Phono stages, SUTs, and cartridge loading question for those with experience...
- Thread starter Bobvin
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Hi Erik, I'm finishing up setting up the AMG Turbo... the house was full of Christmas decorations waiting to be repackaged for another year, meaning my turntable work areas were not available. Fortunately my Benz fits into the AMG alignment jig, and I'll have my analog buddy stop by and verify with his protractor.
Wrt XLR from the turntable, yes, I had the original AMG reference cable done up in XLR. (Another of the problems of using an SUT, most are single ended inputs, requiring a change of cable.)
Wrt XLR from the turntable, yes, I had the original AMG reference cable done up in XLR. (Another of the problems of using an SUT, most are single ended inputs, requiring a change of cable.)
And a good thought it is! Considering the Yps cost, and added cost of SUT and cables, if I could find a nice price on the Etsuro it would be tempting. The Benz is a nice cartridge indeed, and represents a real value compared to the lofty prices of the top cartridges these days.
There is a pretty comprehensive thread on loading on this site. But here is cartridge loading in a nutshell:
First we'll start with phono sections which do not need an SUT, IOW have enough gain on their own:
The loading resistor is primarily for the benefit of the phono section, not the cartridge (although decreasing the load resistor value will decrease the compliance of the cantilever). The cartridge has an inductance and the tone arm cable (and to a certain extent, also the input of the phono section) has a capacitance. They are in parallel. Anytime you find an inductance and a capacitance in parallel you have a tuned circuit, also known as a 'tank circuit' (as it stores AC energy) which resonates at a certain frequency. BTW an inductance in parallel with a capacitance is the means used to tune a radio to a station.
Since the inductance of a moving coil is usually slight and capacitance of the cable is also low (or should be) the resonant frequency is rather high and can be anywhere from several 100KHz to several MHz. This resonance is a peak and can be as much as 30dB! When the peak is at resonance, the term for this is 'excitation'. The energy of the cartridge itself is enough to drive the tank circuit into excitation.
Essentially as far as the phono preamp is concerned this energy is RFI (Radio Frequency Interference) and is injected directly into the input of the phono section! It can overload the input of the phono section and this can result in a tick or a pop. So to avoid this problem the input of the phono section should be designed with plenty of headroom.
The loading resistor suggested is in parallel with the tank circuit. It acts to detune the tank circuit and prevent resonance. Since a cartridge manufacturer has no idea of how competent the phono section to which the cartridge is connected will be, they often recommend a low loading value to allow that phono section to operate correctly. Its well-known that RFI causes audio electronics to not sound right (often brighter)!
But if the phono section is RFI immune and has good overload margins, the loading resistor simply isn't needed. This allows the cartridge greater compliance and can result in better tracking performance, depending to some extent on the tone arm. Right about here things get a bit complex, but as a general rule of thumb most cartridges are designed to drive 47Kohms, despite the 100 or 500 ohm loading recommendation.
Now with SUTs:
Any transformer does exactly what is says it does- it transforms impedance. This has to be both ways. Now since loading a low output moving coil does nothing for the cartridge itself, it follows that the loading is for something else when a transformer is involved and that something is the transformer.
So as a result you load its output. A transformer inherently limits bandwidth, so you don't have to worry so much about that pesky RFI that is there if the transformer isn't. But now you have to worry about the transformer ringing (which all inductors do; and if you're wondering why LOMC cartridges don't, actually they do but not until you're well outside of the audio band) which is a kind way of saying that is distorts. To avoid that you have to load it at what is called 'critical damping'. Depending on the transformer and the cartridge, this loading value might be the 47K input impedance of the preamp.
But one of the best SUTs out there is made by Jensen, and they are not for a specific cartridge, and since each cartridge has a different impedance, the correctly loading at the output of the transformer is different too. Fortunately Jensen has these loading values available as a pdf which lists the loading for various cartridges and they've tested most high end LOMC cartridges.
One problem with all of this is the tone arm cable, which has a capacitance and is part of the source impedance driving the input side of the transformer. Since each cable has a different capacitance, this is a variable that usually isn't addressed unless the loading of the SUT is done by someone who is hand-picking the needed loading values using test equipment.
The advantage of SUTs is they tend to be low noise. The disadvantage is they limit bandwidth and add a bit of distortion of their own. Nothing comes for free in this world! For these reasons I personally prefer a preamp that has enough gain and does not otherwise have a need for loading. Then its just plug and play, one thing that normally isn't easy with LPs.
A further note- nearly all cartridges are balanced sources. To this end, if you can receive the phono signal in the balanced domain, one serious advantage is that the tone arm cable will be more neutral/more transparent. If there is any place in the system that maximum transparency is important, getting the phono signal right so that it hits the input of the phono preamp intact is probably it. You can't correct for lost information downstream. That is why our preamps have always had a balanced phono connection and for many years were the only preamps that had such a connection.
Now you don't need to be an engineer to get analog right. That is the job of people like me who design this stuff. Unfortunately many designers think that a good phono section only needs correct EQ and enough gain. But that ignores the fact that the cartridge is an inductor and all that comes with that.
So here are a few tips! The best phono preamps that are direct-in (No SUT) will not need a loading resistor to sound right. A further benefit will be that you hear less ticks and pops (see above). IMO about 80-90% of all ticks and pops are caused by the phono section, not the LP itself . Check with the phono preamp manufacturer- do they they require loading? If no, you're good to go. Or you can use an SUT, and deal with the bit of unpredictability that comes with them (also see above) and get good results that way.
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Hi, I am quite keen to hear your impressions of the AMG Turbo. I have the AMG Viella ans have been contemplating the merits of upgrading. Thanks in advance...
Sujay, not quite ready yet for prime time... have had a few things get in the way of dialing in the tonearm, but I hope to have a few cycles to spare this weekend for another go. I'll post when I have things dialed-in. I need to get things right, as I am hoping to acquire a new cartridge soon and will be using this as my learning curve.
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