measuring cartridge inductance

eweitzman · « **on:** 9 Jun 2006, 07:23 am »

Is it possible to measure the inductance of an MC cartridge using a DMM without frying the cartridge? I'd like plug the actual inductance of my cartridge into the calculators in Jim's cartridge loading web page.

- Eric

hagtech · « **Reply #1 on:** 9 Jun 2006, 09:56 pm »

Depends on the DMM. Perhaps a low powered (professional) LCR bridge is the way to go. Besides, the inductance of an MC isn't usually an issue, being relatively low. Any value under 1kohm loading will likely eliminate any resonances.

I would use the loading resistance to tune for sonics instead of maximal bandwidth. This can only be done by ear. Personally, I like to err on the high side. I am not into the dark and mellow sound.

On the other hand, perhaps cartridges are not so fragile. I finally took a FryKleaner to one. I drove the burn-in signal straight into the cartridge. Figured it would act like a motor (opposite of generator) or sonic transducer. Well shoot, I couldn't hear a thing. I even cranked it up to line levels. A full 2V signal on the MM cart. I still couldn't hear a peep. Didn't blow the coil either. Hard to say why. Maybe the inductance was enough to limit input current. That said, I never plugged the cart in to see if it sounded the same. Just thought it was cool that I couldn't fry the coils.

jh

eweitzman · « **Reply #2 on:** 9 Jun 2006, 10:21 pm »

Using the last calculator in your spreadsheet for bandwidth, a change in inductance from 5mH to 1mH moves the 3db point from 3.2k to 15.9k with a 100 ohm load. I thought it would be nice to know if the 100 ohm load is a problem with my cart. The manufacturer recommends 100 or *less*. I can't go up to 500 ohms without a half hour of work cracking the thing open, removing knobs and interior covers. Plus I haven't a clue about reasonable or typical values for cartridge inductance.

I'm trying very hard to find the manual for my DMM to see what it's output level is. It's around here somewhere

I'm a little wary of frying a sensitive expensive little cart in the name of science.

- Eric

eweitzman · « **Reply #3 on:** 12 Jun 2006, 07:42 am »

I measured the inductance and impedance of my Shelter 901 cartridge: L=22uH, R=12 ohm. Also measured the tonearm wire and interconnect C=100pF.

Next I modeled it in CircuitMaker. The circuit shows the cartridge and cable three times, but with three different preamp loads (10, 100, and 1k ohms). It looks like this cartridge really likes the typical 100 ohm load (yellow graph) that most MC preamps are preconfigured with. Bandwidth (f3) is about 700kHz and there is absolutely no HF peaking.

See Jim's page at http://www.hagtech.com/loading.html if you want to get the skinny on cartridge loading. Thanks, Jim!

- Eric

GBB · « **Reply #4 on:** 12 Jun 2006, 11:40 am »

OK - any good theories on how a step up transformer changes the optimal loading. I'm asking because I've found that the loading does not change the way I'd expect. My phono stage has enough gain that I can put an MC directly into it. When I run it this way, I find that 100 ohms sounds best.
Using an S&B TX-103 (burned in with my Frykleaner Pro - see there is a Hagtech connection) wired with a 1:5 ratio I would have thought that the optimal loading was 25x 100 ohms or 2500 ohms. I would have been wrong. Instead, things sounded best with the 47K stock input impedance of the phono stage paralleled with 240K. And the sound was extremely sensitive to small changes in loading resistance. Paralleling 47K with 150K sounded a bit over damped. 47K alone sounded a tad bright.
Any ideas?

Thanks,
---Gary

hagtech · « **Reply #5 on:** 12 Jun 2006, 07:52 pm »

Things change a lot once you add in a step-up. Remember, it's not just the resistance that changes. It's the loading capacitance too! That 100pF on the phonostage gets multiplied by 25. Basically, now you have to worry about capacitance too. That's 2.5nF sitting on the cart.

So use very short output cables from the step-up! Now the loading issue becomes much more like the MM case. You are no longer damping LR, but now LCR.

This is why the Trumpet's StepUp has very short 12" output cables. And they thought I did it for aesthetics...

jh

GBB · « **Reply #6 on:** 12 Jun 2006, 08:53 pm »

Jim,
I would think that it gets stepped down by a factor of 25 - just like the resistance does. So 100pf would go down to 4pf. Why should capacitance act differently?
Thanks,
---Gary

hagtech · « **Reply #7 on:** 13 Jun 2006, 05:09 am »

Impedance goes down - meaning capacitance goes up.

jh

GBB · « **Reply #8 on:** 13 Jun 2006, 11:22 am »

Of course - must have been dozing off when I asked that question.

Now I've got to play with the wiring. My first test of the TX-103s was just hard wired with very short leads (3 inches) but those leads were twisted pairs. I didn't think the capacitance would be too high, probably less than 10pf but multiplying that by 25 could have interesting consequences.

One more thing to play with!

By the way, I was shocked at how much of an improvement I heard going to the transformers. Even though I had enough gain for the MC with my phono stage (a highly tweaked old NYAL Super It), it sounded a bit dark. Inserting the TX-103s made everything open up and brought much better soundstaging and dynamics.

---Gary