Calculation verification please


If using a SUT with 1:40 (32dB gain) and a cartridge with an Internal impedance of 1.4Ω going into a 47,000 phono input, what parallel loading RCA resistors on the phono stage would I require to see a 375-ohm load?

sksos1

Showing 2 responses by mulveling

So yeah:

  • For a typical fixed 47Kohm MM input, a 40x SUT gives your cart a 29 ohm load. You CAN’T raise that 29 by paralleling anything. If you try to add resistance in series, you will lose almost all of your actual signal before it hits the MM input (voltage divider). As mentioned earlier, you have to either modify the phono stage to get higher than 47K input (600K = 375 * 40 * 40, as mentioned ealier) or switch to an MM stage that has non-standard loading options above 47K.
  • Rogue Ares has an MM option for 100K and 1 MEG ohm loading in MM mode. Phono stages with this kind of option are few and far between. 1 MEG would give you 625 ohms off a 40x SUT (1 million / 40 / 40). 100K is 62.5 ohms. Still, you target 375 eludes you. 
  • A 1.4 ohm cart will be FINE into 29 ohms. Like lewm, I don’t see much value in going higher. I don’t hear much difference in small gradiations of load, at least until you’ve gone way too far (low ohms) for the coils at hand.
  • Your 1.4 ohms screams "My Sonic Labs". If so, these carts also have high outputs - like 0.5mV. A 40x on this (20mV) is going to push you way towards the overload margins of most phono stages, which will likely have a very negative sonic impact - much more so than your loading concerns.

By normal convention "40/40" means 40 divided by 40, which is 1.

Good point - division is not associative. Indeed I meant ((1 MEG / 40) / 40), same as applying the div operator strictly left-to-right without groupings. 

@sksos1 could you share which cartridge you're using? 1.4 ohm coils are fairly unsual, and "usually" a good match to 40x SUT - but not if it's an MSL, which are extra unusual cartridges.

Perhaps off topic, but does anyone know how MSL gets as much as 0.5 mV from only 1.4 ohm coils? No other cartridge maker gets close to this ratio. MSL's marketing blurb goes like "magic SH-μX core", but it's hard to believe a core material could net that much more generator efficient versus iron-based cores of other manufacturers. Could they possibly be using larger-than-usual gauge coil wire, which nets more mass but lower DC ohms for a given number of turns? If so, that feels like "cheating" laugh