Cartridge loading box for MC

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ryanoceros

New Member
I'm running a Schiit Mani MM/MC preamp and plan to pick up an AT33Sa MC cartridge. The Mani only offers 47 ohms of resistance but the AT33Sa recommended loading is > 100 ohms and some users are reporting preferred performance at greater loads (400 ohms, 1k ohms). So I need to add resistance. I was initially going to add a 100 ohm resistor to each RCA cable, but I think I would like to make this adjustable and dial it in for my system. I'm planning to build a simple box with 4 female RCA plugs and a 1k ohm dual gang potentiometer. The pot gangs would each be run in parallel with the cartridge signals.

Anything I'm missing here? Are there any concerns I should be aware of that may affect sound quality?
 
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Sorry I left out some crucial information. The Mani also offers 47k ohms but it is ordinarily worthless for use with a MC cart. I would use the 47k ohm setting with the 1k ohm pots in parallel to decrease the resistance. I should probably add a 50-100 ohm in series with each pot gang, then we have roughly 100-1000 ohms of range.

Any downside to using a potentiometer over discrete resistors?
 
Noise and tracking between the channels. If you two pots then you will need test records and a very accurate true RMS volt meter to measure and balance the levels after the phono stage. Remember most cartridges are off balance anyway so this will be your chance to correct that too.
 
ryanoceros said:
Sorry I left out some crucial information. The Mani also offers 47k ohms but it is ordinarily worthless for use with a MC cart. I would use the 47k ohm setting with the 1k ohm pots in parallel to decrease the resistance. I should probably add a 50-100 ohm in series with each pot gang, then we have roughly 100-1000 ohms of range.
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That makes a lot more sense than what I thought you were proposing.
 
ryanoceros said:
Any downside to using a potentiometer over discrete resistors?
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Personally, I think that the sensitivity of LOMC cartridges to load impedance is often overstated. Also, I'd be a bit uncomfortable with pots due to their poor resistance tolerance and channel matching, so what I'd probably do is make a temporary adjustable load device with pots and use it to experiment with a range of load impedances. Then I'd either replace it with a fixed resistor, or a set of switchable resistors, or just abandon it altogether depending on the results of the tests with the pots.
 
rothwellaudio said:
Personally, I think that the sensitivity of LOMC cartridges to load impedance is often overstated. Also, I'd be a bit uncomfortable with pots due to their poor resistance tolerance and channel matching, so what I'd probably do is make a temporary adjustable load device with pots and use it to experiment with a range of load impedances. Then I'd either replace it with a fixed resistor, or a set of switchable resistors, or just abandon it altogether depending on the results of the tests with the pots.
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Regarding resistance tolerance I thought maybe using a dual gang potentiometer (separate terminals, same knob) would help this, but in reality it is two pots with one knob and it isn't going to help. in fact with this approach if there was a problem with differences in resistance i wouldn't be able to adjust each channel independently.

I think my best option if I want an experimental device would be to use a number of DIP switches and multiple resistors to allow selection of resistance within 50 ohms. I'll check each channel using my true RMS multimeter.

I might even just buy two RCA splitters and make some load plugs as warrjon suggested and i've seen as the most common solution. I've yet to use the Mani for MC and I might not be satisfied. I've compared it to a few other MM stages and prefer it over all others (Rega Fono, U-turn, a couple others i don't remember the names of at the moment). By the way I've had no hum issues.

Thank you all for sharing your knowledge and experience.
 
Why not just get a proper MC phono stage with the loading you need? or a SUT to go with your Mani?


Audiofreak71
 
ryanoceros said:
I might even just buy two RCA splitters and make some load plugs as warrjon suggested and i've seen as the most common solution.
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Agreed ^^^^^^^^^^. I first did this when I was running a Shure M91ED that needed additional capacitance to perform well but I also used it for MC loading. Simple and cheap. I looked at pots for my Wyn Palmer preamps but if there is one per channel there could be a mismatch. Sure wish I could find the ones Parasound uses for their JC3+. I then looked at a rotary switch and that was better but might be limiting or a hassle to solder in new resistors.
 
AvFan said:
Agreed ^^^^^^^^^^. I first did this when I was running a Shure M91ED that needed additional capacitance to perform well but I also used it for MC loading. Simple and cheap. I looked at pots for my Wyn Palmer preamps but if there is one per channel there could be a mismatch. Sure wish I could find the ones Parasound uses for their JC3+. I then looked at a rotary switch and that was better but might be limiting or a hassle to solder in new resistors.
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By using that solution is the noise level low enough for listening?


Audiofreak71
 
Audiofreak71 said:
Why not just get a proper MC phono stage with the loading you need? or a SUT to go with your Mani?
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What exactly do you mean by proper MC stage?

The Mani supports MC but only offers 47 ohm and 47k ohm loading. The MC cartridges I am considering have recommended loadings of 100 ohms+. I've considered trying a new MC preamp but It makes more sense to try the one I already have, especially considering I've preferred it over several other MM stages that were recommended to me. Ideally I'd like to try several on my system and buy one, but that is not a possibility.
 
ryanoceros said:
What exactly do you mean by proper MC stage?

The Mani supports MC but only offers 47 ohm and 47k ohm loading. The MC cartridges I am considering have recommended loadings of 100 ohms+. I've considered trying a new MC preamp but It makes more sense to try the one I already have, especially considering I've preferred it over several other MM stages that were recommended to me. Ideally I'd like to try several on my system and buy one, but that is not a possibility.
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I’m meaning a MC phono stage with the loading settings you need for the cart/s your using rather than the 47k (which will do in a pinch but won’t give the full potential of your cart) . You could also go the SUT route, headlamp route etc that you can hook up to your existing MM preamp. The route your taking could very well work as well. Have you checked into Graham Slee’s try before you buy program?, it might be worth looking into.


Audiofreak71
 
I've just looked at the specs for the AT33Sa and its coil resistance is only 10 ohms with 22uH inductance. There'll be a difference in volume between a 47 ohm load and a 100 ohm load, but it will be only 0.8dB, so not very much at all. As for a difference in tonal balance, I doubt there'll be much difference. A couple of Y cables which will allow you to plug in different resistance loads will soon tell you if its worth making anything more elaborate.
 
rothwellaudio said:
I've just looked at the specs for the AT33Sa and its coil resistance is only 10 ohms with 22uH inductance. There'll be a difference in volume between a 47 ohm load and a 100 ohm load, but it will be only 0.8dB, so not very much at all. As for a difference in tonal balance, I doubt there'll be much difference. A couple of Y cables which will allow you to plug in different resistance loads will soon tell you if its worth making anything more elaborate.
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I too think that MC cartridge loading is a solution looking for a problem. Try the cartridge with the mani at 47k and see how it sounds. If it is too bright then you can add parallel resistance.

The need for MC cart loading historically is due to MC cart ultrasonic resonance overloading poorly designed phono preamps.

Here’s some simple circuit math to help make my point.

The inductive reactance of a 22 uH inductance at 20 kHz is

Xl=2*pi*f*l
Xl=2*3.14*2e4*22e-6
Xl=2.76 Ohms

So the difference between 47k and 47 ohms at 20 kHz will be small, and even smaller at frequencies you can hear. Yet many folks find cartridge loading makes a big difference so something else is going on, and that something else is ultrasonic overload.

Anyway, have fun experimenting and enjoy your new cart.
 
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