Cartridge Loading- Low output M/C

Well, if the simulations are to be believed the results are quite interesting, if hardly entirely unexpected.
I'm simulating an opamp based phono stage with near perfect 20-20kHz RIAA compliance, with both active FB and passive RIAA implementations. The feedback design has the extra HF pole.
 If you load the cartridge with 47k the input rings at the resonant frequency of the input network (>4MHz) when you hit the RIAA preemphasis network with a 2KHz square wave and it lasts for 10s of us. 
When the load is reduced to 1k the ringing is damped and it ends in a few us. 
Into 100 ohms the response is well damped with a small over shoot and after 1us it tracks the input perfectly. At 400 ohms there is just a small amount of ringing.
Adding 0.1uF to the 100ohm load noticeably slows the edge of the feedback RIAA preamp output square wave compared to 85pF. Increasing the R to 400 ohms, and keeping the 85pF shows the slight ringing on the output response but doesn't have a reduced rise time, increasing the R to 47k shows significant near-oscillation at the output of the preamp.
The passive design shows none of these pathologies with the change in load R, and always has a significantly slower and essentially constant, risetime, so clearly the RIAA de-emphasis producing opamps are reacting to the HF signal.
So, perhaps the answer to the loading question is, no matter how unlikely it seems- it depends on the architecture of your phono amplifer!

Great info, Wyn; thanks! And thanks also to Catcher10 for his inputs.

So, perhaps the answer to the loading question is, no matter how unlikely it seems- it depends on the architecture of your phono amplifier!

This is exactly what Ralph (Atmasphere) has said in more than a few prior threads here, as well as in this one, that optimal loading is primarily dependent on the design of the phono stage.

BTW, the Herron phono stage I own, which as I previously mentioned I (and also some other members here) run with essentially no resistive load whatsoever (just the input resistance of a FET stage, which is nearly infinite) uses passive RIAA equalization. As do the phono stages that are built into Ralph’s preamps.

Thanks again. Best regards,
-- Al

We have arguments about this all the time as he feels so sure that his A810 with NE5532 opamps galore in the playback path is so superior to LPs and he often cites the limitations described to him by the mastering engineers.
I assume that the 42kHz cut off isn’t a single pole- and I also assume that it uses some kind of relatively benign analog filter like a butterworth or gaussian.

Actually the 42KHz is just handled by a single choke in series with the cutter head!

So, perhaps the answer to the loading question is, no matter how unlikely it seems- it depends on the architecture of your phono amplifer!

+1

I’ve always preferred passive EQ as it seems to result in a more stable preamp. I’ve maintained that a lot of ticks and pops heard in many phono sections are actually the phono section misbehaving (due to oscillation) and not actually audible ticks and pops on the LP surface; your modeling *seems* to confirm that (please correct me if I am misinterpreting your data). To that end, I’ve often recommended that one look into the matter of cartridge loading; if the preamp does not seem to need the input load and is alright with the resulting (RF) peak, then a pretty important side benefit will be less ticks and pops.

In non-opamp circuits, the presence of stopping resistors at all input nodes of the active devices in the circuit seems to play a role (many Japanese phono equalizers from the 60s-80s had no stopping resistors at all; not surprisingly they seem to exhibit more ticks and pops). So I don’t think that feedback is the destabilizing factor as I know of phono equalizers that are well behaved (no need of loading, no ticks and pops) that employ feedback EQ. I too feel that topology plays a huge role!

catcher10 (and other experts here)
In absolute terms, does a Phono pre with multiple gain/loading options only sound it's absolute best at 1 permutation of settings?
I ask because I once read a ARC Ref 2 Phono review and it was said that it sounded better at the lower gain setting. Then in my pre (an older ARC Ref) the loading involves soldering resistors onto some taps. To my ears this always sounds more veiled than without the resistors with the 5 carts of different recommended loadings that I have owned.

No, you are interpreting what I say about the simulations quite right.
It definitely seems that if you want to terminate the cartridge in a high impedance- and it hardly matters whether it's 47kohm or 1Gohm, you'd better be using a passive RIAA stage, or possibly an inverting RIAA stage- I haven't checked that out yet. 
The non-inverting active RIAA can be 50-90dB worse than the passive design in the several MHz regions where the resonances and RFI reside if the extra pole is not included,  and 20-60dB if it is included. Loading the cartridge with 100 ohms gives you most of the difference back, but the passive is still c. 20dB better.
The non-inverting active, however, has superior square wave response than the passive- due to having higher frequency correct amplitude harmonic content- under low load R conditions- but who knows if that matters.
I wonder if this correlates against the reports from various reviewers concerning their preferred load impedances.
Back to the cutter- could you possibly please tell me a few more pieces of information- like the inductance you use and the C/R of the head input?