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Impedance Misunderstandings

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Graham Slee View Drop Down
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    Posted: 23 Apr 2008 at 9:27am
My inbox often features a query relating to the impedance matching of a moving coil cartridge to a phono preamp or head amp input.

It usually goes "my cartridge stipulates an impedance of 2 Ohms but the lowest your EXP offers is 23!"

First thing to check here is the cartridge specification. We find it doesn't "stipulate" at all. What it does tell us is that the cartridge impedance is 2 Ohms.

That doesn't mean it should be loaded by 2 Ohms!

If the cartridge impedance is 2 Ohms, that is the impedance of the coils. Think of that impedance as being the driving impedance (which it surely is).

If you loaded 2 Ohms with 2 Ohms you'd divide the signal in half.

It is the practice to make the input impedance (the load impedance the driving impedance "sees") several times higher so as not to attenuate the signal.

However, step-up transformers do their job of stepping up by impedance ratio, and to end up with a low enough driving impedance for the 47k input phono preamp, their output winding impedance needs to be about 1,000 Ohms. If the transformer needs to step-up by a factor of ten, then the input winding impedance ends up being 100 Ohms in theory.

But as usual in electronics, nothing's that simple. The transformer also steals some cartridge current to excite or magnetize its core. Therefore the input winding can end up being 50 Ohms.

Therefore you often find a cartridge manufacturer makes a moving coil cartridge compatible with the input of a step-up transformer, and will quote the lowest load impedance it will satisfactorily drive in his specification.

But look for the "more than" sign - it looks like this...  >

Unless somebody has done a typo and missed it off, the load impedance should read...
" > 50 Ohms "

And that simply means that any load impedance above 50 Ohms will be OK, even 47k!

There is one final factor however. Most MCs have a rising high frequency response (and hence a rising high frequency impedance) and the way to tame this is through loading and by choosing a load impedance that intercepts this, the rising response is tamed. The most common load impedance for this job is 100 Ohms - a sort of industry standard. However, there are the oddballs - they're usually the expensive ones, and a head amp like the Elevator EXP covers sufficient bases here.

The values above are arbitrary, and although common may not be exactly the same as specifications you see, but armed with the above knowledge you should be able to make the correct assumption.

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Analog Kid View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Analog Kid Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 2008 at 3:31pm
Very interesting.

A specific example...Dynavector states for their 17D3 moving coil cartridge:

DC resistance: 38 ohms
Recommended load resistance: >100 ohms



By "DC resistance" is that the coil impedance they mean or is it something else?
And with a cartridge like that, how will I know which loading value above 100 ohms to use?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote ServerBaboon Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 2008 at 5:40pm
Stick to MM and never have to think about it!! Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 2008 at 7:20pm
Originally posted by Analog Kid Analog Kid wrote:

Very interesting.

A specific example...Dynavector states for their 17D3 moving coil cartridge:

DC resistance: 38 ohms
Recommended load resistance: >100 ohms


By "DC resistance" is that the coil impedance they mean or is it something else?
And with a cartridge like that, how will I know which loading value above 100 ohms to use?



There's always one! LOL

I guess a couple of inches of 0.012mm dia copper wire (the coil winding) is 38 Ohms though I've not measured any lately.

Impedance (the electrical term for resistance in the AC or signal domain) will always be higher than DC resistance due to a thing called inductance.

Therefore the coil impedance will be greater than 38 Ohms (the driving, or source impedance).

100 Ohms as a suitable load sounds a little on the low side to me, but if it delivers a flat output as specified then that's all we really need to know. As I said, the industry standard is 100 Ohms and most MC phono preamps, head amps and some transformers will cater for that.
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Tony View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tony Quote  Post ReplyReply Direct Link To This Post Posted: 19 Apr 2011 at 7:29pm
Hi
 
Question:- my cartridge is a van den hul colibri which states 100 to 500 ohms load, my phono is an Audio Research PH5 and does not properly amplify the 0.28mV the colibri outputs so I bought a new elevator which I have just installed and am busy running in.
Do I set the load on the elevator and keep the PH5 at 47K or do I keep the elev at 47k and load the PH5?
 
 
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 2011 at 9:27pm
The cartridge is connected to the Elevator so I would set it on 100 ohms and leave the PH5 at 47k. It may be worth trying different load settings on the Elevator like 840 ohms.
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote errollgarner Quote  Post ReplyReply Direct Link To This Post Posted: 29 Apr 2011 at 5:30am
That is one of my concern. I have a Grado Sonata low output (0.5 mv) . It is a MM cartridge with an impedance of 47K.
I was wondering if there is a phono stage which would handle this cartridge as I plan to replace it with another grdo low output or and MC cartidge.
As my preamp has a good valve phono stage, I was wondering if a step up like the elavator wouldn't be a better choice.
Any advice welcome. Thanx.
Cheers
Philippe
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