Audio Note IO Sut

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NickC
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Post by NickC »

Thanks for your original write-up, Dave - very informative and helpful!

I'm looking for a SUT (pair) to match a low-impedance and very low o/p cartridge (Audio Note IO) to a MM phono stage. Specs are:

Cart internal impedance 1 ohm, matching impedance 3-4 ohms, output 0.04 mV; Phono amp input impedance 47k, sensitivity 2.15 mV for 775 mV out

The phono amp is working fine with a moving iron cartridge with nominally 4 mV output, so I think the numbers actually match very well if I aim at a turns ratio of 1:110 (hence impedance ratio of 1:12.1k). That would give me a nominal output of 4.4 mV, with cartridge loading at 3.88 ohms. Assuming linear behaviour from the phono amp, the maximum output voltage should be 4.4/2.15*0.775 = 1.586V

Does this all look right to you?

I suspect the hard part will be finding SUTs with this exact ratio; would you be able to wind a pair for me? (email also sent to establish comms)
dave slagle
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Post by dave slagle »

Hey,

The 1Ω impedance and low output of the SUT makes it well suited for higher step up ratios and I have done as high as 1:100 of this cart with great results. The limiting factor here becomes the reflected load from the typical 47K input impedance of a phono stage. Also the input capacitance comes into play here as well with the high ratio SUT's.

A 1:100 into a 47K will reflect back 4.7 ohms which is at the edge of what many like and if there is 100pf of capacitance in the cabling and inout stage that becomes 1µf at the cartridge which @20kHz is 8Ω in parallel with the reflected 4.7Ω which becomes 3Ω.

There are two simple solutions to this issue.

-Keep the capacitance low and increase the input resistance to 100K or more.

-Drop the Ratio to 1:50

It is interesting to note that the cart into a 1:100 feeding a 47K will show about 3dB of voltage loss due to the source to load ratio (assuming 1Ω of tonearm cabling DCR) so figure 37dB from a 1:100 into 47K and 38.5dB into 100K and dropping the ratio to 1:50 nets 33.5dB of gain into 47K and 34dB into 100K. This essentially means that doubling the turns ratio which sin a perfect world should vive 6dB of gain will only net you 3dB due to the loading match.

I Do have mumetal shielding cans can can help you through getting them put in a proper enclosure and It would be cool to document the process here for others.

dave
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NickC
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Joined: Wed Jun 26, 2019 8:07 pm

Post by NickC »

Hi Dave,

The phono preamp is an older Audio Innovations P2, which has loading in the form of a 47 kohm resistor and a 51 pF cap in parallel on the input to the first tube (ECC83 half), so I guess it's likely to be fairly close to your estimate of the 1:100 case. The cart is recommended to be given a 3-4 ohm load so we're in the right ball park on paper - but does that sound too low based on your experience with these carts?

Where I get a bit lost is in trying to choose between a 1:50 ratio, which I guess should work pretty well with no other changes, and the 1:100 which in theory should suit the cart better, but would probably need a different resistor on the input to the phono amp. That would be an easy change, but wouldn't be necessary if the real-world performance of the 1:50 SUT is going to be very little different from the 1:100 + 100 kohm input.

Could you comment on the likely differences in behaviour between these 2 approaches?

The mumetal shielding cans sound great, and I'd be glad of any advice you can give on enclosures too.

Nick
dave slagle
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Post by dave slagle »

the 47K can be increased and the 51pf removed. This still leaves the 200pf of the ECC83 to drag down the top end by reflecting back 2µf to the cart.

I have no direct experience with the Io but do know that manufacturer load recommendations should be taken with a grain of salt and need to be determined by listening. The 1:100 would tie your hands for an additional 3dB of gain. I would go with a slightly smaller ratio and still increase the input resistor to a higher value. maybe a 1:70 and 100K would be a good compromise. You could even go for 200K at the input since the only purpose of this resistor is stability if the input is left open.

dave
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NickC
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Joined: Wed Jun 26, 2019 8:07 pm

Post by NickC »

Having checked with other users of IOs, they do apparently like to be loaded close to or within the recommended range, so ideally I'd like to go with 1:100.

That way, I can try the original 47k input with the cap removed to apply a nominal 4.7 ohm load without too much capacitance effect and without having to add a loading resistor to the SUT input. The gain is also not too high, and I still have the option to change the resistor to get a higher loading figure if I want to - 100k would reflect 10, and 200k = 20 (before taking the ECC83 into account), which should give me enough adjustment if I feel I need it.

Can you let me know what happens next -payment options etc? Please plan on installing the transformers in mumetal screening cans, as you suggested earlier.

Many thanks,

Nick
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