I opened up the Mani for a look inside, the components on the circuit board are really tiny so I don’t think removing and replacing the input resistors is really a viable possibility. I’ve sent a message to Audio Technica UK to query the use of the ATF2 or ATF7 into a impedance of 47ohms, as yet I’ve not had any response.
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With a 47 ohm load there will be a 2dB reduction in signal amplitude compared to the open circuit voltage and a 1dB reduction in signal amplitude compared to the signal amplitude into a 100 ohm load, ie not a lot to worry about.
As for any change to the frequency response, I doubt there will be any (because the source impedance, particularly inductance, is so low). No doubt others on the forum will disagree and swear that small changes in load impedance produce audible results. In my experience the load impedance isn't that significant.
Is this graph rubbish then ?
I certainly wouldn’t be happy with either of the lower 2 response curves
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Not necessarily complete rubbish, but maybe not applicable to the AT cartridges the OP is considering.
The graph shows in general terms what happens when a source with a resistive and inductive output impedance is fed into different load impedances. However, with a resistance of only 12 ohms and an inductance of only 25uH the curves won't be anything like as bad as the ones in the graph. I'll check the figures and get back to you.
This page illustrates what's going on with an MC cartridge and loading:
http://www.hagtech.com/loading.html
However, his examples use cartridges with 500mH, 50mH and 5mH inductance. Most LOMCs have much lower inductance. The Audio Technica cartridge has an inductance of only 25 micro Henries which is 200 times less than the lowest inductance in the Hagerman examples.
Yes, the graph shows the correct form of the frequency response curves but putting in realistic values gives much flatter curves than those.
It certainly is possible, just fiddly.
The circuit uses a low noise op-amp as the input stage so the loading will be just in parallel with the input.
Trying the unit with whatever cartridge you get is the best approach, the resistor change is still on the cards should you wish.
With all the variables it looks as though it’s almost impossible to predict the outcome with any particular MC cartridge. The Youtube video I saw when a MC was tried with the Mani was with a Dynavector xx2 cartridge, this is a £1k cartridge, it also has a low coil impedance of 6ohms, so should work better into the Mani’s 47ohms input impedance, unfortunately the reviewer did not consider the combination to be optimal.
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