Balancing output of turntable

Binaural · 7th January 2009, 07:20 AM

I have been flirting with the idea of transformer balancing the output of my Technics SL-1650. I figure I could use a 1:2 or maybe a 1:4. This would step up the output signal a bit, get rid of the separate ground connection, and keep the noise as low as possible on the way to the preamp. I didn't know if it was as easy as simply connecting a input transformer to the output or if it would require some additional circuitry. Any ideas on this? Is it a good idea?

Steerpike · 7th January 2009, 09:57 AM

1) What cartridge have you got: cartridges are inherently balanced sources, though some manufacturers choose to ground (connect to body) one pin, which slightly messes up using them in this way. You an often get around it though.

2) Does your preamplifier have balanced inputs? If NOT, then the entire operation will serve little purpose.

3) just adding a transformer will affect the load seen by the cartridge - unless its carefully chosen & compensated.

analog_sa · 7th January 2009, 10:55 AM

If your cart is MM the transformer will likely add too much capacitance. It only makes sense with MC carts and many of the step-up transformers are capable of balanced outputs (are center-tapped).

EC8010 · 8th January 2009, 09:16 PM

Quote:

Originally posted by analog_sa
... many of the step-up transformers are capable of balanced outputs (are center-tapped).

No, no, this is a common misconception. "Balanced" means balanced impedances to earth - a centre tap is entirely unnecessary and actually a bad idea. What is required is a transformer with balanced stray capacitances and leakage inductances to connect to the cartridge, which is, as pointed out earlier, inherently balanced. Sometimes, a transformer with such balancing is referred to as having a balanced geometry. Anything that upsets the balance of the impedances degrades common-mode rejection.

A transformer only makes sense with moving coil cartridges - the stray capacitances in a transformer would react unfavourably with a moving magnet cartridge. "Electronic" (op-amp techniques) balancing would work with MM but the inevitable 3dB noise degradation makes it questionable.

Steerpike · 8th January 2009, 09:21 PM

>>>a centre tap is entirely unnecessary and actually a bad idea.

And THREE CHEERS TO YOU!
You have no idea (or maybe you do!) how many people I have tried to explain that concept to... and they will NOT have it.

scott wurcer · 8th January 2009, 09:30 PM

I run a Grado balanced into a true differential amp (FET) configuration. You can probably avoid the 3dB noise degradation with any MM or MI by having the thermal noise of the cartridge dominate. The Grado is 400 Ohms resistive (2.5nV) and a 1nV diff amp is doable. It should be even easier with a MM (stick with FETs to avoid the bipolar current noise). There is a National app note from 1977 on MM noise that shows this. Unfortunately at the time there were no low noise FET amps.

Binaural · 9th January 2009, 06:05 AM

Well to clarify further, the preamp that I'm currently using is a John Broskie Aikido design (his phono pre board) which is going to have a switchable RIAA option and balanced I/O for microphones and I figured hey, why not balance my turntable. I haven't seen it before but i was hoping there wasn't a well known reason for it (to hide my ignorance haha.) I know impedence is different than resistance, but i measured the cartridge out with a VOM in ohms mode and it was a perfect 1k! I know this wouldn't be hard to find on a transformer primary.

The cartridge I have now is the Shure SC35C and it works for most things but is by no means a "reference" product. I do have an interest in purchasing a MC cartridge though. Should I wait until I have the MC cartridge to make this mod? I have a couple of Beyerdynamic TR/BV 351 015 006 1:15 transformers. Would these do well for an MC cartridge?

Also what is a decent lower cost (less than $200) MC cartridge. I know there aren't many but would a MM cartridge in the $200 price range be better than a MC in the same range?

Steerpike · 9th January 2009, 12:06 PM

You need to decide WHY you want it balanced.
The only point of balanced lines is to reject noise picked up BY THE CABLE.
If that is not a problem at present, you will get no advantage - and possibly crate some problems - converting to balanced.

analog_sa · 9th January 2009, 02:41 PM

Quote:

Originally posted by Steerpike
>>>a centre tap is entirely unnecessary and actually a bad idea.

And THREE CHEERS TO YOU!
You have no idea (or maybe you do!) how many people I have tried to explain that concept to... and they will NOT have it.

Interestingly, my AN Japan step-up has a center-tapped primary. Maybe no one has explained this futility to Kondo San.

Steerpike · 9th January 2009, 04:35 PM

The centre tap is a convenient point to connect the cable screen, but generally it ought not to be connected at both ends.
Its a complex subject, and there are many variations on a theme; different topologies can be applied to different situations, and HAVING a centre tap doesn't mean it ha to be used in all circumstances.

7th January 2009, 07:20 AM	#1
Binaural diyAudio Member Join Date: Oct 2005 Location: Monroe, NC (SE of Charlotte)	Balancing output of turntable I have been flirting with the idea of transformer balancing the output of my Technics SL-1650. I figure I could use a 1:2 or maybe a 1:4. This would step up the output signal a bit, get rid of the separate ground connection, and keep the noise as low as possible on the way to the preamp. I didn't know if it was as easy as simply connecting a input transformer to the output or if it would require some additional circuitry. Any ideas on this? Is it a good idea?

8th January 2009, 09:30 PM	#6
scott wurcer diyAudio Member Join Date: Jan 2004 Location: cambridge ma	I run a Grado balanced into a true differential amp (FET) configuration. You can probably avoid the 3dB noise degradation with any MM or MI by having the thermal noise of the cartridge dominate. The Grado is 400 Ohms resistive (2.5nV) and a 1nV diff amp is doable. It should be even easier with a MM (stick with FETs to avoid the bipolar current noise). There is a National app note from 1977 on MM noise that shows this. Unfortunately at the time there were no low noise FET amps. __________________ 2012, our time is running out.

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7th January 2009, 09:57 AM	#2
Steerpike diyAudio Member Join Date: Jan 2006 Location: Johannesburg, South Africa	1) What cartridge have you got: cartridges are inherently balanced sources, though some manufacturers choose to ground (connect to body) one pin, which slightly messes up using them in this way. You an often get around it though. 2) Does your preamplifier have balanced inputs? If NOT, then the entire operation will serve little purpose. 3) just adding a transformer will affect the load seen by the cartridge - unless its carefully chosen & compensated.

7th January 2009, 10:55 AM	#3
analog_sa diyAudio Member Join Date: Aug 2002 Location: Sofia	If your cart is MM the transformer will likely add too much capacitance. It only makes sense with MC carts and many of the step-up transformers are capable of balanced outputs (are center-tapped).

8th January 2009, 09:21 PM	#5
Steerpike diyAudio Member Join Date: Jan 2006 Location: Johannesburg, South Africa	>>>a centre tap is entirely unnecessary and actually a bad idea. And THREE CHEERS TO YOU! You have no idea (or maybe you do!) how many people I have tried to explain that concept to... and they will NOT have it.

9th January 2009, 06:05 AM	#7
Binaural diyAudio Member Join Date: Oct 2005 Location: Monroe, NC (SE of Charlotte)	Well to clarify further, the preamp that I'm currently using is a John Broskie Aikido design (his phono pre board) which is going to have a switchable RIAA option and balanced I/O for microphones and I figured hey, why not balance my turntable. I haven't seen it before but i was hoping there wasn't a well known reason for it (to hide my ignorance haha.) I know impedence is different than resistance, but i measured the cartridge out with a VOM in ohms mode and it was a perfect 1k! I know this wouldn't be hard to find on a transformer primary. The cartridge I have now is the Shure SC35C and it works for most things but is by no means a "reference" product. I do have an interest in purchasing a MC cartridge though. Should I wait until I have the MC cartridge to make this mod? I have a couple of Beyerdynamic TR/BV 351 015 006 1:15 transformers. Would these do well for an MC cartridge? Also what is a decent lower cost (less than $200) MC cartridge. I know there aren't many but would a MM cartridge in the $200 price range be better than a MC in the same range?

9th January 2009, 12:06 PM	#8
Steerpike diyAudio Member Join Date: Jan 2006 Location: Johannesburg, South Africa	You need to decide WHY you want it balanced. The only point of balanced lines is to reject noise picked up BY THE CABLE. If that is not a problem at present, you will get no advantage - and possibly crate some problems - converting to balanced.

9th January 2009, 04:35 PM	#10
Steerpike diyAudio Member Join Date: Jan 2006 Location: Johannesburg, South Africa	The centre tap is a convenient point to connect the cable screen, but generally it ought not to be connected at both ends. Its a complex subject, and there are many variations on a theme; different topologies can be applied to different situations, and HAVING a centre tap doesn't mean it ha to be used in all circumstances.

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