Howto calculate load for MC cartridge using transformer

[karsten21]

Dear Folks

I have a nice MC step up 1:10 transformer which I will use as an input transformer of my new RIAA phono stage. The general math is clear so far but I do not know on which "side" it is wise to adjust to get the right load impedance for the MC system.

Resistor parallel at primary side.
Rg // Cmiller as impedance Zsec will be transformed as Zprim = N² * Zsec.
Now calculate the parallel resistor to retrieve the load of the MC like Rin= (Zprim * Zload)/(Zprim - Zload). For different MC systems I have to calculate/Switch different Rin on primary side.

Rg // Cmiller at secondary side.
This is in general the same. Calculating necessary impedance at secondary side Zsec = Zload / N². Then calculate Rg take Cmiller in account: Rg = (Xc * Zsec) / (XC - Zsec). Now I have to choose different Rg to get the right load of the MC system.

So what is the "right" or best way here?

The next thing is to calculate the possible zobel network. No idea here! All I know is, that it may necessary to damping the "ringing" but I do not found any hint how to calculate it or how to start with an emperimental approach.
Any hint on this toppic are warm wellcome!

Thanks a lot for all your help!
Karsten

 

[pieter t]

You could work your way through this to gain some insight:
http://www.diyaudio.com/forums/anal...ding-audio-technica-oc-09-using-1-10-sut.html

 

[karsten21]

hey peter t,
Thanks a lot! I will read the whole and inside linked thread carefully! I´ve just overseen this therad..🙁

Karsten

 

[karsten21]

Pieter!
Sorry I do not write you name correctly!

I read the linked thread....
Well done! I bow down pieter!
So, using a resistor on secondary side is the right way. I complete missed the DC resistance in my calculation. I wonder if it make sens to take the reactance of the capacity Cmiller + Ccable +Cstray in account.
But I follow the recommendation to let the Ears decide.

So now open is the question about the zobel network. If it is necessary and how to determine it ??

 

[andyr]

Why not simply use a (ss) 'head amp' instead of a SUT?

With this, there will typically be a 47K default load, which you can reduce by using parallel load resistors. No stuffing around with coil ratios or winding resistances! 🙂

The killer when using a SUT is ...if your cart sounds best at 1K ohms - or even 4K7, like my Benz LP - how do you achieve this when using a SUT?

Andy

 

[karsten21]

Hello andyr,

Using a SUT as input for a tube base phono preamp seemed to me the best solution. Other solutions need special tubes or a lot of effort to get the small signal out of the mud.
I do not understand what do you mean with "a (ss) 'head amp' ". What is (ss) in this case?

Thanks to Pieter´s description it´s now easy to calculate the correct load for the MC system ( or several once using a rotating switch ). As I develop the phono stage I´m free to choose the input impedance of the first tube stage.

You might be right if you want to use an external SUT. But at least this is the same story with the drawback that you are not able to increase the load to the MC above the reflected impedance which is resutled by the fix 47K input of your MM Phono stage.
Take my transformer in account this means the maximum load impedance with a fixed 47K input impedance will be 495.5 Ohm. Using a parallel resistor provide the possebility to align the load impedance below the maximum.

My transformer
Zsec = 47K
Rdc_prim = 19 Ohm
Rdc_sec = 647 Ohm
N = 10 ( 1:10)

Calculation
Radd = (N²* Rdc_prim) + Rdc_sec
Zload = (Zsec + Radd) / N²

Best Regards
Karsten

 

[merlin el mago]

What's your target of load for your cartridge?

 

[karsten21]

Hello merlin,

The first test will be done with an simple DENON DL-103R MC system. The recommendation for Zload using a transformer is 40 Ohms. Minimum is described with 100 Ohm.
Later on I think 5 or 6 switching load impedances are necessary to get a good averange. The first planed steps are 40, 100, 150, 200, 300 and 400 Ohms.

best Regards
Karsten

 

[andyr]

Hello andyr,

Using a SUT as input for a tube based phono preamp seemed to me the best solution. Other solutions need special tubes or a lot of effort to get the small signal out of the mud.
I do not understand what do you mean with "a (ss) 'head amp' ". What is (ss) in this case?

Best Regards
Karsten

Hi Karsten,

My reading of it is that this is a 'religious' argument; folk who use a tube phono stage cannot accept the concept of dreaded ss (solid state) gear in their signal chain. So they automatically reach for a SUT ... and get caught up in the resulting problems for getting the optimal cart loading.

(Because you certainly don't want to use a tube-based head amp - far too noisy! 🙁

Note: if you have a cart whose optimal loading is, say, 100 ohms ... you have no issue; you can achieve this when using a SUT. But if you have a cart which likes, say, 800 ohms loading - or several K, like my Benz LP, then you cannot achieve this with a SUT.

But you can with a solid stage (typically JFET-based) 'head amp' by using parallel load resistors in conjunction with the default 47K load.

Thanks to Pieter´s description it´s now easy to calculate the correct load for the MC system ( or several once using a rotating switch ).

I suggest using a switch at such low signal levels will result in signal degradation.

Andy

 

[pieter t]

Hi Karsten,

My reading of it is that this is a 'religious' argument; folk who use a tube phono stage cannot accept the concept of dreaded ss (solid state) gear in their signal chain. So they automatically reach for a SUT ... and get caught up in the resulting problems for getting the optimal cart loading.

(Because you certainly don't want to use a tube-based head amp - far too noisy! 🙁

Note: if you have a cart whose optimal loading is, say, 100 ohms ... you have no issue; you can achieve this when using a SUT. But if you have a cart which likes, say, 800 ohms loading - or several K, like my Benz LP, then you cannot achieve this with a SUT.

But you can with a solid stage (typically JFET-based) 'head amp' by using parallel load resistors in conjunction with the default 47K load.



I suggest using a switch at such low signal levels will result in signal degradation.

Andy

Well that's a very narrow minded opinion, Andy.
I respect your obvious preference for solid state; numerous vinyl adepts however prefer tubes, for various reasons.
SUT's are very well up to the task; one of the advantages is that they don't introduce noise; even the best ss head amps do as they are active stages.

It is very well possible to achieve whatever loading with a SUT: just adapt the standard 47k input resistor of the preamp to the higher needed value.
This is basic stuff Andy; no need to argue.

The load resistors are parallel; a switch (rotary or dip) does not harm.

 

[karsten21]

Hi andyr,

Well, using tubes instead "sand" MAY a decision by emotion. There are a lot of very good reasons why to use high integrated modern semicondutros. But if I decide to use tubes by some reason, it make no sens to me to mix it up with semiconductors ( as long as take all part in account which "working" on the signal way. ( This discussion is a little bit off topic for my point of view )

To support cartirdges which need 800 Ohm or more load is absolute no problem as I try do describe. If you fixed to an existing system which can´t be changed like a standard 47K input impedance you are right. But if you are free to defin your input impedance ( as log as the following stage can support this ) it´s absolute no problem to support 800Ohm with an SUT.

I do not see any drawback to use a good rotation switch or DIP switch to choose different impedance loads to a MC cartridge. A "bad" switch may add 0.5 Ohms. For sure: You should not switch if you amp is on which lead to a big "plopp" noise...

Karsten

 

[pieter t]

I suggest using a switch at such low signal levels will result in signal degradation.

Andy

Works great:

 

[karsten21]

Hi Peter,
Looks very nice! Whcih type of transformer do you use here ?

 

[andyr]

Well that's a very narrow minded opinion, Andy.

No more narrow minded than people who assume you must use a SUT when you have a tube MM phono stage, IMO. 😀

SUT's are very well up to the task; one of the advantages is that they don't introduce noise; even the best ss head amps do as they are active stages.

The "best ss head amp" I am talking about (SLA-powered, JFET-based AKSA 'Paris') has a noise floor of -140dB ... as measured by some notable from Germany.

I suggest that is so low, you would certainly not be able to hear it in your system.

It is very well possible to achieve whatever loading with a SUT: just adapt the standard 47k input resistor of the preamp to the higher needed value.
This is basic stuff Andy; no need to argue.

OK, so if one is using a 1;10 SUT, you need the 47K default load resistor in the MM phono stage increased to 80K, right? (To deliver an 800ohm load.)

That is certainly feasible - but what if you want a 4K7 load? (Like my Benz LP does.) 4,700 x 100 = 470K. You cannot just go on increasing the default load resistor in front of the 1st gain stage in a phono stage without sonic implications.

Andy

 

[andyr]

Works great:

Of course it "works great"! (As a switch.) 🙄

To me, the issue is ... is the sound with a switch as good as it would have been without the switch.

Andy

 

[pieter t]

Andy, please keep in mind the subject of this thread.
Open a thread when you feel the need to promote the superiority of ss head amps over SUTs.

By the way, it seems you have a commercial motive to promote the AKSA "Paris" head amp.
No problem with that, but you should not do that here but where it belongs:
http://www.diyaudio.com/forums/aksa...amp-aka-head-amp-available-fully-made-up.html

 

[andyr]

By the way, it seems you have a commercial motive to promote the AKSA "Paris" head amp.
No problem with that, but you should not do that here but where it belongs:
http://www.diyaudio.com/forums/aksa...amp-aka-head-amp-available-fully-made-up.html

Good research, Pieter. I had thought that if I didn't name the head amp, you - or someone else reading this thread - would ask me for the name. But in future I shall keep stumm! 😀

Andy, please keep in mind the subject of this thread.
Open a thread when you feel the need to promote the superiority of ss head amps over SUTs.

Sure, I know what the title of the thread is, Pieter. My interjection was prompted by what I thought was a complex solution to a simple issue (getting the optimal load for a cartridge. 🙂

Regards,
Andy

 

[6L6]

andyr, the point of this thread is to help the OP determine the optimal loading of his cartridge, in his phonostage, whilst using a 1:10 SUT. Wether or not this is a solution you agree with, or if you think it's too complicated is immaterial.

You are welcome to offer different solutions, even encouraged to, but going on and on about it looks somewhat like threadjacking. I'm sure that's not your intention.

My interjection was prompted by what I thought was a complex solution to a simple issue (getting the optimal load for a cartridge.

/hat

Anyway, here are a few links relevant to the original question, I'm sure they may be of use -

http://www.diyaudio.com/forums/anal...r-load-resistance-mc-step-up-transformer.html

This has some calculators as well as very good information -

mh-audio.nl - Home

And if you've ever wanted to know more about noise in phono systems, SY's article about his tube MC pnonostage is a treasure trove of fantastic information.

http://www.diyaudio.com/forums/diya...oise-thoroughly-modern-tube-phono-preamp.html

 

[mkane77g]

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