SUT's and Capacitors

Ratcatcher

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A friend recently bought an artemis phono stage and was asking me about capacitor loading which is mentioned in the manual. As im only just getting my head round impedance matching/loading with SUT's the capacitor thing had me stumped. I remember my old Naim Superline used to have plugs for both impedance and capacitance but tbh, i didnt really understand it.

So, whats the theory behind it? and how do you work out the correct size capacitors to use? and which capacitors work best?

 

Ratcatcher

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Not sure what you mean there. The artemis manual refers to capacitance loading for mc carts, and exactly like resistors they're used across the secondary of the SUT.

 

SergeAuckland

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MM cartridges have a high DC resistance as there's lots of wire in their coils and all tose turns also gives the cartridge a high inductance. That makes the cartridge pretty sensitive to the external load. Resistance-wise, the load on a phono cartridge has been standardised at 47kohms for very many years, although possibly some cartridges might benefit from a different load. Nevertheless, 47k it is. As far as capacitance goes, theoretically, the lower the better, however, some cartridges assume a certain amount of external capacitance, and this should be noted in their manual, so you need to match the capacitative load the manufacturer recommends. As this capacitance is the sum of the capacitance in the arm wiring and phono input, a phono ampwith switchable capacitance is very useful, and allows the frequency response of the cartridge to be set, either to what the manufacturer recommends, or to what the user prefers.

Low outputMC cartridges have very small coils, made of only a few turns of wire so have low DC resistance and inductance. That makes them pretty immune to external loading. Provided the external resistive load is something in excess of 10x the cartidge's opwn DC resistance, then there is very little change that takes place in the frequency response or output voltage with changing load. Indeed, there's a school of thought that prefers to run even Low Output MC cartridges into 47kohms. I do this for my TSD15. Reducing the impedance below some 10x the DC resistance of the cartridge will reduce the cartridge's output voltage and possibly (but unlikely) to increase distortion. There's also a school of thought that prefers to run a low output MC cartridge into a short-circuit (usually a virtual earth) and use the cartridge's current output rather than voltage output. I have no experience of this, but unless someone has a good explanation of the benefits, it seems to me being different for the sake of it.

As to capacitative loading with low output MCs, the inductance is so low that it would take huge amounts of external capacitance to have much of an effect. If one wants to change the cartridge's frequency response, and adjustable RIAA equaliser seems to me the better way, in other words, a tone control on the phono stage.

S.

 

Ratcatcher

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Thanks Serge, glad someone took the time to answer.

I think the SUT in the Artemis is a Sowter 1:10 so presumably that would be geared more towards high output MC's, hence the mention of capacitance loading in the manual. How would you go about deciding what value of capacitor to add? is there a relationship between the primary/secondary (as in impedance matching) that needs to calculated when adding caps?

What sort/brand of cap would you recommend?

 

SergeAuckland

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Not necessarily. A 10:1 transformer is aimed more at a low output MC Of something like 300uV to perhaps 500uV.

It will also present an impedance of around 470 ohms from a 47k input as the 10:1 transformation gives a100:1 impedance transformation. This pretty typical for MC cartridges. Some, like classic Ortofons need more than 10:1 as do Audionote cartridges, but later Ortofons and many others are happy with 10:1(20dB) of step-up. Many MM stages give around 40dB (100x) gain so an additional 20dB is about right.

S

Edit: As to capacitors, I wouldn't add any to a MC cartridge as they're not sensitive to capacitance.

 
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