Balanced input stage beneficial for unbalanced turntables?

[Charles Hansen]

CMRR ~90dB (thus some 40dB down from the practical limit), as per nomogram (Fig.4) in the Whitlock paper.

But the problem of a coax on an diff-input is elsewhere.... the different stray pickup.

Thanks on behalf of Steve Eddy -- you did his homework for him. Now we all know that his suspicions were *well* off the mark....

And I agree that it is obvious that the hum picked up by the shield of a coaxial cable will be radically different from the hum picked up by the center conductor. And I further agree that is the problem of trying to use a co-axial cable with a balanced input.

 

[KBK]

I'm getting the impression that no-one has anything useful to add to the conversation.

To say that the conversation (thread) over at the 'vinyl engine' was about 1000% more useful--would be a gross understatement.

I could learn more in one offpost in that thread than 1.5 pages of crap here.

Which is sad.

So... the consensus in the vinyl engine thread was that large gain on a single stage is a no-no. (duh).

I'm curious if it is possible to build a test rig to easily test ad grade both halves of dual op amp for matching the aspects of said amps..and then use the select units as the inputs..then go to single op-amp and a dual stage passive RIAA. IE, one dual opamp on the balanced input with 'middling' gain (not 1:1) and then passive RIAA stages.

Whether this would beat out a single ended phono stage of 'similar expense' is the deal here. Would a well constructed cheapo balanced input (lets say at $25 to construct) sonically beat out a $25 single ended design?

That's the kind of question I'm looking to address. I mean, throwing money at a shot at a good design can and many times does win out sonically. As they say in the world of loudspeaker design-any buffoon with unlimited funds can make a great speaker.

Now, make one cheap. That's a difficult task.

Same question here, is my angle.

I will freely admit I know squat about the idea of balanced phono stages, but I'd hazard a guess that only the input is balanced and the rest is single ended. I can't possibly imagine the benefit of balanced RIAA EQ. Sounds like a recipe for disaster to me. I could be wrong, but hey, slap me in the forehead here, tell me something I don't know.

 

[leadbelly]

KBK, WTF kind of a post is that? You are trying to hijack my thread, knock this forum, and can't be bothered to do a basic search on what a balanced phonostage looks like?

 

[KBK]

Apologies. I'll cease and desist. I thought the topic was balanced phono. I went looking. I didn't find all that much. This is one of the few things I found. I was attempting to keep it on topic, I thought, but Charles and Steve seem to insist on 'having at' one another.

 

[SY]

I will freely admit I know squat about the idea of balanced phono stages, but I'd hazard a guess that only the input is balanced and the rest is single ended.

It can be done either way. Looking at one excellent design (Morgan Jones's balanced RIAA stage from "Valve Amplifiers" 3rd ed.), it's balanced all the way through. So is Allen Wright's preamp, shown here.

My own stage is on the lines you describe, using an input transformer with excellent balance, and the remainder single-ended. I'm not sold on the need for balanced other than in low level transmission over a meter or more.

 

[leadbelly]

It may be just nitpicking, but I don't think it can be called a balanced phonostage if it isn't balanced throughout. Of course the input stage is where high CMRR will show the most improvement.

I would be interested in finding out if there are tube and discrete SS analogs to Whitlock's circuit for opamps that I posted. So far, all I know for best CMRR is LTP + CCS, both tubes and discrete.

 

[SY]

I'll take one thing back- I do see the virtue of balanced operation for a cascoded circuit like Allen's. Supply rejection is very critical there.

 

[Brett]

I'll take one thing back- I do see the virtue of balanced operation for a cascoded circuit like Allen's. Supply rejection is very critical there.

Yes, you need a well implemented and very quiet PS to bring out the best in the RTP3C front end.

 

[Conrad Hoffman]

Never tried one for phono, but I can't imagine any better scheme for reducing hum than a balanced input. I do have a suspicion, however, that a differential amplifier will never quite equal the noise performance that can be achieved with a single ended amplifier because you need twice the active devices.

 

[EC8010]

I do have a suspicion, however, that a differential amplifier will never quite equal the noise performance that can be achieved with a single ended amplifier because you need twice the active devices.

That's right, the noise goes up by 3dB.

 

[Onvinyl]

Every real world phonoamp I have seen has more issues due to noise pick-up through the cartridge and the cabling then due to inherent noise of the input diff.

What I will try one day, when other projects are finished, is a p-jfet diff pair, ccs'd, and folded-cascoded with two tubes. These should allow quite some current, so one could probably parallel the fet's. The next stage shall change the signal to single-ended and have the riaa. The output stage should be again with tubes, for to take the huge signal swing when the vinyl pops due to dirt.

I disagree wrt the gain, I think the first stage should provide as miuch gain as possible both to feed the riaa network with enough current, good control of overload (due to popping, again) and good noise performance (the input stage should have the best input noise performance)

I never build with tubes so far, so it will take some time till I'll try.

Rüdiger

 

[mlloyd1]

Originally posted by Onvinyl
Every real world phonoamp I have seen has more issues due to noise pick-up through the cartridge and the cabling than due to inherent noise of the input diff.

this has been my experience, too. consequently, i am less concerned by the 3dB noise penalty of diff input versus single ended.

mlloyd1

 

[GlidingDutchman]

You can do a balanced pre-amplification setup with two Single-Ended phono stages. Search the Van Den Hul site for the explination by mr AJ vd Hul himself... very innovative!

D

 

[EC8010]

If you're using a moving coil cartridge you can have your cake and eat it. You wire your arm wiring to be balanced (twisted pair, plus external screen) and take it via a balanced connector of your choice into a transformer that has been wired to be balanced (no input terminals connected to earth). Even better, you use a transformer that was intended to be used this way and that has balanced impedances to earth, such as the Sowter 8055 or (I believe) the Jensen JT-346. The output can then go to a conventional single-ended amplifier and avoid the 3dB differential pair noise issue. This is how microphone amplifiers in professional mixing desks work.

 

[forr]

Hi EC8010,

===quote:
Originally posted by Conrad Hoffman
I do have a suspicion, however, that a differential amplifier will never quite equal the noise performance that can be achieved with a single ended amplifier because you need twice the active devices.===

---That's right, the noise goes up by 3dB---


I am in difficulty here.
3 dB more noise, yes but 6 dB more amplification, I think.
So maybe less noise in the end. Am I wrong ?
Some good and very silent mic preamps are buit with a differential input pair, aren't they ?

 

[Conrad Hoffman]

forr, that's why I still have some doubts, but in a real world design I did (not a phono preamp) that was near theoretical noise limits, going to a differential input always cost me a noise penalty. OTOH, I absolutely agree that hum is a bigger problem in phono preamps than noise, so I'm just bringing up the point, not suggesting that differential is a bad thing to do. Obviously signal to noise ratio is set in the first gain stage, and nothing that comes after can improve it, so you have to look at gain distribution (most of it up front) and the performance of that first stage.

 

[SY]

I think the first stage should provide as miuch gain as possible both to feed the riaa network with enough current, good control of overload (due to popping, again) and good noise performance (the input stage should have the best input noise performance)

Fully agree. In my phono stage, the first stage has close to 60dB gain, just to get stuff out of the muck. The Sowter 8055 that EC8010 mentioned is responsible for 20 of those dB, and its input balance is so good that hum pickup really is much lower than stage noise. It's a remarkable transformer.

 

[EC8010]

3 dB more noise, yes but 6 dB more amplification, I think.
So maybe less noise in the end. Am I wrong?

I was assuming that the gain had remained the same. You'd expect the noise to double because each input has only half of the input signal, but when the noise from the two sources (each input) is summed, it only goes up by 3dB because the noise sources are uncorrelated, whereas the signal goes up by 6dB (because those signals are correlated), so the total degradation is 3dB.