Phono stages: variable loading v load rejecting designs

[Berty bass]

I currently have a NVA Phono 2 (with dual PSUs), which I've been very happily using for a few years now. Being technically inept though I do find myself occasionally wondering about the difference between variable loading and load rejecting (like the Phone 2) phono stage designs - what the difference is, whether one is inherently 'better' (whatever that means!) than the other and why designers might reject one design in favour of the other. It seems there are far more of the former on the market than there are the latter, so I assume it is an easier brief to deliver on? Does that make it inferior though? Anyone with the technical know-how and time to spare care to enlighten me? I ask out of total ignorance, you understand, so please don't infer any mischief on my part.

[Geoff.R.G]

You might try a search using your thread title, I got these results.
[url]https://www.vinylengine.com › turntable_forum › viewtopic.php?t=6674[/url]
[url]https://www.monoandstereo.com › 2015 › 02 › phono-stage-phono-loading.html[/url]
[url]https://www.audio-talk.co.uk › phpBB3 › viewtopic.php?t=8107[/url]
As examples, I haven't opened any of them but they should give you some guidance.

[Berty bass]

Lots about it in the archive, here. Search 'load rejecting'. Or start here: viewtopic.php?f=3&t=45558&p=70253&hilit ... ing#p70253

The NVA does not need variable loading settings because it has bags of reserve power, due to it's massive power supply; it is a proper load rejecting design.

Audiophile anxiety about loading values is caused by two things:

1. Inferior phono stages that require loading settings to handle non-standard cartridges.
2. Bullshit on (other) forums.

[Vinyl-ant]

Its not really to do with power, it is to do with the fact that the input creates a tuned circuit when a cart is connected. The bit of the cart that is connected is a coil which has inductance and resistance, (and some low pf capacitance) the input has a load resistor between the signal and ground, 100r ish for mc 47k ish for mm, and capacitance either on the input itself or on the front of the op amp, transistor or valve.

Mm carts have more turns in the coil to get more output voltage so have a higher L, mc have fewer turns so lower L, so the lcr ratio between the cart coil L, input r and c alters more with the change of input r in the case of an mc cart, so its easier to alter its frequency response by changing the input r

The amplifying device sees an lcr circuit infront of it. This combination creates a filter that will affect the f/r of the circuit and subtly alter that quoted f/r curve of the cart.you will see generally, but not limited to, a rising response in the case of an mc cart between 15 and 40k. Mm doesnt care much because the 47k is usually a large enough value that the differences in the L of the cart only change the tuning in a very small way. Mc because the load is only 100r will see a bigger difference as the L of the cart changes, and conversely changing the input r wil have a bigger impact on the f/r when the cart stays the same.

On a technical basis, there is a perfectly valid argument for variable loading, all the change is, is an alteration of the load resistor value which is easily done with an in line phono plug with a resistor in it. This resistor is in parallel with the one in the phono stage, so wether a stage has variable load is irrelevant. Its just convenience because it can be done to any phono stage.

Wether it makes any difference to change the load is entirely subjective, the usual rising mc response can be flattened out by altering the load or made more pronounced. Thing is, there are no rule of thumb loads other than the pretty standard 100r mc and 47k mm, and you need to know the input capacitance of the stage and the L of the cart to work it out properly which arent always quoted or available

Personally i dont bother because i think there are other more important things to be looking at, and the lack of info about coil inductance and total input capacitance makes trying to get an optimal load resistor a stab in the dark, and the plot for the cart doesnt generally tell you what load they ran the cart into when it was recorded

[Vinyl-ant]

This, was Richard's view:

"Now well designed solid state active phono stages (very few are well designed). You can build in massive amounts or power and headroom relative to the jobs being done. the door is pushed and swung around very easily and the speed it can change and react to amplitude and speed changes is instantanious - reason is that that large headroom means there is no need to perfectly match, the door is just bashed open and closed by power. This is referred to as load rejecting because matching is no longer a large consideration, it just becomes a very minor musical balance preference. BUT we still have all the forum bullshit about cartridge matching that only applies to SUTs, even some active phono stage makers have pandered to it by giving load adjustments switches for the customer, a waste of time and the switch contacts themselves damage the music at that low signal level."

[Vinyl-ant]

This, was Richard's view:

"matching is no longer a large consideration, it just becomes a very minor musical balance preference. BUT we still have all the forum bullshit about cartridge matching that only applies to SUTs, even some active phono stage makers have pandered to it by giving load adjustments switches for the customer, a waste of time and the switch contacts themselves damage the music at that low signal level."

This is the bit that i find relevant, because matching is subtle enough to be a balance change. Headroom in my view is much more important. The amount of headroom a phono stage actually needs is in my view vastly underestimated. The current demands from damage transients from clicks and pops are huge relative to the actual music transients, the power supply needs to be much more capable in terms of peak current than people think. I agree with richard on that one.
Not sure about the switch contacts effect being particularly detrimental, unless said switch is particularly crap, but the point stands that there are bigger fish to fry before going to the nth degree of optimising the load.
The power richard was talking about is not load rejecting, it is just not load optimising. and in my view that headroom is more important.
An optimised load will not make up for a weedy power supply

[Berty bass]

Fascinating - thank you for the responses so far. Really interesting and enlightening.

[TheRealAleman]

An optimised load will not make up for a weedy power supply

That is ultimately what put me off of buying into the Naim "club"

Talking to Trevor about the SuperNait 3 ... "It's a great little amp, but really needs a power supply" The same applied to the Naim phono stages.