Balanced input stage beneficial for unbalanced turntables?

[leadbelly]

I often see the recommendation to switch stock unbalanced turntable wiring to balanced. I understand how this, coupled with a preamp with good CMRR, would minimize hum.

However, in most the few turntables I have taken apart, even though the wiring is unbalanced, there is no path to ground, i.e. both wires float.

So, if you build your balanced phono input stage to terminate this unbalanced connection properly, won't you reduce hum (compared to a unbalanced input stage) since at least some of the noise in the TT wiring will be common mode? I understand that you won't get the full benefit of balanced wiring.

[SY]

There is absolutely an advantage to making that preamp input balanced if you can rewire the turntable cables.

In my setup, the cables are a pair of twisted pairs inside a single shield. The shield can be connected at either or both the turntable ground end and the phono preamp chassis. The preamp input is a DIN plug with all signal leads floating and connected to the primary of an input transformer. Despite the VERY low output of the cartridge, the setup seems to be quite insensitive to hum pickup.

[KBK]

Just the subject I was getting onto, SY! Excellent timing.

Any DIY 'balanced phono stage' ideas here on the board?

PS. I don't eat packet soup.

[Steve Eddy]

Originally posted by leadbelly
I often see the recommendation to switch stock unbalanced turntable wiring to balanced. I understand how this, coupled with a preamp with good CMRR, would minimize hum.

However, in most the few turntables I have taken apart, even though the wiring is unbalanced, there is no path to ground, i.e. both wires float.

So, if you build your balanced phono input stage to terminate this unbalanced connection properly, won't you reduce hum (compared to a unbalanced input stage) since at least some of the noise in the TT wiring will be common mode? I understand that you won't get the full benefit of balanced wiring.

Except that the wiring isn't unbalanced.

se

[leadbelly]

Originally posted by Steve Eddy

Except that the wiring isn't unbalanced.
se

This comment is too subtle for my understanding. Do you mean to say that if you terminate a pair of floating wires in a balanced fashion (i.e. both with a termination resistor to ground) that this is now balanced wiring?

[Steve Eddy]

Originally posted by leadbelly
This comment is too subtle for my understanding. Do you mean to say that if you terminate a pair of floating wires in a balanced fashion (i.e. both with a termination resistor to ground) that this is now balanced wiring?

Basically what I'm saying is that the wiring itself doesn't know balanced or unbalanced from a hole in the ground.

For example, a simple twisted pair of wires is a simple twisted pair of wires whether they're used to connect unbalanced interfaces or balanced interfaces.

se

[Charles Hansen]

Originally posted by Steve Eddy
Basically what I'm saying is that the wiring itself doesn't know balanced or unbalanced from a hole in the ground.

Gotta disagree with you on this one.

Having built and sold hundreds (if not thousands) of balanced phono stages, we have found that it is critical that the wires do not introduce any imbalance into the system.

The balanced circuit can only reject hum if it is common to both signals.

Most turntables only have a pair of coaxial cables, one for each channel. If you run this into a balanced input, you are going to get hum out the wazoo because the shielded conductor (no longer connected to ground, but instead to either the inverting or the non-inverting input) is going to pick up far more hum than the center conductor of the same cable.

The hum is no longer common mode and cannot be rejected.

If you want to use a balanced setup for your phono stage, you need to have balanced (ie, symmetrical) wires in place.

Originally posted by Steve Eddy
For example, a simple twisted pair of wires is a simple twisted pair of wires whether they're used to connect unbalanced interfaces or balanced interfaces.

Yep, a simple twisted pair would be a good example of balanced (symmetrical) cables that would work well with a balanced phono stage.

I would recommend shielding the twisted pair (or twisted quad if both channels are in the same cable) just in case the CMRR of the phono stage is less-than-perfect, but I've seen it work without shielding also.

[KBK]

With what I'm doing these days with cables....shielding is the worst kind of signal damage I have ever come across. For most other folk, carry on - shielding is likely a good idea.

[Steve Eddy]

Originally posted by Charles Hansen
Gotta disagree with you on this one.

Okie doke.

Having built and sold hundreds (if not thousands) of balanced phono stages, we have found that it is critical that the wires do not introduce any imbalance into the system.

Agreed.

The balanced circuit can only reject hum if it is common to both signals.

Ditto.

Most turntables only have a pair of coaxial cables, one for each channel.

Yeah? Mmmm. You talking about cheap turntables that use the same cheap spaghetti wires for the cable that are given away with most other mass market stuff?

Most decent turntables that I'm aware of use twisted pairs or quads from the cartridge to either a DIN plug or RCAs for the outputs.

If you run this into a balanced input, you are going to get hum out the wazoo because the shielded conductor (no longer connected to ground, but instead to either the inverting or the non-inverting input) is going to pick up far more hum than the center conductor of the same cable.

The hum is no longer common mode and cannot be rejected.

Far as I'm aware, a coaxial geometry is, as with a twisted pair, self-shielding by way of cancellation.

I think the problem isn't so much that the outer conductor picks up more hum, but rather that in a typical coax, the resistance of the center conductor isn't the same as the resistance as the outer conductor.

Bill Whitlock has demonstrated that in twisted pair cables, just the difference in wire resistance due to manufacturing tolerances is sufficient enough to significantly degrade common-mode rejection in electronically balanced inputs due to source impedance imbalances

Yep, a simple twisted pair would be a good example of balanced (symmetrical) cables that would work well with a balanced phono stage.

Yes. But, in my opinion, because the conductor resistances are the same.

I would recommend shielding the twisted pair (or twisted quad if both channels are in the same cable) just in case the CMRR of the phono stage is less-than-perfect, but I've seen it work without shielding also.

Well, I wouldn't recommend coaxial cables in a balanced system either, but for the reason I gave above.

se

[Steve Eddy]

Originally posted by KBK
With what I'm doing these days with cables....shielding is the worst kind of signal damage I have ever come across. For most other folk, carry on - shielding is likely a good idea.

Hehehe. Not a terribly big fan of shielded cables myself.

I built some unshielded phono cables for a friend of mine and even with a low output moving coil into a 60dB phono stage and 104dB speakers, there was no sign of hum.

se