Vintage Mic Transformer for MC step-up - gain-phase and impedance measurements

[Cqwet Dbdfte]

If your setup maxes out at -55dBu noise, (1mV rms) the cart is not the main issue, however the measurement is done. The problem source should be tracked with a wideband diff probe. Line freq noise from rectifiers would be high on the list of suspects. A spectrum analyzer would help greatly to find the nature of the noise. All electronics in the chain is suspect, along with wiring, power supplies, motors, or anything generating magnetic fields.
As to transformer core size, there are simple formulas for figuring this out, the core inside the carts is a clue, as is commercial offerings from Jensen and others. If transformer is placed very close to the cart, and diff wiring follows, and the noise persists, the problem is not the carts wiring, but everything after it.
I get 1mV of broadband rms noise "needle-up", at my speaker terminals with gain set for a very loud output. I stand by my measurements.

 

[CJ]

Core loss increases with weight. Imagine an MC cart trying to pass signal through a nickel core the size of a Hoover dam transformer. Nothing would come out the other end. So you want the core to be as small as possible. Saturation will not be an issue at phono cart levels..


A larger core might mean fewer turns but because the turns are larger, the capacitance will be just as high if not higher than a small core with more turns.

There is no free lunch with a Zobel network. Your response might get flatter but the source will have to supply extra current due to the added load. What will this do to your sound?

That is why people pay 5 grand for a WE 618b input that is flat to 60 kc with no network.

 

[JohnRoberts]

Phono preamp designers weren’t thinking too much about it back then either. You might think a designer would be interested in the actual performance of the device. Apparently not. It appears to be news that the real world performance of a phono system has a noise floor of about -55dBu dominated by dirty nasty hash.

Circuit designers focus on things that they have control over, like playback electronic circuitry.

JR

PS: I am glad to leave vinyl in the rear view mirror, after the technology was eclipsed by digital media, decades ago.

 

[Cqwet Dbdfte]

Vinyl has seen some resurgence, I guess the mp3 generation has discovered grunge.
Some on these forums have commented on the limitations of vinyls program content, compared to digital, which may be a reason for vinyls advantage, simplicity.
More is not always better. While CDs were touted as long lived, I have found the opposite, 1950's vinyl sounding OK, and 1990's CDs having "CD-rot", i e. porosity in lacquer layer allowing air to oxidize aluminum layer to form holes. Gold sputtering of CDs is of course better than aluminum and avoids this, as would really thick sealing of the metal layer.
The limitations of vinyl would actually make it better due to the contents restrictions.
I have not made the leap to hi-rez digital but I would think it being superior to 16bit 44.1KHz contrived in the mid 1970s, and lower rez formats.
I do not look forward to replacing my ~ 500 records with 24/96 etc.
The digital domain opens up a different can of worms, records I can play with only four active devices in the signal chain, but I have six, between my ears and the vinyl.

 

[My3gger]

There is no free lunch with a Zobel network. Your response might get flatter but the source will have to supply extra current due to the added load. What will this do to your sound?

That is why people pay 5 grand for a WE 618b input that is flat to 60 kc with no network.

I thought we use a cap between the load and zobel filter to avoid additional loading of the mic.
In BA-2 tests one new mic input similar to Jensen came close to 50kHz unloaded, i can repeat the test to see if it goes higher.

 

[Gold]

No one has argued a noise floor of -55dBu is sufficient for record reproduction. That surely would have been the answer in the 1960’s. It shouldn’t be the answer now.

 

[JohnRoberts]

No one has argued a noise floor of -55dBu is sufficient for record reproduction. That surely would have been the answer in the 1960’s. It shouldn’t be the answer now.

In the 1980s CBS designed and promoted a vinyl companding NR system, CX to expand the dynamic range of recordings.

My kit company was granted a license and designed a record playback decoder that we ran on the cover of Popular Electronics magazine. I still have a few CX encoded records in my closet. The system added a couple tens of dB downward expansion to the noise floor.

They tried to make it operate relative to a fixed threshold, hoping consumers would accept the extra compression during playback without decoders (kind of like Dolby tape NR).

That was just one of several weaknesses in the CBS program and the technology/program failed.

JR

 

[Gold]

A balanced interface would solve the problem. It wouldn’t have to cost more either. It could be done with STP ethernet cable and RJ45 connectors on the cheap. And of course you could use very expensive stuff too. Far be it from me to deprive anyone from a living.

 

[JohnRoberts]

A balanced interface would solve the problem. It wouldn’t have to cost more either. It could be done with STP ethernet cable and RJ45 connectors on the cheap. And of course you could use very expensive stuff too. Far be it from me to deprive anyone from a living.

My 1980 phono preamp kit had a balanced input topology and some puke from TX came after me because he managed to convince the USPTO to give him a patent on the concept. Junior, wet behind the ears, patent examiners generally couldn't find their butt in the dark.

I sent the puke in TX a zerox schematic of a tube RIAA amp with a transformer (balanced) input, and told him to stop bothering me. I put a wire jumper in one input to avoid any legal liability however unfounded.

I returned to using an unbalanced input topology for my last (final) phono preamp (with no customer complaints).

JR

 

[Gold]

(with no customer complaints).

JR

That's because apparently everyone believes the fake noise numbers and no one bothered to actually measure (I'm not picking on you specifically). I'm sure the original designers of the interface knew exactly what was going on and deemed the noise floor sufficient for most users. A noise floor of -55dBu is probably sufficient for the average living room listener. It is woefully inadequate for a professional setup.

The most common reason no one says anything is that everyone thinks, "it must be me". Even Abbey made a half hearted excuse that his setup was a humble consumer setup. The humble consumer setup is no different than the expensive consumer setup or a professional setup.

 

[Gold]

There are ways around the problem but it requires jumping through hoops. With an MC cartridge you could run shielded twisted pair from the cart to the step up transformer. Locate the transformer about an inch from the phono input jacks. That lowers the noise floor significantly.

Then there is the driven shield approach that Neumann used. Impractical and somewhat confusing and difficult to implement. The PTS easily beats the noise floor of this approach.

 

[CJ]

what about opamp in the tonearm?

 

[JohnRoberts]

what about opamp in the tonearm?

Audio Measurement Resources

asked and answered...

JR

 

[Gold]

what about opamp in the tonearm?

That means reengineering every tonearm. Wires for power would have to be run too. You could have a balanced interface next to an unbalanced interface. It would be backwards compatible. The worst thing that would happen if you get the connection wrong and short one side of the balanced connection to ground is you's loose the noise advantage.

 

[CJ]

oh well,

https://www.diyaudio.com/community/...the-tonearm-just-behind-the-cartridge.361143/

 

[JohnRoberts]

that was easy....

JR

 

[CJ]

I lived in Googleville for a while (Mt. View)

they put wifi transceivers on top of all the street lamps so the whole town got free wifi.

too slow so i put a coffee can with a piece of 1 5/8" copper in the middle, stuck it up a redwood tree and aimed it at the main antenna on top of their building which increased speed tenfold.

 

[Gold]

I stand by my measurements.

My phono playback system has a noise floor of -70dBu. You said your system has a noise floor of about -45dbu in one channel and -55dbu in the other. Those numbers are bogus but let’s say they aren’t for the moment. Certainly you should be able to get closer to my number by following your advice to find the source of the noise. Let. Us know when you have a noise floor of -70dBu. With the cartridge hanging in free air.

 

[Cqwet Dbdfte]

At which point are you measuring your noise?

 

[Gold]

At which point are you measuring your noise

Cartridge hanging in free air. Measure the line output of the phono preamp.