High Impedance cartridges and SUT's

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dave slagle
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High Impedance cartridges and SUT's

Post by dave slagle »

I have noticed a trend lately of people asking for high ratio SUTs for higher impedance cartridges. The current request was for a 1:50 for a 22Ω cartridge and the following is why I think that is a bad idea. There are two different places where you can get into trouble here and they both have to do with the reflected load. Lets start out with the ideal with a mythical 22Ω cartridge feeding a 1:20 (green) 1:40 (Blue) and a 1:60 (red) into an open secondary where the full expected gain of the SUT will be seen.
Screen Shot 2022-07-28 at 10.19.30 AM.png
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The gains are the expected 26dB, 32dB and 35.6dB and this is about as far as the average SUT user gets. It is critical to dig a bit deeper. All MM phono stages have a 47kΩ input resistor and when we add this to the mix the problem of the reflected resistive load becomes apparent. The addition of the 47kΩ resistive load causes all three of the outputs to drop with the 1:60 actually giving less output than the 1:40! This all makes perfect sense when you consider that the 1:60 will reflect back a 13Ω load to the 22Ω cartridge which quickly explains where all the gain goes. For a more thorough explanation have a look at: SUT Design
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The simple solution to this is to understand that the typical 47kΩ input resistor is there for MM cartridges and can only be a liability for a properly wound SUT. Simply increasing the value to 300kΩ goes a long way to get closer to the gain predicted by the turns ratio.
Screen Shot 2022-07-28 at 10.40.39 AM.png
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Unfortunately the reflected resistive load is only 1/2 of the battle and we also need to look at the reflected capacitive load. The input of a typical mm phono stage also has a fair bit of capacitance in order to deal with resonant behavior at the top of the audio band. This too is a liability for a SUT and the capacitance (both cable and phono input) should be kept to an absolute minimum. The next set of plots shows what happens when we simply add 100pF of capacitance to the 300KΩ resistive load. It is the reflected capacitance that has a severe impact on the high frequency extension. This is what limits my suggested maximum step up ratio to 1:20 for this case.
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To finally complete things, lets look at this situation for a typical MM phono input of 47kΩ + 200pF of input+cable capacitance.
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In this 'worst case' scenario the 1:20 is -1dB @ 50kHz which is right at the edge of what I would call acceptable.

dave
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Stegie
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Re: High Impedance cartridges and SUT's

Post by Stegie »

Hi Dave,

how about low impedance cartridges? I am using a MySonicLab Ultra Eminent with 0,3 mV output and 0,6 ohm impedance. Is it possible to design a step-up that puts this cartridge in its sweet-spot?

A ratio of 1:15 seems about right, but how about a 1:12? I rather have a bit less gain and future ability to use another cartridge, like the MySoniclab Platinum 0,5 mV/1,2 ohm.

Regarding silver, there is much hearsay that is can sound bright and harsh. What is your experience and what would you recommend?

Best Erik
dave slagle
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Re: High Impedance cartridges and SUT's

Post by dave slagle »

The achilles heel of a 0.6Ω cartridge is the tonearm wiring is often double that value effectively turning it into a 2Ω cartridge.

I would confirm the tonearm DCR prior to committing to a design.

dave
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Stegie
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Re: High Impedance cartridges and SUT's

Post by Stegie »

I had not thought of that.

Measurement of the cartridge including the tonearm-wiring shows 2,4 ohm and 2,3 ohm on respectively the left and right channel. My interconnect ads another 0,1 ohm. My multimeter test-probe leads show a resistance of 0,3 ohm which should be subtracted.

All in all a step-up would see 2,2 ohm and 2,1 ohm on respectively the left and right channel.

Erik
dave slagle
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Re: High Impedance cartridges and SUT's

Post by dave slagle »

right.... and from the perspective of the SUT it is being driven from a 2Ω source or 3X the published 0.6Ω. I used to do SUT's specific to the sub 1Ω carts until I realized that the tonearm DCR has a dominant effect on the situation.
dave
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Stegie
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Re: High Impedance cartridges and SUT's

Post by Stegie »

my output is 0,3 mV and a 1:15 step-up seems right. But how about a 1:12 step-up so I would also be able to use a o,4 or 0,5 mV cartridge in the future.
Is it possible for this range to design a step-up in its sweet spot?

Erik
dave slagle
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Re: High Impedance cartridges and SUT's

Post by dave slagle »

The difference between 12 and 15 on the step up is only 2dB so for the most part I would consider them interchangeable.

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ckniker
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Re: High Impedance cartridges and SUT's

Post by ckniker »

Stegie wrote: Wed Sep 21, 2022 9:26 am Measurement of the cartridge including the tonearm-wiring shows 2,4 ohm and 2,3 ohm on respectively the left and right channel. My interconnect ads another 0,1 ohm. My multimeter test-probe leads show a resistance of 0,3 ohm which should be subtracted.

All in all a step-up would see 2,2 ohm and 2,1 ohm on respectively the left and right channel.
How do you measure the output impedance of your cartridge?

Frankly, I'd be terrified about hooking up a multimeter to my low-Z cartridge for fear that something might get blown. Am I being too cautious?

In a separate, off line conversation I had with someone, it was suggested I do the following: "you can use a test record and load the cartridge directly with a low value resistor, record the drop in output, then calculate what the effective impedance is."
dave slagle
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Re: High Impedance cartridges and SUT's

Post by dave slagle »

this is a "loaded" question.

My answer for myself is I always simply use a DVM to measure the resistance.

HOWEVER!

I am not going to ever suggest that to anyone else due to the small chance that something goes wrong in the process with a multi-thousand dollar cartridge. and I get blamed.

That said, the two concerns that are often voiced are:

The meter will blow up the tiny wire in the cartridge.

to which I answer the denon 103 is wound with what appears to be #54 wire and represents the thinnest wire I have seen in a cart. #54 will easily handle 1ma of current all day long without issue and 1ma is more that 100X the current that a DVM applies to a cartridge and the cartridge output itself is typically several multiples of what a meter applies so the "blowing up" just ain't gonna happen.

To test for DCR, meters put a DC potential across the cartridge that will permanently magnetize the cartridge.

by necessity, the working bits of a cartridge are all submerged in a fixed magnetic field which means they are already magnetized! For reference to fully magnetize an alnico magnet from a 103 takes 50,000µF charged to 100Vdc discharged through 10 turns of wire. This is the kind of current that will vaporize wire and if this is what is required to magnetize the circuit a similar amount is required to also demagnetize it.

If you do decide to simply put the DVM probes across the pins of a cartridge I say you will be fine but my lawyers insist I cannot say this in public so you didn't hear that from me :-) I will say that the "resistive loading" method is equally valid and the only caveat there is you are measuring the impedance of the cartridge + tonearm cables. Unknown to most, the tonearm wiring can add up to 2Ω into the mix which can be substantial for some of the super low Z carts coming out of Japan. This brings up the question of how you measure the tonearm cable DCR without permanently magnetizing it ;-)

dave
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