this is the assumption that the cartridge is floating, whereas in reality it will have some capacitance to the arm and cable screen. Fortunately most of the magnetic induction will be at 50Hz where the stray capacitance has high reactance.

[Steve Eddy]

Originally Posted by DF96

The result is that you can run a balanced cable into an unbalanced input and still get low noise.

But an unbalanced input won't have nearly the common-mode rejection that a balanced input does. Remember, it's the input where common-mode noise is rejected.

se

[atmasphere]

Originally Posted by DF96

Actually, in a typical domestic audio setup, the cartridge input is about the only place where balance is helpful (except for the likely 3dB noise penalty). Fortunately, pseudo balance is good enough here: balanced cable to an unbalanced phono input.

There is advantage to be balanced differential not just at the input but throughout the preamp. With each stage of gain, you can get 6db less noise than you might get with the same topology in the single-ended domain. That adds up as you add gain stages, although due to the random nature of noise, you won't get 12 db in two stages, but it will nonetheless be significant.

Also, a fully differential circuit will have distortion cancellation with each gain stage. So such a preamp will have very low even-ordered harmonics. This is another way of saying it will lack the 'caramel' 'tube sound'. Distortion can mask low level detail, so by eliminating it the preamp will be more detailed as well.

Finally, the circuit will have greater immunity to power supply noise.

Contrary to popular myth, going fully-differential will not double the number of parts.