moving coil loading

ok, moving coil cartridges are said to be less susceptible to capacitive loading than mm's.

But the coil inductance will still resonate with the lead and input cap., which the loading resistor damps.

so we should vary the load resistor to get a flat response.

Not much is said about the lead or input cap tho', surely to get the correct value resistor, we need the correct value cap. as well, so how do we work this out. We should know our cartridge inductance, so we can calculate damping resistor by square root of (L divided by C)

And why do mc stages still have input caps, surely they could just use lead capacitance?

And what's all this about using resistors to do mechanical damping, too? bit confused....

 

This is due to moving coils having less coils inside the cartridge. The coil is conected to the cantilever and it moves inside a magnetic field. With this low inductance a wideband width can be had. The capacitance would have to be very large to effect into the audio range.

Yes they do. Depending on the inductance and the capacitance, you may get sharp resonance peak. The peak can usually be outside the audio range but the rise could start in the audio range. To damp this peak, you need to alter the load resistance. For mm, this is set to 47K but if this is lowered then the resonance peak is damped.

Yes. worth a try.

A lot of phono stages have low input capacitance so most of the capacitance come from the cables from the cartridge to the phono stage.

If you can reduce the capacitance, the more the better but as mentioned the having a low inductance helps. The calculations do take into account the cable capacitance.



This is the basic equation used. You need to know the inducatance of the cartridge and the capciatance of the cable and input. The is the basic model.



Rc is the cartridge resistance Lc is inductance of the cartridge, Ra is the load resistance & Ca is the Capacitance of the load input and cables.

To recap, low inductance and low capacitance will give you a wide frequency range. With a moving coil, you may get a sharp resonance peak. This can be damped by changing the loading resistor. Some coils are not fussy about loading. This may be due to any resonance peak being well above the audio range so the effect of changing the load resistor is not seen in the audio range.

The load resistors are to change electrical damping not mechanical. There is a whole set of different factors that affect these.

I hope this helps.

SCIDB

 

I've no idea what the coil inductance of a moving coil is but it must be somewhere between naff all and sod all ~ a few turns of coil = microhenries [ballpark]

work out the res freq if you know L.
What is L reactance at res freq? Presumably much lower than the series resistance recommended [res of coil,leads and headamp/transformer/gizmo, all in series] so resonance is damped for practical input resistance values. If you lowered input res to zero with say a virtual earth input you would probably knacker the response anyway as you electrically damp the mechanical movement [except, as you said at resonance where it would bounce happily along..]

edit:

Damn, I can't compete with that one Dean..I must learn to type faster!

 

hmmm, strictly speaking(it may be neglible, ) but one would have to add the cable inducance and dc resistance to get a complete model.

still, we must know the capacitance if we are to get the right damping resistor, seems like some of us MAY be using wrongly set input stages, but as Dean says, it may not be inside the audio band, even so, there is phase shift to take into account, which may be inside the audio band.

the intricacies of audio eh? I suspect the makers now don't bother much and treat us like idiots, which largely, we are, a lot more to it than meets the eye...

I have drawn a more complete model below. There may even be capacitance in the coil section, and series inducance and resistance in the preamp input section, perhaps so low it can be safely ignored.






The coil inductance and lead/preamp input inductance add up, the coil resistance and lead resistance add, and the lead capacitance and loading capacitance add.

Thus we get a 2nd order filter, rather like the hi cut section of a speaker bass unit filter in a xover, (which like all filters will

1/ have a cutoff frequency (-3db down, mc gives this very hi up, as the ind. and cap. are hopefully low)
2/ a slope, 2nd order rolloff slope, and
3/ an ELECTRICAL resonant(ringing) frequency. like Dean has said above.(to give the mechanical analogy, to have resonance(ringing) we need mass, and compliance (the lo freq resonance of a turntable is mechanical and is given by the mass of the arm/cart. and the spring of the cartridge rubber, this needs damping by various techinques, too)

To damp this, we need a load resistor equal to square root of the (total series inducance divided by the total parallel capacitance(which adds))

but we MUST know our values!!!

This is the thoery behind cartridge loading, with mc transformers, its more complex.

Someone may be able to add more, as despite what looks clever above, I am not totally comfortable with electrical models, and find it conceptually difficult.

With an mm, i am as sure, the inducance(coil) is bigger, more turns, means more wire, means more dc resistance, too.

this gives a lower frequency hi cut, and a bigger resonance, which needs to be damped via capacitance I am told. they usually work into a 47kish preamp resistor, and put out around 10 times the voltage of an mc. so vary caps on mm, and resistor on mc

but we still need all the caps and inducatance values for a totally accurate loaded stage.

I hope this is understandable and enlightening

 

Hi Ian,

The resistance in the lead will be very small and so would the inductance as a rule. The cable capacitance would be much larger than the input capacitance as a rule. Hence my simpler model. It willl have an effect, very small one.

To get your values, you need to measure them in your own setup. For a ball park figure, use the supplied figures. A lot of arm cables have a capactance of under 200pF. Moving Coil cartridges have inductances of less than 5mH, a number are a lot less.

Put the value into the equation and you will get a figure. This figure may not be that low.

A mistake I did make in the last post was about mechanicall damping. Some input resistances are lowered to aid mechanical damping.
As mentioned before a low inductance cartridge is going to have a wide bandwidth. The capacitances you get with cables isn't going to alter thing that much.

Try subbing in figures into the basic equations.

A the end of the day, you do need to suck and see on this one. There are many other factors that will influence sound quality. mechanical damping and personal preference are just two. The basic equation will give a good ball park to aim at. But for critical use, you will need to do by trial and error like most things in hifi.

SCIDB

 

A bit strong. Why do you think that? A lot of the data is there when you buy a cartridge. Not everyone has the skills and understanding to follow to the nth degree. The basic model and or fine tuning is good enough to get you going.

That is correct. There is a magnet in a field of coils. The magnet is conected to the cantilever and this moves, hence the name.

That is more or less correct. If you lower the capacitance then you will damp the resonance peak. 47K is an industry standard for mm cartridges. MM give much more output than MC cartridges as a rule.

Yes but there are many other variables so you do need to suck and see. Ideally you need to measure the values or rely on manufacturers values.


SCIDB

 

the top point was refenence to the fact that with mm coils having different inductance values, there can not possibly be one size fits all for a loading cap, and with vinyl LARGELY a minority compared to cd and the heyday back in the 70s/80s, we do get pretty paltry options on phonostages, as was commented in hifi world that phonostages can be just add ons as opposed to outright stand alones.

trouble is with lower inductance mm coils, the required cap value into 47 k may be too small, so you have to lower the 47 k resistor.

as the generator inductance is fixed, we have the 2 things to play with given by:

(amp input R load) squared = cart induc. divided by loading cap.

[ or R= sqrt (L/C) ]

I think its time I learned to use a scope to do freq. responses, looking at a transformer model(my step up isn't sounding totally satisfactory in the top end, bit silvery), it gets much more complex with values, and they don't give all the needed data, like you say, it all may be fairly negligible and we should use the makers info( which is rather a general guideline, depends how fastidious you are.)

all good fun....

do you play much Dean with loading by ear or do it mathematically or use recommended values?

What do you do about the low end and mechanical tuning?

ps I am playing with transformers a little bit, too,.

I recall you saying you used the audio innovations step ups, I have tried these, not very good in the bass, can be significantly bettered, but nice ambient mid/top.

Have you tried any others?

 

LCD,

You know which Phono stage I'm using . I have just switched carts and am now trying out a brand new Goldring Eroica LX MC cart, in place of my Shure xMR. The Goldring instructions state: "Cartridge load should be 100 Ohms and the total capacitance in parallel with this, including arm lead and amplifier capacitance should not exceed 500pF"

I can't say that I've ever really noticed this before, usually i just see the 100 Ohm spec for MC's.

Initial impressions where that the Eroica LX was certainly louder than my previous two carts used on this amp, glad I've got that extra gain . I'll freely admit that I wasn't blown away though, perhaps I'd read too much for too long and was expecting too much ? I've only played two sides of AC/DC hIghway to hell so far though . I've also still got the AD743 Op Amps fitted and not tried the 797's as yet, perhaps that will improve things a little, (that is why i spent £16 buying the little buggers after all). Any thoughts ? or is the fact that it sounds 'basically' OK enough and not to worry about capacitence and stuff ( which i probably couldn't even measure anyway)...

 

No but moving coils have very low inductances. the capactances is mainly set by the cable. So you are changing one variable, the loading resistance.

A lot of phono sections in the past didn't have much adjustment. Some offered different RIAA equalisation curves. Many phono stages in recent times do have some adjustment. Only a very few will have the full all singing all dancing adjustment.

If you know L and know C then R can be calculated. If you change R you are changing only one variable. You can change many things but you are best to change one at a time.

If you what to go fully objective you will to measure everything if the data is not available. But a lot of data is there.

Can be.

Mainly by ear. After all you have to listen to when playing records. I have messed around with setting to see which i prefer.

I do look at the compliance values and masses of arm and cartridges. Again I play by ear. I also use test records any adjustment is done by ear.

The step ups i use are inside the P2. The overall sound is governed by you input. The quality of the turntable, arm and cartridge. I'm getting very bass quality. This is due to the front end.

I hope to try some different transformers. There are a few companies make transformers that will make for a good project.

I have tried in the past Audio Innovations, Audionote, EAR and the Blackhead. I haven't tried these at the same time. All these were capable of showing changes in the front end.

SCIDB

 

Thanks Dean, intereseting stuff.

Sid, if you want to play, I have found this for the goldring, dunno if its right....

https://shop.mantra-audio.co.uk/acatalog/Online_Catalogue_Goldring_7.html

ok coil inuctance is 12 micro henries which is pretty low. It says load cap 200-1000pf.

your cap on the rotel is 100 pico farads, add this to your arm lead and you get your input capacitance. Say 200 pf max?

now you can calculate the resonant frequency of the cap and coil.

From Dean's equation, its MILES out of the audio band, so far not even to worry about load capacitance one iota.

From the equation for the loading resistor, r squared equals coil inducance divide by load cap, I get a resistor value of around 240 ohms, but that's based on my guess of the lead capacitance.

AS the cap value is not really relevant for moving coil, you could remove the loading cap, and you would just have the lead capacitance, and I reckon you could use any load resistor from 250-500 ohms.

I heard the goldring is slightly soft sounding, but nice, so probably not an upgrade from the shure

Try an audiotechnica oc9, dynavector dv20 or my tip, denon 304 for a real mc upgrade.

I am not sure the 797 will make much of a difference, as its isolated from the cartridge by that transistor section, if your cart was seeing the opamps, you would likely see a difference.

as an experiment, try getting a cheapish moving coild transformer off ebay, and plug it into the mm section, then compare with the rotels mc section, you may be surprised.

The rotel can be bettered quite easily sadly, nice design, if you want an upgrade, try a trichord dino and one of the carts above.

Or there is a dynavector p75 on ebay, not mine, and its one of the very very best, should only go for around £300, jumpers a tad fiddly tho.

cant' recommend highly enough, everyone, buy these for your vinyl, they are SO GOOD. better than just about anything

 

LCD,

I've already bought and fitted the Eroica LX.

It arrived on Saturday ( from Mantra). When i first listened to it i wasn't too impressed , however in saying that the Shure V15VxMR and the Denon DL103 that preceded it where both very good.

I've fitted the 797 Op Amps and thought that i did notice a small difference between it and the 743's previously fitted , psyco- accoustics could be playing a part here though.

The first thing that struck me when i listened to the Eroica LX was that the background 'hum' type noise that i first noticed with the Shure had gone, the Eroica has an output of 0.5mV which I'm sure helps.

I'm really trying at the moment to just listen to my music and not do too much Hi-Fi 'analysis' . The Goldring maybe a little soft in the bass, especially compared to the shure which has awsome bass , however the treble detail is excellent and I'm quite enjoying my tunes at the momnent , even Oasis sounds good, I've got everybody's favorite Coldplay LP 'X & Y' on at the moment and am happy with the sound it sounds quite 'zippy'

Thanks for your efforts , i'll try to decipher the maths later on if i can be arsed to get the circuit diagram out.....