[Housteau]

Most cartridges MM and MC have a recommended resistance and capacitance to be inline with them before the phono preamp, usually in parallel.  How does the R value alter the sound as opposed to the C value.  In other words, what should one experience as the two values are adjusted up and down away from their recommended values with regards to a MM cartridge? 

[baldrick]

People will know more about this than I do, but the load resistor and load capacitor do two (or more) things.
The first is that they form an R-C filter, and will affect frequency response.
Also they will damp the output of the cartridge so that you do not get wild peaks or dips in frequency output of the cartridge. A perfectly loaded cartridge will have a smooth frequency response from 20Hz up to 20KHz or 30KHz.
Typically, improperly loaded cartridges will exhibit a large ultrasonic peak (or more than one) which may cause your amplification system downstream to go into oscillation or to drive it into clipping.  Either is very bad.
One thing that I have experienced over the years is that many cartridge manufacturers that specify a 47K ohm loading actually have it wrong and the correct loading is much lower - like 20 K ohm or less, even. 
The best thing I can say is to try adjusting the loading of the cartridge and then listen.  Trust your ears.
Cartridge loading is a funny business.  I have an Audio Technical OC-9 ML that is happiest at a super low 20 ohm loading, while my Grado cartridge likes a 10 K ohm load best.

[Housteau]

My reason for posting is that I am trying to understand and accept what I am experiencing.  I am using an old Cotter phono preamp that when used with a MC cartridge requires no loading.  However, for a MM one is supposed to place 52.3K resistors in parallel with the tone arm leads to create a total of 47K. 

A fellow I know that used my exact same LP front end never did that and almost always had extremely good sound.  He always had the capacitance as recommended though.  In my room I am experiencing the same thing.  I have the C correct, but have left out the 52.3K resistors.  When I add them in to achieve the 'ideal' 47K, the sound becomes dull and without everything that makes it glorious otherwise.

What troubles me is that the natural input of the preamp must be close to 500K to have a 52.3K R in parallel with it to bring it to 47K.

[baldrick]

Measure the input resistance of the preamp and you'll know for sure what the internal loading of the preamp is.
I find it hard to believe that it requires no loading with a MC cartridge since many MCs typically need 100 ohms of loading.

[Housteau]

What they mean is that with a MC you do not need to add any external, or extra loading.  It has internal loading as fixed values.  I just found some literature that states an internal load of 40 pf and 500K.

[baldrick]

My wager is that the internal loading of 500K is way too high for a typical MC.(I'd still measure the input resistance with an ohm-meter to verify it.)
Then add load resistors in parallel to get to what the proper loading should be for your cartridge.

[Housteau]

My wager is that the internal loading of 500K is way too high for a typical MC.

That is exactly my point. 

Then add load resistors in parallel to get to what the proper loading should be for your cartridge.

As I mentioned, when that is done most of the great qualities of my LP front end evaporate.

I did measure the input and it is not 500K.  It is 1K which makes less sense.  I have a second phono preamp here that I can set to 1K and when I do that it sounds like there is a great mismatch with something definately wrong.  When I set that one to 47K it sounds normal, but not nearly as good as the Cotter at its measured 1K input.  When I add the loading resistors to the Cotter its quality drops down below the other one. 

This second preamp is acting in a manner that makes sense to me.  There must be something else going on within the Cotter that allows it to perform as it does?

[baldrick]

Open it up and find out what's inside.  You should be able to spot the input loading resistors right away. 
I read the owners manual for the Cotter PSC-2 (assuming that this is what you might have) and it explains the internal loading of 500K.  It is to match the MC step up transformers' secondaries.  The MC step up transformers' primaries are usually of a lower impedance that will match the MC cartridge more closely.
The question now is what setup are you using?  MM or MC?

[blakep]

Open it up and find out what's inside.  You should be able to spot the input loading resistors right away. 
I read the owners manual for the Cotter PSC-2 (assuming that this is what you might have) and it explains the internal loading of 500K.  It is to match the MC step up transformers' secondaries.  The MC step up transformers' primaries are usually of a lower impedance that will match the MC cartridge more closely.
The question now is what setup are you using?  MM or MC?

Yes, the cartridge(s) being used is very important and I can't see any mention of it in this thread or the OP's system profile page.

Bear in mind that many vinyl users utilize loading with MC's or capacitance with MM's to "tailor" the sound. In other words, there are many factors at play here, not the least of which might be system synergy or room setup and its effect on sound quality, a listener's preference for a brighter, livelier sound vs. somthing more laid back, etc. etc.

Also bear in mind that each time you change the loading on a MC cartridge you are changing to some extent (small or large depending on how large the loading change is) its output and its relationship, for lack of a better word, with the phono preamp gain. Proper matchups with gain, particularly with low output MC's are generally speaking going to give you much better sound quality so this is a very large contributing factor to sound quality as well. 

[Housteau]

I had missed the MC in your statemet "My wager is that the internal loading of 500K is way too high for a typical MC.", and so my response was about a MM.  All of my observations using the Cotter PSC-2 are about MMs as that is what I am using.

This is a low gain design, but even though the volume on my line level preamp needs to be a good bit higher, the stage remains fairly quiet.

[neobop]

Most cartridges MM and MC have a recommended resistance and capacitance to be inline with them before the phono preamp, usually in parallel.  How does the R value alter the sound as opposed to the C value.  In other words, what should one experience as the two values are adjusted up and down away from their recommended values with regards to a MM cartridge?

With a MM cart, as you lower the resistance, the high end rolls off. If a cart is too bright, lowering this R value tones it down. The opposite is also true. If a cart is too mellow, raising the resistance will make it brighter.

Excessive capacitance will lower the high frequency resonance peak. It depends on the HF response and the inductance of the cart. Some carts recommend a certain amount of capacitance to do just that, and brighten up the treble. IMO, it's better to increase R rather than use C. Sometimes C can be used to roll off the extreme high end. It really depends on the cart. In general, keep C as low as possible for best performance.

You can't measure all phono stage's impedance by measuring DC resistance. It depends on the circuit topology. 

[Housteau]

You can't measure all phono stage's impedance by measuring DC resistance. It depends on the circuit topology.

That would explain the strange reading that I got for it.  The former owner bought it new and it was never modified, and so I will assume the literature is correct at 500K.  That is a big step up from 47K and I would have thought it would be far too bright.  It certainly is open and clean, but I think it sounds really good and not over the top.  I have been playing around with the old and venerable Sure Type 15, and a perfect match for a vintage preamp it seems.

[baldrick]

Here's the rub...
The Shure V15 has an internal resistance of 720 ohms and an internal inductance of 630 mH.

With a typical capacitance loading of 250 pF (including tonearm and interconnect cable) the proper loading for good balance would be about 28 K ohms.  Oddly, Shure recommends 47k ohms like 99% of MM cartridges manufacturers do.  Shure also recommends a pretty high capacitance loading of 400 pF for this older cartridge.

The newer V15VXMR has an internal resistance of 1000 ohms and an internal inductance of 425 mH.  Assuming the same capacitance loading of 250 pF (Shure recommends 250pF in this case) the proper loading would be 20.3 K ohm.  (Shure recommends 47k ohm).

If your Cotter is loaded at 500 K ohm then either cartridge would sound pretty hot and bright (not in a good way) in the top end and could drive your preamp into clipping during loud passages.I'd experiment with lower loading values and evaluate the sound.

Get the loading in the ballpark, listen and trust your ears.

[Housteau]

If your Cotter is loaded at 500 K ohm then either cartridge would sound pretty hot and bright (not in a good way) in the top end and could drive your preamp into clipping during loud passages.I'd experiment with lower loading values and evaluate the sound.

Exactly.  It should sound horrible, but it doesn't.  However, it does sound pretty lifeless with the correct R loading.  These units were press fit together.  There are no screws for an easy disassembly.  I prefer not to break into it.  I feel it would be like waking someone up to give them a sleeping pill.  It is working great and I don't want to take a chance on damaging it.  I was aware of the specs for my cartridge and what is recommended.  That is why I was confused on why I am getting what I consider pretty decent sound when the numbers say I shouldn't.  However, I do notic a positive improvement with the correct pF loading. 

[neobop]

Hmm.... this doesn't make a whole lot of sense. 52K in parallel with 500K would give you approx 47K. I doubt if the cart is being loaded, where it sounds good, at 500K. So I believe your pre is special order, but with a Cotter piece, you never know. Whatever you do, don't try to open it. Cotter used to fill them with expanding foam, so his circuitry wouldn't be stolen.

High inductance carts (> 400 or 500 mH) generally need capacitance as low as possible and more than normal resistance. The overly mellow Shure M97 benefits from loading around 60K. But electrical loading models for phono carts are somewhat useless because the mechanical aspect is not taken into account. Here's a lengthy explanation:
http://www.vinylengine.com/phpBB2/viewtopic.php?t=6674&highlight=cartridge+loading+explained

[Housteau]

It may very well be a special unit.  That is the only explanation that makes sense to me as well.  It helps me knowing that this was the way this unit has always been used by the previous owner, who has always been known for the good sound he achieves no matter what gear he uses.  He has the knack for getting the very most out of what he uses.

I have a second preamp here to experiment with.  It is a Pro-ject tube stage where I can select loading.  As a make the selections they are directly measureable on the input jacks.  I can also set that unit where it reads zero.  From there I can start fresh adding in my own external resistors with a Y-cable and see if I can match what the Cotter is doing internally.

[baldrick]

Here's why your Shure sounds terrible at 47K loading:
The inductance of the Shure V15 is 630 mH and it has an internal resistance of 720 ohms.

Here's a spice model of the Shure V15 loaded down to 47K and 250 pF capacitance.  Notice the significant HF rolloff beginning at 10KHz:



Now look at it with 100K loading and 250pF capacitance: 

Still not right.  You get a small peak around 12 KHz followed by a strong rolloff above that, but still within the audible range.

Now lower the loading to 80K and lower the capacitance to 100 pF (about the lowest you can possibly go with good interconnects and good tonearm wiring):




Much better.   Now you're only down about 2dB at 22KHz. 
Of course this is an ideal model.  Chances are that your Shure may not be as responsive in the high end in which case adjusting the loading higher may brighten the top end somewhat.

[Housteau]

Thank you for those graphs.  I like it when things start to come together and make sense.  I will be going out of town for a while, but when I get back I will do some real world charting of my own with different loading values and some pink noise to see what the response is in my room.

[exojam]

You may find this looong thread intersting reading.

http://www.vinylengine.com/phpBB2/viewtopic.php?t=6674&highlight=catridge+loading

James

[neobop]

You may find this looong thread intersting reading.

http://www.vinylengine.com/phpBB2/viewtopic.php?t=6674&highlight=catridge+loading

James

Hi James,
Yes, that's the same thread I linked to previously. If you go to page 5 of the thread, Werner (reviewer for TNT) provides graphs of actual measurements, not models of a Shure M97. You can see the effects of loading - capacitance and resistance. It's a very long thread. But it's very enlightening.