SUT Options for Audio Technica AT-ART7 Cartridge

[Decibel_116]

I am looking for some suggestions for properly gain staging this cartridge. I would prefer to use an SUT with it because of the extremely low output voltage (0.12mV). A 1 to 20 SUT would put the output at about 2.4mV, which should be fine with the correct active gain stage behind it. However, the reflected impedance the cartridge will see is slightly less than optimal. The rule of thumb is at least 10x the source impedance. This puts the minimum acceptable at 120 ohms. The SUT puts the impedance at 117 ohms. I could load the secondary side with a network to improve the matching. Has anyone else tried to optimize an SUT for this cartridge?

 

[Ken Boyd]

I can't answer your question, but it would be nice if the manufactures had some recommendations as to best practices to achieve the correct impedance and gain, in respect to each cart they make.

 

[Decibel_116]

I can't answer your question, but it would be nice if the manufactures had some recommendations as to best practices to achieve the correct impedance and gain, in respect to each cart they make.

Yes, for instance a recommended SUT ratio and maybe a circuit schematic for an impedance matching network for proper damping.

 

[bobins08]

I am looking for some suggestions for properly gain staging this cartridge. I would prefer to use an SUT with it because of the extremely low output voltage (0.12mV). A 1 to 20 SUT would put the output at about 2.4mV, which should be fine with the correct active gain stage behind it. However, the reflected impedance the cartridge will see is slightly less than optimal. The rule of thumb is at least 10x the source impedance. This puts the minimum acceptable at 120 ohms. The SUT puts the impedance at 117 ohms. I could load the secondary side with a network to improve the matching. Has anyone else tried to optimize an SUT for this cartridge?

For this application I think you are close enough to call it good.

As you noted, the 10x is a rule of thumb to make sure the majority of the output voltage is dropped on the phono stage input load resistor.

If you go with 1:18 you get 2.1mv and 145 ohm load. Until you hear both, you won’t know what you like better.

Maybe @rothwellaudio will chime in, he knows this stuff better than most.

 

[rothwellaudio]

The rule of thumb is at least 10x the source impedance. This puts the minimum acceptable at 120 ohms. The SUT puts the impedance at 117 ohms. I could load the secondary side with a network to improve the matching.

10x the source impedance is a good rule of thumb and 117 ohms is so close to 120 ohms as to be near spot-on. The difference is only 3 ohms, which is only 0.2dB when expressed as dBs. Really, 3 ohms either way won't make any audible difference and is only slightly higher than the tolerance on 1% resistors.
The only way to get the cartridge to see a higher impedance would be to have a higher input impedance on the phonostage or to add some resistance in series with the phonostage. A higher input impedance on the phonostage may or may not be practical, but in my opinion isn't worth the hassle to increase the load seen by the cartridge by 3 ohms. Adding resistance in series isn't a good idea at all and will only degrade the signal-to-noise ratio.

I agree that 1:20 is a sensible ratio for the ART-7 and 2.4mV into the following phonostage would be enough. You could go for a higher ratio, maybe up to 1:30, at the expense of a lower load impedance. However, with higher turns ratios you may run into other problems.

 

[Wildcat]

The rule of thumb is at least 10x the source impedance. This puts the minimum acceptable at 120 ohms. The SUT puts the impedance at 117 ohms. I could load the secondary side with a network to improve the matching. Has anyone else tried to optimize an SUT for this cartridge?

The ART7 has a minimum impedance spec of 100Ω, and that 10x rule of thumb puts the impedance pretty much in an ideal place with a standard 47kΩ input impedance. Three ohms isn't worth fretting over.

My phono stage has a 49dB gain to it, and a 1:20 ratio is a ~26dB gain, so I am hoping 1:20 would be enough gain for mine (~75dB net). As you may have guessed, I also have an ART7 and am at the same stage in the game as you are. (I was previously using a Phonomena II+, but it is too noisy for such a low output cartridge.)

I'm considering the Lundahl LL9226 which offers me three ratios (1:5, 1:10, 1:20) for future use--I may buy a circuit board made for this transformer so I can make my own case with the quality of components I'm seeking. Or I might choose to get one already in a case from K&K Audio: http://www.kandkaudio.com/moving-coil-phono-step-up-kits/

FYI: Rothwell Audio's MCL has a 1:20 ratio.