SUT - electrical theory and practical experience

@intactaudio 

One "gotcha" many people run into is dynamically overloading the input of the phono.  

With the current trend of higher output cartridges like the Atlas being paired with the typical 1:20+ SUT intended for LOMC's I see this type dynamic overload happening al lot.  

This in my view is more important than loading. most vintage transformers have too much gain for todays phono stages. - especially solid state phono stages where high frequency overload margins can be much lower.

During the covid lockdowns I pulled out all my transformers and ran a series of tests with several cartridges ranging from 3-40 ohms internal impedance with each of the transformers.

The testing confirmed for me -

• Most important is getting the gain structure right for the cartridge and phono. This has more impact on the sound quality than the load the cartridge sees ( assuming the load is reasonable ).
• Each of the transformers had sound characteristics that were easily discernible regardless of what the cartridge internal impedance was.
• The actual load that the cartridge sees is less important than the gain structure and interface between the transformer and phono.
• I do not like any additional loading ( primary or secondary ) on the transformer.
• Cable has a massive impact on sound quality and characteristics.

On a couple of the transformers I was able to reconfigure for a range of gain structures - in each of these instances I preferred the lowest gain setting required to get the phono operating at about 1/3 volume setting. Phono used was a tube phono with 42db gain.

One oddity was that I preferred the cheaper Altec 4722 to the higher spec Altec 4629 on a Denon 103, but this was the one exception - with my other 40 ohm cartridges the 4629's were better.

As an aside one of several SUT's I have is a pair of Altec/Peerless 4629's ( same as 4665 ). Have you ever tested these comparing the Altec 4629/4665 transformers loaded at 47k vs 70/80k as designed ?

The other question I have is the input capacitance of tube phonos - most of which will have 150-200pf - have you found whether this adversely affects the transformers ?

Lastly fyi with a pair of top of the line Jensens which have been designed to be used with a recommended zobel network I have experimented with scoping the load ( using the cartridge resistance as a load ) and tweaking the zobel network for each cartridge - this was beneficial - the improvements were quite audible,

Overall the current sensing step up ( a custom bespoke design from a cartridge manufacturer/designer ) that I have ( into the same 42db tube phono ) outperforms all of the transformers in bass extension, midrange information and transparency regardless of internal impedance - my current sensing unit works with 3 - 40 ohm cartridges no problems, but I have experienced variable results with some of the commercially available units such as the ESE Labs Nibiru and BMC.

One interesting trait is that with the SUT's I can spend hours dialling in the VTA, and still be unsure, whereas if I plug the current sensing unit in VTA is a doddle, the optimum is clearly audible - and then if I plug the SUT back in voila. The transparency of the current sensing unit used in conjunction with a tube phono I have is thus far unassailable - including some other highly regarded phono's such as Klyne System 7/Jadis  etc that I have owned.

@pindac 

You do realise that everything can be demagnetised , including the phono, preamp stages and power amps - all with subtle gains.

If you are going down that road, to the nth degree, - have you checked all your electronics for polarity ( relative to the incoming mains ) in order to minimise DC leakage. Just to make it really interesting, here in New Zealand each alternate power pole has reversed polarity - but the problem is that if there is a power outage or adverse event, the polarity can reverse. So the tests must be done after each power outage or indeed if you move house.

I don't have any detailed knowledge on the current sensing unit. I cant tell you anything about the circuit.

For some years in my system the active high gain phono stage came with a 100 ohm input resistor and through all those years I had not any single trouble ( that I been aware of. ) with several and different LOMC cartridges that I owned and own.

If you never tried any load other than 100 ohm, then you cannot know if there was a problem due to loading at 100 ohm .

My point about conjugating the imaginary (reactive) impedance of the cart is to cancel out any inductive or capacitive reactance so the cart sees a purely resistive load which will give the flatest frequency response with no peaking at high frequencies. 

@phoenixengr 

Flattest frequency response or best square wave ? Are these codependent ?

With Jensen SUT's that are designed to be used with a zobel network  they suggest  adjusting the network to the source load by scoping a square wave.

@holmz 

this one -

• whether they generated a square wave with the same output impedance as the cart and fed the SUT with that

Dear @dover  : Yes I did it during the " thousands " of tests with the prototypes of the phonolinepreamp and through measures and listening tests the only parameter we can been aware was changes in SPL but not in FR but additional to that normally LOMC cartridges come with the " rigth " internal resistance to work very good looking those 100 ohms. Clearaudio is something espcial with those 50 Ohms.

There are plenty of MC's up to 40 ohms ( eg Benz, Dynavector, Sony to name a few ) for which 100 ohms loading in my view is killing the cat.

If your phono linear were stable you would be able to load at any value.

It would seem that any observations or cartridge evaluations posted by you with this phono stage would be jaundiced by improper loading.