moving coil loading

The preamp only supply 6 sets of loading resistors, that is the reason I recommend the metal film resistor kit, it contains 1280 pieces 64-value ±1% metal film resistors for $10. My suggestion is start with 82Ω or 100Ω, and try different values ​​​​to find what sounds best to you.

https://www.amazon.com/Resistor-Assorted-Resistors-Assortment-Experiments

Wow so much info, making my head spin. I was going to try copy and paste pages 4+5 from the manual which discusses mc loading, but if you follow the link above you can look at it directly. I just sent the preamp out to be checked out and potentially upgraded, that's why I wanted to research the loading issue and figured this is the best time to get it right.

I do appreciate all the comments and help

@atmasphere 

Thank you for the circuit analysis, greatly appreciated.

@imhififan If that schematic is correct, that preamp is susceptible to RFI injected directly into its input via the tonearm cable. So the 30dB peak that can result from certain LOMC cartridges would likely cause the phono section to make distortion- which would likely cause it to sound bright.

If you look at the left channel input, you'll see that the input isn't directly inside the feedback network. So when the peak is active (which can be energized and set into oscillation by the cartridge signal) it can overload the input, creating a distortion similar to 'transient intermodulation distortion' that occurred with some high feedback amplifiers made about the same time.

Loading is probably the most practical solution; If the input were modified with RF protection (and stopping resistors in the circuit) it could be rendered immune. But most phono section designers didn't take the significance of putting an inductor in parallel with a capacitance... they figured if they had enough gain and proper EQ they were home free.

I would start with something a lot higher than just 10X (maybe start with 1000Ohms) since what you're up to is killing the RFI generated. The highest value that can kill it will likely also be the best sounding.

If the phono section is resistant to RFI at its input, its plug and play with no worries about loading

@atmasphere ,

That’s the problem!

We don’t know how resistant the Acoustat Trans nova preamp MC phono section is to RFI.

IMO, a load resistance of 10 times the coil impedance or higher will not severely load down the cartridge, and my advice to the OP is to try different values ​​and find what works best for him.

BTW, you can find the schematic of Acoustat Trans nova preamp HERE in page 8 and 9.

Please correct me if I'm wrong.

@vtacoustatx You are not loading the cartridge to any effect you can hear until the load is extremely low.

The loading resistor is there for the benefit of the phono preamp, which apparently is sensitive to Radio Frequency Interference (RFI).

The cartridge is an inductor and the tonearm cable has capacitance so they have a resonant frequency which is often 1MHz or higher. When energized, this resonance is essentially RFI that can mess with the input of many phono sections; hence the resistor, which detunes the resonance.

So ignore the stuff you commonly see about '10x the impedance of the cartridge'. All that's important for your phono section is the elimination of RFI. So the highest resistor value that does that is the right value.

BTW, when you load the cartridge you make the cantilever stiffer since you are asking more work of the cartridge. The industry standard for cartridge loading is 47,000 Ohm (47K); when you make it drive 400 Ohms you are asking two orders magnitude more work and that has to come from somewhere.

That additional stiffness is decreased compliance so the cartridge may not track as expected in the arm. It also reduces the ability of the cartridge to trace higher frequencies (both measurable and audible) which means there is a risk of the loading resistor becoming a tone control.

If the phono section is resistant to RFI at its input, its plug and play with no worries about loading (this isn't true for MM high output cartridges; that's an entirely different topic).

I went from knowing nothing to learning here, then researching ... for myself and friends.

I made a virtual system with copies of SUT pages I have gathered

sut info

you will need to increase your screen’s zoom to read some charts

..................................

SUT indications can be quite confusing, i.e.

my Fidelity Research FRT-4, in round numbers:

setting 3 ohms is x Factor 36. (+31 db)

setting 10 ohms is x factor 21 (+26db) (possibly what your #10 is referring to?)

setting 30 ohms is x factor 18 (+25db)

setting 100 ohms is x factor 11 (+20db)

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a borderline low signal strength needs to be checked against your preamp’s mm phono sensitivity, i.e. my mx110z preamp's phono sensitivity is 3.0 mv. thus the calculated 3.2mv above would not be my choice.

must be an error, load impedance guidance: probably 100 ohms, not 10.

gain and load are inverse. high gain/low load. low gain/high load

yours is a difficult combination of very low signal strength and 8 ohm resistance.

an X Factor of 25 would boost 0.16mv to 4.0mv, I like that.

X factor 25 squared = 625. 47000 / 625 = 75 ohm load, a bit low ....

X factor 20 boost’s 0.16mv to 3.2mv weaker than I would want.

x factor 20 squared = 400. 47000 / 400 = 118 ohm load, good.

don’t forget, there is a slight loss of signal strength, i.e. calculated 4.0mv might actually be 3.8mv; 3.2mv might actually be 3.0mv, just guessing here.

.........................

solution: for cartridges with these difficult relationships: a phono stage with separate gain choices and separate load choices. opens up the choices to nearly anything. they get expensive.

Or, change to a cartridge with more compatible relationship, in your case, lower impedance, or higher output.

A general rule of thumb is that the load resistance is at 10 times or higher than the coil impedance.

My suggestion is buy a metal film resistor kit, starting with 82Ω, and try different values ​​to find what works best for you.