Step Up Transformer Question


Ok, so bear with me as this is new terrain for me. I was quite happy with how things were sounding and then I accidentally bumped the stylus of my Cadenza Black and snapped the cantilever clean off. I did some research and ultimately decided to have Steve at VAS at fix my cartridge but it was going to take a few weeks and there was going to be no way to play the Christmas vinyl that my wife loves so much. She told me to buy another cartridge so we could have Christmas records. I was stunned. I found a great deal on a Winfeld Ti on this site and pulled the trigger. I've got it on my table and it sounds amazing. 

After speaking with Steve though, I'm going to trade in my Cadenza and have him build me a wood body cartridge to have something different. It will also be 0.2mv output. I think I've got the 2nd arm situation sorted (gonna buy a Wand and a pod to mount it on) which leaves the phono pre. I love my Rogers PA-2 and it is switchable between MC and MM with 2 different inputs in the back. Finally got the SUT part. I've been looking and trying to figure things out. I spoke with Roger and he said the transformer built into the PA-2 is 1:10 and with that I can achieve 70 or so dB of gain. I did a lot of reading and looking and while I'm sure a 4k+ SUT is amazing that's just not where I'm at with all the other expenses at the moment. I saw the Bellari MT502 was a stereophole recommended component for many years and Amazon had one on sale for like $349 or something. Figured can't hurt to try it, it's Amazon so I can always return it. I plugged it in with some admittedly questionable interconnects I had lying around and when I put the phono stage k there was a bit more hum than I'm used to but I said let's have a listen. Holy shit. This little thing blew my mind. Better impact, tighter bass, more space around instruments so better staging, improved transients. This puts me in an interesting place. 

This is an extremely inexpensive piece and it has changed my system quite a bit. I think the little extra gain I get even at 1:12 brings the cartridge to life. My question is where do I go from here? Bob's makes the sky 30 which is switchable from 1:15 to 1:30 which I like because I can play with the gain tubes in my preamp and potentially go to a quieter 12au7 than the 12ax7 that's currently in there. Will that be a real improvement? Less hum? Had also considered ordering a Rothwell from jolly old England but can't find much on them. Thoughts? I'd like to keep this sub 1k or so for now. Maybe I'll save up for a big boy SUT later. 

rmdmoore

You could do worse than ask Steve Leung whether his VAS transformer would suit your cartridges. I have the MkI version (there is now a MkII) and I have four low output cartridges feeding it through a switch (LP-S, Ruby 3 mono conversion, Ortofon Kontrapunkt C and a Soundsmith Sussurro MkII ES - the last is a low output MI cartridge). I'm very happy with it, even if I don't begin to understand how to calculate whether it is the "right" SUT for any of them.

Good point on asking Steve Leung, there @Dogberry. I also would not underestimate the N Clayton builds- I needed something not spendy for my vintage system based on old Quads and was using a VAS Franken-Denon cartridge Steve put together. Clayton may be taking a few days off for the holidays- but offers blue Cinemags, as well as vintage transformers. There is also a guy on SHF who custom builds some old stock Altec Peerless- he used to work at Altec and had a stash.

I remember first looking at SUTs years ago when I was still running an ARC SP-10 --it had gain, but the preamp was notorious for microphony. At the time, I was trying to find the elusive Expressive Technologies unit- no go.

I use an Allnic phono with built in SUTs on my main system- readily adjustable with the turn of a knob. I like that feature-there are differences in sound based on the settings. My dumbed down approach is to look at the internal impedance of the cartridge and multiply by 10 for the load seen by the cartridge. The secondary (or output) from the SUT will be 47k.

On the cables, I do use the high end spread in the main system, but on the vintage system, use Analysis Plus -- well made, cost effective, and suitable for a system of that period. 

Good hunting.

 

What has always boggled my mind is that SUT manufacturers so rarely publish the turns ratio(s) of their products. Which would make it very simple indeed to at least begin to match a SUT to a particular cartridge. Instead, and especially in the golden era of vinyl, most SUTs were given a value in ohms (see Elliot’s compendium above). Typically, the value in ohms is meant to indicate the recommended internal impedance of the cartridge to which you want to match the SUT. For example a "3 ohm" SUT is meant to be used with cartridges having an internal impedance close to 3 ohms (probably 2 to 5 ohms cartridges would work). It also means that the turns ratio of that cartridge is such that when mated to that group of cartridges will result in an acceptable ratio between the internal R of the cartridge and the input impedance that will be seen by the cartridge when using that SUT hooked up to the standard MM stage with a 47K ohm input impedance.

The turns ratio is equal to the voltage step-up ratio. A SUT with a turns ratio of 1:10 will increase the output signal V of the cartridge by 10-fold. Etc. The resulting impedance seen by the cartridge will be equal to the input Z of the phono stage (47K ohms) divided by the square of the turns ratio (100, in this case). So the cartridge sees 470 ohms (47,000/100). Since you want the ratio of the internal R of the cartridge to be about 1/10 of the input impedance of the phono, you can see that any cartridge with an internal R of less than 47 can drive a 1:10 SUT. Nothing explodes if you break the rule of thumb. The actual loss of gain and high frequency response that occurs when the match gives you a ratio less than 1:10 is on a continuum, losses get gradually worse as the ratio approaches 1:1. At a ratio of 1:1, 50% of cartridge output goes to ground instead of driving the phono stage, and there is audible loss of HF response, due to input capacitance.

Back to our "3 ohm" SUT. A theoretical cartridge with an internal R of 3 ohms can drive a net 30 ohm load before you cross the line of the 1/10 rule of thumb. In order to create a 30 ohm load, you just have to work the formula for the load seen by the cartridge backward. I calculate 1:40. A cartridge with a 3 ohm internal R can drive a SUT with a step-ratio (voltage gain) of 1:40 into a 47K load, if you want to rigidly obey the 1/10 rule of thumb.

By the way also, an ideal SUT or any transformer does not have an impedance per se. Transformers merely reflect the impedances from secondaries to primary windings, or vice-versa. I wondered that Elliot mentioned there are SUTs that let you choose both voltage gain and impedance. That can only be done if the SUT has built in resistors typically across the secondary, which would be in parallel with the standard 47K load.

Whart, you wrote, "My dumbed down approach is to look at the internal impedance of the cartridge and multiply by 10 for the load seen by the cartridge. The secondary (or output) from the SUT will be 47k."  In your first sentence, I suppose you mean that you multiply by 10 to determine the (minimum) load that should be seen by the cartridge.  I agree. In your second sentence, you may be confusing others.  The secondary is connected to a 47K standard load at the input of an MM stage. The SUT acts on that impedance to reduce it as seen by the cartridge, in proportion to the square of the turns ratio of the cartridge.  I am guessing you know this but there is a semantic disconnect in what you wrote. I point it out only so others will not be confused.

4 lines from the bottom of my post I wrote “in proportion to the square of the turns ratio of the cartridge “. I of course meant “in proportion to the square of the turns ratio of the SUT”.