Resistive Loading SUT or Nah?

[watchnerd]

After re-trying my VM540ML for a while and disliking what was happening in the highs (not the cart's fault -- the default input pF of my amp is 320 pf, plus cables), I'm going back to MC for a while.

Mounted the ART9XA. Relevant specs for this discussion:

AT-ART9XA

Although conventional non-magnetic cores struggle with output voltage, the AT-ART9XA features a newly designed coil armature with an improved output voltage of 0.2 mV (1 kHz, 5 cm/sec). The dual m ...

www.audio-technica.com

Recommended Load Impedance	Min. 100 ohms (when head amplifier connected)

Coil Impedance	12 ohms (1 kHz)
DC Resistance	12 ohms

Output Voltage	0.2 mV (1 kHz, 5 cm/sec.)

The .2 mV has always been a bit below the .3 mV MC input spec for my Luxman L-590AXII, so I decided to dust off the old Hashimoto HM-7 SUT. The HM-7 can be used at low 15:1 / 23 dB gain or high 30:1 / 30 dB gain.

Napkin math for low gain tells me:

Output Voltage: 0.2mV x 15 = 3.0mV

[this is fine, equal to my MP-500, and above 2.5mV input rating of amp]

Reflected load (at 47k MM input): 47,000 ohm / (15^2) = 209 ohms

[SWAG using the "10x rule of thumb" suggests this should be okay, given it is 17x coil impedance and >100 ohms minimum suggested loading]

So we're at a decent starting position in terms of cartridge loading and output voltage.

I could, however, use plug resistors to tweak this, and drop the reflected resistance down a little. For example:

2 x 62k ohm resistor plugs in parallel -> 47k ohm MM input load -> 26.4k ohm = 117.3 ohm reflected to cart


Is there much mileage in doing so?

Or would we expect a ~92 ohm change in cartridge loading to have almost impact?

 

[JP]

An LOMC cart is practically just DCR so the resistive loading will only affect signal level as in a practical sense you're just dealing with a resistor divider. Loading may affect the performance of the SUT.

 

[watchnerd]

An LOMC cart is practically just DCR so the resistive loading will only affect signal level as in a practical sense you're just dealing with a resistor divider. Loading may affect the performance of the SUT.

I thought MC resistance loading impacts the generator damping and resonant peak / Q factor of the cartridge, as well?

 

[JP]

I thought MC resistance loading impacts the generator damping and resonant peak / Q factor of the cartridge, as well?

Electrically, no, as that requires the coil to have some meaningful amount of inductance and capacitance. That is the case with MM.

There's an argument of "dynamic braking" of the cantilever due to Lenz's law but that's a tough one for many to swallow due to how inefficient the generator is - there's a lot of mechanical force at the far end of the stick. The original case and the one reproduced case that I know of are Swiss cheese, and I've done experimentation here where no meaningful change could be found, as have others.

 

[watchnerd]

Electrically, no, as that requires the coil to have some meaningful amount of inductance and capacitance. That is the case with MM.

There's an argument of "dynamic braking" of the cantilever due to Lenz's law but that's a tough one for many to swallow due to how inefficient the generator is - there's a lot of mechanical force at the far end of the stick. The original case and the one reproduced case that I know of are Swiss cheese, and I've done experimentation here where no meaningful change could be found, as have others.

Yes, it's obvious with MM and easy to hear.

But I've read "audio wisdom" that said things like this (that Rothwell is also questioning) with regard to MCs:

"Does the cartridge's tonal balance change with load impedance? It certainly does if the cartridge is a moving magnet type, but low output moving coil cartridges are much less sensitive to changes in the load impedance. Users sometimes claim that higher load impedances produce a brighter sound than lower ones, but cartridge manufacturers tend be non-specific about recommended load impedances, often recommending a wide range or simply anything above a minimum impedance.
The recommendation of Rothwell Audio Products is in line with Ortofon, Audio Technica and most other cartridge manufacturers - that 100 ohms is a good value for most cartridges, and that the exact value is not critical as long as it is well above the cartridge's source impedance."

mc step-up transformers explained

Maybe this is just urban legend confusing MM vs MC, or massively overstating the mechanical braking case you mention.

Sounds like I shouldn't bother with making resistors plugs and whatnot.

 

[JP]

First MC Cartridge

Use the 10 Hertz compliance number - 16 - to see the resonance for the Traveler. Looks like both are in the green! i also use the korf graphs ... http://korfaudio.com/calculator i don't know if it's better or not, but Korf have objective arguments about it, worth reading.

www.audiosciencereview.com

 

[Balle Clorin]

That a look at my plots here , English legend should be understandable with some imagination, RS is source resistance.

if the source impedence is not optimal -like 40 ohm fit the 40ohm SUT tap- the SUT may ring and need a RC load across the secondary- like Rothwell explain.

I also found that using the 3 ohm tap on a 10ohm source caused an early high end roll off at47k load…. And so on

Step-up for dummies

Det der ser ikke riktig ut i det hele.... Det ligger to små DIP releer mellom SUT-ene, som nok skal står for å switche mellom MM og MC... det ser ut som bryteren er moddet til å gi ground lift fra chassis i stedet.... har du kjøpt denne brukt?? EDIT: Til venstre for den ene trafoen hvor den...

www.hifisentralen.no

 

[watchnerd]

That a look at my plots here , English legend should be understandable with some imagination, RS is source resistance.

if the source impedence is not optimal -like 40 ohm fit the 40ohm SUT tap- the SUT may ring and need a RC load across the secondary- like Rothwell explain.

Step-up for dummies

Det der ser ikke riktig ut i det hele.... Det ligger to små DIP releer mellom SUT-ene, som nok skal står for å switche mellom MM og MC... det ser ut som bryteren er moddet til å gi ground lift fra chassis i stedet.... har du kjøpt denne brukt?? EDIT: Til venstre for den ene trafoen hvor den...

www.hifisentralen.no

Is there a way to estimate if the source impedance is optimal or likely to produce SUT ringing?

 

[Balle Clorin]

The best is to stay close to the tap nominal ohm tap

I do not know how to calculate the optimal, l spent a weekend experimenting and measuring. A function generator with an added 1800 ohm series resistance and a 10 ohm in parallel; This gives about 10 ohm series resistance: 1/10 +1/1800=1/RS. (This also reduces the voltage so the SUT can take it. ) Then connect the SUT with an Oscilloscope with 10x probe In parallel with the RIAA. Look for a square wave without too much ringing.

I tried to match results in LTspice, not perfect but can be used to see sensitivities.
'

Or you could just run your MC with n without SUT and plolt the difference( I A I / I B I in REW and see what you get , use the ortofon or some other 50khz record

Simulate 3 (Blue)-6-10-12-20 40 source resistance for a given SUT

Spoiler: some more stuff

some unreliable REW sweeps..of Ortofon T-5 SUT. REW and USB DAC give wrong loading of sut and it rolls off compared to proper load ,see upper right

With proper termination via my RIAA (Pufin line in) the sweep is more correct

Experimentering in LTspice

 

[watchnerd]

The best is to stay close to the tap nominal ohm tap

I do not know how to calculate the optimal, l spent a weekend experimenting and measuring. A function generator with an added 1800 ohm series resistance and a 10 ohm in parallel; This gives about 10 ohm series resistance: 1/10 +1/1800=1/RS. (This also reduces the voltage so the SUT can take it. ) Then connect the SUT with an Oscilloscope with 10x probe In parallel with the RIAA. Look for a square wave without too much ringing.

I tried to match results in LTspice, not perfect but can be used to see sensitivities.
'

Or you could just run your MC with n without SUT and plolt the difference( I A I / I B I in REW and see what you get , use the ortofon or some other 50khz record


View attachment 452552

Simulate 3 (Blue)-6-10-12-20 40 source resistance for a given SUT
View attachment 452554

Spoiler: some more stuff

some unreliable REW sweeps..of Ortofon T-5 SUT. REW and USB DAC give wrong loading of sut and it rolls off compared to proper load ,see upper right
View attachment 452548

With proper termination via my RIAA (Pufin line in) the sweep is more correct
View attachment 452551


Experimentering in LTspice
View attachment 452549

My SUT doesn't have a nominal ohm tap, unfortunately.

Hmmm. Maybe I'll just start, like you said, with some resistors so I don't fry the SUT from the signal generator.

 

[watchnerd]

First MC Cartridge

Use the 10 Hertz compliance number - 16 - to see the resonance for the Traveler. Looks like both are in the green! i also use the korf graphs ... http://korfaudio.com/calculator i don't know if it's better or not, but Korf have objective arguments about it, worth reading.

www.audiosciencereview.com

Well, I see what you mean

 

[egellings]

I thought MC resistance loading impacts the generator damping and resonant peak / Q factor of the cartridge, as well?

MM carts can have several millihenries of coil inductance due to the large number of turns in the coils, whereas the MC cart has relatively few turns, and inductance is instead measured in microhenries. That smaller inductance is much less affected by loading, compared to the much larger MM amount of inductance.

 

[lini]

MM carts can have several millihenries of coil inductance (...)

More like several hundred mH, I'd say.

Greetings from Munich!

Manfred / lini

 

[SIY]

Is there a way to estimate if the source impedance is optimal or likely to produce SUT ringing?

External stepups with no optimization for source and load (impractical) or a Zobel on the secondary (almost never done) will almost always have either overshoot or droop.

 

[watchnerd]

External stepups with no optimization for source and load (impractical) or a Zobel on the secondary (almost never done) will almost always have either overshoot or droop.

In some ways SUTs are a PITA

 

[SIY]

In some ways SUTs are a PITA

They are very particular, but optimization isn't too hard if you have basic test equipment (square wave generator and oscilloscope. But yeah, chances of getting it right at random are near zero.

That said, if you set things up properly, the high common mode noise rejection and (nearly) noise-free gain of a good transformer are massive advantages.

 

[watchnerd]

They are very particular, but optimization isn't too hard if you have basic test equipment (square wave generator and oscilloscope. But yeah, chances of getting it right at random are near zero.

That said, if you set things up properly, the high common mode noise rejection and (nearly) noise-free gain of a good transformer are massive advantages.

I do have an o scope and function generator

 

[dlaloum]

After re-trying my VM540ML for a while and disliking what was happening in the highs (not the cart's fault -- the default input pF of my amp is 320 pf, plus cables), I'm going back to MC for a while.

Mounted the ART9XA. Relevant specs for this discussion:

AT-ART9XA

Although conventional non-magnetic cores struggle with output voltage, the AT-ART9XA features a newly designed coil armature with an improved output voltage of 0.2 mV (1 kHz, 5 cm/sec). The dual m ...

www.audio-technica.com

Recommended Load Impedance	Min. 100 ohms (when head amplifier connected)

Coil Impedance	12 ohms (1 kHz)
DC Resistance	12 ohms

Output Voltage	0.2 mV (1 kHz, 5 cm/sec.)

The .2 mV has always been a bit below the .3 mV MC input spec for my Luxman L-590AXII, so I decided to dust off the old Hashimoto HM-7 SUT. The HM-7 can be used at low 15:1 / 23 dB gain or high 30:1 / 30 dB gain.

Napkin math for low gain tells me:

Output Voltage: 0.2mV x 15 = 3.0mV

[this is fine, equal to my MP-500, and above 2.5mV input rating of amp]

Reflected load (at 47k MM input): 47,000 ohm / (15^2) = 209 ohms

[SWAG using the "10x rule of thumb" suggests this should be okay, given it is 17x coil impedance and >100 ohms minimum suggested loading]

So we're at a decent starting position in terms of cartridge loading and output voltage.

I could, however, use plug resistors to tweak this, and drop the reflected resistance down a little. For example:

2 x 62k ohm resistor plugs in parallel -> 47k ohm MM input load -> 26.4k ohm = 117.3 ohm reflected to cart


Is there much mileage in doing so?

Or would we expect a ~92 ohm change in cartridge loading to have almost impact?

To get the best from a high inductance cartridge (most MM's) you need to have adjustable C & R loading, and low C cabling is a must have.

Adjusting R loading can be done only DOWN from the built in load and adjusting C loading can only be done UP from the built in load.

Some time back I switched the TT cabling to drop its C load down to around 60pf (using BlueJeans low C cables) - and I have a Phono stage, with no additional C loading - I can then add load by adding cables, or plug adapters, etc...
The Phono stage was customised with a 100kOhm base R load - so using plugs I could then adjust it, and I soldered myself a collection of R and C plugs which allowed me to adjust from around 60pf to 700pf C load and from 27kOhm to 80kOhm.

AT MM designs have always done best at low to very low C loads so you need to start with something under 200pf (preferably circa 100pf) - then you can tweak with the R loading.

MC cartridges for the most part, have such low inductance, that they are effectively load insensitive - you cannot tune them via the load, what you get is what you get. (although in this day and age, an ADC, with some EQ filtering is not difficult to set up if needed)

 

[SIY]

I do have an o scope and function generator

Morgan Jones did some nice measurements and tuning demonstration that was on Sowter's website and (if memory serves) in the 3rd edition of Valve Amplifiers. I described my version in my article on the His Master's Noise preamp.
Links:

Sowter Type 8055

His Master's Noise: A Thoroughly Modern Tube Phono Preamp

After more than 25 years of faithful service, it seemed that it might be time to redo my phono system. After all, I like to think that I've picked up a few tricks in the intervening years... The old system consisted of a VPI HW17-II, a Linn Ittok LVII tonearm, and a Troika cartridge. The Troika...

www.diyaudio.com

 

[Balle Clorin]

External stepups with no optimization for source and load (impractical) or a Zobel on the secondary (almost never done) will almost always have either overshoot or droop.

Yes, but if I keep the source resistance close to the nominal tap values (3 and 40 ohm) in my Denon AU-320 it is very flat.
Using 10 ohm source on 40 ohm tap gives an overshoot…
The bump below 100hz seems to be a measuring artifact when using the oscilloscope, not there when I use other methods

With a Zobel I can correct the overshoot, I cannot really hear any difference…For maximum flatness and extension I need to allow a small overshoot on my Oscilloscope..